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+The Project Gutenberg EBook of Outlines of Dairy Bacteriology, 8th edition, by 
+H. L. Russell
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
+
+Title: Outlines of Dairy Bacteriology, 8th edition
+       A Concise Manual for the Use of Students in Dairying
+
+Author: H. L. Russell
+
+Release Date: January 11, 2009 [EBook #27778]
+
+Language: English
+
+Character set encoding: ISO-8859-1
+
+*** START OF THIS PROJECT GUTENBERG EBOOK DAIRY BACTERIOLOGY, 8TH EDITION ***
+
+
+
+
+Produced by Mark C. Orton, Linda McKeown, Josephine Paolucci
+and the Online Distributed Proofreading Team at
+https://www.pgdp.net.
+
+
+
+
+
+
+
+OUTLINES
+
+OF
+
+DAIRY BACTERIOLOGY
+
+A CONCISE MANUAL FOR THE USE OF STUDENTS IN DAIRYING
+
+BY
+
+H. L. RUSSELL
+
+DEAN OF THE COLLEGE OF AGRICULTURE, UNIVERSITY OF WISCONSIN
+
+EIGHTH EDITION
+THOROUGHLY REVISED
+
+MADISON, WISCONSIN
+H. L. RUSSELL
+1907
+
+
+Copyrighted 1905
+BY
+H. L. RUSSELL
+
+
+STATE JOURNAL PRINTING COMPANY,
+Printers And Stereotypers,
+Madison, Wis.
+
+Transcriber's note:
+
+For Text: A word surrounded by a cedilla such as ~this~ signifies that
+the word is bolded in the text. A word surrounded by underscores like
+_this_ signifies the word is italics in the text. For numbers and
+equations, underscores before bracketed numbers in equations denote a
+subscript.
+
+Minor typos have been corrected.
+
+
+
+
+PREFACE.
+
+
+Knowledge in dairying, like all other technical industries, has grown
+mainly out of experience. Many facts have been learned by observation,
+but the _why_ of each is frequently shrouded in mystery.
+
+Modern dairying is attempting to build its more accurate knowledge upon
+a broader and surer foundation, and in doing this is seeking to
+ascertain the cause of well-established processes. In this, bacteriology
+is playing an important rôle. Indeed, it may be safely predicted that
+future progress in dairying will, to a large extent, depend upon
+bacteriological research. As Fleischmann, the eminent German dairy
+scientist, says: "The gradual abolition of uncertainty surrounding dairy
+manufacture is the present important duty which lies before us, and its
+solution can only be effected by bacteriology."
+
+It is therefore natural that the subject of Dairy Bacteriology has come
+to occupy an important place in the curriculum of almost every Dairy
+School. An exposition of its principles is now recognized as an integral
+part of dairy science, for modern dairy practice is rapidly adopting the
+methods that have been developed as the result of bacteriological study.
+The rapid development of the subject has necessitated a frequent
+revision of this work, and it is gratifying to the writer that the
+attempt which has been made to keep these Outlines abreast of
+bacteriological advance has been appreciated by students of dairying.
+
+While the text is prepared more especially for the practical dairy
+operator who wishes to understand the principles and reasons underlying
+his art, numerous references to original investigations have been added
+to aid the dairy investigator who wishes to work up the subject more
+thoroughly.
+
+My acknowledgments are due to the following for the loan of
+illustrations: Wisconsin Agricultural Experiment Station; Creamery
+Package Mfg. Co., Chicago, Ill.; and A. H. Reid, Philadelphia, Pa.
+
+        H. L. Russell.
+           University of Wisconsin.
+
+
+
+
+CONTENTS.
+
+
+CHAPTER I. Structure of the bacteria and conditions governing
+their development and distribution                                   1
+
+CHAPTER II. Methods of studying bacteria                            13
+
+CHAPTER III. Contamination of milk                                  19
+
+CHAPTER IV. Fermentations in milk and their treatment               62
+
+CHAPTER V. Relation of disease-bacteria to milk                     82
+
+  Diseases transmissible from animal to man
+  through diseased milk                                             84
+
+  Diseases transmissible to man through infection
+  of milk after withdrawal                                          94
+
+CHAPTER VI. Preservation of milk for commercial purposes           102
+
+CHAPTER VII. Bacteria and butter making                            134
+
+  Bacterial defects in butter                                      156
+
+CHAPTER VIII. Bacteria in cheese                                   160
+
+  Influence Of bacteria in normal cheese processes                 160
+
+  Influence of bacteria in abnormal cheese processes               182
+
+
+
+
+CHAPTER I.
+
+STRUCTURE OF THE BACTERIA AND CONDITIONS GOVERNING THEIR DEVELOPMENT AND
+DISTRIBUTION.
+
+
+Before one can gain any intelligent conception of the manner in which
+bacteria affect dairying, it is first necessary to know something of the
+life history of these organisms in general, how they live, move and
+react toward their environment.
+
+~Nature of Bacteria.~ Toadstools, smuts, rusts and mildews are known to
+even the casual observer, because they are of evident size. Their
+plant-like nature can be more readily understood from their general
+structure and habits of life. The bacteria, however, are so small, that
+under ordinary conditions, they only become evident to our unaided
+senses by the by-products of their activity.
+
+When Leeuwenhoek (pronounced Lave-en-hake) in 1675 first discovered
+these tiny, rapidly-moving organisms he thought they were animals.
+Indeed, under a microscope, many of them bear a close resemblance to
+those minute worms found in vinegar that are known as "vinegar-eels."
+The idea that they belonged to the animal kingdom continued to hold
+ground until after the middle of the nineteenth century; but with the
+improvement in microscopes, a more thorough study of these tiny
+structures was made possible, and their vegetable nature demonstrated.
+The bacteria as a class are separated from the fungi mainly by their
+method of growth; from the lower algae by the absence of chlorophyll,
+the green coloring matter of vegetable organisms.
+
+~Structure of bacteria.~ So far as structure is concerned the bacteria
+stand on the lowest plane of vegetable life. The single individual is
+composed of but a single cell, the structure of which does not differ
+essentially from that of many of the higher types of plant life. It is
+composed of a protoplasmic body which is surrounded by a thin membrane
+that separates it from neighboring cells that are alike in form and
+size.
+
+~Form and size.~ When a plant is composed of a single cell but little
+difference in form is to be expected. While there are intermediate
+stages that grade insensibly into each other, the bacteria may be
+grouped into three main types, so far as form is concerned. These are
+spherical, elongated, and spiral, and to these different types are given
+the names, respectively, _coccus_, _bacillus_ and _spirillum_ (plural,
+_cocci_, _bacilli_, _spirilla_) (fig. 1). A ball, a short rod, and a
+corkscrew serve as convenient models to illustrate these different
+forms.
+
+[Illustration: FIG. 1. Different forms of bacteria. _a_, _b_, _c_,
+represent different types as to form: _a_, coccus, _b_, bacillus, _c_,
+spirillum; _d_, diplococcus or twin coccus; _e_, staphylococcus or
+cluster coccus; _f_ and _g_, different forms of bacilli, _g_ shows
+internal endospores within cell; _h_ and _i_, bacilli with motile organs
+(cilia).]
+
+In size, the bacteria are the smallest organisms that are known to
+exist. Relatively there is considerable difference in size between the
+different species, yet in absolute amount this is so slight as to
+require the highest powers of the microscope to detect it. As an average
+diameter, one thirty-thousandth of an inch may be taken. It is difficult
+to comprehend such minute measurements, but if a hundred individual
+germs could be placed side by side, their total thickness would not
+equal that of a single sheet of paper upon which this page is printed.
+
+~Manner of Growth.~ As the cell increases in size as a result of growth,
+it elongates in one direction, and finally a new cell wall is formed,
+dividing the so-called mother-cell into two, equal-sized daughter-cells.
+This process of cell division, known as _fission_, is continued until
+growth ceases and is especially characteristic of bacteria.
+
+~Cell Arrangement.~ If fission goes on in the same plane continually, it
+results in the formation of a cell-row. A coccus forming such a chain of
+cells is called _strepto-coccus_ (chain-coccus). If only two cells
+cohere, it is called a _diplo-coccus_ (twin-coccus). If the second cell
+division plane is formed at right angles to the first, a _cell surface_
+or _tetrad_ is formed. If growth takes place in three dimensions of
+space, a _cell mass_ or _sarcina_ is produced. Frequently, these cell
+aggregates cohere so tenaciously that this arrangement is of value in
+distinguishing different species.
+
+~Spores.~ Some bacteria possess the property of forming _spores_ within
+the mother cell (called _endospores_, fig. 1g) that are analogous in
+function to the seeds of higher plants and spores of fungi. By means of
+these structures which are endowed with greater powers of resistance
+than the vegetating cell, the organism is able to protect itself from
+the effect of an unfavorable environment. Many of the bacilli form
+endospores but the cocci do not. It is these spore forms that make it
+so difficult to thoroughly sterilize milk.
+
+~Movement.~ Many bacteria are unable to move from place to place. They
+have, however, a vibrating movement known as the _Brownian_ motion that
+is purely physical. Many other kinds are endowed with powers of
+locomotion. Motion is produced by means of fine thread-like processes of
+protoplasm known as _cilia_ (sing. _cilium_) that are developed on the
+outer surface of the cell. By means of the rapid vibration of these
+organs, the cell is propelled through the medium. Nearly all cocci are
+immotile, while the bacilli may or may not be. These cilia are so
+delicate that it requires special treatment to demonstrate their
+presence.
+
+~Classification.~ In classifying or arranging the different members of any
+group of living objects, certain similarities and dissimilarities must
+be considered. These are usually those that pertain to the structure and
+form, as such are regarded as most constant. With the bacteria these
+differences are so slight that they alone do not suffice to distinguish
+distinctly one species from another. As far as these characters can be
+used, they are taken, but in addition, many characteristics of a
+physiological nature are added. The way that the organism grows in
+different kinds of cultures, the by-products produced in different
+media, and effect on the animal body when injected into the same are
+also used as data in distinguishing one species from another.
+
+~Conditions favoring bacterial growth.~ The bacteria, in common with all
+other living organisms are affected by external conditions, either
+favorably or unfavorably. Certain conditions must prevail before
+development can occur. Thus, the organism must be supplied with an
+adequate and suitable food supply and with moisture. The temperature
+must also range between certain limits, and finally, the oxygen
+requirements of the organism must be considered.
+
+~Food supply.~ Most bacteria are capable of living on dead, inert, organic
+matter, such as meats, milk and vegetable material, in which case, they
+are known as _saprophytes_. In contradistinction to this class is a
+smaller group known as _parasites_, which derive their nourishment from
+the living tissues of animals or plants. The first group comprise by far
+the larger number of known organisms which are concerned for the most
+part in the decomposition of organic matter. The parasitic group
+includes those which are the cause of various communicable diseases.
+Between these two groups there is no sharp line of division, and in some
+cases, certain species possess the faculty of growing either as
+parasites or saprophytes, in which case they are known as _facultative_
+parasites or saprophytes.
+
+The great majority of bacteria of interest in dairying belong to the
+saprophytic class; only those species capable of infecting milk through
+the development of disease in the animal are parasites in the strict
+sense of the term. Most disease-producing species, as diphtheria or
+typhoid fever, while parasitic in man lead a saprophytic method of life
+so far as their relation to milk is concerned.
+
+Bacteria require for their growth, nitrogen, hydrogen, carbon, oxygen,
+together with a limited amount of mineral matter. The nitrogen and
+carbon are most available in the form of organic compounds, such as
+albuminous material. Carbon in the form of carbohydrates, as sugar or
+starch, is most readily attacked by bacteria.
+
+Inasmuch as the bacteria are plant-cells, they must imbibe their food
+from material in solution. They are capable of living on solid
+substances, but in such cases, the food elements must be rendered
+soluble, before they can be appropriated. If nutritive liquids are too
+highly concentrated, as in the case of syrups and condensed milk,
+bacteria cannot grow therein, although all the necessary ingredients may
+be present. Generally, bacteria prefer a neutral or slightly alkaline
+medium, rather than one of acid reaction; but there are numerous
+exceptions to this general rule, especially among the bacteria found in
+milk.
+
+~Temperature.~ Growth of bacteria can only occur within certain
+temperature limits, the extremes of which are designated as the
+_minimum_ and _maximum_. Below and above these respective limits, life
+may be retained in the cell for a time, but actual cell-multiplication
+is stopped. Somewhere between these two cardinal temperature points, and
+generally nearer the maximum limit is the most favorable temperature for
+growth, known as the _optimum_. The temperature zone of most dairy
+bacteria in which growth occurs ranges from 40°-45° F. to somewhat above
+blood-heat, 105°-110° F., the optimum being from 80°-95° F. Many
+parasitic species, because of their adaptation to the bodies of
+warm-blooded animals, generally have a narrower range, and a higher
+optimum, usually approximating the blood heat (98°-99° F). The broader
+growth limits of bacteria in comparison with other kinds of life explain
+why these organisms are so widely distributed in nature.
+
+~Air supply.~ Most bacteria require as do the green plants and animal
+life, the free oxygen of the air for their respiration. These are called
+_aerobic_. Some species, however, and some yeasts as well possess the
+peculiar property of taking the oxygen which they need from organic
+compounds such as sugar, etc., and are therefore able to live and grow
+under conditions where the atmospheric air is excluded. These are known
+as _anaerobic_. While some species grow strictly under one condition or
+the other, and hence are _obligate_ aerobes or anaerobes, others possess
+the ability of growing under either condition and are known as
+_facultative_ or optional forms. The great majority of milk bacteria are
+either obligate or facultative aerobes.
+
+~Rate of growth.~ The rate of bacterial development is naturally very much
+affected by external conditions, food supply and temperature exerting
+the most influence. In the neighborhood of the freezing point but little
+growth occurs. The rate increases with a rise in temperature until at
+the _optimum_ point, which is generally near the blood heat or slightly
+below (90°-98° F.), a single cell will form two cells in 20 to 30
+minutes. If temperature rises much above blood heat rate of growth is
+lessened and finally ceases. Under ideal conditions, rapidity of growth
+is astounding, but this initially rapid rate of development cannot be
+maintained indefinitely, for growth is soon limited by the accumulation
+of by-products of cell activity. Thus, milk sours rapidly at ordinary
+temperatures until the accumulation of acid checks its development.
+
+~Detrimental effect of external conditions.~ Environmental influences of a
+detrimental character are constantly at work on bacteria, tending to
+repress their development or destroy them. These act much more readily
+on the vegetating cell than on the more resistant spore. A thorough
+knowledge of the effect of these antagonistic forces is essential, for
+it is often by their means that undesirable bacteria may be killed out.
+
+~Effect of cold.~ While it is true that chilling largely prevents
+fermentative action, and actual freezing stops all growth processes,
+still it does not follow that exposure to low temperatures will
+effectually destroy the vitality of bacteria, even in the vegetative
+condition. Numerous non-spore-bearing species remain alive in ice for a
+prolonged period, and recent experiments with liquid air show that even
+a temperature of -310° F. for hours does not effectually kill all
+exposed cells.
+
+~Effect of heat.~ High temperatures, on the other hand, will destroy any
+form of life, whether in the vegetative or latent stage. The temperature
+at which the vitality of the cell is lost is known as the _thermal death
+point_. This limit is not only dependent upon the nature of the
+organism, but varies with the time of exposure and the condition in
+which the heat is applied. In a moist atmosphere the penetrating power
+of heat is great; consequently cell-death occurs at a lower temperature
+than in a dry atmosphere. An increase in time of exposure lowers the
+temperature point at which death occurs.
+
+For vegetating forms the thermal death point of most bacteria ranges
+from 130°-140° F. where the exposure is made for ten minutes which is
+the standard arbitrarily selected. In the spore stage resistance is
+greatly increased, some forms being able to withstand steam at 210°-212°
+F. from one to three hours. If dry heat is employed, 260°-300° F. for an
+hour is necessary to kill spores. Where steam is confined under
+pressure, a temperature of 230°-240° F. for 15-20 minutes suffices to
+kill all spores.
+
+~Drying.~ Spore-bearing bacteria like anthrax withstand drying with
+impunity; even tuberculous material, although not possessing spores
+retains its infectious properties for many months. Most of the dairy
+bacteria do not produce spores, and yet in a dry condition, they retain
+their vitality unimpaired for considerable periods, if they are not
+subjected to other detrimental influences.
+
+~Light.~ Bright sunlight exerts on many species a powerful disinfecting
+action, a few hours being sufficient to destroy all cells that are
+reached by the sun's rays. Even diffused light has a similar effect,
+although naturally less marked. The active rays in this disinfecting
+action are those of the chemical or violet end of the spectrum, and not
+the heat or red rays.
+
+~Influence of chemical substances.~ A great many chemical substances exert
+a more or less powerful toxic action of various kinds of life. Many of
+these are of great service in destroying or holding bacterial growth in
+check. Those that are toxic and result in the death of the cell are
+known as _disinfectants_; those that merely inhibit, or retard growth
+are known as _antiseptics_. All disinfectants must of necessity be
+antiseptic in their action, but not all antiseptics are disinfectants
+even when used in strong doses. Disinfectants have no place in dairy
+work, except to destroy disease bacteria, or preserve milk for
+analytical purposes. Corrosive sublimate or potassium bichromate are
+most frequently used for these purposes. The so-called chemical
+preservatives used to "keep" milk depend for their effect on the
+inhibition of bacterial growth. With a substance so violently toxic as
+formaldehyde (known as formalin, freezene) antiseptic doses are likely
+to be exceeded. In this country most states prohibit the use of these
+substances in milk. Their only function in the dairy should be to check
+fermentative or putrefactive processes outside of milk and so keep the
+air free from taints.
+
+~Products of growth.~ All bacteria in their development form certain more
+or less characteristic by-products. With most dairy bacteria, these
+products are formed from the decomposition of the medium in which the
+bacteria may happen to live. Such changes are known, collectively, as
+fermentations, and are characterised by the production of a large amount
+of by-products, as a result of the development of a relatively small
+amount of cell-life. The souring of milk, the formation of butyric acid,
+the making of vinegar from cider, are all examples of fermentative
+changes.
+
+With many bacteria, especially those that affect proteid matter,
+foul-smelling gases are formed. These are known as putrefactive changes.
+All organic matter, under the action of various organisms, sooner or
+later undergoes decay, and in different stages of these processes,
+acids, alkalies, gases and numerous other products are formed. Many of
+these changes in organic matter occur only when such material is brought
+in direct contact with the living bacterial cell.
+
+In other instances, soluble, non-vital ferments known as _enzyms_ are
+produced by the living cell, which are able to act on organic matter, in
+a medium free from live cells, or under conditions where the activity of
+the cell is wholly suspended. These enzyms are not confined to bacteria
+but are found throughout the animal and plant world, especially in those
+processes that are concerned in digestion. Among the better known of
+these non-vital ferments are rennet, the milk-curdling enzym; diastase
+or ptyalin of the saliva, the starch-converting enzym; pepsin and
+trypsin, the digestive ferments of the animal body.
+
+Enzyms of these types are frequently found among the bacteria and yeasts
+and it is by virtue of this characteristic that these organisms are
+able to break down such enormous quantities of organic matter. Most of
+these enzyms react toward heat, cold and chemical poisons in a manner
+quite similar to the living cells. In one respect they are readily
+differentiated, and that is, that practically all of them are capable of
+producing their characteristic chemical transformations under
+anaesthetic conditions, as in a saturated ether or chloroform
+atmosphere.
+
+~Distribution of bacteria.~ As bacteria possess greater powers of
+resistance than most other forms of life, they are to be found more
+widely distributed than any other type. At the surface of the earth,
+where conditions permit of their growth, they are found everywhere,
+except in the healthy tissues of animals and plants. In the superficial
+soil layers, they exist in myriads, as here they have abundance of
+nourishment. At the depth of several feet however, they diminish rapidly
+in numbers, and in the deeper soil layers, from six to ten feet or more,
+they are not present, because of the unsuitable growth conditions.
+
+The bacteria are found in the air because of their development in the
+soil below. They are unable to grow even in a moist atmosphere, but are
+so readily dislodged by wind currents that over land areas the lower
+strata of the air always contain them. They are more numerous in summer
+than in winter; city air contains larger numbers than country air.
+Wherever dried fecal matter is present, as in barns, the air contains
+many forms.
+
+Water contains generally enough organic matter in solution, so that
+certain types of bacterial life find favorable growth conditions. Water
+in contact with the soil surface takes up many impurities, and is of
+necessity rich in microbes. As the rain water percolates into the soil,
+it loses its germ content, so that the normal ground water, like the
+deeper soil layers, contains practically no bacterial life. Springs
+therefore are relatively deficient in germ life, except as they become
+infected with soil organisms, as the water issues from the soil. Water
+may serve to disseminate certain infectious diseases as typhoid fever
+and cholera among human beings, and a number of animal maladies.
+
+While the inner tissues of healthy animals are free from bacteria, the
+natural passages as the respiratory and digestive tracts, being in more
+direct contact with the exterior, become more readily infected. This is
+particularly true with reference to the intestinal tract, for in the
+undigested residue, bacterial activity is at a maximum. The result is
+that fecal matter contains enormous numbers of organisms so that the
+possibility of pollution of any food medium such as milk with such
+material is sure to introduce elements that seriously affect the quality
+of the product.
+
+
+
+
+CHAPTER II.
+
+METHODS OF STUDYING BACTERIA.
+
+
+~Necessity of bacterial masses for study.~ The bacteria are so extremely
+small that it is impossible to study individual germs separately without
+the aid of first-class microscopes. For this reason, but little advance
+was made in the knowledge of these lower forms of plant life, until the
+introduction of culture methods, whereby a single organism could be
+cultivated and the progeny of this cell increased to such an extent in a
+short course of time, that they would be visible to the unaided eye.
+
+This is done by growing the bacteria in masses on various kinds of food
+media that are prepared for the purpose, but inasmuch as bacteria are so
+universally distributed, it becomes an impossibility to cultivate any
+special form, unless the medium in which they are grown is first freed
+from all pre-existing forms of germ life. To accomplish this, it is
+necessary to subject the nutrient medium to some method of
+sterilization, such as heat or filtration, whereby all life is
+completely destroyed or eliminated. Such material after it has been
+rendered germ-free is kept in sterilized glass tubes and flasks, and is
+protected from infection by cotton stoppers.
+
+~Culture media.~ For culture media, many different substances are
+employed. In fact, bacteria will grow on almost any organic substance
+whether it is solid or fluid, provided the other essential conditions of
+growth are furnished. The food substances that are used for culture
+purposes are divided into two classes; solids and liquids.
+
+Solid media may be either permanently solid like potatoes, or they may
+retain their solid properties only at certain temperatures like gelatin
+or agar. The latter two are of utmost importance in bacteriological
+research, for their use, which was introduced by Koch, permits the
+separation of the different forms that may happen to be in any mixture.
+Gelatin is used advantageously because the majority of bacteria present
+wider differences due to growth upon this medium than upon any other. It
+remains solid at ordinary temperatures, becoming liquid at about 70° F.
+Agar, a gelatinous product derived from a Japanese sea-weed, has a much
+higher melting point, and can be successfully used, especially with
+those organisms whose optimum growth point is above the melting point of
+gelatin.
+
+Besides these solid media, different liquid substances are extensively
+used, such as beef broth, milk, and infusions of various vegetable and
+animal tissues. Skim-milk is of especial value in studying the milk
+bacteria and may be used in its natural condition, or a few drops of
+litmus solution may be added in order to detect any change in its
+chemical reaction due to the bacteria.
+
+[Illustration: FIG. 2. A gelatin plate culture showing appearance of
+different organisms in a sample of milk. Each mass represents a
+bacterial growth (colony) derived from a single cell. Different forms
+react differently toward the gelatin, some liquefying the same, others
+growing in a restricted mass. _a_, represents a colony of the ordinary
+bread mold; _b_, a liquefying bacterium; _c_, and _d_, solid forms.]
+
+~Methods of isolation.~ Suppose for instance one wishes to isolate the
+different varieties of bacteria found in milk. The method of procedure
+is as follows: Sterile gelatin in glass tubes is melted and cooled down
+so as to be barely warm. To this gelatin which is germ-free a drop of
+milk is added. The gelatin is then gently shaken so as to thoroughly
+distribute the milk particles, and poured out into a sterile flat glass
+dish and quickly covered. This is allowed to stand on a cool surface
+until the gelatin hardens. After the culture plate has been left for
+twenty-four to thirty-six hours at the proper temperature, tiny spots
+will begin to appear on the surface, or in the depth of the culture
+medium. These patches are called _colonies_ and are composed of an
+almost infinite number of individual germs, the result of the continued
+growth of a single organism that was in the drop of milk which was
+firmly held in place when the gelatin solidified. The number of these
+colonies represents approximately the number of germs that were present
+in the milk drop. If the plate is not too thickly sown with these germs,
+the colonies will continue to grow and increase in size, and as they do,
+minute differences will begin to appear. These differences may be in the
+color, the contour and the texture of the colony, or the manner in
+which it acts toward gelatin. In order to make sure that the seeding in
+not too copious so as to interfere with continued study, an
+_attenuation_ is usually made. This consists in taking a drop of the
+infected gelatin in the first tube, and transferring it to another tube
+of sterile media. Usually this operation is repeated again so that these
+culture plates are made with different amounts of seed with the
+expectation that in at least one plate the seeding will not be so thick
+as to prevent further study. For transferring the culture a loop made of
+platinum wire is used. By passing this through a gas flame, it can be
+sufficiently sterilized.
+
+[Illustration: FIG. 3. Profile view of gelatin plate culture; _b_, a
+liquefying form that dissolves the gelatin; _c_ and _d_, surface
+colonies that do not liquefy the gelatin.]
+
+To further study the peculiarities of different germs, the separate
+colonies are transferred to other sterile tubes of culture material and
+thus _pure cultures_ of the various germs are secured. These cultures
+then serve as a basis for continued study and must be planted and grown
+upon all the different kinds of media that are obtainable. In this way
+the slight variations in the growth of different forms are detected and
+the peculiar characteristics are determined, so that the student is able
+to recognize this form when he meets it again.
+
+These culture methods are of essential importance in bacteriology, as it
+is the only way in which it is possible to secure a quantity of germs of
+the same kind.
+
+~The microscope in bacterial investigation.~ In order to verify the purity
+of the cultures, the microscope is in constant demand throughout all the
+different stages of the isolating process. For this purpose, it is
+essential that the instrument used shall be one of strong magnifying
+powers (600-800 diameters), combined with sharp definition.
+
+[Illustration: FIG. 4. Pure cultures of different kinds of bacteria in
+gelatin tubes. _a_, growth slight in this medium; _b_, growth copious at
+and near surface. Fine parallel filaments growing out into medium
+liquefying at surface; _c_, a rapid liquefying form; _d_, a
+gas-producing form that grows equally well in lower part of tube as at
+surface (facultative anaerobe); _e_, an obligate anaerobe, that develops
+only in absence of air.]
+
+The microscopical examination of any germ is quite as essential as the
+determination of culture characteristics; in fact, the two must go hand
+in hand. The examination reveals not only the form and size of the
+individual germs, but the manner in which they are united with each
+other, as well as any peculiarities of movement that they may possess.
+
+In carrying out the microscopical part of the work, not only is the
+organism examined in a living condition, but preparations are made by
+using solutions of anilin dyes as staining agents. These are of great
+service in bringing out almost imperceptible differences. The art of
+staining has been carried to the highest degree of perfection in
+bacteriology, especially in the detection of germs that are found in
+diseased tissues in the animal or human body.
+
+In studying the peculiarities of any special organism, not only is it
+necessary that these cultural and microscopical characters should be
+closely observed, but special experiments must be carried out along
+different lines, in order to determine any special properties that the
+germ may possess. Thus, the ability of any form to act as a fermentative
+organism can be tested by fermentation experiments; the property of
+causing disease, studied by the inoculation of pure cultures into
+animals. A great many different methods have been devised for the
+purpose of studying special characteristics of different bacteria, but a
+full description of these would necessarily be so lengthy that in a work
+of this character they must be omitted. For details of this nature
+consult standard reference books on bacteriological technique.
+
+
+
+
+CHAPTER III.
+
+CONTAMINATION OF MILK.
+
+
+No more important lesson is to be learned than that which relates to the
+ways in which milk is contaminated with germ life of various kinds; for
+if these sources of infection are thoroughly recognized they can in
+large measure be prevented, and so the troubles which they engender
+overcome. Various organisms find in milk a congenial field for
+development. Yeasts and some fungi are capable of growth, but more
+particularly the bacteria.
+
+~Milk a suitable bacterial food.~ The readiness with which milk undergoes
+fermentative changes indicates that it is well adapted to nourish
+bacterial life. Not only does it contain all the necessary nutritive
+substances but they are diluted in proper proportions so as to render
+them available for bacterial as well as mammalian life.
+
+Of the nitrogenous compounds, the albumen is in readily assimilable
+form. The casein, being insoluble, is not directly available, until it
+is acted upon by proteid-dissolving enzyms like trypsin which may be
+secreted by bacteria. The fat is relatively resistant to change,
+although a few forms are capable of decomposing it. Milk sugar, however,
+is an admirable food for many species, acids and sometimes gases being
+generally produced.
+
+~Condition when secreted.~ When examined under normal conditions milk
+always reveals bacterial life, yet in the secreting cells of the udder
+of a healthy cow germ life is not found. Only when the gland is diseased
+are bacteria found in any abundance. In the passage of the milk from
+the secreting cells to the outside it receives its first infection, so
+that when drawn from the animal it generally contains a considerable
+number of organisms.
+
+[Illustration: FIG. 5. Microscopic appearance of milk showing relative
+size of fat globules and bacteria.]
+
+~Contamination of milk.~ From this time until it is consumed in one form
+or another, it is continually subjected to contamination. The major part
+of this infection occurs while the milk is on the farm and the degree of
+care which is exercised while the product is in the hands of the milk
+producer is the determining factor in the course of bacterial changes
+involved. This of course does not exclude the possibility of
+contamination in the factory, but usually milk is so thoroughly seeded
+by the time it reaches the factory that the infection which occurs here
+plays a relatively minor rôle to that which happens earlier. The great
+majority of the organisms in milk are in no wise dangerous to health,
+but many species are capable of producing various fermentative changes
+that injure the quality of the product for butter or cheese. To be able
+to control abnormal changes of an undesirable character one must know
+the sources of infection which permit of the introduction of these
+unwelcome intruders.
+
+~Sources of infection.~ The bacterial life that finds its way into milk
+while it is yet on the farm may be traced to several sources, which may
+be grouped under the following heads: unclean dairy utensils, fore milk,
+coat of animal, and general atmospheric surroundings. The relative
+importance of these various factors fluctuates in each individual
+instance.
+
+~Dairy utensils.~ Of first importance are the vessels that are used during
+milking, and also all storage cans and other dairy utensils that come in
+contact with the milk after it is drawn. By unclean utensils, actually
+_visible_ dirt need not always be considered, although such material is
+often present in cracks and angles of pails and cans. Unless cleansed
+with especial care, these are apt to be filled with foul and decomposing
+material that suffices to seed thoroughly the milk. Tin utensils are
+best. Where made with joints, they should be well flushed with solder so
+as to be easily cleaned (see Fig. 6). In much of the cheaper tin ware on
+the market, the soldering of joints and seams is very imperfect,
+affording a place of refuge for bacteria and dirt.
+
+Cans are often used when they are in a condition wholly unsuitable for
+sanitary handling of milk. When the tin coating becomes broken and the
+can is rusty, the quality of the milk is often profoundly affected.
+Olson[1] of the Wisconsin Station has shown that the action of rennet is
+greatly impaired where milk comes in contact with a rusty iron surface.
+
+[Illustration: FIG. 6.]
+
+With the introduction of the form or hand separator a new milk utensil
+has been added to those previously in use and one which is very
+frequently not well cleaned. Where water is run through the machine to
+rinse out the milk particles, gross bacterial contamination occurs and
+the use of the machine much increases the germ content of the milk.
+Every time the separator is used it should be taken apart and thoroughly
+cleaned and dried before reassembling.[2]
+
+~Use of milk-cans for transporting factory by-products.~ The general
+custom of using the milk-cans to carry back to the farm the factory
+by-products (skim-milk or whey) has much in it that is to be deprecated.
+These by-products are generally rich in bacterial life, more especially
+where the closest scrutiny is not given to the daily cleaning of the
+vats and tanks. Too frequently the cans are not cleaned immediately upon
+arrival at the farm, so that the conditions are favorable for rapid
+fermentation. Many of the taints that bother factories are directly
+traceable to such a cause. A few dirty patrons will thus seriously
+infect the whole supply. The responsibility for this defect should,
+however, not be laid entirely upon the shoulders of the producer. The
+factory operator should see that the refuse material does not accumulate
+in the waste vats from day to day and is not transformed into a more or
+less putrid mass. A dirty whey tank is not an especially good object
+lesson to the patron to keep his cans clean.
+
+It is possible that abnormal fermentations or even contagious diseases
+may thus be disseminated.
+
+Suppose there appears in a dairy an infectious milk trouble, such as
+bitter milk. This milk is taken to the factory and passes unnoticed into
+the general milk-supply. The skim-milk from the separator is of course
+infected with the germ, and if conditions favor its growth, the whole
+lot soon becomes tainted. If this waste product is returned to the
+different patrons in the same cans that are used for the fresh milk, the
+probabilities are strongly in favor of some of the cans being
+contaminated and thus infecting the milk supply of the patrons. If the
+organism is endowed with spores so that it can withstand unfavorable
+conditions, this taint may be spread from patron to patron simply
+through the infection of the vessels that are used in the transportation
+of the by-products. Connell has reported just such a case in a Canadian
+cheese factory where an outbreak of slimy milk was traced to infected
+whey vats. Typhoid fever among people, foot and mouth disease and
+tuberculosis among stock are not infrequently spread in this way. In
+Denmark, portions of Germany and some states in America, compulsory
+heating of factory by-products is practiced to eliminate this danger.[3]
+
+~Pollution of cans from whey tanks.~ The danger is greater in cheese
+factories than in creameries, for whey usually represents a more
+advanced stage of fermentation than skim-milk. The higher temperature at
+which it is drawn facilitates more rapid bacterial growth, and the
+conditions under which it is stored in many factories contribute to the
+ease with which fermentative changes can go on in it. Often this
+by-product is stored in wooden cisterns or tanks, situated below ground,
+where it becomes impossible to clean them out thoroughly. A custom that
+is almost universally followed in the Swiss cheese factories, here in
+this country, as in Switzerland, is fully as reprehensible as any dairy
+custom could well be. In Fig. 7 the arrangement in vogue for the
+disposal of the whey is shown. The hot whey is run out through the
+trough from the factory into the large trough that is placed over the
+row of barrels, as seen in the foreground. Each patron thus has allotted
+to him in his individual barrel his portion of the whey, which he is
+supposed to remove day by day. No attempt is made to clean out these
+receptacles, and the inevitable result is that they become filled with a
+foul, malodorous liquid, especially in summer. When such material is
+taken home in the same set of cans that is used to bring the fresh milk
+(twice a day as is the usual custom in Swiss factories), it is no wonder
+that this industry is seriously handicapped by "gassy" fermentations
+that injure materially the quality of the product. Not only is the above
+danger a very considerable one, but the quality of the factory
+by-product for feeding purposes, whether it is skim-milk or whey, is
+impaired through the development of fermentative changes.
+
+[Illustration: FIG. 7. Swiss cheese factory (Wisconsin), showing
+careless way in which whey is handled. Each patron's share is placed in
+a barrel, from which it is removed by him. No attempt is made to cleanse
+these receptacles.]
+
+~Improved methods of disposal of by-products.~ The difficulties which
+attend the distribution of these factory by-products have led to
+different methods of solution. One is to use another separate set of
+receptacles to carry back these products to the farm. This method has
+been tried, and while it is deemed impracticable by many to handle two
+sets of vessels, yet some of the most progressive factories report
+excellent results where this method is in use.
+
+Large barrels could be used for this purpose to economize in wagon
+space.
+
+Another method that has met with wider acceptance, especially in
+creameries, is the custom of pasteurizing or scalding the skim-milk
+immediately after it is separated, so that it is returned to the farmer
+in a hot condition. In factories where the whole milk is pasteurized,
+further treatment of the by-product is not necessary. In most factories
+steam, generally exhaust, is used directly in the milk, and experience
+has shown that such milk, without any cooling, will keep sweet for a
+considerable number of hours longer than the untreated product. It is
+noteworthy that the most advanced and progressive factories are the ones
+that appreciate the value of this work, and although it involves some
+time and expense, experience has shown the utility of the process in
+that a better grade of milk is furnished by the patrons of factories
+which follow this practice.[4] The exclusion of all danger of animal or
+human disease is also possible in this way.
+
+~Cleaning dairy utensils.~ The thorough cleaning of all dairy apparatus
+that in any way comes in contact with the milk is one of the most
+fundamental and important problems in dairying. All such apparatus
+should be so constructed as to permit of easy cleaning. Tinware,
+preferably of the pressed variety, gives the best surface for this
+purpose and is best suited for the handling of milk.
+
+Milk vessels should never be allowed to become dry when dirty, for dried
+particles of milk residue are extremely difficult to remove. In cleaning
+dairy utensils they should first be rinsed in lukewarm instead of hot
+water, so as to remove organic matter without coagulating the milk. Then
+wash thoroughly in hot water, using a good washing powder. The best
+washing powders possess considerable disinfecting action.[5] Strong
+alkalies should not be used. After washing rinse thoroughly in clean hot
+water. If steam is available, as it always is in creameries, cans and
+pails should be turned over jet for a few moments. While a momentary
+exposure will not suffice to completely sterilize such a vessel, yet
+many bacteria are killed in even a short exposure, and the cans dry more
+thoroughly and quickly when heated by steam.
+
+Not only should the greatest care be paid to the condition of the cans
+and milk-pails, but all dippers, strainers, and other utensils that come
+in contact with the milk must be kept equally clean. Cloth strainers,
+unless attended to, are objectionable, for the fine mesh of the cloth
+retains so much moisture that they become a veritable hot-bed of
+bacterial life, unless they are daily boiled or steamed.
+
+The inability to thoroughly render vessels bacteria-free with the
+conveniences which are generally to be found on the farm has led in some
+cases to the custom of washing and sterilizing the milk cans at the
+factory.
+
+~Germ content of milk utensils.~ Naturally the number of bacteria found in
+different milk utensils after they have received their regular cleaning
+will be subject to great fluctuations; but, nevertheless, such
+determinations are of value as giving a scientific foundation for
+practical methods of improvement. The following studies may serve to
+indicate the relative importance of the utensils as a factor in milk
+contamination.
+
+Two cans were taken, one of which had been cleaned in the ordinary way,
+while the other was sterilized by steaming. Before milking, the udder
+was thoroughly cleaned and special precautions taken to avoid raising of
+dust; the fore milk was rejected. Milk drawn into these two cans showed
+the following germ content:
+
+                      No. bacteria       Hours before
+                        per cc.           souring.
+
+    Steamed pail          165              28-1/2
+    Ordinary pail        4265              23
+
+Harrison[6] has shown how great a variation is in the bacterial content
+in milk cans. The utensils were rinsed with 100 cc. of sterile water and
+numerical determinations of this rinsing water made. In poorly cleaned
+cans, the average germ content was 442,000; in cans washed in tepid
+water and then scalded--the best farm practice--54,000, and in cans
+carefully washed and then steamed for five minutes, 880.
+
+Another method used by the writer is to wash the utensil with 100 cc.
+sterile wash water, using a sterile swab to remove dirt. Then repeat the
+process twice or more with fresh rinsing waters, making plate cultures
+from each. The following data were obtained from three such
+determinations:
+
+No. bacteria in different washings.     Total No.
+   I.           II.         III.        bacteria.
+7,800,000    1,450,000     49,000       9,299,000
+  283,000       43,400     35,000         361,400
+1,685,000      105,000     61,400       1,851,400
+
+~Infection of milk in udder cavity.~ A frequently neglected but
+considerable factor of infection is that which is attributable to the
+bacteria which are present in the udder and which are removed in large
+numbers during the milking process. An examination of the fore milk, i.
+e., the first few streams from each teat, and that which is subsequently
+withdrawn, generally reveals a very much larger number of organisms in
+the fore milk.[7] Not infrequently will this part of the milk when drawn
+under as careful conditions as possible, contain several score thousand
+organisms per cc. If successive bacterial determinations are made at
+different periods of the milking, as shown in the following experiment,
+a marked diminution is to be noted after the first portion of the milk
+is removed:
+
+    _Bacterial content at different periods of milking._
+
+                Fore     200th    2000th    4300th    6500th   Strippings.
+                milk.     cc.      cc.       cc.       cc.
+Expt. 1        6,500     1,700     475      220        75         5
+Expt. 2        8,100     1,650     400      240        50        10
+
+By some observers it has been claimed that it is possible to secure
+absolutely sterile milk in the strippings but this is rarely so. It is
+quite probable that such reported results are due to the fact that too
+small quantities of milk were used in the examinations and so erroneous
+conclusions were drawn. This marked diminution in numbers indicates that
+the larger proportion of the organisms found in the fore milk are
+present in the lower portion of the udder and milk ducts. When
+consideration is given to the structure of the udder, it is readily
+apparent that infection will be greater here than above.
+
+[Illustration: FIG. 8. Sectional view of udder showing teat with milk
+duct connecting exterior with the milk cistern. Milk sinuses are mostly
+shown in cross section interspersed and below the secreting tissue
+(Moore and Ward).]
+
+The udder is composed of secreting tissue (_gland cells_) held in place
+by fibrous connective tissue. Ramifying throughout this glandular
+structure are numerous channels (_milk sinuses_) that serve to convey
+the milk from the cells where it is produced into the _milk cistern_, a
+common receptacle just above the teats. This cavity is connected with
+the exterior through the milk duct in the teat, which is more or less
+tightly closed by the circular sphincter muscles, thus preventing the
+milk from flowing out. The mucous membranes of the milk duct and cistern
+are naturally moist. The habits of the animal render it impossible to
+prevent infection of the external opening at the end of the teat and
+there is no mechanical reason why bacteria cannot readily find their way
+along the moist lining membrane for some distance. If organisms are
+adapted to this kind of an environment, ideal conditions exist for their
+multiplication, as moisture, warmth and suitable food supply are
+present. The question arises how far up into this organ is penetration
+possible? Within late years numerous observations have been made on the
+presence of organisms in the upper portion of the udder in contact with
+the secreting tissue.[8]
+
+These investigations prove that bacteria are distributed throughout the
+whole of the udder, although numerically they are much less abundant in
+the region of the milk-secreting tissue than in the lower portion.
+Ward's conclusions are "that milk when secreted by the glands of a
+healthy udder is sterile. It may however, immediately become
+contaminated by the bacteria which are normally present in the smaller
+milk ducts of the udder."
+
+~Nature of bacteria in fore milk.~ Generally speaking the number of
+different species found in the fore milk is not large, and of those
+which do appear many occur at only occasional intervals. Moore[9] in the
+examination of 9 udders found 20 different forms, and of these only 3
+species predominated, all of which proved to be micrococci. Streptococci
+have also been quite frequently reported. Freudenreich[10] found the
+most common types to be cocci, belonging to both the liquefying and
+non-liquefying class.
+
+Peptonizing[11] and spore-bearing[12] species have also been reported as
+well as gas-producing[13] forms allied to the colon bacillus. Such
+findings are, however, due in all probability to accidental invasion.
+Most investigators report the absence of the distinctively lactic-acid
+group of organisms.[14]
+
+~Origin of bacteria in udder.~ There is no question but that many of the
+types of bacteria found in the udder gain access from the outside. Those
+belonging to the spore-bearing, digesting and intestinal types have such
+a favorable opportunity for introduction from outside and are so
+unlikely to have come directly from the body of the animal, that the
+external source of infection is much more probable. Whether this
+explanation answers the origin of the cocci that are so generally found
+in the upper portion of the udder is questionable. The statement is
+ordinarily made that the inner tissues of healthy organs are
+bacteria-free, but the studies of Ford[15] seem to indicate that 70 per
+cent. of such organs, removed under aseptic conditions from guinea pigs,
+rabbits, dogs and cats contained living organisms. Others have reported
+similar results in which cocci have been found[16] very similar to those
+occurring in the udder. These findings increase the probability that the
+origin of this type is from the blood. The persistence of certain
+species in the udder for months as noted by Ward indicates possibility
+of growth of some forms at least. Stocking[17] has shown where cows are
+not milked clean that the germ content of succeeding milkings is greatly
+increased.
+
+~Artificial introduction of bacteria into udder.~ If bacteria are capable
+of actually developing in the udder proper, it ought to be possible to
+easily demonstrate this by the artificial introduction of cultures. In a
+number of cases[18] such experiments have been made with various
+saprophytic forms, such as _B. prodigiosus_, lactic acid bacilli and
+others. In no case has it appeared evident that actual growth has
+occurred, although the introduced organism has been demonstrated in
+diminishing numbers for 5-6 days. Even the common lactic acid germ and a
+yellow liquefying coccus isolated from the fore milk failed to persist
+for more than a few days when thus artificially introduced. This failure
+to colonize is indeed curious and needs explanation. Is it due to
+unsuitable environmental conditions or attributable to the germicidal
+influence of the milk?
+
+Various body fluids are known to possess the property of destroying
+bacteria and it is claimed by Fokker[19] that this same property was
+found in freshly drawn milk. This peculiarity has also been investigated
+by Freudenreich,[20] and Hunziker[21] who find a similar property.
+
+No material increase in germ content takes place in milk for several
+hours when chilled to 40°-70° F.; on the other hand an actual, but
+usually not a marked decrease is observed for about 6 hours. This
+phenomenon varies with the milk of different cows. Nothing is known as
+to the cause of this apparent germicidal action. The question is yet by
+no means satisfactorily settled, although the facts on which the
+hypothesis is based are not in controversy. If such a peculiarity
+belongs to milk, it is not at all improbable that it may serve to keep
+down the germ content in the udder. Freudenreich[22] found that udders
+which were not examined for some time after death showed abundant
+growth, which fact he attributed to the loss of this germicidal
+property.
+
+The infection of the whole milk can be materially reduced by rejecting
+the fore milk, but it is questionable whether such rejection is worth
+while, except in the case of "sanitary" dairies where milk is produced
+with as low a germ content as possible. The intrinsic loss in butter fat
+in the fore milk is inconsiderable as the first few streams contain only
+about one-fifth the normal fat content.
+
+~Infection of milk after withdrawal from animal.~ The germ content of the
+milk, when it is being drawn from the animal is immediately increased
+upon contact with the atmosphere. These organisms are derived from the
+surrounding air and the utensils in which the milk is received and
+stored. The number of organisms which find their way into the milk
+depends largely upon the character of the surroundings. Bacteria are so
+intimately associated with dirt, dust and filth of all kinds that
+wherever the latter are found, the former are sure to be present in
+abundance.
+
+The most important factors in the infection of the milk after withdrawal
+are the pollution which is directly traceable to the animal herself and
+the condition of the milk utensils. Fortunately both of these sources of
+contamination are capable of being greatly minimized by more careful
+methods of handling.
+
+~Infection directly from the cow.~ It is a popular belief that the
+organisms found in milk are derived from the feed and water which the
+cow consumes, the same passing directly from the intestinal tract to the
+milk by the way of the blood circulation. Such a view has no foundation
+in fact as bacteria absorbed into the circulation are practically all
+destroyed in the tissues by the action of the body fluids and cells.[23]
+While organisms cannot pass readily from the intestine to the udder, yet
+this must not be interpreted as indicating that no attention should be
+given to the bacterial character of the material consumed. The water
+supply given should be pure and wholesome and no decomposed or spoiled
+food should be used.
+
+The infection traceable directly to the cow is modified materially by
+the conditions under which the animal is kept and the character of the
+feed consumed. The nature of the fecal matter is in part dependent upon
+the character of the food. The more nitrogenous rations with which
+animals are now fed leads to the production of softer fecal discharges
+which is more likely to soil the coat of the animal unless care is
+taken. The same is true with animals kept on pasture in comparison with
+those fed dry fodder.
+
+Stall-fed animals, however, are more likely to have their flanks fouled,
+unless special attention is paid to the removal of the manure. All dairy
+stalls should be provided with a manure drop which should be cleaned as
+frequently as circumstances will permit.
+
+[Illustration: FIG. 9. Showing the bacterial contamination arising from
+hair. These hairs were allowed to fall on a sterile gelatin surface. The
+adherent bacteria developed readily in this medium, and the number of
+bacteria thus introduced into the milk from these hairs can be estimated
+by the number of developing colonies.]
+
+The animal herself contributes materially to the quota of germ life
+finding its way into the milk through the dislodgment of dust and filth
+particles adhering to its hairy coat. The nature of this coat is such as
+to favor the retention of these particles. Unless care is taken the
+flanks and udder become polluted with fecal matter, which upon drying is
+displaced with every movement of the animal. Every hair or dirt particle
+so dislodged and finding its way into the milk-pail adds its quota of
+organisms to the liquid. This can be readily demonstrated by placing
+cow's hairs collected with care on the moist surface of gelatin culture
+plates. Almost invariably, bacteria will be found in considerable
+numbers adhering to such hairs as is indicated in Fig. 9. Dirt particles
+are even richer in germ life. Not only is there the dislodgment of
+hairs, epithelial scales and masses of dirt and filth, but during the
+milking process, as at all other times, every motion of the animal is
+accompanied by a shower of _invisible_ particles more or less teeming
+with bacterial life.
+
+The amount of actual impurities found in milk is often considerable and
+when it is remembered that about one-half of fresh manure dissolves in
+milk,[24] and thus does not appear as sediment, the presence of this
+undissolved residue bespeaks filthy conditions as to milking. From
+actual tests made, it is computed that the city of Berlin, Germany
+consumes about 300 pounds of such dirt and filth daily. Renk has laid
+down the following rule with reference to this insoluble dirt: If a
+sample of milk shows any evidence of impurity settling on a transparent
+bottom within two hours, it should be regarded as too dirty for use.
+
+While the number of organisms here introduced is at all times large, the
+character of the species is of even greater import. Derived primarily
+from dirt and fecal matter, it is no wonder that such forms are able to
+produce very undesirable fermentative changes.
+
+~Influence of milker.~ The condition of the milker is not to be ignored in
+determining all possible factors of infection, for when clothed in the
+dust-laden garments that have been worn all day, a favorable opportunity
+for direct contamination is possible. The filthy practice of wetting the
+hands with milk just before milking is to be condemned. The milker's
+hands should be washed immediately before milking in clean water and
+dried. A pinch of vaseline on hands is sometimes used to obtain a firmer
+grasp and prevents the ready dislodgment of scales.[25] It must also be
+borne in mind that the milker may spread disease through the milk. In
+typhoid fever and diphtheria, the germs often remain in the system for
+weeks and thus make infection possible. Stocking[26] has shown that the
+individual milker exerts a potent influence on the total germ content of
+milk, even where the procedure is quite the same. In sanitary dairies
+milkers are usually clad in white duck suits.
+
+~Milking by machinery.~ Several mechanical devices have been invented for
+milking, some of which have been tested bacteriologically as to their
+efficiency. Harrison[27] has examined the "Thistle" machine but found a
+much higher germ content than with hand-drawn milk. The recent
+introduction of the Burrel-Lawrence-Kennedy machine has led to numerous
+tests in which very satisfactory results have been obtained. If the
+rubber parts of the milker are thoroughly cleaned and kept in lime water
+solution, they remain nearly sterile. When milk is properly handled, the
+germ content may be greatly reduced.
+
+~Reduction in dirt and adherent bacteria.~ No factor of contamination is
+so susceptible of improvement as that which relates to the reduction in
+filth and dirt which gains access during and immediately subsequent to
+the milking. The care which is taken to keep the coat of the animal
+clean by grooming lessens very much the grosser portion of such
+contamination, but with a dry, hairy coat, fine scales and dust
+particles must of necessity be dislodged.[28] Ordinarily the patron
+thinks all evidence of such dirt is removed if the milk is strained, but
+this process only lessens the difficulty; it does not overcome it.
+Various methods are in use, the effectiveness of which is subject to
+considerable variation. Some of these look to the elimination of the
+bacteria after they are once introduced into the milk; others to the
+prevention of infection in the first place.
+
+_1. Straining the milk._ Most of the visible, solid particles of filth,
+such as hairs, dirt particles, etc., can be removed by simple straining,
+the time-honored process of purification. As ordinarily carried out,
+this process often contributes to instead of diminishing the germ life
+in milk. The strainer cloths unless washed and thoroughly sterilized by
+boiling harbor multitudes of organisms from day to day and may thus
+actually add to the organisms present. Various methods have been
+suggested for this simple process, but the most practical and efficient
+strainer is that made of fine wire gauze to which is added 3-4 layers of
+cheese cloth, the whole to set over the storage milk can.
+
+_2. Filtration._ In Europe especially, the system of cleaning milk by
+filtration through sand, gravel and other substances has been quite
+extensively used. These filters are built in sections and the milk
+passes from below upward. The filtering substance is washed in hot water
+immediately after use and then steamed and finally baked. While it is
+possible to remove the solid impurities in this way, the germ content
+cannot be greatly reduced.[29] Cellulose filters have also been
+suggested[30] as an improvement over the sand filters. Methods of
+filtration of this character have not been used under commercial
+conditions here in this country.
+
+_3. Clarification in separator._ Within recent years the custom has
+grown of clarifying milk or removing the visible dirt by passing the
+milk through a centrifugal separator the cream and skim milk being
+remixed after separation. This process naturally removes the solid
+impurities as dirt, hairs, epithelial scales and cells, also some of the
+casein, making what is known as centrifuge slime. This conglomerate mass
+is incomparably rich in germ life and the natural inference would be
+that the bacterial content of the milk would be greatly reduced by this
+procedure. Eckles and Barnes[31] noted a reduction of 37 to 56 per cent.
+of the bacteria but others have failed to observe such reductions.[32]
+This condition is explained by the more thorough breaking up of the
+bacterial masses in the process, thus apparently not reducing them in
+numbers.
+
+It is somewhat surprising that in spite of the elimination of much
+organic matter and bacteria, such clarified milk sours as rapidly as the
+untreated product.[33]
+
+The mechanical shock of separation ruptures the clusters of fat globules
+and so delays creaming and also lessens the consistency of cream derived
+from such milk. This practical disadvantage together with the increased
+expense of the operation and the failure to materially enhance the
+keeping quality of the product outweigh the advantage which might come
+from removal of solid impurities which can be largely accomplished on
+the farm by efficient straining.
+
+_4. Washing the udder._ If a surface is moist, bacteria adherent to it
+cannot be dislodged by ordinary movements. Thus the air over
+snow-covered mountains or oceans is relatively devoid of germ life. The
+method of moistening the udder is applied with success to the hairy coat
+of the animal thus subserving the double purpose of cleaning the animal
+and preventing in large measure the continual dislodgment of dust
+particles. After these parts have been well carded to remove loose hairs
+and dirt particles, the skin should be thoroughly moistened with clean
+water and then wiped. It has been urged that this procedure lessens the
+yield of milk but Eckles[34] concludes from experiments that when the
+animal becomes accustomed to this treatment, no noticeable change in
+amount of milk or butter-fat is produced.
+
+The effectiveness of this method in reducing the actual amount of dirt
+and filth introduced into the milk as well as the great diminution in
+germ life is shown by the instructive experiments of Fraser[35] who
+found that the actual amount of dirt dislodged from udders of apparently
+clean animals during the milking process was three and one-half times as
+much as when the cow's udders were washed. From udders visibly polluted
+one ounce of such filth was removed in 275 pounds of milk, while from
+cows whose udders had been washed, the same amount of dirt was
+distributed through 24,030 pounds.
+
+Fraser observed as a result of 420 examinations that the average germ
+content of 4-inch culture dishes under clean but unwashed udders was
+578, while under washed animals it was reduced to 192. From numerous
+tests made in the writer's laboratory, it is evident that the germ
+content of the milk in the pail is increased from 20,000-40,000 bacteria
+_per minute_ during the milking period. By far the larger part of this
+pollution can be easily prevented by cleaning and dampening the udder.
+
+_5. Diminishing exposed surface of pail._ The entrance of organisms into
+the milk can be greatly reduced by lessening the area of the milk pail
+directly exposed to the dust shower. A number of so-called sanitary or
+hygienic milk pails have been devised for this purpose. In one case the
+pail is smaller at the top than bottom, but in most of them the common
+form is kept and the exposed area is lessened by means of a cover, the
+milk being received through a narrower opening. In some cases, strainers
+are also interposed so as to remove more effectually the coarse
+particles. It is necessary to have these covers and strainers
+constructed in such a way so they can be easily removed and cleaned.
+
+[Illustration: FIG. 10. Sanitary milk pails designed to diminish the
+introduction of hairs, scales, dirt, etc., into milk.]
+
+Stocking tested one of these pails (A, Fig. 10) and found that 63 per
+cent of the dirt and 29 per cent. of the bacteria were prevented from
+passing into the milk. Eckles examined one in which the germ content was
+found to be 3200 per cc. as against 43200 per cc. in a common open
+pail. This milk did not sour until it was 64 hours old in the first case
+while in the latter it curdled in 43 hours.
+
+~Air in barn.~ The atmosphere of the barn where the milking is done may
+frequently contribute considerable infection. Germ life is incapable of
+development in the air, but in a dried condition, organisms may retain
+their vitality for long periods. Anything which contributes to the
+production of dust in the stable and aids in the stirring up of the same
+increases the number of organisms to be found in the air (Fig. 11).
+Thus, the feeding of dry fodder and the bedding of animals with straw
+adds greatly to the germ life floating in the air. Dust may vary much in
+its germ content depending upon its origin. Fraser found the dust from
+corn meal to contain only about one-sixth to one-eighth as much germ
+life as that from hay or bran.[36] In time most of these dust particles
+settle to the floor, but where the herd is kept in the barn, the
+constant movement of the animals keeps these particles more or less in
+motion. Much can be done by forethought to lessen the germ content of
+stables. Feeding dry feed should not be done until after milking.[37] In
+some of the better sanitary dairies, it is customary to have a special
+milking room that is arranged with special reference to the elimination
+of all dust. In this way this source of infection may be quite obviated
+as the air of a clean, still room is relatively free from bacteria,
+especially if the floor is moistened. It has often been noted that the
+milk of stall-fed animals does not keep as well as that milked out of
+doors, a condition in part attributable to the lessened contamination.
+
+[Illustration: FIG. 11. Effect of contaminated air. The number of spots
+indicate the colonies that have developed from the bacteria which fell
+in 30 seconds on the surface of the gelatin plate (3 inches in
+diameter). This exposure was made at time the cows were fed.]
+
+~Relative importance of different sources of infection.~ It is impossible
+to measure accurately the influence of the different sources of
+infection as these are continually subject to modification in each and
+every case. As a general rule, however, where milk is drawn and handled
+without any special care, the utensils and the animal contribute the
+larger proportion of dirt and bacteria that find their way into milk.
+Where the manner of milking and handling is designed to exclude the
+largest number of organisms possible, the bacteria appearing in the fore
+milk make up the major portion remaining. By putting into practice the
+various suggestions that have been made with reference to diminishing
+the bacterial content of milk, it is possible to greatly reduce the
+number of organisms found therein, and at the same time materially
+improve the keeping quality of the milk. Backhaus[38] estimates that
+the germ life in milk can be easily reduced to one-two thousandth of its
+original number by using care in milking. He reports a series of
+experiments covering two years in which milk was secured that averaged
+less than 10,000 bacteria per cc., while that secured under ordinary
+conditions averaged over 500,000.
+
+[Illustration: FIG. 12. Bacterial content of milk handled in ordinary
+way. Each spot represents a colony growing on gelatin plate. Compare
+with Fig. 13, where same quantity of milk is used in making culture.
+Over 15,000 bacteria per cc. in this milk.]
+
+Fig. 13 gives an illustration as to what care in milking will do in the
+way of eliminating bacteria. Fig. 12 shows a gelatin plate seeded with
+the same quantity of milk that was used in making the culture indicated
+by Fig. 13. The first plate was inoculated with milk drawn under good
+conditions, the germ content of which was found to be 15,500 bacteria
+per cc., while the sample secured under as nearly aseptic conditions as
+possible (Fig. 13) contained only 330 organisms in the same volume.
+
+[Illustration: FIG. 13. Bacterial content of milk drawn with care.
+Diminished germ content is shown by smaller number of colonies (330
+bacteria per cc.). Compare this culture with that shown in Fig. 12.]
+
+~"Sanitary" or "certified" milk.~ Within recent years there has been more
+or less generally introduced into many cities, the custom of supplying
+high grade milk that has been handled in a way so as to diminish its
+germ content as much as possible. Milk of this character is frequently
+known as "sanitary," "hygienic" or "certified," the last term being used
+in connection with a certification from veterinary authorities or boards
+of health as to the freedom of animals from contagious disease.
+Frequently a numerical bacterial standard is exacted as a pre-requisite
+to the recommendation of the board of examining physicians. Thus, the
+Pediatric Society of Philadelphia requires all children's milk that
+receives its recommendation to have not more than 10,000 bacteria per
+cc. Such a standard has its value in the scrupulous cleanliness that
+must prevail in order to secure these results. This in itself is
+practically a guarantee of the absence of those bacteria liable to
+produce trouble in children. The number of organisms found in such milks
+is surprisingly low when compared with ordinary milk. Naturally, there
+is considerable fluctuation from day to day, and occasionally the germ
+content is increased to a high figure without any apparent reason. The
+average results though, show a greatly reduced number of organisms. De
+Schweinitz[39] found in a Washington dairy in 113 examinations extending
+throughout a year, an average of 6,485 bacteria per cc. The daily
+analyses made of the Walker-Gordon supply sold in Philadelphia for an
+entire year, showed that the milk almost always contained less than
+5,000 bacteria per cc. and on 120 days out of the year the germ content
+was 1,000 organisms per cc. or less.
+
+From a practical point of view, the improvement in quality of sanitary
+milk, in comparison with the ordinary product is seen in the enhanced
+keeping quality. During the Paris Exposition in 1900, milk and cream
+from several such dairies in the United States were shipped to Paris,
+arriving in good condition after 15 to 18 days transit. When milk has
+been handled in such a way, it is evident that it is much better suited
+to serve as a food supply than where it has undergone the fermentative
+changes incident to the development of myriads of organisms.
+
+~Application of foregoing precautions to all milk producers.~ Milk is so
+susceptible to bacterial changes that it is necessary to protect it from
+invasion, if its original purity is to be maintained, and yet, from a
+practical point of view, the use to which it is destined has much to do
+with the care necessary to take in handling. The effect of the bacterial
+contamination of milk depends largely upon the way in which the product
+is used. To the milk-man engaged in the distribution of milk for direct
+consumption, all bacterial life is more or less of a detriment, while to
+the butter-maker and cheese-maker some forms are a direct necessity. It
+is unnecessary and impracticable to require the same degree of care in
+handling milk destined to be worked up into factory products as is done,
+for instance, in sanitary milk supplies, but this fact should not be
+interpreted to mean that the care of milk for factories is a matter of
+small consequence. In fact no more important dairy problem exists, and
+the purer and better the quality of the raw material the better the
+product will be. Particularly is this true with reference to
+cheese-making.
+
+Dairymen have learned many lessons in the severe school of experience,
+but it is earnestly to be hoped that future conditions will not be
+summed up in the words of the eminent German dairy scientist, Prof.
+Fleischmann, when he says that "all the results of scientific
+investigation which have found such great practical application in the
+treatment of disease, in disinfection, and in the preservation of
+various products, are almost entirely ignored in milking."
+
+~Growth of bacteria in milk.~ Milk is so well suited as a medium for the
+development of germ life that it might be expected that all
+microörganisms would develop rapidly therein, and yet, as a matter of
+fact, growth does not begin at once, even though the milk may be richly
+seeded. At ordinary temperatures, such as 70° F., no appreciable
+increase is to be noted for a period of 6-9 hours; at lower temperatures
+(54° F.) this period is prolonged to 30-40 hours or even longer. After
+this period has elapsed, active growth begins and continues more or less
+rapidly until after curdling.
+
+The cause of this suspended development is attributed to the germicidal
+properties inherent to the milk.[40]
+
+Milk is of course seeded with a considerable variety of organisms at
+first. The liquefying and inert species are the most abundant, the
+distinctively lactic acid class occurring sparsely, if at all. As milk
+increases in age, germ growth begins to occur. More or less development
+of all types go on, but soon the lactic species gain the ascendency,
+owing to their being better suited to this environment; they soon
+outstrip all other species, with the result that normal curdling
+generally supervenes. The growth of this type is largely conditioned by
+the presence of the milk sugar. If the sugar is removed from milk by
+dialysis, the liquid undergoes putrefactive changes due to the fact that
+the putrefactive bacteria are able to grow if no acid is produced.
+
+~Relation of temperature to growth.~ When growth does once begin in milk,
+the temperature at which it is stored exerts the most profound effect on
+the rate of development. When milk is not artificially cooled, it
+retains its heat for some hours, and consequently the conditions become
+very favorable for the rapid multiplication of the contained organisms,
+as is shown in following results obtained by Freudenreich[41]:
+
+    _No. of bacteria per cc. in milk kept at different temperatures._
+
+                                 77° F.       95° F.
+     5 hrs. after milking       10,000        30,000
+     8  "     "    "            25,000    12,000,000
+    12  "     "    "            46,000    35,280,000
+    26  "     "    "         5,700,000    50,000,000
+
+[Illustration: FIG. 14. Effect of cooling milk on the growth of
+bacteria.]
+
+Conn[42] is inclined to regard temperature of more significance in
+determining the keeping quality than the original infection of the milk
+itself. Milk which curdled in 18 hours at 98° F., did not curdle in 48
+hours at 70°, and often did not change in two weeks, if the temperature
+was kept at 50° F.
+
+Where kept for a considerable period at this low temperature, the milk
+becomes filled with bacteria of the undesirable putrefactive type, the
+lactic group being unable to form acid in any appreciable amounts.
+Running well water can be used for cooling, if it is possible to secure
+it at a temperature of 48°-50° F. The use of ice, of course, gives
+better results, and in summer is greatly to be desired. The influence of
+these lowered temperatures makes it possible to ship milk long
+distances[43] by rail for city supplies, if the temperature is kept low
+during transit.
+
+~Mixing night and morning milk.~ Not infrequently it happens when old milk
+is mixed with new, that the course of the fermentative changes is more
+rapid than would have been the case if the two milks had been kept
+apart. Thus, adding the cooled night milk to the warm morning milk
+sometimes produces more rapid changes in both. The explanation for this
+often imperfectly understood phenomenon is that germ growth may have
+gone on in the cooled milk, and when this material is added to the
+warmer, but bacteria-poor, fresh milk, the temperature of the whole mass
+is raised to a point suitable for the more rapid growth of all bacteria
+than would have occurred if the older milk had been kept chilled.
+
+~Number of bacteria in milk.~ The number of organisms found in milk
+depends upon (1) the original amount of contamination, (2) the age of
+the milk, and (3) the temperature at which it has been held. These
+factors all fluctuate greatly in different cases; consequently, the germ
+life is subject to exceedingly wide variations. Here in America, milk
+reaches the consumer with less bacteria than in Europe, although it may
+often be older. This is due largely to the more wide-spread use of ice
+for chilling the milk _en route_ to market. Examinations have been made
+of various supplies with the following results: Sedgwick and Batchelder
+found in 57 tests of Boston milk from 30,000-4,220,000 per cc. Jordan
+and Heineman found 30% of samples of Chicago milk to range from 100,000
+to 1,000,000 while nearly one half were from 1-20,000,000 per cc. The
+germ content of city milks increase rapidly in the summer months.
+Park[44] found 250,000 organisms per cc. in winter, about 1,000,000 in
+cool weather and 5,000,000 per cc. in hot summer weather. Knox and
+Bassett in Baltimore report 1,500,000 in spring and nearly 4,500,000 in
+summer. Eckles[45] studied milk under factory conditions. He finds from
+1,000,000 to 5,000,000 per cc. in winter, and in summer from 15-30
+millions.
+
+~Bacterial standards for city supplies.~ It would be very desirable to
+have a hygienic standard for city milk supplies, as there is a butter
+fat and milk-solid test, but the wide spread variation in germ content
+and the impracticability of utilizing ordinary bacterial determinations
+(on account of time required) makes the selection of such a standard
+difficult. Some hold, as Park, that such a standard is feasible. The New
+York City Milk commission has set a standard of 30,000 bacteria per cc.
+for their certified milk and 100,000 per cc. for inspected milk.
+Rochester, N. Y. has attempted the enforcement of such a standard
+(limit, 100,000 per cc.) with good results it is claimed while Boston
+has placed the legal limit at 500,000 per cc. Quantitative standards
+would seem more applicable to "certified" or sanitary supplies than to
+general city supplies, where the wide range in conditions lead to such
+enormous variations that the bacterial standard seems too refined a
+method for practical routine inspection.
+
+~Other tests.~ Any test to be of much service must be capable of being
+quickly applied. The writer believes for city milk inspectors that the
+acid test would serve a very useful purpose. This test measures the
+acidity of the milk. There is, of course, no close and direct
+relationship between the development of acidity and the growth of
+bacteria, yet in a general way one follows the other at normal
+temperatures. Where the temperature is kept rather low, bacterial growth
+might go on without much acid development, but in the great majority of
+cases a high degree of acidity means either old milk, in which there has
+been a long period of incubation, or high temperature, where rapid
+bacterial growth has been possible. Either of these conditions
+encourages germ growth and thus impairs the quality of the milk.
+
+The rapid determination of acidity may be made in an approximate manner
+so as to serve as a test at the weigh-can or intake. The test is best
+made by the use of the well known alkaline tablet which is composed of a
+solid alkali, and the indicator, phenolphthalein. The tablets are
+dissolved in water, one to each ounce used. A number of white cups are
+filled with the proper quantity of the solution necessary to neutralize
+say, 0.2 per cent. lactic acid. Then, as the milk is delivered, the
+proper quantity is taken from each can to which is added the tablet
+solution. A retention of the pink color shows that there is not
+sufficient acid in the milk to neutralize the alkali used; a
+disappearance of color indicates an excess of acid. The standard
+selected is of course arbitrarily chosen. In our experience, 0.2 per
+cent. acidity (figured as lactic), has proven a satisfactory point. With
+carefully handled milk the acidity ought to be reduced to about 0.15 per
+cent. The acidity of the milk may be abnormally reduced if milk is kept
+in rusty cans, owing to action of acid on the metal.
+
+[Illustration: FIG. 15. Apparatus used in making rapid acid test. A
+definite quantity of the alkali solution and indicator is placed in the
+white tea cup. To this is added the quantity of milk by means of the
+cartridge measure which would just be neutralized if the acidity was 0.2
+per cent. A retention of the pink color shows a low acid milk; its
+disappearance, a high acid milk.]
+
+~Kinds of bacteria in milk.~ The number of bacteria in milk is not of so
+much consequence as the kinds present. With reference to the number of
+different species, the more dirt and foreign matter the milk contains,
+the larger the number of varieties found in the same. While milk may
+contain forms that are injurious to man, still the great majority of
+them have no apparent effect on human health. In their effect on milk,
+the case is much different. Depending upon their action in milk, they
+may be grouped into three classes:
+
+1. Inert group, those producing no visible change in the milk.
+
+2. Sour milk forms, those breaking up the milk sugar with or without the
+formation of gas.
+
+3. Digesting or peptonizing group, those capable of rendering the casein
+of milk soluble and more or less completely digested.
+
+A surprisingly large number of bacteria that are found in milk belong to
+the first class. Undoubtedly they affect the chemical characteristics of
+the milk somewhat, but not to the extent that it becomes physically
+perceptible. Eckles[46] reports in a creamery supply from 20 to 55 per
+cent. of entire flora as included in this class.
+
+By far the most important group is that embraced under the second head.
+It includes not only the true lactic acid types in which no gas is
+formed, but those species capable of producing gases and various kinds
+of acids. These organisms are the distinctively milk bacteria, although
+they do not predominate when the milk is first drawn. Their adaptation
+to this medium is normally shown, however, by this extremely rapid
+growth, in which they soon gain the ascendency over all other species
+present. It is to this lactic acid class that the favorable
+flavor-producing organisms belong which are concerned in butter-making.
+They are also indispensable in cheese-making.
+
+The third class represents those capable of producing a liquefied or
+digested condition on gelatin or in milk. They are usually the
+spore-bearing species which gain access from filth and dirt. Their high
+powers of resistance due to spores makes it difficult to eradicate this
+type, although they are materially held in subjection by the lactic
+bacteria. The number of different kinds that have been found in milk is
+quite considerable, something over 200 species having been described
+more or less thoroughly. In all probability, however, many of these
+forms will be found to be identical when they are subjected to a more
+critical study.
+
+~Direct absorption of taints.~ A tainted condition in milk may result from
+the development of bacteria, acting upon various constituents of the
+milk, and transforming these in such a way as to produce by-products
+that impair the flavor or appearance of the liquid; or it may be
+produced by the milk being brought in contact with any odoriferous or
+aromatic substance, under conditions that permit of the direct
+absorption of such odors.
+
+This latter class of taints is entirely independent of bacterial action,
+and is largely attributable to the physical property which milk
+possesses of being able to absorb volatile odors, the fat in particular,
+having a great affinity for many of these substances. This direct
+absorption may occur before the milk is withdrawn from the animal, or
+afterwards if exposed to strong odors.
+
+It is not uncommon for the milk of animals advanced in lactation to have
+a more or less strongly marked odor and taste; sometimes this is apt to
+be bitter, at other times salty to the taste. It is a defect that is
+peculiar to individual animals and is liable to recur at approximately
+the same period in lactation.
+
+The peculiar "cowy" or "animal odor" of fresh milk is an inherent
+peculiarity that is due to the direct absorption of volatile elements
+from the animal herself. This condition is very much exaggerated when
+the animal consumes strong-flavored substances as garlic, leeks, turnips
+and cabbage. The volatile substances that give to these vegetables their
+characteristic odor are quickly diffused through the system, and if such
+foods are consumed some few hours before milking, the odor in the milk
+will be most pronounced. The intensity of such taints is diminished
+greatly and often wholly disappears, if the milking is not done for some
+hours (8-12) after such foods are consumed.
+
+This same principle applies in lesser degree to many green fodders that
+are more suitable as feed for animals, as silage, green rye, rape, etc.
+Not infrequently, such fodders as these produce so strong a taint in
+milk as to render it useless for human use. Troubles from such sources
+could be entirely obviated by feeding limited quantities of such
+material immediately after milking. Under such conditions the taint
+produced is usually eliminated before the next milking. The milk of
+swill-fed cows is said to possess a peculiar taste, and the urine of
+animals fed on this food is said to be abnormally acid. Brewers' grains
+and distillery slops when fed in excess also induce a similar condition
+in the milk.
+
+Milk may also acquire other than volatile substances directly from the
+animal, as in cases where drugs, as belladonna, castor oil, sulfur,
+turpentine, jalap, croton oil, and many others have been used as
+medicine. Such mineral poisons as arsenic have been known to appear
+eight hours after ingestion, and persist for a period of three weeks
+before being eliminated.
+
+~Absorption of odors after milking.~ If milk is brought in contact with
+strong odors after being drawn from the animal, it will absorb them
+readily, as in the barn, where frequently it is exposed to the odor of
+manure and other fermenting organic matter.
+
+It has long been a popular belief that milk evolves odors and cannot
+absorb them so long as it is warmer than the surrounding air, but from
+experimental evidence, the writer[47] has definitely shown that the
+direct absorption of odors takes place much more rapidly when the milk
+is warm than when cold, although under either condition, it absorbs
+volatile substances with considerable avidity. In this test fresh milk
+was exposed to an atmosphere impregnated with odors of various essential
+oils and other odor-bearing substances. Under these conditions, the
+cooler milk was tainted very much less than the milk at body temperature
+even where the period of exposure was brief. It is therefore evident
+that an exposure in the cow barn where the volatile emanations from the
+animals themselves and their excreta taint the air will often result in
+the absorption of these odors by the milk to such an extent as to
+seriously affect the flavor.
+
+The custom of straining the milk in the barn has long been deprecated as
+inconsistent with proper dairy practice, and in the light of the above
+experiments, an additional reason is evident why this should not be
+done.
+
+Even after milk is thoroughly cooled, it may absorb odors as seen where
+the same is stored in a refrigerator with certain fruits, meats, fish,
+etc.
+
+~Distinguishing bacterial from non-bacterial taints.~ In perfectly fresh
+milk, it is relatively easy to distinguish between taints caused by the
+growth of bacteria and those attributable to direct absorption.
+
+If the taint is evident at time of milking, it is in all probability due
+to character of feed consumed, or possibly to medicines. If, however,
+the intensity of the taint grows more pronounced as the milk becomes
+older, then it is probably due to living organisms, which require a
+certain period of incubation before their fermentative properties are
+most evident.
+
+Moreover, if the difficulty is of bacterial origin, it can be frequently
+transferred to another lot of milk (heated or sterilized is preferable)
+by inoculating same with some of the original milk. Not all abnormal
+fermentations are able though to compete with the lactic acid bacteria,
+and hence outbreaks of this sort soon die out by the re-establishment of
+more normal conditions.
+
+~Treatment of directly absorbed taints.~ Much can be done to overcome
+taints of this nature by exercising greater care in regard to the feed
+of animals, and especially as to the time of feeding and milking. But
+with milk already tainted, it is often possible to materially improve
+its condition. Thorough aeration has been frequently recommended, but
+most satisfactory results have been obtained where a combined process of
+aeration and pasteurization was resorted to. Where the milk is used in
+making butter, the difficulty has been successfully met by washing the
+cream with twice its volume of hot water in which a little saltpeter has
+been dissolved (one teaspoonful per gallon), and then separating it
+again.[48]
+
+The treatment of abnormal conditions due to bacteria has been given
+already under the respective sources of infection, and is also still
+further amplified in following chapter.
+
+~Aeration.~ It is a common belief that aeration is of great aid in
+improving the quality of milk, yet, when closely studied, no material
+improvement can be determined, either where the milk is made into butter
+or sold as milk. Dean in Canada and Storch in Denmark have both
+experimented on the influence of aeration in butter making, but with
+negative results. Marshall and Doane failed to observe any material
+improvement in keeping quality, but it is true that odors are eliminated
+from the milk during aeration. The infection of the milk during aeration
+often more than counterbalances the reputed advantage. Especially is
+this so if the aeration is carried out in an atmosphere that is not
+perfectly clean and pure.
+
+In practice aeration differs greatly. In some cases, air is forced into
+the milk; in others, the milk is allowed to distribute itself in a thin
+sheet over a broad surface and fall some distance so that it is brought
+intimately in contact with the air. This latter process is certainly
+much more effective if carried out under conditions which preclude
+infection. It must be remembered that aeration is frequently combined
+with cooling, in which case the reputed advantages may not be entirely
+attributable to the process of aeration.
+
+~Infection of milk in the factory.~ The problem of proper handling of milk
+is not entirely solved when the milk is delivered to the factory or
+creamery, although it might be said that the danger of infection is much
+greater while the milk is on the farm.
+
+In the factory, infection can be minimized because effective measures of
+cleanliness can be more easily applied. Steam is available in most
+cases, so that all vats, cans, churns and pails can be thoroughly
+scalded. Special emphasis should be given to the matter of cleaning
+pumps and pipes. The difficulty of keeping these utensils clean often
+leads to neglect and subsequent infection. In Swiss cheese factories,
+the custom of using home-made rennet solutions is responsible for
+considerable factory infection. Natural rennets are soaked in whey which
+is kept warm in order to extract the rennet ferment. This solution when
+used for curdling the milk often adds undesirable yeasts and other
+gas-generating organisms, which are later the cause of abnormal ferment
+action in the cheese (See page 186).
+
+The influence of the air on the germ content of the milk is, as a rule,
+overestimated. If the air is quiet, and free from dust, the amount of
+germ life in the same is not relatively large. In a creamery or factory,
+infection from this source ought to be much reduced, for the reason that
+the floors and wall are, as a rule, quite damp, and hence germ life
+cannot easily be dislodged. The majority of organisms found under such
+conditions come from the person of the operators and attendants. Any
+infection can easily be prevented by having the ripening cream-vats
+covered with a canvas cloth. The clothing of the operator should be
+different from the ordinary wearing-apparel. If made of white duck, the
+presence of dirt is more quickly recognized, and greater care will
+therefore be taken than if ordinary clothes are worn.
+
+The surroundings of the factory have much to do with the danger of germ
+infection. Many factories are poorly constructed and the drainage is
+poor, so that filth and slime collect about and especially under the
+factory. The emanations from these give the peculiar "factory odor" that
+indicates fermenting matter. Not only are these odors absorbed
+directly, but germ life from the same is apt to find its way into the
+milk. Connell[49] has recently reported a serious defect in cheese that
+was traced to germ infection from defective factory drains.
+
+The water supply of a factory is also a question of prime importance.
+When taken from a shallow well, especially if surface drainage from the
+factory is possible, the water may be contaminated to such an extent as
+to introduce undesirable bacteria in such numbers that the normal course
+of fermentation may be changed. The quality of the water, aside from
+flavor, can be best determined by making a curd test (p. 76) which is
+done by adding some of the water to boiled milk and incubating the same.
+If "gassy" fermentations occur, it signifies an abnormal condition. In
+deep wells, pumped as thoroughly as is generally the case with factory
+wells, the germ content should be very low, ranging from a few score to
+a few hundred bacteria per cc. at most.
+
+Harrison[50] has recently traced an off-flavor in cheese in a Canadian
+factory to an infection arising from the water-supply. He found the same
+germ in both water and cheese and by inoculating a culture into
+pasteurized milk succeeded in producing the undesirable flavor. The
+danger from ice is much less, for the reason that good dairy practice
+does not sanction using ice directly in contact with milk or cream.
+Then, too, ice is largely purified in the process of freezing, although
+if secured from a polluted source, reliance should not be placed in the
+method of purification; for even freezing does not destroy all
+vegetating bacteria.
+
+FOOTNOTES:
+
+[1] Olson. 24 Rept. Wis. Expt. Stat., 1907.
+
+[2] Erf and Melick Bull. 131, Kan. Expt. Stat., Apr. 1905.
+
+[3] Storch (40 Rept. Danish Expt. Stat., Copenhagen, 1898) has devised a
+test whereby it can be determined whether this treatment has been
+carried out or not: Milk contains a soluble enzym known as galactase
+which has the property of decomposing hydrogen peroxid. If milk is
+heated to 176° F. (80° C.) or above, this enzym is destroyed so that the
+above reaction no longer takes place. If potassium iodid and starch are
+added to unheated milk and the same treated with hydrogen peroxid, the
+decomposition of the latter agent releases oxygen which acts on the
+potassium salt, which in turn gives off free iodine that turns the
+starch blue.
+
+[4] McKay, N. Y. Prod. Rev., Mch. 22, 1899.
+
+[5] Doane, Bull. 79, Md. Expt. Stat., Jan. 1902.
+
+[6] Harrison, 22 Rept. Ont. Agr'l Coll., 1896, p. 113.
+
+[7] Moore and Ward, Bull. 158, Cornell Expt. Stat., Jan. 1899; Ward,
+Bull. 178, Cornell Expt. Stat., Jan. 1900.
+
+[8] Harrison, 22 Rept. Ont. Agr. Coll., 1896, p. 108; Moore, 12 Rept.
+Bur. Animal Ind., U. S. Dept. Ag., 1895-6, p. 261.
+
+[9] Moore, Bacteria in Milk, N. Y. Dept. Ag., 1902.
+
+[10] Freudenreich, Cent. f. Bakt., II Abt., 10: 418, 1903.
+
+[11] Harrison, 22 Rept. Ont. Agr. Coll., 1896, p. 108.
+
+[12] Marshall, Bull. 147, Mich. Expt. Stat., p. 42.
+
+[13] Moore and Ward, Bull. 158, Cornell Expt. Stat., Jan. 1899.
+
+[14] Burr, R. H. Cent. f. Bakt., II Abt., 8: 236, 1902. Freudenreich, l.
+c. p. 418. Ward, Bull. 178, Cornell Expt. Stat., p. 277. Bolley (Cent.
+f. Bakt., II Abt., 1: 795, 1895), in 30 experiments found 12 out of 16
+species to belong to lactic class. Harrison (Trans. Can. Inst., 7: 474,
+1902-3) records the lactic type as most commonly present.
+
+[15] Ford, Journ. of Hyg., 1901, 1: 277.
+
+[16] Freudenreich, l. c. p. 421.
+
+[17] Stocking, Bull. 42, Storrs Expt. Stat., June, 1906.
+
+[18] Dinwiddie, Bull, 45 Ark. Expt. Stat., p. 57. Ward, Journ. Appld.
+Mic. 1: 205, 1898. Appel, Milch Zeit., No. 17, 1900. Harrison and
+Cumming, Journ. Appld. Mic. 5: 2087. Russell and Hastings, 21 Rept. Wis.
+Expt. Stat., 158, 1904.
+
+[19] Fokker, Zeit. f. Hyg., 9: 41, 1890.
+
+[20] Freudenreich, Ann. de Microg., 3: 118, 1891.
+
+[21] Hunziker, Bull. 197, Cornell Expt. Stat., Dec. 1901.
+
+[22] Freudenreich, Cent. f. Bakt., II Abt., 10: 417, 1903.
+
+[23] This general statement is in the main correct, although Ford
+(Journ. of Hyg., 1: 277, 1901) claims to have found organisms sparingly
+present in healthy tissues.
+
+[24] Backhaus, Milch Zeit., 26: 357, 1897.
+
+[25] Freudenreich, Die Bakteriologie, p. 30.
+
+[26] Stocking, Bull. 42, Storrs Expt. Stat., June 1906.
+
+[27] Harrison, Cent. f. Bakt., II Abt., 5: 183, 1899.
+
+[28] Drysdale, Trans. High. and Agr. Soc. Scotland. 5 Series, 10: 166,
+1898.
+
+[29] Schuppan, (Cent. f. Bakt., 13: 155, 1893) claims to have found a
+reduction of 48 per cent. in the Copenhagen filters while in the more
+extended work of Dunbar and Kister (Milch Zeit., pp. 753, 787, 1899) the
+bacterial content was higher in the filtered milk in 17 cases out of 22.
+
+[30] Backhaus and Cronheim, Journ. f. Landw., 45: 222, 1897.
+
+[31] Eckles and Barnes, Bull. 159 Iowa Expt. Stat., Aug. 1901.
+
+[32] Dunbar and Kister, Milch Zeit., p. 753, 1899. Harrison and Streit,
+Trans. Can. Inst., 7: 488, 1902-3.
+
+[33] Doane, Bull. 88 Md. Expt. Stat., May 1903.
+
+[34] Eckles, Hoard's Dairyman, July 8, 1898.
+
+[35] Fraser, Bull. 91, Ill. Expt. Stat.
+
+[36] Fraser, Bull. 91, Ill. Expt. Stat., Dec. 1903.
+
+[37] Stocking, Bull. 42, Storrs Expt. Stat., June, 1906.
+
+[38] Backhaus. Ber. Landw. Inst. Univ. Königsberg 2: 12, 1897.
+
+[39] De Schweinitz, Nat. Med. Rev., April, 1899.
+
+[40] Conn, Proc. Soc. Amer. Bacteriologists, 1902.
+
+[41] Freudenreich, Ann. de Microg., 2:115, 1890.
+
+[42] Conn, Bull. 26, Storrs Expt. Stat.
+
+[43] New York City is supplied with milk that is shipped 350 miles.
+
+[44] Park, N. Y. Univ. Bull., 1: 85, 1901.
+
+[45] Eckles, Bull. 59, Iowa Expt. Stat., Aug. 1901.
+
+[46] Eckles, Bull. 59, Iowa Expt. Stat., Aug. 1901.
+
+[47] Russell, 15 Rept. Wis. Expt. Stat. 1898, p. 104.
+
+[48] Alvord, Circ. No. 9, U. S. Dept. Agric. (Div. of Bot.).
+
+[49] Connell, Rept. of Commissioner of Agr., Canada, 1897, part XVI, p.
+15.
+
+[50] Harrison, Hoard's Dairyman, March 4, 1898.
+
+
+
+
+CHAPTER IV.
+
+FERMENTATIONS IN MILK AND THEIR TREATMENT.
+
+
+Under the conditions in which milk is drawn, it is practically
+impossible to secure the same without bacterial contamination. The
+result of the introduction of these organisms often changes its
+character materially as most bacteria cause the production of more or
+less pronounced fermentative processes. Under normal conditions, milk
+sours, i. e., develops lactic acid, but at times this more common
+fermentation may be replaced by other changes which are marked by the
+production of some other more or less undesirable flavor, odor or change
+in appearance.
+
+In referring to these changes, it is usually customary to designate them
+after the most prominent by-product formed, but it must be kept in mind
+that generally some other decomposition products are usually produced.
+Whether the organisms producing this or that series of changes prevail
+or not depends upon the initial seeding, and the conditions under which
+the milk is kept. Ordinarily, the lactic acid organisms grow so
+luxuriantly in the milk that they overpower all competitors and so
+determine the nature of the fermentation; but occasionally the milk
+becomes infected with other types of bacteria in relatively large
+numbers and the conditions may be especially suitable to the development
+of these forms, thereby modifying the course of the normal changes that
+occur.
+
+The kinds of bacteria that find it possible to develop in milk may be
+included under two heads:
+
+1. Those which cause no appreciable change in the milk, either in taste,
+odor or appearance. While these are frequently designated as the inert
+bacteria, it must not be supposed that they have absolutely no effect on
+milk. It is probably true in most cases that slight changes of a
+chemical nature are produced, but the nature of the changes do not
+permit of ready recognition.
+
+2. This class embraces all those organisms which, as a result of their
+growth, are capable of producing evident changes. These transformations
+may be such as to affect the taste, as in the sour milk or in the bitter
+fermentations, or the odor, as in some of the fetid changes, or the
+appearance of the milk, as in the slimy and color changes later
+described.
+
+~Souring of milk.~ Ordinarily if milk is allowed to stand for several days
+at ordinary temperatures it turns sour. This is due to the formation of
+lactic acid, which is produced by the decomposition of the milk-sugar.
+While this change is well nigh universal, it does not occur without a
+pre-existing cause, and that is the presence of certain living bacterial
+forms. These organisms develop in milk with great rapidity, and the
+decomposition changes that are noted in souring are due to the
+by-products of their development.
+
+The milk-sugar undergoes fermentation, the chief product being lactic
+acid, although various other by-products, as other organic acids
+(acetic, formic and succinic), different alcohols and gaseous products,
+as CO_{2}, H, N and methane (CH_{4}) are produced in small amounts.
+
+In this fermentation, the acidity begins to be evident to the taste when
+it reaches about 0.3 per cent., calculated as lactic acid. As the
+formation of acid goes on, the casein is precipitated and incipient
+curdling or lobbering of the milk occurs. This begins to be apparent
+when the acidity is about 0.4 per cent., but the curd becomes more solid
+with increasing acidity. The rapidity of curdling is also dependent upon
+the temperature of the milk. Thus milk which at ordinary temperatures
+might remain fluid often curdles when heated. The growth of the bacteria
+is continued until about 0.8 to 1.0 per cent. acid is formed, although
+the maximum amount fluctuates considerably with different lactic acid
+species. Further formation then ceases even though all of the milk-sugar
+is not used up, because of the inability of the lactic bacteria to
+continue their growth in such acid solutions.
+
+As this acidity is really in the milk serum, cream never develops so
+much acid as milk, because a larger proportion of its volume is made up
+of butter-fat globules. This fact must be considered in the ripening of
+cream in butter-making where the per cent. of fat is subject to wide
+fluctuations.
+
+The formation of lactic acid is a characteristic that is possessed by a
+large number of bacteria, micrococci as well as bacilli being numerously
+represented. Still the preponderance of evidence is in favor of the view
+that a few types are responsible for most of these changes. The most
+common type found in spontaneously soured milk changes the milk-sugar
+into lactic acid without the production of any gas. This type has been
+described by various workers on European as well as American milks, and
+is designated by Conn as the _Bact. lactis acidi_ type.[51] It is
+subject to considerable variation under different conditions.
+
+Curiously enough if milk which has been drawn with special care is
+examined immediately after milking, the lactic organisms are not usually
+found. They are incapable of development in the udder itself, as shown
+by injections into the milk cistern. They abound, however, on hay, in
+dust, in the barn air, on the hairy coat of the animal, and from these
+sources easily gain access to the milk. In this medium they find an
+exceptionally favorable environment and soon begin a very rapid growth,
+so that by the time milk is consumed, either in the form of milk or milk
+products, they make up numerically the larger portion of the bacteria
+present.
+
+Another widely disseminated, although numerically less prevalent, type
+is _B. lactis aerogenes_. This type forms gas in milk so that the soured
+milk is torn by the presence of gas bubbles. It also grows more
+luxuriantly in contact with the air.
+
+Other types occur more or less sporadically, some of which are capable
+of liquefying the casein of milk while at the same time they also
+develop lactic acid. Conn and Aikman refer to the fact that over one
+hundred species capable of producing variable quantities of lactic acid
+are already known. It is fair to presume, however, that a careful
+comparative study of these would show that simply racial differences
+exist in many cases, and therefore, that they are not distinct species.
+
+As a group these bacteria are characterized by their inability to
+liquefy gelatin or develop spores. On account of this latter
+characteristic they are easily destroyed when milk is pasteurized. They
+live under aerobic or anaerobic conditions, many of them being able to
+grow in either environment, although, according to McDonnell,[52] they
+are more virulent when air is not excluded.
+
+While growth of these lactic forms may go on in milk throughout a
+relatively wide range in temperature, appreciable quantities of acid are
+not produced except very slowly at temperatures below 50° F.[53]
+
+From the standpoint of frequency the most common abnormal changes that
+occur in milk are those in which gases of varying character are
+developed in connection with acids, from the milk sugar. Other volatile
+products imparting bad flavors usually accompany gas production. These
+fermentations are of most serious import in the cheese industry, as they
+are especially prone to develop in the manufacture of milk into certain
+types of cheese. Not often is their development so rapid that they
+appear in the milk while it is yet in the hands of the milk producer,
+but almost invariably the introduction of the causal organisms takes
+place while the milk is on the farm. Numerous varieties of bacteria
+possess this property of producing gas (H and CO_{2} are most common
+although N and methane (CH_{4}) are sometimes produced). The more common
+forms are those represented by _B. lactis aerogenes_ and the common
+fecal type, _B. coli commune_. The ordinary habitat of this type is dirt
+and intestinal filth. Hence careless methods of milk handling invite
+this type of abnormal change in milk.
+
+It is a wide-spread belief that thunder storms cause milk to sour
+prematurely, but this idea has no scientific foundation. Experiments[54]
+with the electric spark, ozone and loud detonations show no effect on
+acid development, but the atmospheric conditions usually incident to a
+thunder storm are such as permit of a more rapid growth of organisms.
+There is no reason to believe but that the phenomenon of souring is
+wholly related to the development of bacteria. Sterile milks are never
+affected by the action of electric storms.
+
+~"Gassy" milks.~ Where these gas bacteria abound, the amount of lactic
+acid is generally reduced, due to the splitting up of some of the sugar
+into gaseous products. This type of germ life does not seem to be able
+to develop well in the presence of the typical lactic acid non
+gas-forming bacteria.
+
+[Illustration: FIG. 16. Cheese made from "gassy" milk.]
+
+~"Sweet curdling" and digesting fermentations.~ Not infrequently milk,
+instead of undergoing spontaneous souring, curdles in a weakly acid or
+neutral condition, in which state it is said to have undergone "sweet
+curdling." The coagulation of the milk is caused by the action of enzyms
+of a rennet type that are formed by the growth of various species of
+bacteria. Later the whey separates more or less perfectly from the curd,
+producing a "wheyed off" condition. Generally the coagulum in these
+cases is soft and somewhat slimy. The curd usually diminishes in bulk,
+due to the gradual digestion or peptonization of the casein by
+proteid-dissolving enzyms (tryptic type) that are also produced by the
+bacteria causing the change.
+
+A large number of bacteria possess the property of affecting milk in
+this way. So far as known they are able to liquefy gelatin (also a
+peptonizing process) and form spores. The Tyrothrix type of bacteria (so
+named by Duclaux on account of the supposed relation to cheese ripening)
+belongs to this class. The hay and potato forms are also digesters.
+Organisms of this type are generally associated with filth and manure,
+and find their way into the milk from the accumulations on the coat of
+the animal.
+
+Conn[55] has separated the rennet enzym from bacterial cultures in a
+relatively pure condition, while Fermi[56] has isolated the digestive
+ferment from several species.
+
+Duclaux[57] has given to this digesting enzym the name _casease_ or
+cheese ferment. These isolated ferments when added to fresh milk possess
+the power of causing the characteristic curdling and subsequent
+digestion quite independent of cell development. The quantity of ferment
+produced by different species differs materially in some cases. In these
+digestive fermentations, the chemical transformations are profound, the
+complex proteid molecule being broken down into albumoses, peptones,
+amido-acids (tyrosin and leucin) and ammonia as well as fatty acids.
+
+Not infrequently these fermentations gain the ascendency over the normal
+souring change, but under ordinary conditions they are held in abeyance,
+although this type of bacteria is always present to some extent in milk.
+When the lactic acid bacteria are destroyed, as in boiled, sterilized
+or pasteurized milk, these rennet-producing, digesting species develop.
+
+~Butyric acid fermentations.~ The formation of butyric acid in milk which
+may be recognized by the "rancid butter" odor is not infrequently seen
+in old, sour milk, and for a long time was thought to be a continuation
+of the lactic fermentation, but it is now believed that these organisms
+find more favorable conditions for growth, not so much on account of the
+lactic acid formed as in the absence of dissolved oxygen in the milk
+which is consumed by the sour-milk organisms.
+
+Most of the butyric class of bacteria are spore-bearing, and hence they
+are frequently present in boiled or sterilized milk. The by-products
+formed in this series of changes are quite numerous. In most cases,
+butyric acid is prominent, but in addition to this, other organic acids,
+as lactic, succinic, and acetic, are produced, likewise different
+alcohols. Concerning the chemical origin of butyric acid there is yet
+some doubt. Duclaux[58] affirms that the fat, sugar and casein are all
+decomposed by various forms. In some cases, the reaction of the milk is
+alkaline, with other species it may be neutral or acid. This type of
+fermentation has not received the study it deserves.
+
+In milk these organisms are not of great importance, as this
+fermentation does not readily gain the ascendency over the lactic
+bacteria.
+
+~Ropy or slimy milk.~ The viscosity of milk is often markedly increased
+over that which it normally possesses. The intensity of this abnormal
+condition may vary much; in some cases the milk becoming viscous or
+slimy; in others stringing out into long threads, several feet in
+length, as in Fig. 17. Two sets of conditions are responsible for these
+ropy or slimy milks. The most common is where the milk is clotted or
+stringy when drawn, as in some forms of garget. This is generally due to
+the presence of viscid pus, and is often accompanied by a bloody
+discharge, such a condition representing an inflamed state of the udder.
+Ropiness of this character is not usually communicable from one lot of
+milk to another.
+
+[Illustration: FIG. 17. Ropy milk.]
+
+The communicable form of ropy milk only appears after the milk has been
+drawn from the udder for a day or so, and is caused by the development
+of various species of bacteria which find their way into the milk after
+it is drawn. These defects are liable to occur at any season of the
+year. Their presence in a dairy is a source of much trouble, as the
+unsightly appearance of the milk precludes its use as food, although
+there is no evidence that these ropy fermentations are dangerous to
+health.
+
+There are undoubtedly a number of different species of bacteria that are
+capable of producing these viscid changes,[59] but it is quite probable
+that they are not of equal importance in infecting milk under natural
+conditions.
+
+In the majority of cases studied in this country,[60] the causal
+organism seems to be _B. lactis viscosus_, a form first found by Adametz
+in surface waters.[61] This organism possesses the property of
+developing at low temperatures (45°-50° F.), and consequently it is
+often able in winter to supplant the lactic-acid forms. Ward has found
+this germ repeatedly in water tanks where milk cans are cooled; and
+under these conditions it is easy to see how infection of the milk might
+occur. Marshall[62] reports an outbreak which he traced to an external
+infection of the udder; in another case, the slime-forming organism was
+abundant in the barn dust. A defect of this character is often
+perpetuated in a dairy for some time, and may therefore become
+exceedingly troublesome. In one instance in the writer's experience, a
+milk dealer lost over $150 a month for several months from ropy cream.
+Failure to properly sterilize cans, and particularly strainer cloths, is
+frequently responsible for a continuance of trouble of this sort.
+
+The slimy substance formed in milk comes from various constituents of
+the milk, and the chemical character of the slime produced also varies
+with different germs. In some cases the slimy material is merely the
+swollen outer cell membrane of the bacteria themselves as in the case of
+_B. lactis viscosus_; in others it is due to the decomposition of the
+proteids, but often the chief decomposition product appears to come from
+a viscous fermentation of the milk-sugar.
+
+An interesting case of a fermentation of this class being utilized in
+dairying is seen in the use of "lange wei" (long or stringy whey) which
+is employed as a starter in Holland to control the gassy fermentations
+in Edam cheese. This slimy change is due to the growth of
+_Streptococcus Hollandicus_.[63]
+
+~Alcoholic fermentations.~ Although glucose or cane-sugar solutions are
+extremely prone to undergo alcoholic fermentation, milk sugar does not
+readily undergo this change. Where such changes are produced it is due
+to yeasts. Several outbreaks attributable to such a cause have been
+reported.[64] Russell and Hastings[65] have found these milk-sugar
+splitting yeasts particularly abundant in regions where Swiss cheese is
+made, a condition made possible by the use of whey-soaked rennets in
+making such cheese.
+
+Kephir and Koumiss are liquors much used in the Orient which are made
+from milk that has undergone alcoholic fermentation. Koumiss was
+originally made from mare's milk but is now often made from cows' milk
+by adding cane sugar and yeast. In addition to the CO_{2} developed,
+alcohol, lactic acid, and casein-dissolving ferments are formed. Kephir
+is made by adding to milk Kephir grains, which are a mass of yeast and
+bacterial cells. The yeasts produce alcohol and CO_{2} while the
+bacteria change the casein of milk, rendering it more digestible. These
+beverages are frequently recommended to persons who seem to be unable to
+digest raw milk readily. The exact nature of the changes produced are
+not yet well understood.[66]
+
+~Bitter milk.~ The presence of bitter substances in milk may be ascribed
+to a variety of causes. A number of plants, such as lupines, ragweed and
+chicory, possess the property of affecting milk when the same are
+consumed by animals. At certain stages in lactation, a bitter salty
+taste is occasionally to be noted that is peculiar to individual
+animals.
+
+A considerable number of cases of bitter milk have, however, been traced
+to bacterial origin. For a number of years the bitter fermentation of
+milk was thought to be associated with the butyric fermentation, but
+Weigmann[67] showed that the two conditions were not dependent upon each
+other. He found that the organism which produced the bitter taste acted
+upon the casein.
+
+Conn[68] observed a coccus form in bitter cream that was able to impart
+a bitter flavor to milk. Sometimes a bitter condition does not develop
+in the milk, but may appear later in the milk products, as in the case
+of a micrococcus which Freudenreich[69] found in cheese.
+
+Harrison[70] has traced a common bitter condition in Canadian milk to a
+milk-sugar splitting yeast, _Torula amara_ which not only grows rapidly
+in milk but produces an undesirable bitterness in cheddar cheese.
+
+Cream ripened at low temperatures not infrequently develops a bitter
+flavor, showing that the optimum temperature for this type of
+fermentation is below the typical lactic acid change.
+
+Milk that has been heated often develops a bitter condition. The
+explanation of this is that the bacteria producing the bitter substances
+usually possess endospores, and that while the boiling or sterilizing of
+milk easily kills the lactic acid germs, these forms on account of their
+greater resisting powers are not destroyed by the heat.
+
+~Soapy milk:~ A soapy flavor in milk was traced by Weigmann and Zirn[71]
+to a specific bacillus, _B. lactis saponacei_, that they found gained
+access to the milk in one case from the bedding and in another instance
+from hay. A similar outbreak has been reported in this country,[72] due
+to a germ acting on the casein and albumen.
+
+~Red milk.~ The most common trouble of this nature in milk is due to
+presence of blood, which is most frequently caused by some wound in the
+udder. The ingestion of certain plants as sedges and scouring rushes is
+also said to cause a bloody condition; madders impart a reddish tinge
+due to coloring matter absorbed. Defects of this class can be readily
+distinguished from those due to germ growth because they are apparent at
+time of milking. Where blood is actually present, the corpuscles settle
+out in a short time if left undisturbed.
+
+There are a number of chromogenic or color-producing bacteria that are
+able to grow in milk, but their action is so slow that generally they
+are not of much consequence. Moreover their development is usually
+confined to the surface of the milk as it stands in a vessel. The most
+important is the well-known _B. prodigiosus_. Another form found at
+times in milk possessing low acidity[73] is _B. lactis erythrogenes_.
+This species only develops the red color in the dark. In the light, it
+forms a yellow pigment. Various other organisms have been reported at
+different times.[74]
+
+~Blue milk.~ Blue milk has been known for many years, its communicable
+nature being established as long ago as 1838. It appears on the surface
+of milk first as isolated particles of bluish or grey color, which
+later become confluent, the blue color increasing in intensity as the
+acidity increases. The causal organism, _B. cyanogenes_, is very
+resistant toward drying,[75] thus accounting for its persistence. In
+Mecklenberg an outbreak of this sort once continued for several years.
+It has frequently been observed in Europe in the past, but is not now so
+often reported. Occasional outbreaks have been reported in this country.
+
+~Other kinds of colored milk.~ Two or three chromogenic forms producing
+still other colors have occasionally been found in milk. Adametz[76]
+discovered in a sample of cooked milk a peculiar form (_Bacillus
+synxanthus_) that produced a citron-yellow appearance which precipitated
+and finally rendered soluble the casein. Adametz, Conn, and List have
+described other species that confer tints of yellow on milk. Some of
+these are bright lemon, others orange, and some amber in color.
+
+Still other color-producing bacteria, such as those that produce violet
+or green changes in the milk, have been observed. In fact, almost any of
+the chromogenic bacteria are able to produce their color changes in milk
+as it is such an excellent food medium. Under ordinary conditions, these
+do not gain access to milk in sufficient numbers so that they modify the
+appearance of it except in occasional instances.
+
+~Treatment of abnormal fermentations.~ If the taint is recognized as of
+bacterial origin (see p. 57) and is found in the mixed milk of the herd,
+it is necessary to ascertain, first, whether it is a general trouble, or
+restricted to one or more animals. This can sometimes be done by
+separating the milk of the different cows and noting whether any
+abnormal condition develops in the respective samples.
+
+~Fermentation tests.~ The most satisfactory way to detect the presence of
+the taints more often present is to make a fermentation test of one kind
+or another. These tests are most frequently used at the factory, to
+enable the maker to detect the presence of milk that is likely to prove
+unfit for use, especially in cheese making. They are based upon the
+principle that if milk is held at a moderately high temperature, the
+bacteria will develop rapidly. A number of different methods have been
+devised for this purpose. In Walther's lacto-fermentator samples of milk
+are simply allowed to stand in bottles or glass jars until they sour.
+They are examined at intervals of several hours. If the curdled milk is
+homogeneous and has a pure acid smell, the milk is regarded as all
+right. If it floats in a turbid serum, is full of gas or ragged holes,
+it is abnormal. As generally carried out, no attempt is made to have
+these vessels sterile. Gerber's test is a similar test that has been
+extensively employed in Switzerland. Sometimes a few drops of rennet are
+added to the milk so as to curdle the same, and thus permit of the more
+ready detection of the gas that is evolved.
+
+~Wisconsin curd test.~ The method of testing milk described below was
+devised at the Wisconsin Experiment Station in 1895 by Babcock, Russell
+and Decker.[77] It was used first in connection with experimental work
+on the influence of gas-generating bacteria in cheese making, but its
+applicability to the detection of all taints in milk produced by
+bacteria makes it a valuable test for abnormal fermentations in general.
+
+In the curd test a small pat of curd is made in a glass jar from each
+sample of milk. These tests may be made in any receptacle that has been
+cleaned in boiling water, and to keep the temperature more nearly
+uniform these jars should be immersed in warm water, as in a wash tub or
+some other receptacle. When the milk is about 95° F., about ten drops of
+rennet extract are added to each sample and mixed thoroughly with the
+milk. The jars should then remain undisturbed until the milk is
+completely curdled; then the curd is cut into small pieces with a case
+knife and stirred to expel the whey. The whey should be poured off at
+frequent intervals until the curd mats. If the sample be kept at blood
+heat (98° F.) for six to eight hours, it will be ready to examine.
+
+[Illustration: FIG. 18. Improved bottles for making curd test. _A_, test
+bottle complete; _B_, bottle showing construction of cover; _S_, sieve
+to hold back the curd when bottle is inverted; _C_, outer cover with _(D
+H)_ drain holes to permit of removal of whey.]
+
+More convenient types of this test than the improvised apparatus just
+alluded to have been devised by different dairy manufacturers.
+Generally, they consist of a special bottle having a full-sized top,
+thus permitting the easy removal of the curd. The one shown in Fig. 18
+is provided with a sieve of such construction that the bottles will
+drain thoroughly if inclined in an inverted position.
+
+~Interpretation of results of test.~ The curd from a good milk has a firm,
+solid texture, and should contain at most only a few small pin holes. It
+may have some large, irregular, "mechanical" holes where the curd
+particles have failed to cement, as is seen in Fig. 19. If gas-producing
+bacteria are very prevalent in the milk, the conditions under which the
+test is made cause such a rapid growth of the same that the evidence of
+the abnormal fermentation may be readily seen in the spongy texture of
+the curd (Fig. 20). If the undesirable organisms are not very abundant
+and the conditions not especially suited to their growth, the "pin
+holes" will be less frequent.
+
+[Illustration: FIG. 19. Curd from a good milk. The large irregular holes
+are mechanical.]
+
+Sometimes the curds show no evidence of gas, but their abnormal
+condition can be recognized by the "mushy" texture and the presence of
+"off" flavors that are rendered more apparent by keeping them in closed
+bottles. This condition is abnormal and is apt to produce quite as
+serious results as if gas was formed.
+
+~Overcoming taints by use of starters.~ Another method of combatting
+abnormal fermentations that is often fruitful, is that which rests upon
+the inability of one kind of bacteria to grow in the same medium in
+competition with certain other species.
+
+Some of the undesirable taints in factories can be controlled in large
+part by the introduction of starters made from certain organisms that
+are able to obtain the ascendency over the taint-producing germ. Such a
+method is commonly followed when a lactic ferment, either a commercial
+pure culture, or a home-made starter, is added to milk to overcome the
+effect of gas-generating bacteria.
+
+[Illustration: FIG. 20. Curd from a badly tainted milk. Large ragged
+holes are mechanical; numerous small holes due to gas. This curd was a
+"floater."]
+
+A similar illustration is seen in the case of the "lange wei" (slimy
+whey), that is used in the manufacture of Edam cheese to control the
+character of the fermentation of the milk.
+
+This same method is sometimes applied in dealing with certain abnormal
+fermentations that are apt to occur on the farm. It is particularly
+useful with those tainted milks known as "sweet curdling." The ferment
+organisms concerned in this change are unable to develop in the
+presence of lactic acid bacteria, so the addition of a clean sour milk
+as a starter restores the normal conditions by giving the ordinary milk
+bacteria the ascendency.
+
+~Chemical disinfection.~ In exceptional instances it may be necessary to
+employ chemical disinfectants to restore the normal conditions. Of
+course with such diseases as tuberculosis, very stringent measures are
+required, as they are such a direct menace to human life, but with these
+abnormal or taint-producing fermentations, care and cleanliness, well
+directed, will usually overcome the trouble.
+
+If it becomes necessary to employ chemical substances as disinfecting
+agents, their use should always be preceded by a thorough cleansing with
+hot water so that the germicide may come in direct contact with the
+surface to be disinfected.
+
+It must be borne in mind that many chemicals act as deodorants, _i.e._,
+destroy the offensive odor, without destroying the cause of the trouble.
+
+_Sulfur_ is often recommended as a disinfecting agent, but its use
+should be carefully controlled, otherwise the vapors have but little
+germicidal power. The common practice of burning a small quantity in a
+room or any closed space for a few moments has little or no effect upon
+germ life. The effect of sulfur vapor (SO_{2}) alone upon germ life is
+relatively slight, but if this gas is produced in the presence of
+moisture, sulfurous acid (H_{2}SO_{3}) is formed, which is much more
+efficient. To use this agent effectively, it must be burned in large
+quantities in a moist atmosphere (three lbs. to every 1,000 cubic feet
+of space), for at least twelve hours. After this operation, the space
+should be thoroughly aired.
+
+_Formalin_, a watery solution of a gas known as formaldehyde, is a new
+disinfectant that recent experience has demonstrated to be very useful.
+It may be used as a gas where rooms are to be disinfected, or applied as
+a liquid where desired. It is much more powerful in its action than
+sulfur, and it has a great advantage over mercury and other strong
+disinfectants, as it is not so poisonous to man as it is to the lower
+forms of life.
+
+_Bleaching powder or chloride of lime_ is often recommended where a
+chemical can be advantageously used. This substance is a good
+disinfectant as well as a deodorant, and if applied as a wash, in the
+proportion of four to six ounces of the powder to one gallon of water,
+it will destroy most forms of life. In many cases this agent is
+inapplicable on account of its odor.
+
+_Corrosive sublimate_ (HgCl_{2}) for most purposes is a good
+disinfectant, but it is such an intense poison that its use is dangerous
+in places that are at all accessible to stock.
+
+For the disinfection of walls in stables and barns, common thin _white
+wash_ Ca(OH)_{2} is admirably adapted if made from freshly-burned quick
+lime. It possesses strong germicidal powers, increases the amount of
+light in the barn, is a good absorbent of odors, and is exceedingly
+cheap.
+
+Carbolic acid, creosote, and such products, while excellent
+disinfectants, cannot well be used on account of their odor, especially
+in factories.
+
+For gutters, drains, and waste pipes in factories, _vitriol salts_
+(sulfates of copper, iron and zinc) are sometimes used. These are
+deodorants as well as disinfectants, and are not so objectionable to use
+on account of their odor.
+
+These suggestions as to the use of chemicals, however, only apply to
+extreme cases and should not be brought into requisition until a
+thorough application of hot water, soap, a little soda, and the
+scrubbing brush have failed to do their work.
+
+FOOTNOTES:
+
+[51] Günther and Thierfelder, Arch. f. Hyg., 25:164, 1895; Leichmann,
+Cent. f. Bakt., 2:281, 1896; Esten, 9 Rept. Storrs Expt. Stat., p. 44,
+1896; Dinwiddie, Bull. 45, Ark. Expt. Stat., May, 1897; Kozai, Zeit. f.
+Hyg., 38:386, 1901; Weigmann, Hyg. Milk Congress, Hamburg, 1903, p. 375.
+
+[52] McDonnell, Inaug. Diss., Kiel. 1899, p. 39.
+
+[53] Kayser, Cent. f. Bakt. II. Abt. 1:436.
+
+[54] Treadwell, Science, 1894, 17:178.
+
+[55] Conn, 5 Rept. Storrs Expt. Stat., 1892, p. 396.
+
+[56] Fermi, Arch. f. Hyg., 1892, 14:1.
+
+[57] Duclaux, Le Lait, p. 121.
+
+[58] Duclaux, Principes de Laiterie, p. 67.
+
+[59] Guillebeau (Milch Zeit., 1892, p. 808) has studied over a dozen
+different forms that possess this property.
+
+[60] Ward, Bull. 165, Cornell Expt. Stat., Mch., 1899; also Bull. 195,
+Ibid., Nov., 1901.
+
+[61] Adametz, Landw. Jahr., 1891, p. 185.
+
+[62] Marshall, Mich. Expt. Stat., Bull. 140.
+
+[63] Milch Zeit., 1899, p. 982.
+
+[64] Duclaux, Principes de Laiterie, p. 60. Heinze and Cohn, Zeit. f.
+Hyg., 46: 286, 1904.
+
+[65] Bull. 128, Wis. Expt. Stat., Sept. 1905.
+
+[66] Freudenreich, Landw. Jahr. d. Schweiz, 1896, 10; 1.
+
+[67] Weigmann, Milch Zeit., 1890, p. 881.
+
+[68] Conn, 3 Rept. Storrs Expt. Stat., 1890, p. 158.
+
+[69] Freudenreich, Fühl. Landw. Ztg. 43: 361.
+
+[70] Harrison, Bull. 120 Ont. Agr'l. Coll., May, 1902.
+
+[71] Milch Zeit. 22:569.
+
+[72] Marshall, Bull. 146, Mich. Expt. Stat., p. 16.
+
+[73] Grotenfelt, Milch Zeit., 1889, p. 263.
+
+[74] Menge, Cent. f. Bakt., 6:596; Keferstein, Cent. f. Bakt., 21:177.
+
+[75] Heim, Arb. a. d. Kais. Gesundheitsamte, 5:578.
+
+[76] Adametz, Milch Zeit., 1890, p. 225.
+
+[77] 12 Rept. Wis. Expt. Stat., 1895, p. 148; also Bull. 67, Ibid.,
+June, 1898.
+
+
+
+
+CHAPTER V.
+
+RELATION OF DISEASE-BACTERIA TO MILK.
+
+
+Practical experience with epidemic disease has abundantly demonstrated
+the fact that milk not infrequently serves as a vehicle for the
+dissemination of contagion. Attention has been prominently called to
+this relation by Ernest Hart,[78] who in 1880 compiled statistical
+evidence showing the numerous outbreaks of various contagious diseases
+that had been associated with milk infection up to that time. Since
+then, further compilations have been made by Freeman,[79] and also by
+Busey and Kober,[80] who have collected the data with reference to
+outbreaks from 1880 to 1899.
+
+These statistics indicate the relative importance of milk as a factor in
+the dissemination of disease.
+
+The danger from this source is much intensified for the reason that
+milk, generally speaking, is consumed in a raw state; and also because a
+considerable number of disease-producing bacteria are able, not merely
+to exist, but actually thrive and grow in milk, even though the normal
+milk bacteria are also present. Moreover the recognition of the presence
+of such pathogenic forms is complicated by the fact that often they do
+not alter the appearance of the milk sufficiently so that their
+presence can be detected by a physical examination. These facts which
+have been experimentally determined, coupled with the numerous clinical
+cases on record, make a strong case against milk serving as an agent in
+the dissemination of disease.
+
+~Origin of pathogenic bacteria in milk.~ Disease-producing bacteria may be
+grouped with reference to their relation toward milk into two classes,
+depending upon the manner in which infection occurs:
+
+Class I. Disease-producing bacteria capable of being transmitted
+directly from a diseased animal to man through the medium of infected
+milk.
+
+Class II. Bacteria pathogenic for man but not for cattle which are
+capable of thriving in milk after it is drawn from the animal.
+
+In the first group the disease produced by the specific organism must be
+common to both cattle and man. The organism must live a parasitic life
+in the animal, developing in the udder, and so infect the milk supply.
+It may, of course, happen that diseases toward which domestic animals
+alone are susceptible may be spread from one animal to another in this
+way without affecting human beings.
+
+In the second group, the bacterial species lives a saprophytic
+existence, growing in milk, if it happens to find its way therein. In
+such cases milk indirectly serves as an agent in the dissemination of
+disease, by giving conditions favorable to the growth of the disease
+germ.
+
+By far the most important of diseases that may be transmitted directly
+from animal to man through a diseased milk supply is tuberculosis, but
+in addition to this, foot and mouth disease (aphthous fever in
+children), anthrax and acute enteric troubles have also been traced to a
+similar source of infection.
+
+The most important specific diseases that have been disseminated through
+subsequent pollution of the milk are typhoid fever, diphtheria, scarlet
+fever and cholera, but, of course, the possibility exists that any
+disease germ capable of living and thriving in milk may be spread in
+this way. In addition to these diseases that are caused by the
+introduction of specific organisms (the causal organism of scarlet fever
+has not yet been definitely determined), there are a large number of
+more or less illy-defined troubles of an intestinal character that occur
+especially in infants and young children that are undoubtedly
+attributable to the activity of microörganisms that gain access to milk
+during and subsequent to the milking, and which produce changes in milk
+before or after its ingestion that result in the formation of toxic
+products.
+
+
+DISEASES TRANSMISSIBLE FROM ANIMAL TO MAN THROUGH DISEASED MILK.
+
+~Tuberculosis.~ In view of the wide-spread distribution of this disease in
+both the human and the bovine race, the relation of the same to milk
+supplies is a question of great importance. It is now generally admitted
+that the different types of tubercular disease found in different kinds
+of animals and man are attributable to the development of the same
+organism, _Bacillus tuberculosis_, although there are varieties of this
+organism found in different species of animals that are sufficiently
+distinct to permit of recognition.
+
+The question of prime importance is, whether the bovine type is
+transmissible to the human or not. Artificial inoculation of cattle with
+tuberculous human sputum as well as pure cultures of this variety show
+that the human type is able to make but slight headway in cattle. This
+would indicate that the danger of cattle acquiring the infection from
+man would in all probability be very slight, but these experiments offer
+no answer as to the possibility of transmission from the bovine to the
+human. Manifestly it is impossible to solve this problem by direct
+experiment upon man except by artificial inoculation, but comparative
+experiments upon animals throw some light on the question.
+
+Theo. Smith[81] and others[82] have made parallel experiments with
+animals such as guinea pigs, rabbits and pigeons, inoculated with both
+bovine and human cultures of this organism. The results obtained in the
+case of all animals tested show that the virulence of the two types was
+much different, but that the bovine cultures were much more severe.
+While of course this does not prove that transmission from bovine to
+human is possible, still the importance of the fact must not be
+overlooked.
+
+In a number of cases record of accidental infection from cattle to man
+has been noted.[83] These have occurred with persons engaged in making
+post-mortem examinations on tuberculous animals, and the tubercular
+nature of the wound was proven in some cases by excision and
+inoculation.
+
+In addition to data of this sort that is practically experimental in
+character, there are also strong clinical reasons for considering that
+infection of human beings may occur through the medium of milk.
+Naturally such infection should produce intestinal tuberculosis, and it
+is noteworthy that this phase of the disease is quite common in
+children especially between the ages of two and five.[84] It is
+difficult to determine, though, whether primary infection occurred
+through the intestine, for, usually, other organs also become involved.
+In a considerable number of cases in which tubercular infection by the
+most common channel, inhalation, seems to be excluded, the evidence is
+strong that the disease was contracted through the medium of the milk,
+but it is always very difficult to exclude the possibility of pulmonary
+infection.
+
+Tuberculosis as a bovine disease has increased rapidly during recent
+decades throughout many portions of the world. This has been most marked
+in dairy regions. Its extremely insidious nature does not permit of an
+early recognition by physical means, and it was not until the
+introduction of the tuberculin test[85] in 1892, as a diagnostic aid
+that accurate knowledge of its distribution was possible. The quite
+general introduction of this test in many regions has revealed an
+alarmingly large percentage of animals as affected. In Denmark in 1894
+over forty per cent were diagnosed as tubercular. In some parts of
+Germany almost as bad a condition has been revealed. Slaughter-house
+statistics also show that the disease has increased rapidly since 1890.
+In this country the disease on the average is much less than in Europe
+and is also very irregularly distributed. In herds where it gained a
+foothold some years ago, often the majority of animals are frequently
+infected; many herds, in fact the great majority, are wholly free from
+all taint. The disease has undoubtedly been most frequently introduced
+through the purchase of apparently healthy but incipiently affected
+animals. Consequently in the older dairy regions where stock has been
+improved the most by breeding, more of the disease exists than among the
+western and southern cattle.
+
+[Illustration FIG. 21: Front view of a tuberculous udder, showing extent
+of swelling in single quarter.]
+
+~Infectiousness of milk of reacting animals.~ Where the disease appears in
+the udder the milk almost invariably contains the tubercle organism.
+Under such conditions the appearance of the milk is not materially
+altered at first, but as the disease progresses the percentage of fat
+generally diminishes, and at times in the more advanced stages where the
+physical condition of the udder is changed (Fig. 21), the milk may
+become "watery"; but the percentage of animals showing such udder
+lesions is not large, usually not more than a few per cent. (4 per cent.
+according to Ostertag.)
+
+On the other hand, in the earlier phases of the disease, where its
+presence has been recognized solely by the aid of the tuberculin test,
+before there are any recognizable physical symptoms in any part of the
+animal, the milk is generally unaffected. Between these extremes,
+however, is found a large proportion of cases, concerning which so
+definite data are not available. The results of investigators on this
+point are conflicting and further information is much desired. Some have
+asserted so long as the udder itself shows no lesions that no tubercle
+bacilli would be present,[86] but the findings of a considerable number
+of investigators[87] indicate that even when the udder is apparently not
+diseased the milk may contain the specific organism as revealed by
+inoculation experiments upon animals. In some cases, however, it has
+been demonstrated by post-mortem examination that discoverable udder
+lesions existed that were not recognizable before autopsy was made. In
+the experimental evidence collected, a varying percentage of reacting
+animals were found that gave positive results; and this number was
+generally sufficient to indicate that the danger of using milk from
+reacting animals was considerable, even though apparently no disease
+could be found in the udder.
+
+The infectiousness of milk can also be proven by the frequent
+contraction of the disease in other animals, such as calves and pigs
+which may be fed on the skim milk. The very rapid increase of the
+disease among the swine of Germany and Denmark,[88] and the frequently
+reported cases of intestinal infection of young stock also attest the
+presence of the organism in milk.
+
+The tubercle bacillus is so markedly parasitic in its habits, that,
+under ordinary conditions, it is incapable of growing at normal air
+temperatures. There is, therefore, no danger of the germ developing in
+milk after it is drawn from the animal, unless the same is kept at
+practically blood heat.
+
+Even though the milk of some reacting animals may not contain the
+dangerous organism at the time of making the test, it is quite
+impossible to foretell how long it will remain free. As the disease
+becomes more generalized, or if tuberculous lesions should develop in
+the udder, the milk may pass from a healthy to an infectious state.
+
+This fact makes it advisable to exclude from milk supplies intended for
+human use, all milk of animals that respond to the tuberculin test; or
+at least to treat it in a manner so as to render it safe. Whether it is
+necessary to do this or not if the milk is made into butter or cheese is
+a somewhat different question. Exclusion or treatment is rendered more
+imperative in milk supplies, because the danger is greater with children
+with whom milk is often a prominent constituent of their diet, and also
+for the reason that the child is more susceptible to intestinal
+infection than the adult.
+
+The danger of infection is much lessened in butter or cheese, because
+the processes of manufacture tend to diminish the number of organisms
+originally present in the milk, and inasmuch as no growth can ordinarily
+take place in these products the danger is minimized. Moreover, the fact
+that these foods are consumed by the individual in smaller amounts than
+is generally the case where milk is used, and also to a greater extent
+by adults, lessens still further the danger of infection.
+
+Notwithstanding this, numerous observers[89] especially in Germany have
+succeeded in finding the tubercle bacillus in market butter, but this
+fact is not so surprising when it is remembered that a very large
+fraction of their cattle show the presence of the disease as indicated
+by the tuberculin test, a condition that does not obtain in any large
+section in this country.
+
+The observations on the presence of the tubercle bacillus in butter have
+been questioned somewhat of late[2] by the determination of the fact
+that butter may contain an organism that possesses the property of being
+stained in the same way as the tubercle organism. Differentiation
+between the two forms is rendered more difficult by the fact that this
+tubercle-like organism is also capable of producing in animals lesions
+that stimulate those of tuberculosis, although a careful examination
+reveals definite differences. Petri[90] has recently determined that
+both the true tubercle and the acid-resisting butter organism may be
+readily found in market butter.
+
+In the various milk products it has been experimentally determined that
+the true tubercle bacillus is able to retain its vitality in butter for
+a number of months and in cheese for nearly a year.
+
+~Treatment of milk from tuberculosis cows.~ While it has been shown that
+it is practically impossible to foretell whether the milk of any
+reacting animal actually contains tubercle bacilli or not, still the
+interests of public health demand that no milk from such stock be used
+for human food until it has been rendered safe by some satisfactory
+treatment.
+
+_1. Heating._ By far the best treatment that can be given such milk is
+to heat it. The temperature at which this should be done depends upon
+the thermal death point of the tubercle bacillus, a question concerning
+which there has been considerable difference of opinion until very
+recently. According to the work of some of the earlier investigators,
+the tubercle bacillus in its vegetative stage is endowed with powers of
+resistance greater than those possessed by any other pathogenic
+organism. This work has not been substantiated by the most recent
+investigations on this subject. In determining the thermal death point
+of this organism, as of any other, not only must the temperature be
+considered, but the period of exposure as well, and where that exposure
+is made in milk, another factor must be considered, viz., the presence
+of conditions permitting of the formation of a "scalded layer," for as
+Smith[91] first pointed out, the resistance of the tubercle organism
+toward heat is greatly increased under these conditions. If tuberculous
+milk is heated in a closed receptacle where this scalded membrane cannot
+be produced, the tubercle bacillus is killed at 140° F. in 15 to 20
+minutes. These results which were first determined by Smith, under
+laboratory conditions, and confirmed by Russell and Hastings,[92] where
+tuberculous milk was heated in commercial pasteurizers, have also been
+verified by Hesse.[93] A great practical advantage which accrues from
+the treatment of milk at 140° F. is that the natural creaming is
+practically unaffected. Of course, where a higher temperature is
+employed, the period of exposure may be materially lessened. If milk is
+momentarily heated to 176° F., it is certainly sufficient to destroy the
+tubercle bacillus. This is the plan practiced in Denmark where all skim
+milk and whey must be heated to this temperature before it can be taken
+back to the farm, a plan which is designed to prevent the dissemination
+of tuberculosis and foot and mouth disease by means of the mixed
+creamery by-products. This course renders it possible to utilize with
+perfect safety, for milk supplies, the milk of herds reacting to the
+tuberculin test, and as butter of the best quality can be made from
+cream or milk heated to even high temperatures,[94] it thus becomes
+possible to prevent with slight expense what would otherwise entail a
+large loss.
+
+_2. Dilution._ Another method that has been suggested for the treatment
+of this suspected milk is dilution with a relatively large volume of
+perfectly healthy milk. It is a well known fact that to produce
+infection, it requires the simultaneous introduction of a number of
+organisms, and in the case of tuberculosis, especially that produced by
+ingestion, this number is thought to be considerable. Gebhardt[95] found
+that the milk of tuberculous cows, which was virulent when injected by
+itself into animals, was innocuous when diluted with 40 to 100 times its
+volume of healthy milk. This fact is hardly to be relied upon in
+practice, unless the proportion of reacting to healthy cows is
+positively known.
+
+It has also been claimed in the centrifugal separation of cream from
+milk[96] that by far the larger number of tubercle bacilli were thrown
+out with the separator slime. Moore[97] has shown that the tubercle
+bacilli in an artificially infected milk might be reduced in this way,
+so as to be no longer microscopically demonstrable, yet the purification
+was not complete enough to prevent the infection of animals inoculated
+with the milk.
+
+Another way to exclude all possibility of tubercular infection in milk
+supplies is to reject all milk from reacting animals. This method is
+often followed where pasteurization or sterilization is not desired. In
+dairies where the keeping quality is dependent upon the exclusion of
+bacteria by stringent conditions as to milking and handling ("sanitary"
+or "hygienic" milk), the tuberculin test is frequently used as a basis
+to insure healthy milk.
+
+~Foot and mouth disease.~ The wide-spread extension of this disease
+throughout Europe in recent years has given abundant opportunity to show
+that while it is distinctively an animal malady, it is also
+transmissible to man, although the disease is rarely fatal. The causal
+organism has not been determined with certainty, but it has been shown
+that the milk of affected animals possesses infectious properties[98]
+although appearing unchanged in earlier phases of the disease.
+
+Hertwig showed the direct transmissibility of the disease to man by
+experiments made on himself and others. By ingesting milk from an
+affected animal, he was able to produce the symptoms of the disease, the
+mucous membrane of the mouth being covered with the small vesicles that
+characterize the malady. It has also been shown that the virus of the
+disease may be conveyed in butter.[99] This disease is practically
+unknown in this country, although widely spread in Europe.
+
+There are a number of other bovine diseases such as anthrax,[100]
+lockjaw,[101] and hydrophobia[102] in which it has been shown that the
+virus of the disease is at times to be found in the milk supply, but
+often the milk becomes visibly affected, so that the danger of using the
+same is greatly minimized.
+
+There are also a number of inflammatory udder troubles known as garget
+or mammitis. In most of these, the physical appearance of the milk is so
+changed, and often pus is present to such a degree as to give a very
+disagreeable appearance to the milk. Pus-forming bacteria (staphylococci
+and streptococci) are to be found associated with such troubles. A
+number of cases of gastric and intestinal catarrh have been reported as
+caused by such milks.[103]
+
+
+DISEASES TRANSMISSIBLE TO MAN THROUGH INFECTION OF MILK AFTER
+WITHDRAWAL.
+
+Milk is so well adapted to the development of bacteria in general, that
+it is not surprising to find it a suitable medium for the growth of many
+pathogenic species even at ordinary temperatures. Not infrequently,
+disease-producing bacteria are able to grow in raw milk in competition
+with the normal milk bacteria, so that even a slight contamination may
+suffice to produce infection.
+
+The diseases that are most frequently disseminated in this way are
+typhoid fever, diphtheria, scarlet fever and cholera, together with the
+various illy-defined intestinal troubles of a toxic character that occur
+in children, especially under the name of cholera infantum, summer
+complaint, etc.
+
+Diseases of this class are not derived directly from animals because
+cattle are not susceptible to the same.
+
+~Modes of infection.~ In a variety of ways, however, the milk may be
+subject to contaminating influences after it is drawn from the animal,
+and so give opportunity for the development of disease-producing
+bacteria. The more important methods of infection are as follows:
+
+_1. Infection directly from a pre-existing case of disease on premises._
+Quite frequently a person in the early stage of a diseased condition may
+continue at his usual vocation as helper in the barn or dairy, and so
+give opportunity for direct infection to occur. In the so-called cases
+of "walking typhoid," this danger is emphasized. It is noteworthy in
+typhoid fever that the bacilli frequently persist in the urine and in
+diphtheria they often remain in the throat until after convalescence. In
+some cases infection has been traced to storage of the milk in rooms in
+the house where it became polluted directly by the emanations of the
+patient.[104] Among the dwellings of the lower classes where a single
+room has to be used in common this source of infection has been most
+frequently observed.
+
+_2. Infection through the medium of another person._ Not infrequently
+another individual may serve in the capacity of nurse or attendant to a
+sick person, and also assist in the handling of the milk, either in
+milking the animals or caring for the milk after it has been drawn.
+Busey and Kober report twenty-one outbreaks of typhoid fever in which
+dairy employees also acted in the capacity of nurses.
+
+_3. Pollution of milk utensils._ The most frequent method of infection
+of cans, pails, etc., is in cleaning them with water that may be
+polluted with disease organisms. Often wells may be contaminated with
+diseased matter of intestinal origin, as in typhoid fever, and the use
+of water at normal temperatures, or even in a lukewarm condition, give
+conditions permitting of infection. Intentional adulteration of milk
+with water inadvertently taken from polluted sources has caused quite a
+number of typhoid outbreaks.[105] Sedgwick and Chapin[106] found in the
+Springfield, Mass., epidemic of typhoid that the milk cans were placed
+in a well to cool the milk, and it was subsequently shown that the well
+was polluted with typhoid fecal matter.
+
+_4. Pollution of udder_ of animal _by wading in infected water_, or by
+washing same with contaminated water. This method of infection would
+only be likely to occur in case of typhoid. An outbreak at the
+University of Virginia in 1893[107] was ascribed to the latter cause.
+
+_5. Pollution of creamery by-products, skim-milk, etc._ Where the milk
+supply of one patron becomes infected with pathogenic bacteria, it is
+possible that disease may be disseminated through the medium of the
+creamery, the infective agent remaining in the skim milk after
+separation and so polluting the mixed supply. This condition is more
+likely to prevail with typhoid because of the greater tolerance of this
+organism for acids such as would be found in raw milk. The outbreaks at
+Brandon,[108] England, in 1893, Castle Island,[109] Ireland, and
+Marlboro,[110] Mass., in 1894, were traced to such an origin.
+
+While most outbreaks of disease associated with a polluted milk supply
+originate in the use of the milk itself, yet infected milk may serve to
+cause disease even when used in other ways. Several outbreaks of typhoid
+fever have been traced to the use of ice cream where there were strong
+reasons for believing that the milk used in the manufacture of the
+product was polluted.[111] Hankin[112] details a case of an Indian
+confection made largely from milk that caused a typhoid outbreak in a
+British regiment.
+
+Although the evidence that milk may not infrequently serve as an agent
+in spreading disease is conclusive enough to satisfactorily prove the
+proposition, yet it should be borne in mind that the organism of any
+specific disease in question has rarely ever been found. The reasons for
+this are quite the same as those that govern the situation in the case
+of polluted waters, except that the difficulties of the problem are much
+greater in the case of milk than with water. The inability to readily
+separate the typhoid germ, for instance, from the colon bacillus, an
+organism frequently found in milk, presents technical difficulties not
+easily overcome. The most potent reason of failure to find disease
+bacteria is the fact that infection in any case must occur sometime
+previous to the appearance of the outbreak. Not only is there the usual
+period of incubation, but it rarely happens that an outbreak is
+investigated until a number of cases have occurred. In this interim the
+original cause of infection may have ceased to be operative.
+
+~Typhoid fever.~ With reference to the diseases likely to to be
+disseminated through the medium of milk, infected after being drawn from
+the animal, typhoid fever is the most important. The reason for this is
+due (1) to the wide spread distribution of the disease; (2) to the fact
+that the typhoid bacillus is one that is capable of withstanding
+considerable amounts of acid, and consequently finds even in raw milk
+containing the normal lactic acid bacteria conditions favorable for its
+growth.[113] Ability to grow under these conditions can be shown not
+only experimentally, but there is abundant clinical evidence that even a
+slight infection often causes extensive outbreaks, as in the Stamford,
+Conn., outbreak in 1895 where 386 cases developed in a few weeks, 97 per
+cent. of which occurred on the route of one milk-man. In this case the
+milk cans were thoroughly and properly cleaned, but were rinsed out with
+_cold_ water from a shallow well that was found to be polluted.
+
+The most common mode of pollution of milk with typhoid organisms is
+where the milk utensils are infected in one way or another.[114] Second
+in importance is the carrying of infection by persons serving in the
+dual capacity of nurse and dairy attendant.
+
+~Cholera.~ This germ does not find milk so favorable a nutrient medium as
+the typhoid organism, because it is much more sensitive toward the
+action of acids. Kitasato[115] found, however, that it could live in
+raw milk from one to four days, depending upon the amount of acid
+present. In boiled or sterilized milk it grows more freely, as the
+acid-producing forms are thereby eliminated. In butter it dies out in a
+few days (4 to 5).
+
+On account of the above relation not a large number of cholera outbreaks
+have been traced to milk, but Simpson[116] records a very striking case
+in India where a number of sailors, upon reaching port, secured a
+quantity of milk. Of the crew which consumed this, every one was taken
+ill, and four out of ten died, while those who did not partake escaped
+without any disease. It was later shown that the milk was adulterated
+with water taken from an open pool in a cholera infected district.
+
+~Diphtheria.~ Milk occasionally, though not often, serves as a medium for
+the dissemination of diphtheria. Swithinbank and Newman[117] cites four
+cases in which the causal organism has been isolated from milk. It has
+been observed that growth occurs more rapidly in raw than in sterilized
+milk.[118]
+
+Infection in this disease is more frequently attributable to direct
+infection from patient on account of the long persistence of this germ
+in the throat, or indirectly through the medium of an attendant.
+
+~Scarlet fever.~ Although it is more difficult to study the relation of
+this disease to contaminated milk supplies, because the causal germ of
+scarlet fever is not yet known, yet the origin of a considerable number
+of epidemics has been traced to polluted milk supplies. Milk doubtless
+is infected most frequently from persons in the earlier stages of the
+disease when the infectivity of the disease is greater.
+
+~Diarrhoeal diseases.~ Milk not infrequently acquires the property of
+producing diseases of the digestive tract by reason of the development
+of various bacteria that form more or less poisonous by-products. These
+troubles occur most frequently during the summer months, especially with
+infants and children, as in cholera infantum and summer complaint. The
+higher mortality of bottle-fed infants[119] in comparison with those
+that are nursed directly is explicable on the theory that cows' milk is
+the carrier of the infection, because in many cases it is not consumed
+until there has been ample time for the development of organisms in it.
+Where milk is pasteurized or boiled it is found that the mortality among
+children is greatly reduced. As a cause of sickness and death these
+diseases exceed in importance all other specific diseases previously
+referred to. These troubles have generally been explained as produced by
+bacteria of the putrefactive class which find their way into the milk
+through the introduction of filth and dirt at time of milking.[120]
+Flügge[121] has demonstrated that certain peptonizing species possess
+toxic properties for animals. Recent experimental inquiry[122] has
+demonstrated that the dysentery bacillus (Shiga) probably bears a causal
+relation to some of these summer complaints.
+
+~Ptomaine poisoning.~ Many cases of poisoning from food products are also
+reported with adults. These are due to the formation of various toxic
+products, generally ptomaines, that are produced as a result of
+infection of foods by different bacteria. One of these substances,
+_tyrotoxicon_, was isolated by Vaughan[123] from cheese and various
+other products of milk, and found to possess the property of producing
+symptoms of poisoning similar to those that are noted in such cases. He
+attributes the production of this toxic effect to the decomposition of
+the elements in the milk induced by putrefactive forms of bacteria that
+develop where milk is improperly kept.[124] Often outbreaks of this
+character[125] assume the proportions of an epidemic, where a large
+number of persons use the tainted food.
+
+FOOTNOTES:
+
+[78] Hart, Trans. Int. Med. Cong., London, 1881, 4:491-544.
+
+[79] Freeman, Med. Rec., March 28, 1896.
+
+[80] Busey and Kober, Rept. Health Off. of Dist. of Col., Washington, D.
+C., 1895, p. 299. These authors present in this report an elaborate
+article on morbific and infectious milk, giving a very complete
+bibliography of 180 numbers. They append to Hart's list (which is
+published in full) additional outbreaks which have occurred since,
+together with full data as to extent of epidemic, circumstances
+governing the outbreak, as well as name of original reporter and
+reference.
+
+[81] Smith, Theo., Journ. of Expt. Med., 1898, 3:451.
+
+[82] Dinwiddie, Bull. 57, Ark. Expt. Stat., June, 1899; Ravenel, Univ.
+of Penn. Med. Bull., Sept. 1901.
+
+[83] Ravenel, Journ. of Comp. Med. & Vet. Arch., Dec. 1897; Hartzell,
+Journ. Amer. Med. Ass'n, April 16, 1898.
+
+[84] Stille, Brit. Med. Journ., Aug. 19, 1899.
+
+[85] This test is made by injecting into the animal a small quantity of
+tuberculin, which is a sterilized glycerin extract of cultures of the
+tubercle bacillus. In a tuberculous animal, even in the very earliest
+phases of the disease, tuberculin causes a temporary fever that lasts
+for a few hours. By taking the temperature a number of times before and
+after injection it is possible to readily recognize any febrile
+condition. A positive diagnosis is made where the temperature after
+inoculation is at least 2.0° F. above the average normal, and where the
+reaction fever is continued for a period of some hours.
+
+[86] Martin, Brit. Med. Journ. 1895, 1:937; Nocard, Les Tuberculoses
+animales, 1895.
+
+[87] C. O. Jensen, Milch Kunde und Milch hygiene, p. 69.
+
+[88] Ostertag, Milch Zeit., 22:672.
+
+[89] Obermüller, Hyg. Rund., 1897, p. 712; Petri, Arb. a. d. Kais. Ges.
+Amte, 1898, 14: 1; Hormann und Morgenroth, Hyg. Rund., 1898, p. 217.
+
+[90] Rabinowitsch, Zeit. f. Hyg., 1897, 26: 90.
+
+[91] Th. Smith. Journ. of Expt. Med., 1899, 4:217.
+
+[92] Russell and Hastings, 18 Rept. Wis. Expt. Stat., 1901.
+
+[93] Hesse, Zeit. f. Hyg., 1900, 34:346.
+
+[94] Practically all of the finest butter made in Denmark is made from
+cream that has been pasteurized at temperatures varying from 160°-185°
+F.
+
+[95] Gebhardt, Virch. Arch., 1890, 119:12.
+
+[96] Scheurlen, Arb. a. d. k. Ges. Amte, 1891, 7:269; Bang, Milch Zeit.,
+1893, p. 672.
+
+[97] Moore, Year Book of U. S. Dept. Agr., 1895, p. 432.
+
+[98] Weigel and Noack, Jahres. d. Ges. Med., 1890, p. 642; Weissenberg,
+Allg. med. Cent. Zeit., 1890, p. 1; Baum, Arch. f. Thierheilkunde, 1892,
+18:16.
+
+[99] Schneider, Münch, med. Wochenschr., 1893, No. 27; Fröhner, Zeit f.
+Fleisch u. Milchhygiene, 1891, p. 55.
+
+[100] Feser, Deutsche Zeit. f. Thiermed., 1880, 6:166.
+
+[101] Nocard, Bull. Gén., 1885, p. 54.
+
+[102] Deutsche Viertelsjahr. f. offentl. Gesundheitspflege, 1890,
+20:444.
+
+[103] Zeit. f. Fleisch und Milch hygiene, 11:114.
+
+[104] E. Roth, Deutsche Vierteljahresschr. f. offentl. Gesundheitspfl.,
+1890, 22:238
+
+[105] S. W. North, London Practitioner, 1889, 43:393.
+
+[106] Sedgwick and Chapin, Boston Med. & Surg. Journ., 1893, 129:485.
+
+[107] Dabney, Phila. Med. News, 1893, 63:630.
+
+[108] Welphy, London Lancet, 1894, 2:1085.
+
+[109] Brit. Med. Journ., 1894, 1:815.
+
+[110] Mass. Bd. Health Rept., 1894, p. 765.
+
+[111] Turner, London Practitioner, 1892, 49:141; Munro, Brit. Med.
+Journ., 1894, 2:829.
+
+[112] Hankin, Brit. Med. Journ., 1894, 2:613.
+
+[113] Heim (Arb. a. d. Kais. Gesundheitsamte, 1889, 5:303) finds it
+capable of living from 20-30 days in milk.
+
+[114] Schüder (Zeit. f. Hyg., 1902, 38:34) examined the statistics of
+638 typhoid epidemics. He found 71 per cent. due to infected drinking
+water, 17 per cent. to infected milk, and 3.5 per cent. caused by other
+forms of food.
+
+[115] Kitasato. Arb. a. d. Kais. Gesundheitsamte, 1:470.
+
+[116] Simpson, London Practitioner, 1887, 39:144.
+
+[117] Swithinbank and Newman, Bacteriology of Milk, p. 341.
+
+[118] Schottelius and Ellerhorst. Milch Zeit., 1897, pp. 40 and 73.
+
+[119] Baginsky, Hyg. Rund., 1895, p. 176.
+
+[120] Gaffky, Deutsch. med. Wochen., 18:14.
+
+[121] Flügge. Zeit., f. Hyg., 17:272, 1894.
+
+[122] Duval and Bassett, Studies from the Rockefeller Inst. for Med.
+Research, 2:7, 1904.
+
+[123] Zeit. f. physiol. Chemie, 10:146; 9 Intern. Hyg. Cong. (London),
+1891, p. 118.
+
+[124] Vaughan and Perkins, Arch. f. Hyg., 27:308.
+
+[125] Newton and Wallace (Phila. Med. News, 1887, 50:570) report three
+outbreaks at Long Branch, N. J., two of which occurred in summer hotels.
+
+
+
+
+CHAPTER VI.
+
+BACTERIA AND MILK SUPPLIES WITH ESPECIAL REFERENCE TO METHODS OF
+PRESERVATION.
+
+
+To the milk dealer or distributor, bacteria are more or less of a
+detriment. None of the organisms that find their way into milk, nor the
+by-products formed by their growth, improve the quality of milk
+supplies. It is therefore especially desirable from the milk-dealer's
+point of view that these changes should be held in abeyance as much as
+possible. Then too, the possibility that milk may serve as a medium for
+the dissemination of disease-breeding bacteria makes it advisable to
+protect this food supply from all possible infection from suspicious
+sources.
+
+In considering, therefore, the relation of bacteria to general milk
+supplies, the _economic_ and the _hygienic_ standpoints must be taken
+into consideration. Ordinarily much more emphasis is laid upon the first
+requirement. If the supply presents no abnormal feature as to taste,
+odor and appearance, unfortunately but little attention is paid to the
+possibility of infection by disease germs. The methods of control which
+are applicable to general milk supplies are based on the following
+foundations: (1) the exclusion of all bacterial life, as far as
+practicable, at the time the milk is drawn, and the subsequent storage
+of the same at temperatures unfavorable for the growth of the organisms
+that do gain access; (2) the removal of the bacteria, wholly or in part,
+after they have once gained access.
+
+Until within comparatively recent years, practically no attention was
+given to the character of milk supplies, except possibly as to the
+percentage of butter fat, and sometimes the milk solids which it
+contained. So long as the product could be placed in the hands of the
+consumer in such shape as not to be rejected by him as unfit for food,
+no further attention was likely to be given to its character. At
+present, however, much more emphasis is being given to the quality of
+milk, especially as to its germ content; and the milk dealer is
+beginning to recognize the necessity of a greater degree of control.
+This control must not merely concern the handling of the product after
+it reaches him, but should go back to the milk producer on the farm.
+Here especially, it is necessary to inculcate those methods of
+cleanliness which will prevent in large measure the wholesale infection
+that ordinarily occurs.
+
+The two watch words which are of the utmost importance to the milk
+dealer are _cleanliness_ and _cold_. If the milk is properly drawn from
+the animal in a clean manner and is immediately and thoroughly chilled,
+the dealer has little to fear as to his product. Whenever serious
+difficulties do arise, attributable to bacterial changes, it is because
+negligence has been permitted in one or both directions. The influence
+of cleanliness in diminishing the bacterial life in milk and that of low
+temperatures in repressing the growth of those forms which inevitably
+gain access has been fully dealt with in preceding chapters. It is of
+course not practicable to take all of these precautions to which
+reference has been made in the securing of large supplies of market milk
+for city use, but great improvement over existing conditions could be
+secured if the public would demand a better supervision of this
+important food article. Boards of health in our larger cities are
+awakening to the importance of this question and are becoming
+increasingly active in the matter of better regulations and the
+enforcement of the same.
+
+New York City Board of Health has taken an advanced position in
+requiring that all milk sold in the city shall be chilled down to 45° F.
+immediately after milking and shall be transported to the city in
+refrigerator cars.
+
+Reference has already been made to the application of the acid test
+(page 52) in the inspection of city milk supplies, and it is the opinion
+of the writer that the curd test (see page 76) could also be used with
+advantage in determining the sanitary character of milk. This test
+reveals the presence of bacteria usually associated with dirt and
+permits of the recognition of milks that have been carelessly handled.
+From personal knowledge of examinations made of the milk supplies in a
+number of Wisconsin cities it appears that this test could be utilized
+with evident advantage.
+
+~"Sanitary" or "certified" milk supplies.~ In a number of the larger
+cities, the attempt has been made to improve the quality of the milk
+supplies by the installation of dairies in which is produced an
+especially high grade of milk. Frequently the inspection of the dairy as
+well as the examination of the milk at stated intervals is under the
+control of milk commissions or medical societies and as it is customary
+to distribute the certificate of the examining board with the product,
+such milks are frequently known as "certified." In such dairies the
+tuberculin test is used at regular intervals, and the herd inspected
+frequently by competent veterinarians. The methods of control
+inaugurated as to clean milking and subsequent handling are such as to
+insure the diminution of the bacteria to the lowest possible point. The
+bacterial limit set by the Pediatric Society of Philadelphia is 10,000
+organisms per cc. Often it is possible to improve very materially on
+this standard and not infrequently is the supply produced where it
+contains only a few thousand organisms per cc. Where such a degree of
+care is exercised, naturally a considerably higher price must be paid
+for the product,[126] and it should be remembered that the development
+of such a system is only possible in relatively large centers where the
+dealer can cater to a selected high-class trade. Moreover, it should
+also be borne in mind that such a method of control is only feasible in
+dairies that are under individual control. The impossibility of
+exercising adequate control with reference to the milking process and
+the care which should be given the milk immediately thereafter, when the
+same is produced on different farms under various auspices is evident.
+
+
+PRESERVATION OF MILK SUPPLIES.
+
+While much can be done to improve the quality of milk supplies by
+excluding a large proportion of the bacteria which normally gain access
+to the milk, and preventing the rapid growth of those that do find their
+way therein, yet for general municipal purposes, any practical method of
+preservation[127] that is applicable on a commercial scale must rest
+largely upon the destruction of bacteria that are present in the milk.
+
+The two possible methods by which bacteria can be destroyed after they
+have once gained access is (1) by the use of chemical preservatives; (2)
+by the aid of physical methods.
+
+~Chemical preservatives.~ Numerous attempts have been made to find some
+chemical substance that could be added to milk which would preserve it
+without interfering with its nutritive properties, but as a general rule
+a substance that is toxic enough to destroy or inhibit the growth of
+bacterial life exerts a prejudicial effect on the tissues of the body.
+The use of chemicals, such as carbolic acid, mercury salts and mineral
+acids, that are able to entirely destroy all life, is of course
+excluded, except when milk is preserved for analytical purposes; but a
+number of milder substances are more or less extensively employed,
+although the statutes of practically all states forbid their use.
+
+The substances so used may be grouped in two classes:
+
+1. Those that unite chemically with certain by-products of bacterial
+growth to form inert substances. Thus bicarbonate of soda neutralizes
+the acid in souring milk, although it does not destroy the lactic acid
+bacteria.
+
+2. Those that act directly upon the bacteria in milk, restraining or
+inhibiting their development. The substances most frequently utilized
+are salicylic acid, formaldehyde and boracic acid. These are nearly
+always sold to the milk handler, under some proprietary name, at prices
+greatly in excess of what the crude chemicals could be bought for in the
+open market. Formaldehyde has been widely advertised of late, but its
+use is fraught with the greatest danger, for it practically renders
+insoluble all albuminous matter and its toxic effect is greatly
+increased in larger doses.
+
+These substances are generally used by milk handlers who know nothing of
+their poisonous action, and although it may be possible for adults to
+withstand their use in dilute form, without serious results, yet their
+addition to general milk supplies that may be used by children is
+little short of criminal. The sale of these preparations for use in milk
+finds its only outlet with those dairymen who are anxious to escape the
+exactions that must be met by all who attempt to handle milk in the best
+possible manner. Farrington has suggested a simple means for the
+detection of preservalin (boracic acid).[128] When this substance is
+added to fresh milk, it increases the acidity of milk without affecting
+its taste. As normal milk tastes sour when it contains about 0.3 per
+cent lactic acid, a milk that tests as much or more than this without
+tasting sour has been probably treated with this antiseptic agent.
+
+~Physical methods of preservation.~ Methods based upon the application of
+physical forces are less likely to injure the nutritive value of milk,
+and are consequently more effective, if of any value whatever. A number
+of methods have been tried more or less thoroughly in an experimental
+way that have not yet been reduced to a practical basis, as electricity,
+use of a vacuum, and increased pressure.[129] Condensation has long been
+used with great success, but in this process the nature of the milk is
+materially changed. The keeping quality in condensed milk often depends
+upon the action of another principle, viz., the inhibition of bacterial
+growth by reason of the concentration of the medium. This condition is
+reached either by adding sugar and so increasing the soluble solids, or
+by driving off the water by evaporation, preferably in a vacuum pan.
+Temperature changes are, however, of the most value in preserving milk,
+for by a variation in temperature all bacterial growth can be brought to
+a standstill, and under proper conditions thoroughly destroyed.
+
+~Use of low temperatures.~ The effect of chilling or rapid cooling on the
+keeping quality of milk is well known. When the temperature of milk is
+lowered to the neighborhood of 45° F., the development of bacterial life
+is so slow as to materially increase the period that milk remains sweet.
+Within recent years, attempts have been made to preserve milk so that it
+could be shipped long distances by freezing the product, which in the
+form of milk-ice could be held for an indefinite period without
+change.[130] A modification of this process known as Casse's system has
+been in use more or less extensively in Copenhagen and in several places
+in Germany. This consists of adding a small block of milk-ice (frozen
+milk) to large cans of milk (one part to about fifty of milk) which may
+or may not be pasteurized.[131] This reduces the temperature so that the
+milk remains sweet considerably longer. Such a process might permit of
+the shipment of milk for long distances with safety but as a matter of
+fact, the system has not met with especial favor.
+
+[Illustration: FIG. 22. Microscopic appearance of normal milk showing
+the fat-globules aggregated in clusters.]
+
+~Use of high temperatures.~ Heat has long been used as a preserving agent.
+Milk has been scalded or cooked to keep it from time immemorial. Heat
+may be used at different temperatures, and when so applied exerts a
+varying effect, depending upon temperature employed. All methods of
+preservation by heat rest, however, upon the application of the heat
+under the following conditions:
+
+1. A temperature above the maximum growing-point (105°-115° F.) and
+below the thermal death-point (130°-140° F.) will prevent further
+growth, and consequently fermentative action.
+
+2. A temperature above the thermal death-point destroys bacteria, and
+thereby stops all changes. This temperature varies, however, with the
+condition of the bacteria, and for spores is much higher than for
+vegetative forms.
+
+Attempts have been made to employ the first principle in shipping milk
+by rail, viz., prolonged heating above growing temperature, but when
+milk is so heated, its physical appearance is changed.[132] The methods
+of heating most satisfactorily used are known as sterilization and
+pasteurization, in which a degree of temperature is used approximating
+the boiling and scalding points respectively.
+
+[Illustration: FIG. 23. Microscopic appearance of milk heated above 140°
+F., showing the homogeneous distribution of fat-globules. The physical
+change noted in comparison with Fig. 22 causes the diminished
+consistency of pasteurized cream.]
+
+~Effect of heat on milk.~ When milk is subjected to the action of heat, a
+number of changes in its physical and chemical properties are to be
+noted.
+
+_1. Diminished "body."_ When milk, but more especially cream, is heated
+to 140° F. or above, it becomes thinner in consistency or "body," a
+condition which is due to a change in the grouping of the fat globules.
+In normal milk, the butter fat for the most part is massed in
+microscopic clots as (Fig. 22). When exposed to 140° F. or above for ten
+minutes these fat-globule clots break down, and the globules become
+homogeneously distributed (Fig. 23). A _momentary_ exposure to heat as
+high as 158°-160° may be made without serious effect on the cream lime;
+but above this the cream rises so poorly and slowly that it gives the
+impression of thinner milk.
+
+_2. Cooked Taste._ If milk is heated for some minutes to 160° F., it
+acquires a cooked taste that becomes more pronounced as the temperature
+is further raised. Milk so heated develops on its surface a pellicle or
+"skin." The cause of this change in taste is not well known. Usually it
+has been explained as being produced by changes in the nitrogenous
+elements in the milk, particularly in the albumen. Thoerner[133] has
+pointed out the coincidence that exists between the appearance of a
+cooked taste and the loss of certain gases that are expelled by heating.
+He finds that the milk heated in closed vessels from which the gas
+cannot escape has a much less pronounced cooked flavor than if heated in
+an open vessel. The so-called "skin" on the surface of heated milk is
+not formed when the milk is heated in a tightly-closed receptacle. By
+some[134] it is asserted that this layer is composed of albumen, but
+there is evidence to show that it is modified casein due to the rapid
+evaporation of the milk serum at the surface of the milk.
+
+_3. Digestibility._ Considerable difference of opinion has existed in
+the minds of medical men as to the relative digestibility of raw and
+heated milks. A considerable amount of experimental work has been done
+by making artificial digestion experiments with enzyms, also digestion
+experiments with animals, and in a few cases with children. The results
+obtained by different investigators are quite contradictory, although
+the preponderance of evidence seems to be in favor of the view that
+heating does impair the digestibility of milk, especially if the
+temperature attains the sterilizing point.[135] It has been observed
+that there is a noteworthy increase in amount of rickets,[136] scurvy
+and marasmus in children where highly-heated milks are employed. These
+objections do not obtain with reference to milk heated to moderate
+temperatures, as in pasteurization, although even this lower temperature
+lessens slightly its digestibility. The successful use of pasteurized
+milks in children's hospitals is evidence of its usefulness.
+
+_4. Fermentative changes._ The normal souring change in milk is due to
+the predominance of the lactic acid bacteria, but as these organisms as
+a class do not possess spores, they are readily killed when heated above
+the thermal death-point of the developing cell. The destruction of the
+lactic forms leaves the spore-bearing types possessors of the field, and
+consequently the fermentative changes in heated milk are not those that
+usually occur, but are characterized by the curdling of the milk from
+the action of rennet enzyms.
+
+_5. Action of rennet._ Heating milk causes the soluble lime salts to be
+precipitated, and as the curdling of milk by rennet (in cheese-making)
+is dependent upon the presence of these salts, their absence in heated
+milks greatly retards the action of rennet. This renders it difficult to
+utilize heated milks in cheese-making unless the soluble lime salts are
+restored, which can be done by adding solutions of calcium chlorid.
+
+~Sterilization.~ As ordinarily used in dairying, sterilization means the
+application of heat at temperatures approximating, if not exceeding,
+212° F. It does not necessarily imply that milk so treated is sterile,
+i. e., germ-free; for, on account of the resistance of spores, it is
+practically impossible to destroy entirely _all_ these hardy forms. If
+milk is heated at temperatures above the boiling point, as is done where
+steam pressure is utilized, it can be rendered practically germ-free.
+Such methods are employed where it is designed to keep milk sweet for a
+long period of time. The treatment of milk by sterilization has not met
+with any general favor in this country, although it has been more widely
+introduced abroad. In most cases the process is carried out after the
+milk is bottled; and considerable ingenuity has been exercised in the
+construction of devices which will permit of the closure of the bottles
+after the sterilizing process has been completed. Milks heated to so
+high a temperature have a more or less pronounced boiled or cooked
+taste, a condition that does not meet with general favor in this
+country. The apparatus suitable for this purpose must, of necessity, be
+so constructed as to withstand steam pressure, and consequently is
+considerably more expensive than that required for the simpler
+pasteurizing process.
+
+~Pasteurization.~ In this method the degree of heat used ranges from 140°
+to 185° F. and the application is made for only a limited length of
+time. The process was first extensively used by Pasteur (from whom it
+derives its name) in combating various maladies of beer and wine. Its
+importance as a means of increasing the keeping quality of milk was not
+generally recognized until a few years ago; but the method is now
+growing rapidly in favor as a means of preserving milk for commercial
+purposes. The method does not destroy all germ-life in milk; it affects
+only those organisms that are in a growing, vegetative condition; but if
+the milk is quickly cooled, it enhances the keeping quality very
+materially. It is unfortunate that this same term is used in connection
+with the heating of cream as a preparatory step to the use of pure
+cultures in cream-ripening in butter-making. The objects to be
+accomplished vary materially and the details of the two processes are
+also quite different.
+
+While pasteurizing can be performed on a small scale by the individual,
+the process can also be adapted to the commercial treatment of large
+quantities of milk. The apparatus necessary for this purpose is not
+nearly so expensive as that used in sterilizing, a factor of importance
+when other advantages are considered. In this country pasteurization has
+made considerable headway, not only in supplying a milk that is designed
+to serve as children's food, but even for general purposes.
+
+~Requirements essential in pasteurization.~ While considerable latitude
+with reference to pasteurizing limits is permitted, yet there are
+certain conditions which should be observed, and these, in a sense, fix
+the limits that should be employed. These may be designated as (1) the
+_physical_, and (2) the _biological_ requirements.
+
+~Physical requirements.~ _1. Avoidance of scalded or cooked taste._ The
+English and American people are so averse to a scalded or cooked flavor
+in milk that it is practically impossible for a highly heated product to
+be sold in competition with ordinary raw milk. In pasteurization then,
+care must be taken not to exceed the temperature at which a permanently
+cooked flavor is developed. As previously observed, this point varies
+with the period of exposure. A momentary exposure to a temperature of
+about 170° F. may be made without any material alteration, but if the
+heat is maintained for a few minutes (ten minutes or over), a
+temperature of 158° to 160° F. is about the maximum that can be employed
+with safety.
+
+_2. Normal creaming of the milk._ It is especially desirable that a
+sharp and definite cream line be evident on the milk soon after
+pasteurization. If this fails to appear, the natural inference of the
+consumer is that the milk is skimmed. If the milk be heated to a
+temperature sufficiently high to cause the fat-globule clusters to
+disintegrate (see Figs. 22 and 23), the globules do not rise to the
+surface as readily as before and the cream line remains indistinct.
+Where the exposure is made for a considerable period of time (10 minutes
+or more), the maximum temperature which can be used without producing
+this change is about 140° F.; if the exposure is made for a very brief
+time, a minute or less, the milk may be heated to 158°-160 F.° without
+injuring the creaming property.
+
+_3. No diminution in cream "body."_ Coincident with this change which
+takes place in the creaming of the milk is the change in body or
+consistency which is noted where cream is pasteurized at too high a
+temperature. For the same reason as given under (2) cream heated above
+these temperatures is reduced in apparent thickness and appears to
+contain less butter-fat. Of course the pasteurizing process does not
+change the fat content, but its "body" is apparently so affected. Thus a
+25 per cent. cream may seem to be no thicker or heavier than an 18 per
+cent. raw cream. This real reduction in consistency naturally affects
+the readiness with which the cream can be whipped.
+
+~Biological requirements.~ _1. Enhanced keeping quality._ In commercial
+practice the essential biological requirement is expressed in the
+enhanced keeping quality of the pasteurized milk. This expresses in a
+practical way the reduction in germ life accomplished by the
+pasteurizing process. The improvement in keeping quality depends upon
+the temperature and time of exposure, but fully as much also on the way
+in which the pasteurized product is handled after heating. The lowest
+temperature which can be used with success to kill the active,
+vegetative bacteria is about 140° F., at which point it requires about
+ten minutes exposure. If this period is curtailed the temperature must
+be raised accordingly. An exposure to a temperature of 175° F. for a
+minute has approximately the same effect as the lower degree of heat for
+the longer time.
+
+The following bacteriological studies as to the effect which a variation
+in temperature exerts on bacterial life in milk are of importance as
+indicating the foundation for the selection of the proper limits. In the
+following table the exposures were made for a uniform period (20
+minutes):
+
+_The bacterial content of milk heated at different temperatures._
+
+                          Number of bacteria per cc. in milk.
+                            45° C.     50° C.   55° C.   60° C.   65° C.  70° C.
+               Unheated    113° F.    122° F.  131° F.  140° F.  149° F. 158° F.
+Series I.     2,895,000    ----     1,260,000  798,000  32,000    5,770    3,900
+Series II.      750,000    665,000    262,400   201,000    950      700      705
+Series III.   1,350,000  1,100,000    260,000  215,000     575      610      650
+Series IV.    1,750,000    ----        87,360   ----     4,000     3,500   3,600
+
+It appears from these results that the most marked decrease in
+temperature occurs at 140° F. (60° C.). It should also be observed that
+an increase in heat above this temperature did not materially diminish
+the number of organisms present, indicating that those forms remaining
+were in a spore or resistant condition. It was noted, however, that the
+developing colonies grew more slowly in the plates made from the highly
+heated milk, showing that their vitality was injured to a greater extent
+even though not killed.
+
+_2. Destruction of disease bacteria._ While milk should be pasteurized
+so as to destroy all active, multiplying bacteria, it is particularly
+important to destroy any organisms of a disease nature that might find
+their way into the same. Fortunately most of the bacteria capable of
+thriving in milk before or after it is drawn from the animal are not
+able to form spores and hence succumb to proper pasteurization. Such is
+the case with the diphtheria, cholera and typhoid organisms.
+
+The organism that is invested with most interest in this connection is
+the tubercle bacillus. On account of its more or less frequent
+occurrence in milk and its reputed high powers of resistance, it may
+well be taken as a standard in pasteurizing.
+
+~Thermal death limits of tubercle bacillus.~ Concerning the exact
+temperature at which this germ is destroyed there is considerable
+difference of opinion. Part of this arises from the inherent difficulty
+in determining exactly when the organism is killed (due to its failure
+to grow readily on artificial media), and part from the lack of uniform
+conditions of exposure. The standards that previously have been most
+generally accepted are those of De Man,[137] who found that thirty
+minutes exposure at 149° F., fifteen minutes at 155° F., or ten minutes
+at 167° F., sufficed to destroy this germ.
+
+More recently it has been demonstrated,[138] and these results
+confirmed,[139] that if tuberculous milk is heated in closed receptacles
+where the surface pellicle does not form, the vitality of this disease
+germ is destroyed at 140° F. in 10-15 minutes, while an exposure at 160°
+F. requires only about one minute.[140] If the conditions of heating are
+such that the surface of the milk is exposed to the air, the resistance
+of bacteria is greatly increased. When heated in open vessels Smith
+found that the tubercle organism was not killed in some cases where the
+exposure was made for at least an hour. Russell and Hastings[141] have
+shown an instance where the thermal death-point of a micrococcus
+isolated from pasteurized milk was increased 12.5° F., by heating it
+under conditions that permitted of the formation of the scalded layer.
+It is therefore apparent that apparatus used for pasteurization should
+be constructed so as to avoid this defect.
+
+~Methods of treatment.~ Two different systems of pasteurization have grown
+up in the treatment of milk. One of these has been developed from the
+hygienic or sanitary aspect of the problem and is used more particularly
+in the treatment of cream and relatively small milk supplies. The other
+system has been developed primarily from the commercial point of view
+where a large amount of milk must be treated in the minimum time. In the
+first method the milk is heated for a longer period of time, about
+fifteen minutes at a relatively low temperature from 140°-155° F.; in
+the other, the milk is exposed to the source of heat only while it is
+passing rapidly through the apparatus. Naturally, the exposure under
+such conditions must be made at a considerably higher temperature,
+usually in the neighborhood of 160° F.
+
+The types of apparatus used in these respective processes naturally
+varies. Where the heating is prolonged, the apparatus employed is built
+on the principle of a _tank_ or _reservoir_ in which a given volume of
+milk may be held at any given temperature for any given period of time.
+
+When the heat is applied for a much shorter period of time, the milk is
+passed in a continuous stream through the machine. Naturally the
+capacity of a continuous-flow apparatus is much greater than a machine
+that operates on the intermittent principle; hence, for large supplies,
+as in city distribution, this system has a great advantage. The question
+as to relative efficiency is however one which should be given most
+careful consideration.
+
+~Pasteurizing apparatus.~ The problems to be solved in the pasteurization
+of milk and cream designed for direct consumption are so materially
+different from where the process is used in butter-making that the type
+of machinery for each purpose is quite different. The equipment
+necessary for the first purpose may be divided into two general classes:
+
+1. Apparatus of limited capacity designed for family use.
+
+2. Apparatus of sufficient capacity to pasteurize on a commercial scale.
+
+~Domestic pasteurizers.~ In pasteurizing milk for individual use, it is
+not desirable to treat at one time more than will be consumed in one
+day; hence an apparatus holding a few bottles will suffice. In this case
+the treatment can best be performed in the bottle itself, thereby
+lessening the danger of infection. Several different types of
+pasteurizers are on the market; but special apparatus is by no means
+necessary for the purpose. The process can be efficiently performed by
+any one with the addition of an ordinary dairy thermometer to the common
+utensils found in the kitchen. Fig. 24 indicates a simple contrivance
+that can be readily arranged for this purpose.
+
+The following suggestions indicate the different steps of the process:
+
+1. Use only fresh milk.
+
+2. Place milk in clean bottles or fruit cans, filling to a uniform
+level, closing bottles tightly with a cork or cover. If pint and quart
+cans are used at the same time, an inverted bowl will equalize the
+level. Set these in a flat-bottomed tin pail and fill with warm water to
+same level as milk. An inverted pie tin punched with holes will serve as
+a stand on which to place the bottles during the heating process.
+
+3. Heat water in pail until the temperature of same reaches 155° to 160°
+F.; then remove from source of direct heat, cover with a cloth or tin
+cover, and allow the whole to stand for half an hour. In the preparation
+of milk for children, it is not advisable to use the low-temperature
+treatment (140° F.) that is recommended for commercial city delivery.
+
+[Illustration: FIG. 24. A home-made pasteurizer.]
+
+4. Remove bottles of milk and cool them as rapidly as possible without
+danger to bottles and store in a refrigerator.
+
+~Commercial pasteurizers.~ The two methods of pasteurization practiced
+commercially for the preservation of milk and cream have been developed
+because of the two types of machinery now in use. Apparatus constructed
+on the reservoir or tank principle permits of the retention of the milk
+for any desired period of time. Therefore, a lower temperature can be
+employed in the treatment. In those machines where the milk flows
+through the heater in a more or less continuous stream, the period of
+exposure is necessarily curtailed, thereby necessitating a higher
+temperature.
+
+~Reservoir pasteurizers.~ The simplest type of apparatus suitable for
+pasteurizing on this principle is where the milk is placed in shotgun
+cans and immersed in water heated by steam. Ordinary tanks surrounded
+with water spaces can also be used successfully. The Boyd cream ripening
+vat has also been tried. In this the milk is heated by a swinging coil
+immersed in the vat through which hot water circulates.
+
+In 1894 the writer[142] constructed a tank pasteurizer which consisted
+of a long, narrow vat surrounded by a steam-heated water chamber. Both
+the milk and the water chambers were provided with mechanical agitators
+having a to-and-fro movement.
+
+[Illustration: FIG. 25. Pott's pasteurizer.]
+
+Another machine which has been quite generally introduced is the Potts'
+rotating pasteurizer. This apparatus has a central milk chamber that is
+surrounded with an outer shell containing hot water. The whole machine
+revolves on a horizontal axis, and the cream or milk is thus thoroughly
+agitated during the heating process.
+
+~Continuous-flow pasteurizers.~ The demand for greater capacity than can
+be secured in the reservoir machines has led to the perfection of
+several kinds of apparatus where the milk is heated momentarily as it
+flows through the apparatus. Most of these were primarily introduced for
+the treatment of cream for butter-making purposes, but they are
+frequently employed for the treatment of milk on a large scale in city
+milk trade. Many of them are of European origin although of late years
+several have been devised in this country.
+
+The general principle of construction is much the same in most of them.
+The milk is spread out in a thin sheet, and is treated by passing it
+over a surface, heated either with steam directly or preferably with hot
+water.
+
+Where steam is used directly, it is impossible to prevent the "scalding
+on" of the milk proteids to the heated surface.
+
+In some of these machines (Thiel, Kuehne, Lawrence, De Laval, and
+Hochmuth), a ribbed surface is employed over which the milk flows, while
+the opposite surface is heated with hot water or steam. Monrad, Lefeldt
+and Lentsch employ a centrifugal apparatus in which a thin layer of milk
+is heated in a revolving drum.
+
+In some types of apparatus, as in the Miller machine, an American
+pasteurizer, the milk is forced in a thin sheet between two heated
+surfaces, thereby facilitating the heating process. In the Farrington
+machine heated discs rotate in a reservoir through which the milk flows
+in a continuous stream.
+
+One of the most economical types of apparatus is the regenerator type (a
+German machine), in which the milk passes over the heating surface in a
+thin stream and then is carried back over the incoming cold milk so
+that the heated liquid is partially cooled by the inflowing fresh milk.
+In machines of this class it requires very much less steam to heat up
+the milk than in those in which the cold milk is heated wholly by the
+hot water.
+
+A number of machines have been constructed on the principle of a
+reservoir which is fed by a constantly flowing stream. In some kinds of
+apparatus of this type no attempt is made to prevent the mixing of the
+recently introduced milk with that which has been partially heated. The
+pattern for this reservoir type is Fjord's heater, in which the milk is
+stirred by a stirrer. This apparatus was originally designed as a heater
+for milk before separation, but it has since been materially modified so
+that it is better adapted to the purposes of pasteurization. Reid was
+the first to introduce this type of machine into America.
+
+~Objections to continuous flow pasteurizers.~ In all continuous flow
+pasteurizers certain defects are more or less evident. While they
+fulfill the important requirement of large capacity, an absolute
+essential where large volumes of milk are being handled, it does not of
+necessity follow that they conform to all the hygienic and physical
+requirements that should be kept in mind. The greatest difficulty is the
+shortened period of exposure. The period which the milk is actually
+heated is often not more than a minute or so. Another serious defect is
+the inability to heat _all_ of the milk for a uniform period of time. At
+best, the milk is exposed for an extremely short time, but even then
+portions pass through the machine much more quickly than do the
+remainder. Those portions in contact with the walls of the apparatus are
+retarded by friction and are materially delayed in their passage, while
+the particles in the center of the stream, however thin, flow through
+in the least possible time.
+
+The following simple method enables the factory operator to test the
+period of exposure in the machine: Start the machine full of water, and
+after the same has become heated to the proper temperature, change the
+inflow to full-cream milk, continuing at the same rate. Note the exact
+time of change and also when first evidence of milkiness begins to
+appear at outflow. If samples are taken from first appearance of milky
+condition and thereafter at different intervals for several minutes, it
+is possible, by determining the amount of butter-fat in the same, to
+calculate with exactness how long it takes for the milk to entirely
+replace the water.
+
+Tests made by the writer[143] on the Miller pasteurizer showed, when fed
+at the rate of 1,700 pounds per hour, the minimum period of exposure to
+be 15 seconds, and the maximum about 60-70 seconds, while about
+two-thirds of the milk passed the machine in 40-50 seconds. This
+manifest variation in the rate of flow of the milk through the machine
+is undoubtedly the reason why the results of this type of treatment are
+subject to so much variation. Naturally, even a fatal temperature to
+bacterial life can be reduced to a point where actual destruction of
+even vegetating cells does not occur.
+
+~Bacterial efficiency of reservoir pasteurizers.~ The bacterial content of
+pasteurized milk and cream will depend somewhat on the number of
+organisms originally present in the same. Naturally, if mixed milk
+brought to a creamery is pasteurized, the number of organisms remaining
+after treatment would be greater than if the raw material was fresh and
+produced on a single farm.
+
+An examination of milk and cream pasteurized on a commercial scale in
+the Russell vat at the Wisconsin Dairy school showed that over 99.8 per
+cent of the bacterial life in raw milk or cream was destroyed by the
+heat employed, i. e., 155° F. for twenty minutes duration.[144] In
+nearly one-half of the samples of milk, the germ content in the
+pasteurized sample fell below 1,000 bacteria per cc., and the average of
+twenty-five samples contained 6,140 bacteria per cc. In cream the germ
+content was higher, averaging about 25,000 bacteria per cc. This milk
+was taken from the general creamery supply, which was high in organisms,
+containing on an average 3,675,000 bacteria per cc. De Schweinitz[145]
+has reported the germ content of a supply furnished in Washington which
+was treated at 158° to 160° F. for fifteen minutes. This supply came
+from a single source. Figures reported were from 48-hour-old agar
+plates. Undoubtedly these would have been higher if a longer period of
+incubation had been maintained. The average of 82 samples, taken for the
+period of one year, showed 325 bacteria per cc.
+
+[Illustration: FIG. 26. Effect of pasteurizing on germ content of milk.
+Black square represents bacteria of raw milk; small white square, those
+remaining after pasteurization.]
+
+~Bacterial efficiency of continuous-flow pasteurizers.~ A quantitative
+determination of the bacteria found in milk and cream when treated in
+machinery of this class almost always shows a degree of variation in
+results that is not to be noted in the discontinuous apparatus.
+
+[Illustration: FIG. 27. Reid's Continuous Pasteurizer.]
+
+Harding and Rogers[146] have tested the efficiency of one of the Danish
+type of continuous pasteurizers. These experiments were made at 158°,
+176° and 185° F. They found the efficiency of the machine not wholly
+satisfactory at the lower temperatures. At 158° F. the average of
+fourteen tests gave 15,300 bacteria per cc., with a maximum to minimum
+range from 62,790 to 120. Twenty-five examinations at 176° F. showed an
+average of only 117, with a range from 300 to 20. The results at 185° F.
+showed practically the same results as noted at 176° F. Considerable
+trouble was experienced with the "scalding on" of the milk to the walls
+of the machine when milk of high acidity was used.
+
+Jensen[147] details the results of 139 tests in 1899, made by the
+Copenhagen Health Commission. In 66 samples from one hundred thousand to
+one million organisms per cc. were found, and in 22 cases from one to
+five millions. Nineteen tests showed less than 10,000 per cc.
+
+In a series of tests conducted by the writer[148] on a Miller
+pasteurizer in commercial operation, an average of 21 tests showed
+12,350 bacteria remaining in the milk when the milk was pasteurized from
+156°-164° F. The raw milk in these tests ran from 115,000 to about one
+million organisms per cc.
+
+A recently devised machine of this type (Pasteur) has been tested by
+Lehmann, who found that it was necessary to heat the milk as high as
+176° to 185° F., in order to secure satisfactory results on the
+bacterial content of the cream.
+
+The writer tested Reid's pasteurizer at 155° to 165° F. with the
+following results: in some cases as many as 40 per cent. of the bacteria
+survived, which number in some cases exceeded 2,000,000 bacteria per
+cc.
+
+~Pasteurizing details.~ While the pasteurizing process is exceedingly
+simple, yet, in order to secure the best results, certain conditions
+must be rigidly observed in the treatment before and after the heating
+process.
+
+It is important to select the best possible milk for pasteurizing, for
+if the milk has not been milked under clean conditions, it is likely to
+be rich in the spore-bearing bacteria. Old milk, or milk that has not
+been kept at a low temperature, is much richer in germ-life than
+perfectly fresh or thoroughly chilled milk.
+
+The true standard for selecting milk for pasteurization should be to
+determine the actual number of bacterial _spores_ that are able to
+resist the heating process, but this method is impracticable under
+commercial conditions.
+
+The following method, while only approximate in its results, will be
+found helpful: Assuming that the age or treatment of the milk bears a
+certain relation to the presence of spores, and that the acid increases
+in a general way with an increase in age or temperature, the amount of
+acid present may be taken as an approximate index of the suitability of
+the milk for pasteurizing purposes. Biological tests were carried out in
+the author's laboratory[149] on milks having a high and low acid
+content, and it was shown that the milk with the least acid was, as a
+rule, the freest from spore-bearing bacteria.
+
+This acid determination can be made at the weigh-can by employing the
+Farrington alkaline tablet which is used in cream-ripening. Where milk
+is pasteurized under general creamery conditions, none should be used
+containing more than 0.2 per cent acidity. If only perfectly fresh milk
+is used, the amount of acid will generally be about 0.15 per cent with
+phenolphthalein as indicator.
+
+[Illustration: FIG. 28. Diagram showing temperature changes in
+pasteurizing, and the relation of same to bacterial growth.
+
+Shaded zone represents limits of bacterial growth, 50°-109° F. (10°-43°
+C.), the intensity of shading indicating rapidity of development. The
+solid black line shows temperature of milk during the process. The
+necessity for rapid cooling is evident as the milk falls in temperature
+to that of growing zone.]
+
+Emphasis has already been laid on the selection of a proper limit of
+pasteurizing (p. 114). It should be kept constantly in mind that the
+thermal death-point of any organism depends not alone on the temperature
+used, but on the period of exposure. With the lower limits given, 140°
+F., it is necessary to expose the milk for not less than fifteen
+minutes. If a higher heat is employed (and the cooked flavor
+disregarded) the period of exposure may be curtailed.
+
+~Chilling the milk.~ It is very essential in pasteurizing that the heated
+milk be immediately chilled in order to prevent the germination of the
+resistant spores, for if germination once occurs, growth can go on at
+relatively low temperatures.
+
+The following experiments by Marshall[150] are of interest as showing
+the influence of refrigeration on germination of spores:
+
+Cultures of organisms that had been isolated from pasteurized milk were
+inoculated into bouillon. One set was left to grow at room temperature,
+another was pasteurized and allowed to stand at same temperature, while
+another heated set was kept in a refrigerator. The unheated cultures at
+room temperature showed evidence of growth in thirty trials in an
+average of 26 hours; 29 heated cultures at room temperature all
+developed in an average of 50 hours, while the heated cultures kept in
+refrigerator showed no growth in 45 days with but four exceptions.
+
+Practically all of the rapid-process machines are provided with
+especially constructed cooling devices. In some of them, as in the
+Miller and Farrington, the cooling is effected by passing the milk
+through two separate coolers that are constructed in the same general
+way as the heater. With the first cooler, cold running water is
+employed, the temperature often being lowered in this way to 58° or 60°
+F. Further lessening of the temperature is secured by an additional ice
+water or brine cooler which brings the temperature down to 40°-50° F.
+
+In the economical use of ice the ice itself should be applied as closely
+as possibly to the milk to be cooled, for the larger part of the
+chilling value of ice comes from the melting of the same. To convert a
+pound of ice at 32° F. into a pound of water at the same temperature, if
+we disregard radiation, would require as much heat as would suffice to
+raise 142 pounds of water one degree F., or one pound of water 142° F.
+The absorptive capacity of milk for heat (specific heat) is not quite
+the same as it is with water, being .847 for milk in comparison with 1.0
+for water.[151] Hot milk would therefore require somewhat less ice to
+cool it than would be required by any equal volume of water at the same
+temperature.
+
+~Bottling the product.~ If the milk has been properly pasteurized, it
+should, of course, be dispensed in sterilized bottles. Glass bottles
+with plain pulp caps are best, and these should be thoroughly sterilized
+in steam before using. The bottling can best be done in a commercial
+bottling machine. Care must be taken to thoroughly clean this apparatus
+after use each day. Rubber valves in these machines suffer deterioration
+rapidly.
+
+[Illustration: FIG. 29. Relative consistency of pasteurized cream before
+(A) and after (B) treatment with viscogen as shown by rate of flow down
+inclined glass plate.]
+
+~Restoration of "body" of pasteurized cream.~ The action of heat causes
+the tiny groupings of fat globules in normal milk (Fig. 22) to break up,
+and with this change, which occurs in the neighborhood of 140° F., where
+the milk is heated for about 15 minutes and at about 160-165° F. where
+rapidly heated in a continuous stream, the consistency of the liquid is
+diminished, notwithstanding the fact that the fat-content remains
+unchanged. Babcock and the writer[152] devised the following "cure" for
+this apparent defect. If a strong solution of cane sugar is added to
+freshly slacked lime and the mixture allowed to stand, a clear fluid can
+be decanted off. The addition of this alkaline liquid, which is called
+"viscogen," to pasteurized cream in proportions of about one part of
+sugar-lime solution to 100 to 150 of cream, restores the consistency of
+the cream, as it causes the fat globules to cluster together in small
+groups.
+
+The relative viscosity of creams can easily be determined by the
+following method (Fig. 29):
+
+Take a perfectly clean piece of glass (plate or picture glass is
+preferable, as it is less liable to be wavy). Drop on one edge two or
+three drops of cream at intervals of an inch or so. Then incline piece
+of glass at such an angle as to cause the cream to flow down surface of
+glass. The cream, having the heavier body or viscosity, will move more
+slowly. If several samples of each cream are taken, then the aggregate
+lengths of the different cream paths may be taken, thereby eliminating
+slight differences due to condition of glass.
+
+FOOTNOTES:
+
+[126] From 10 to 16 cents per quart is usually paid for such milks.
+
+[127] Much improvement in quality could be made by more careful control
+of milk during shipment, especially as to refrigeration; also as to the
+care taken on the farms. The use of the ordinary milking machine (see
+page 37), would go far to reduce the germ content of milk.
+
+[128] Farrington, Journ. Amer. Chem. Soc., Sept., 1896.
+
+[129] Hite, Bull. 58, West Va. Expt. Stat., 1899.
+
+[130] Milch Zeit., 1895, No. 9.
+
+[131] Ibid., 1897, No. 33.
+
+[132] Bernstein, Milch Zeit., 1894, pp. 184, 200.
+
+[133] Thoerner, Chem. Zeit., 18:845.
+
+[134] Snyder, Chemistry of Dairying, p. 59.
+
+[135] Doane and Price (Bull. 77, Md. Expt. Stat., Aug. 1901) give quite
+a full resumé of the work on this subject in connection with rather
+extensive experiments made by them on feeding animals with raw,
+pasteurized and sterilized milks.
+
+[136] Rickets is a disease in which the bones lack sufficient mineral
+matter to give them proper firmness. Marasmus is a condition in which
+the ingested food seems to fail to nourish the body and gradual wasting
+away occurs.
+
+[137] De Man, Arch. f. Hyg., 1893, 18:133.
+
+[138] Th. Smith, Journ. of Expt. Med., 1899, 4:217.
+
+[139] Russell and Hastings, 17 Rept. Wis. Expt. Stat., 1900, p. 147.
+
+[140] Russell and Hastings, 21 Rept. Ibid., 1904.
+
+[141] Russell and Hastings, 18 Rept. Ibid., 1901.
+
+[142] Russell, Bull. 44, Wis. Expt. Stat.
+
+[143] Russell, 22 Wis. Expt. Stat. Rept., 1905, p. 232.
+
+[144] Russell, 12 Wis. Expt. Stat. Rept., 1895, p. 160.
+
+[145] De Schweinitz, Nat. Med. Rev., 1899, No. 11.
+
+[146] Harding and Rogers. Bull. 182, N. Y. (Geneva) Expt. Stat., Dec.,
+1899.
+
+[147] Jensen, Milchkunde und Milch Hygiene, p. 132.
+
+[148] 22 Wis. Expt. Stat. Rept., 1905, p. 236.
+
+[149] Shockley, Thesis, Univ. of Wis., 1896.
+
+[150] Marshall, Mich. Expt. Stat., Bull. 147, p. 47.
+
+[151] Fleischmann, Landw. Versuchts Stat., 17:251.
+
+[152] Babcock and Russell, Bull. 54, Wis. Expt. Stat., Aug. 1896.
+
+
+
+
+CHAPTER VII.
+
+BACTERIA AND BUTTER-MAKING.
+
+
+In making butter from the butter fat in milk, it is necessary to
+concentrate the fat globules into cream, preliminary to the churning
+process. The cream may be raised by the gravity process or separated
+from the milk by centrifugal action. In either case the bacteria that
+are normally present in the milk differentiate themselves in varying
+numbers in the cream and the skim-milk. The cream always contains per
+cc. a great many more than the skim-milk, the reason for this being that
+the bacteria are caught and held in the masses of fat globules, which,
+on account of their lighter specific gravity, move toward the surface of
+the milk or toward the interior of the separator bowl. This filtering
+action of the fat globules is similar to what happens in muddy water
+upon standing. As the suspended particles fall to the bottom they carry
+with them a large number of the organisms that are in the liquid.
+
+~Various creaming methods.~ The creaming method has an important bearing
+on the kind as well as the number of the bacteria that are to be found
+in the cream. The difference in species is largely determined by the
+difference in ripening temperature, while the varying number is governed
+more by the age of the milk.
+
+_1. Primitive gravity methods._ In the old shallow-pan process, the
+temperature of the milk is relatively high, as the milk is allowed to
+cool naturally. This comparatively high temperature favors especially
+the development of those forms whose optimum growing-point is near the
+air temperature. By this method the cream layer is exposed to the air
+for a longer time than with any other, and consequently the
+contamination from this source is greater. Usually cream obtained by the
+shallow-pan process will contain a larger number of species and also
+have a higher acid content.
+
+_2. Modern gravity methods._ In the Cooley process, or any of the modern
+gravity methods where cold water or ice is used to lower the
+temperature, the conditions do not favor the growth of a large variety
+of species. The number of bacteria in the cream will depend largely upon
+the manner in which the milk is handled previous to setting. If care is
+used in milking, and the milk is kept so as to exclude outside
+contamination, the cream will be freer from bacteria than if
+carelessness prevails in handling the milk. Only those forms will
+develop in abundance that are able to grow at the low temperature at
+which the milk is set. Cream raised by this method is less frequently
+infected with undesirable forms than that which is creamed at a higher
+temperature.
+
+_3. Centrifugal method._ Separator cream should contain less germ-life
+than that which is secured in the old way. It should contain only those
+forms that have found their way into the milk during and subsequent to
+the milking, for the cream is ordinarily separated so soon that there is
+but little opportunity of infection, if care is taken in the handling.
+As a consequence, the number of species found therein is smaller.
+
+Where milk is separated, it is always prudent to cool the cream so as to
+check growth, as the milk is generally heated before separating in order
+to skim efficiently.
+
+Although cream is numerically much richer in bacteria than milk, yet
+the changes due to bacterial action are slower; hence milk sours more
+rapidly than cream. For this same reason, cream will sour sooner when it
+remains on the milk than it will if it is separated as soon as possible.
+This fact indicates the necessity of early creaming, so as to increase
+the keeping quality of the product, and is another argument in favor of
+the separator process.
+
+~Ripening of cream.~ If cream is allowed to remain at ordinary
+temperatures, it undergoes a series of fermentation changes that are
+exceedingly complex in character, the result of which is to produce in
+butter made from the same the characteristic flavor and aroma that are
+so well known in this article. We are so accustomed to the development
+of these flavors in butter that they are not generally recognized as
+being intimately associated with bacterial activity unless compared with
+butter made from perfectly fresh cream. Sweet-cream butter lacks the
+aromatic principle that is prominent in the ripened product, and while
+the flavor is delicate, it is relatively unpronounced.
+
+In the primitive method of butter-making, where the butter was made on
+the farm, the ripening of cream became a necessity in order that
+sufficient material might be accumulated to make a churning. The
+ripening change occurred spontaneously without the exercise of any
+especial control. With the development of the creamery system came the
+necessity of exercising a control of this process, and therefore the
+modern butter-maker must understand the principles which are involved in
+this series of complex changes that largely give to his product its
+commercial value.
+
+In these ripening changes three different factors are to be taken into
+consideration: the development of acid, flavor and aroma. Much confusion
+in the past has arisen from a failure to discriminate between these
+qualities. While all three are produced simultaneously in ordinary
+ripening, it does not necessarily follow that they are produced by the
+same cause. If the ripening changes are allowed to go too far,
+undesirable rather than beneficial decomposition products are produced.
+These greatly impair the value of butter, so that it becomes necessary
+to know just to what extent this process should be carried.
+
+In cream ripening there is a very marked bacterial growth, the extent of
+which is determined mainly by the temperature of the cream. Conn and
+Esten[153] find that the number of organisms may vary widely in
+unripened cream, but that the germ content of the ripened product is
+more uniform. When cream is ready for the churn, it often contains
+500,000,000 organisms per cc., and frequently even a higher number. This
+represents a germ content that has no parallel in any natural material.
+
+The larger proportion of bacteria in cream as it is found in the
+creamery belong to the acid-producing class, but in the process of
+ripening, these forms seem to thrive still better, so that when it is
+ready for churning the germ content of the cream is practically made up
+of this type.
+
+~Effect on churning.~ In fresh cream the fat globules which are suspended
+in the milk serum are surrounded by a film of albuminous material which
+prevents them from coalescing readily. During the ripening changes, this
+enveloping substance is modified, probably by partial solution, so that
+the globules cohere when agitated, as in churning. The result is that
+ripened cream churns more easily, and as it is possible to cause a
+larger number of the smaller fat-globules to cohere to the butter
+granules, the yield is slightly larger--a point of considerable
+economic importance where large quantities of butter are made.
+
+~Development of acid.~ The result of this enormous bacterial
+multiplication is that acid is produced in cream, lactic being the
+principal acid so formed.
+
+Other organic acids are undoubtedly formed as well as certain aromatic
+products. While the production of acid as a result of fermentative
+activity is usually accompanied with a development of flavor, the flavor
+is not directly produced by the formation of acid. If cream is treated
+in proper proportions with a commercial acid, as hydrochloric,[154] it
+assumes the same churning properties as found in normally ripened cream,
+but is devoid of the desired aromatic qualities. Lactic acid[155] has
+also been used in a similar way but with no better results.
+
+The amount of acidity that should be developed under natural conditions
+so as to secure the optimum quality as to flavor and aroma is the most
+important question in cream ripening. Concerning this there have been
+two somewhat divergent views as to what is best in practice, some
+holding that better results were obtained with cream ripened to a high
+degree of acidity than where a less amount was developed.[156] The
+present tendency seems to be to develop somewhat more than formerly, as
+it is thought that this secures more of the "high, quick" flavor wanted
+in the market. On the average, cream is ripened to about 0.5 to 0.65 per
+cent. acidity, a higher percentage than this giving a strong-flavored
+butter. In the determination of acidity, the most convenient method is
+to employ the Farrington alkaline tablet, which permits of an accurate
+and rapid estimation of the acidity in the ripening cream. The amount of
+acidity to be produced must of necessity be governed by the amount of
+butter-fat present, for the formation of acid is confined to the serum
+of the cream; consequently, a rich cream would show less acid by
+titration than a thinner cream, and still contain really as much acid as
+the other. The importance of this factor is evident in gathered-cream
+factories.
+
+The rate of ripening is dependent upon the conditions that affect the
+rate of growth of bacterial life, such as time and temperature, number
+of organisms in cream and also the per cent of butter fat in the cream.
+Some years ago it was customary to ripen cream at about 50° to 60° F.,
+but more recently better results have been obtained, it is claimed,
+where the ripening temperature is increased and the period of ripening
+lessened. As high a temperature as 70° to 75° F. has been recommended.
+It should be said that this variation in practice may have a valid
+scientific foundation, for the temperature of the ripening cream is
+undoubtedly the most potent factor in determining what kind of bacteria
+will develop most luxuriantly. It is well known that those forms that
+are capable of producing bitter flavors are able to thrive better at a
+lower temperature than some of the desirable ripening species.
+
+The importance of this factor would be lessened where a pure culture was
+used in pasteurized cream, because here practically the selected
+organism alone controls the field.
+
+It is frequently asserted that better results are obtained by stirring
+the cream and so exposing it to the air as much as possible. Experiments
+made at the Ontario Agricultural College, however, show practically no
+difference in the quality of the butter made by these two methods. The
+great majority of the bacteria in the cream belong to the facultative
+class, and are able to grow under conditions where they are not in
+direct contact with the air.
+
+~Flavor and aroma.~ The basis for the peculiar flavor or taste which
+ripened cream-butter possesses is due, in large part, to the formation
+of certain decomposition products formed by various bacteria. Aroma is a
+quality often confounded with flavor, but this is produced by volatile
+products only, which appeal to the sense of smell rather than taste.
+Generally a good flavor is accompanied by a desirable aroma, but the
+origin of the two qualities is not necessarily dependent on the same
+organisms. The quality of flavor and aroma in butter is, of course, also
+affected by other conditions, as, for instance, the presence or absence
+of salt, as well as the inherent qualities of the milk, that are
+controlled, to some extent at least, by the character of the feed which
+is consumed by the animal. The exact source of these desirable but
+evanescent qualities in butter is not yet satisfactorily determined.
+According to Storch,[157] flavors are produced by the decomposition of
+the milk sugar and the absorption of the volatile flavors by the butter
+fat. Conn[158] holds that the nitrogenous elements in cream serve as
+food for bacteria, and in the decomposition of which the desired
+aromatic substance is produced. The change is unquestionably a complex
+one, and cannot be explained as a single fermentation.
+
+There is no longer much doubt but that both acid-forming and
+casein-digesting species can take part in the production of proper
+flavors as well as desirable aromas. The researches of Conn,[159] who
+has studied this question most exhaustively, indicate that both of these
+types of decomposition participate in the production of flavor and
+aroma. He has shown that both flavor and aroma production are
+independent of acid; that many good flavor-producing forms belong to
+that class which renders milk alkaline, or do not change the reaction at
+all. Some of these species liquefied gelatin and would therefore belong
+to the casein-dissolving class. Those species that produced bad flavors
+are also included in both fermentative types. Conn has found a number of
+organisms that are favorable flavor-producers; in fact they were much
+more numerous than desirable aroma-yielding species. None of the
+favorable aroma forms according to his investigations were lactic-acid
+species,--a view which is also shared by Weigmann.[160]
+
+McDonnell[161] has found that the production of aroma in certain cases
+varies at different temperatures, the most pronounced being evolved near
+the optimum growing temperature, which, as a general rule, is too high
+for cream ripening.
+
+The majority of bacteria in ripening cream do not seem to exert any
+marked influence in butter. A considerable number of species are
+positively beneficial, inasmuch as they produce a good flavor or aroma.
+A more limited number are concerned in the production of undesirable
+ripening changes. This condition being true, it may seem strange that
+butter is as good as it is, because so frequently the requisite care is
+not given to the development of proper ripening. In all probability the
+chief reason why this is so is that those bacteria that find milk and
+cream pre-eminently suited to their development, e. g. the lactic-acid
+class, are either neutral or beneficial in their effect on butter.
+
+~Use of starters.~ Experience has amply demonstrated that it is possible
+to control the nature of the fermentative changes that occur in ripening
+cream to such an extent as to materially improve the quality of the
+butter. This is frequently done by the addition of a "starter." While
+starters have been employed for many years for the purpose mentioned, it
+is only recently that their nature has been understood. A starter may be
+selected from widely divergent sources, but in all cases it is sure to
+contain a large number of bacteria, and the presumption is that they are
+of such a nature as to produce desirable fermentative changes in the
+cream.
+
+In the selection of these so-called natural starters, it follows that
+they must be chosen under such conditions as experience has shown to
+give favorable results. For this purpose, whole milk from a single
+animal is often used where the same is observed to sour with the
+production of no gas or other undesirable taint. A skim-milk starter
+from a mixed supply is recommended by many. Butter milk is frequently
+employed, but in the opinion of butter experts is not as suitable as the
+others mentioned.
+
+It not infrequently happens that the practical operator may be misled in
+selecting a starter that is not desirable, or by continuing its use
+after it has become contaminated.
+
+In 1890[162] a new system of cream ripening was introduced in Denmark by
+Storch that possesses the merit of being a truly scientific and at the
+same time practical method. This consisted in the use of pure cultures
+of specific organisms that were selected on account of their ability to
+produce a desirable ripening change in cream. The introduction of these
+so-called culture starters has become universal in Denmark, and in parts
+of Germany. Their use is also rapidly extending in this country,
+Australia and New Zealand.
+
+~Principles of pure-culture cream-ripening.~ In the proper use of pure
+cultures for ripening cream, it is necessary first to eliminate as far
+as possible the bacteria already present in cream before the culture
+starter is added. This result is accomplished by heating the cream to a
+temperature sufficiently high to destroy the vegetating organisms. The
+addition of a properly selected starter will then give the chosen
+organism such an impetus as will generally enable it to gain the
+ascendency over any other bacteria and so control the character of the
+ripening. The principle employed is quite like that practiced in raising
+grain. The farmer prepares his soil by plowing, in this way killing the
+weeds. Then he sows his selected grain, which is merely a pure culture,
+and by the rapid growth of this, other forms are held in check.
+
+The attempt has been made to use these culture starters in raw sweet
+cream, but it can scarcely be expected that the most beneficial results
+will be attained in this way. This method has been justified on the
+basis of the following experiments. Where cream is pasteurized and no
+starter is added, the spore-bearing forms frequently produce undesirable
+flavors. These can almost always be controlled if a culture starter is
+added, the obnoxious form being repressed by the presence of the added
+starter. This condition is interpreted as indicating that the addition
+of a starter to cream which already contains developing bacteria will
+prevent those originally present in the cream from growing.[163] This
+repressive action of one species on another is a well-known
+bacteriological fact, but it must be remembered that such an explanation
+is only applicable in those cases where the culture organism is better
+able to develop than those forms that already exist in the cream.
+
+If the culture organism is added to raw milk or cream which already
+contains a flora that is well suited to develop in this medium, it is
+quite doubtful whether it would gain the supremacy in the ripening
+cream. The above method of adding a culture to raw cream renders
+cream-ripening details less burdensome, but at the same time Danish
+experience, which is entitled to most credence on this question, is
+opposed to this method.
+
+~Reputed advantages of culture starters.~ _1. Flavor and aroma._ Naturally
+the flavor produced by pure-culture ferments depends upon the character
+of the organism used. Those which are most extensively used are able to
+produce a perfectly clean but mild flavor, and a delicate but not
+pronounced aroma. The "high, quick" flavor and aroma that is so much
+desired in the American market is not readily obtained by the use of
+cultures. It is quite problematical whether the use of any single
+species will give any more marked aroma than normally occurs in natural
+ripening.
+
+_2. Uniformity of product._ Culture starters produce a more uniform
+product because the type of fermentation is under more complete control,
+and herein is the greatest advantage to be derived from their use. Even
+the best butter-maker at times will fail to secure uniform results if
+his starter is not perfectly satisfactory.
+
+_3. Keeping quality of product._ Butter made from pasteurized cream to
+which a pure-culture starter has been added will keep much better than
+the ordinary product, because the diversity of the bacterial flora is
+less and the milk is therefore not so likely to contain those organisms
+that produce an "off" condition.
+
+_4. Elimination of taints._ Many defective conditions in butter are
+attributable to the growth of undesirable bacteria in the cream that
+result in the formation of "off" flavors and taints. If cream is
+pasteurized, thereby destroying these organisms, then ripened with pure
+ferments, it is generally possible to eliminate the abnormal
+conditions.[164] Taints may also be present in cream due to direct
+absorption from the cow or through exposure to foul odors.[165] Troubles
+of this sort may thus be carried over to the butter. This is
+particularly true in regions where leeks and wild onions abound, as in
+some of the Atlantic States. The heating of the cream tends to expel
+these volatile taints, so that a fairly good article of butter can be
+made from what would otherwise be a relatively worthless product.
+
+~Characteristics desired in culture starters.~ Certain conditions as the
+following are desirable in starters made from pure cultures:
+
+1. Vigorous growth in milk at ordinary ripening temperatures.
+
+2. Ability to form acid so as to facilitate churning and increase the
+yield of butter.
+
+3. Able to produce a clean flavor and desirable aroma.
+
+4. Impart a good keeping quality to butter.
+
+5. Not easily modified in its flavor-producing qualities by artificial
+cultivation.
+
+These different conditions are difficult to attain, for the reason that
+some of them seem to be in part incompatible. Weigmann[166] found that a
+good aroma was generally an evanescent property, and therefore opposed
+to good keeping quality. Conn has shown that the functions of
+acid-formation, flavor and aroma production are not necessarily related,
+and therefore the chances of finding a single organism that possesses
+all the desirable attributes are not very good.
+
+In all probability no one germ possesses all of these desirable
+qualities, but natural ripening is the resultant of the action of
+several forms.[167] This idea has led to the attempt at mixing selected
+organisms that have been chosen on account of certain favorable
+characteristics which they might possess. The difficulty of maintaining
+such a composite culture in its correct proportions when it is
+propagated in the creamery is seemingly well nigh insuperable, as one
+organism is very apt to develop more or less rapidly than the other.
+
+A very satisfactory way in which these cultures are marketed is to mix
+the bacterial growth with some sterile, inert, dry substance. This is
+the method used in most of the Danish cultures. In this country, some of
+the more prominent cultures employed are marketed in a liquid form.
+
+~Culture vs. home-made starters.~ One great advantage which has accrued
+from the use of culture or commercial starters has been that in
+emphasizing the need of closer control of the ripening process, greater
+attention has been paid to the carrying out of the details. In the
+hands of the better operators, the differences in flavor of butter made
+with a culture or a natural starter are not marked,[168] but in the
+hands of those who fail to make a good product under ordinary
+conditions, an improvement is often secured where a commercial culture
+is used.
+
+~Pasteurization as applied to butter-making.~ This process, as applied to
+butter making, is often confounded with the treatment of milk and cream
+for direct consumption. It is unfortunate that the same term is used in
+connection with the two methods, for they have but little in common
+except in the use of heat to destroy the germ life of the milk. In
+pasteurizing cream for butter-making, it is not necessary to observe the
+stringent precautions that are to be noted in the preservation of milk;
+for the addition of a rapidly developing starter controls at once the
+fermentative changes that subsequently occur. Then again, the physical
+requirement as to the production of a cooked taste is not so stringent
+in butter-making. While a cooked taste is imparted to milk or even cream
+at about 158° F., it is possible to make butter that shows no permanent
+cooked taste from cream that has been raised as high as 185° or even
+195° F. This is due to the fact that the fat does not readily take up
+those substances that give to scalded milk its peculiar flavor.
+
+Unless care is taken in the manipulation of the heated cream, the grain
+or body of the butter may be injured. This tendency can be overcome if
+the ripened cream is chilled to 48° F. for about two hours before
+churning. It is also essential that the heated cream should be quickly
+and thoroughly chilled after being pasteurized.
+
+The Danes, who were the first to employ pasteurization in butter-making,
+used, in the beginning, a temperature ranging from 158° to 167° F., but
+owing to the prevalence of such diseases as tuberculosis and
+foot-and-mouth disease, it became necessary to treat all of the skim
+milk that was returned from the creameries. For this purpose the skim
+milk is heated to a temperature of 176° F., it having been more recently
+determined that this degree of heat is sufficient to destroy the seeds
+of disease. With the use of this higher temperature the capacity of the
+pasteurizing apparatus is considerably reduced, but the higher
+temperature is rendered necessary by the prevailing conditions as to
+disease.
+
+When the system was first introduced in Denmark, two methods of
+procedure were followed: the whole milk was heated to a sufficiently
+high temperature to thoroughly pasteurize it before it was separated, or
+it was separated first, and the cream pasteurized afterwards. In the
+latter case, it is necessary to heat the skim milk after separation to
+destroy the disease organisms, but this can be quickly done by the use
+of steam directly. Much more care must be used in heating the cream in
+order to prevent injury to the grain of the butter. In spite of the
+extra trouble of heating the cream and skim milk separately, this method
+has practically supplanted the single heating. With the continual spread
+of tuberculosis in America the heating of skim milk separately is
+beginning to be introduced.[169]
+
+~Use of starters in pasteurized and unpasteurized cream.~ In order to
+secure the beneficial results presumably attributable to the use of a
+starter, natural as well as a pure culture, it should be employed in
+cream in which the bacteria have first been killed out by
+pasteurization. This is certainly the most logical and scientific method
+and is the way in which the process has been developed in Denmark.
+
+Here in this country, the use of pure cultures has been quite rapidly
+extended, but the system of heating the cream has been used in only a
+slight measure. The increased labor and expense incurred in pasteurizing
+the cream has naturally militated somewhat against the wide-spread use
+of the process, but doubtless the main factor has been the inability to
+secure as high a flavor where the cream was heated as in the unheated
+product. As the demands of the market change from a high, quick flavor
+to one that is somewhat milder but of better keeping quality, doubtless
+pasteurization of the cream will become more and more popular. That such
+a change is gradually occurring is already evident, although as yet only
+a small proportion of butter made in this country is now made in this
+way. Where the cream is unheated, a considerable number of species will
+be found, and even the addition of a pure culture, if that culture is of
+the lactic acid-producing species, will to some extent control the type
+of fermentation that occurs. Such would not be the case with a culture
+composed of the casein-digesting type of bacteria. Only those forms
+could thus be used which are especially well suited to development in
+raw cream. For this reason the pure culture ferments that are generally
+employed in creamery practice are organisms of the lactic acid type,
+able to grow rapidly in cream and produce a pure cream flavor in the
+butter.
+
+~Purity of commercial starters.~ Naturally the butter maker is forced to
+rely on the laboratory for his commercial starter, and the question will
+often arise as to the purity and vigor of the various ferments employed.
+As there is no way for the factory operator to ascertain the actual
+condition of the starter, except by using the same, the greatest care
+should be taken by the manufacturer to insure the absolute purity of the
+seed used.
+
+A bacteriological examination of the various cultures which have been
+placed on the market not infrequently reveals an impure condition. In
+several cases the writer has found a not inconsiderable number of
+liquefying bacteria mixed with the selected organism. Molds not
+infrequently are found in cultures put up in the dry form. Doubtless the
+effect of these accidental contaminations is considerably less in the
+case of a starter composed of a distinctively lactic acid-producing
+organism than with a form which is less capable of thriving vigorously
+in milk, and it should be said that these impurities can frequently be
+eliminated by continued propagation.
+
+The virility and vigor of the starter is also a fluctuating factor,
+dependent in part at least, upon the conditions under which the organism
+is grown. In some cases the germ is cultivated in solutions in which
+acid cannot be formed in abundance. Where the conditions permit of the
+formation of acid, as would be the case if sugar was present with a
+lactic acid-producing species, the vitality of the culture is often
+impaired by the action of the gradually accumulating acid. Some
+manufacturers attempt to minimize this deleterious condition by adding
+carbonate of lime which unites with the acid that is formed.
+
+~Propagation of starters for cream-ripening.~ The preparation and
+propagation of a starter for cream-ripening is a process involving
+considerable bacteriological knowledge, whether the starter is of
+domestic origin or prepared from a pure-culture ferment. In any event,
+it is necessary that the starter should be handled in a way so as to
+prevent the introduction of foreign bacteria as far as possible. It
+should be remembered at all times that the starter is a live thing and
+must be handled throughout its entire history in a way so as to retain
+its vitality and vigor unimpaired. The following points should be taken
+into consideration in growing the starter and transferring it from day
+to day:
+
+1. If a commercial starter is used, see that it is fresh and that the
+seal has not been broken. If the culture is too old, the larger part of
+the organisms may have died out before it is transferred, in which case
+the effect of its addition to the sterilized milk would be of little
+value.
+
+When the commercial ferment is received, it should be stored in the
+refrigerator pending its use so as to retard as much as possible the
+changes that naturally go on in the culture liquid. Be careful that the
+bottle is not exposed to the influence of direct sunlight for in a
+transparent medium the organisms may be readily killed by the
+disinfecting action of the sun's rays.
+
+2. If a home-made starter is employed, use the greatest possible care in
+selecting the milk that is to be used as a basis for the starter.
+
+3. For the propagation and perpetuation of the starter from day to day,
+it is necessary that the same should be grown in milk that is as
+germ-free as it is possible to secure it. For this purpose sterilize
+some fresh skim-milk in a covered can that has previously been well
+steamed. This can be done easily by setting cans containing skim-milk in
+a vat filled with water and heating the same to 180° F. or above for
+one-half hour or more. Steam should not be introduced directly. This
+process destroys all but a few of the most resistant spore-bearing
+organisms. This will give a cooked flavor to the milk, but will not
+affect the cream to which the starter is added. Dairy supply houses are
+now introducing the use of starter cans that are specially made for this
+purpose.
+
+4. After the heated milk is cooled down to about 70° or 80° F., it can
+be inoculated with the desired culture. Sometimes it is desirable to
+"build up" the starter by propagating it first in a smaller volume of
+milk, and then after this has developed, adding it to a larger amount.
+
+This method is of particular value where a large amount of starter is
+needed for the cream-ripening.
+
+5. After the milk has been inoculated, it should be kept at a
+temperature that is suitable for the rapid development of the contained
+bacteria, 65°-75° F., which temperature should be kept as uniform as
+possible.
+
+This can best be done by setting the covered can in a vat filled with
+warm water. The starter cans are often arranged so that temperature can
+be controlled by circulating water.
+
+6. The starter should not be too thoroughly curdled when it is needed
+for use, but should be well soured and only partially curdled for it is
+difficult to break up thoroughly the curd particles if the starter is
+completely curdled. If these curd masses are added to ripening cream,
+white specks may appear in the butter.
+
+7. The vigor of the starter is in all probability stronger when the milk
+is on the point of curdling than it is after the curd has been formed
+some time. The continued formation of lactic acid kills many of the
+bacteria and thus weakens the fermentative action. It is therefore
+highly important that the acidity of the starter should be closely
+watched.
+
+8. Do not refrigerate the starter when it has reached the proper stage
+of development, as this retards the bacterial growth in the same manner
+as cold weather checks the growth of grain. It is preferable to dilute
+the starter, if it cannot be used when ready, with sufficient freshly
+sterilized sweet milk to hold the acidity at the proper point and thus
+keep the bacteria in the starter in a condition which will favor
+vigorous growth.
+
+9. The starter should be propagated from day to day by adding a small
+quantity to a new lot of freshly prepared milk. For this purpose two
+propagating cans should be provided so that one starter may be in use
+while the other is being prepared.
+
+~How long should a starter be propagated?~ No hard-and-fast rule can be
+given for this, for it depends largely upon how carefully the starter is
+handled during its propagation. If the starter is grown in sterilized
+milk kept in steamed vessels and is handled with sterile dippers, it is
+possible to maintain it in a state of relative purity for a considerable
+period of time; if, however, no especial care is given, it will soon
+become infected by the air, and the retention of its purity will depend
+more upon the ability of the contained organism to choke out foreign
+growths than upon any other factor. Experience seems to indicate that
+pure-culture starters "run out" sooner than domestic starters. While it
+is possible, by bacteriological methods, to determine with accuracy the
+actual condition of a starter as to its germ content, still such methods
+are inapplicable in creamery practice. Here the maker must rely largely
+upon the general appearance of the starter as determined by taste and
+smell. The supply houses that deal in cultures of this class generally
+expect to supply a new culture at least every month.
+
+~Bacteria in butter.~ As ripened cream is necessarily rich in bacteria, it
+follows that butter will also contain germ life in varying amounts, but
+as butter-fat is not well adapted for bacterial food, the number of
+germs in butter is usually less than in ripened cream.
+
+Sweet-cream butter is naturally poorer in germ life than that made from
+ripened cream. Grotenfelt reports in sweet-cream butter, the so-called
+"Paris butter," only a few bacteria while in acid cream butter the germ
+content runs from scores to hundreds of thousands.
+
+~Effect of bacteria in wash water.~ An important factor in contamination
+may be the wash water that is used. Much carelessness often prevails
+regarding the location and drainage of the creamery well, and if same
+becomes polluted with organic matter, bacterial growth goes on apace.
+Melick[170] has made some interesting studies on using pasteurized and
+sterilized well waters for washing. He found a direct relation to exist
+between the bacterial content of the wash water and the keeping quality
+of the butter. Some creameries have tried filtered water but under
+ordinary conditions a filter, unless it is tended to with great
+regularity, becomes a source of infection rather than otherwise.
+
+~Changes in germ content.~ The bacteria that are incorporated with the
+butter as it first "comes" undergo a slight increase for the first few
+days. The duration of this period of increase is dependent largely upon
+the condition of the butter. If the buttermilk is well worked out of the
+butter, the increase is slight and lasts for a few days only, while the
+presence of so nutritious a medium as buttermilk affords conditions much
+more favorable for the continued growth of the organisms.
+
+While there may be many varieties in butter when it is fresh, they are
+very soon reduced in kind as well as number. The lactic acid group of
+organisms disappear quite rapidly; the spore-bearing species remaining
+for a somewhat longer time. Butter examined after it is several months
+old is often found to be almost free from germs.
+
+In the manufacture of butter there is much that is dependent upon the
+mechanical processes of churning, washing, salting and working the
+product. These processes do not involve any bacteriological principles
+other than those that are incident to cleanliness. The cream, if ripened
+properly, will contain such enormous numbers of favorable forms that the
+access of the few organisms that are derived from the churn, the air, or
+the water in washing will have little effect, unless the conditions are
+abnormal.
+
+
+BACTERIAL DEFECTS IN BUTTER.
+
+~Rancid change in butter.~ Fresh butter has a peculiar aroma that is very
+desirable and one that enhances the market price, if it can be retained;
+but this delicate flavor is more or less evanescent, soon disappearing,
+even in the best makes. While a good butter loses with age some of the
+peculiar aroma that it possesses when first made, yet a gilt-edged
+product should retain its good keeping qualities for some length of
+time. All butters, however, sooner or later undergo a change that
+renders them worthless for table use. This change is usually a rancidity
+that is observed in all stale products of this class. The cause of this
+rancid condition in butter was at first attributed to the formation of
+butyric acid, but it is now recognized that other changes also enter
+in.[171] Light and especially air also exert a marked effect on the
+flavor of butter. Where butter is kept in small packages it is much more
+prone to develop off flavors than when packed in large tubs. From the
+carefully executed experiments of Jensen it appears that some of the
+molds as well as certain species of bacteria are able to incite these
+changes. These organisms are common in the air and water and it
+therefore readily follows that inoculation occurs.
+
+Practically, rancidity is held in check by storing butter at low
+temperatures where germ growth is quite suspended.
+
+~Lack of flavor.~ Often this may be due to improper handling of the cream
+in not allowing it to ripen far enough, but sometimes it is impossible
+to produce a high flavor. The lack of flavor in this case is due to the
+absence of the proper flavor-producing organisms. This condition can
+usually be overcome by the addition of a proper starter.
+
+~Putrid butter.~ This specific butter trouble has been observed in
+Denmark, where it has been studied by Jensen.[172] Butter affected by it
+rapidly acquires a peculiar putrid odor that ruins it for table use.
+Sometimes, this flavor may be developed in the cream previous to
+churning.
+
+Jensen found the trouble to be due to several different putrefactive
+bacteria. One form which he called _Bacillus foetidus lactis_, a close
+ally of the common feces bacillus, produced this rotten odor and taste
+in milk in a very short time. Fortunately, this organism was easily
+killed by a comparatively low heat, so that pasteurization of the cream
+and use of a culture starter quickly eliminated the trouble, where it
+was tried.
+
+~Turnip-flavored butter.~ Butter sometimes acquires a peculiar flavor
+recalling the order of turnips, rutabagas, and other root crops. Often
+this trouble is due to feeding, there being in several of these crops,
+aromatic substances that pass directly into the milk, but in some
+instances the trouble arises from bacteria that are able to produce
+decomposition products,[173] the odor and taste of which strongly
+recalls these vegetables.
+
+~"Cowy" butter.~ Frequently there is to be noted in milk a peculiar odor
+that resembles that of the cow stable. Usually this defect in milk has
+been ascribed to the absorption of impure gases by the milk as it cools,
+although the gases and odors naturally present in fresh milk have this
+peculiar property that is demonstrable by certain methods of aeration.
+Occasionally it is transmitted to butter, and recently Pammel[174] has
+isolated from butter a bacillus that produced in milk the same peculiar
+odor so commonly present in stables.
+
+~Lardy and tallowy butter.~ The presence of this unpleasant taste in
+butter may be due to a variety of causes. In some instances, improper
+food seems to be the source of the trouble; then again, butter exposed
+to direct sunlight bleaches in color and develops a lardy flavor.[175]
+In addition to these, cases have been found in which the defect has been
+traced to the action of bacteria. Storch[176] has described a
+lactic-acid form in a sample of tallowy butter that was able to produce
+this disagreeable odor.
+
+~Oily butter.~ Jensen has isolated one of the causes of the dreaded oily
+butter that is reported quite frequently in Denmark. The specific
+organism that he found belongs to the sour-milk bacteria. In twenty-four
+hours it curdles milk, the curd being solid like that of ordinary sour
+milk. There is produced, however, in addition to this, an unpleasant
+odor and taste resembling that of machine oil, a peculiarity that is
+transmitted directly to butter made from affected cream.
+
+~Bitter butter.~ Now and then butter develops a bitter taste that may be
+due to a variety of different bacterial forms. In most cases, the bitter
+flavor in the butter is derived primarily from the bacteria present in
+the cream or milk. Several of the fermentations of this character in
+milk are also to be found in butter. In addition to these defects
+produced by a biological cause, bitter flavors in butter are sometimes
+produced by the milk being impregnated with volatile, bitter substances
+derived from weeds.
+
+~Moldy butter.~ This defect is perhaps the most serious because most
+common. It is produced by the development of a number of different
+varieties of molds. The trouble appears most frequently in packed butter
+on the outside of the mass of butter in contact with the tub. Mold
+spores are so widely disseminated that if proper conditions are given
+for their germination, they are almost sure to develop. In some cases
+the mold is due to the growth of the ordinary bread mold, _Penicillium
+glaucum_; in other cases a black mold develops, due often to
+_Cladosporium butyri_. Not infrequently trouble of this character is
+associated with the use of parchment wrappers. The difficulty can easily
+be held in check by soaking the parchment linings and the tubs in a
+strong brine, or paraffining the inside of the tub.
+
+~Fishy butter.~ Considerable trouble has been experienced in Australian
+butter exported to Europe in which a fishy flavor developed. It was
+noted that the production of this defect seemed to be dependent upon the
+storage temperature at which the butter was kept. When the butter was
+refrigerated at 15° F. no further difficulty was experienced. It is
+claimed that the cause of this condition is due to the formation of
+trimethylamine (herring brine odor) due to the growth of the mold fungus
+_Oidium lactis_, developing in combination with the lactic-acid
+bacteria.
+
+A fishy taste is sometimes noted in canned butter. Rogers[177] has
+determined that this flavor is caused by yeasts (_Torula_) which produce
+fat-splitting enzyms capable of producing this undesirable change.
+
+FOOTNOTES:
+
+[153] Conn and Esten, Cent. f. Bakt., II Abt., 1901, 7:746.
+
+[154] Tiemann, Milch Zeit., 23:701.
+
+[155] Milch Zeit., 1889, p. 7; 1894, p. 624; 1895, p. 383.
+
+[156] Dean, Ont. Agr. Coll., 1897, p. 66.
+
+[157] Storch, Nogle, Unders. over Floed. Syrning, 1890.
+
+[158] Conn, 6 Storrs Expt. Stat., 1893, p. 66.
+
+[159] Conn, 9 Storrs Expt. Stat., 1896, p. 17.
+
+[160] Weigmann, Milch Zeit., 1891, p. 793
+
+[161] McDonnell, ü. Milchsäure Bakterien (Diss. Kiel, 1899), p. 43.
+
+[162] Storch, Milch Zeit., 1890, p. 304.
+
+[163] Conn, 9 Storrs Expt. Stat., 1896, p. 25.
+
+[164] Milch Zeit., 1891, p. 122; 1894, p. 284; 1895, p. 56; 1896, p.
+163.
+
+[165] McKay, Bull. 32, Iowa Expt. Stat., p. 47
+
+[166] Weigmann, Landw. Woch. f. Schl. Hol., No. 2, 1890.
+
+[167] Weigmann, Cent. f. Bakt., II Abt., 3:497, 1897.
+
+[168] At the National Creamery Buttermakers' Association for 1901, 193
+out of 240 exhibitors used starters. Of those that employed starters,
+nearly one-half used commercial cultures. There was practically no
+difference in the average score of the two classes of starters, but
+those using starters ranked nearly two points higher in flavor than
+those that did not.
+
+[169] Russell, Bull. 143, Wis. Expt. Stat., Feb. 1907.
+
+[170] Melick, Bull. 138, Kansas Expt. Stat., June 1906.
+
+[171] Reinmann, Cent. f. Bakt., 1900, 6:131; Jensen, Landw. Jahr. d.
+Schweiz, 1901.
+
+[172] Jensen, Cent. f. Bakt., 1891, 11:409.
+
+[173] Jensen, Milch Zeit., 1892, 6, Nos. 5 and 6.
+
+[174] Pammel, Bull. 21, Iowa Expt. Stat., p. 803.
+
+[175] Fischer, Hyg. Rund., 5:573.
+
+[176] Storch, 18 Rept. Danish Agric. Expt. Stat., 1890.
+
+[177] Rogers Bull. 57, B. A. I. U. S. Dept Agric., 1904.
+
+
+
+
+CHAPTER VIII.
+
+BACTERIA IN CHEESE.
+
+
+The art of cheese-making, like all other phases of dairying, has been
+developed mainly as a result of empirical methods. Within the last
+decade or so, the subject has received more attention from the
+scientific point of view and the underlying causes determined to some
+extent. Since the subject has been investigated from the bacteriological
+point of view, much light has been thrown on the cause of many changes
+that were heretofore inexplicable. Our knowledge, as yet, is quite
+meager, but enough has already been determined to indicate that the
+whole industry is largely based on the phenomena of ferment action, and
+that the application of bacteriological principles and ideas is sure to
+yield more than ordinary results, in explaining, in a rational way, the
+reasons underlying many of the processes to be observed in this
+industry.
+
+The problem of good milk is a vital one in any phase of dairy activity,
+but it is pre-eminently so in cheese-making, for the ability to make a
+first-class product depends to a large extent on the quality of the raw
+material. Cheese contains so large a proportion of nitrogenous
+constituents that it is admirably suited, as a food medium, to the
+development of bacteria; much better, in fact, than butter.
+
+
+INFLUENCE OF BACTERIA IN NORMAL CHEESE PROCESSES.
+
+In the manufacture of cheddar cheese bacteria exert a marked influence
+in the initial stages of the process. To produce the proper texture that
+characterizes cheddar cheese, it is necessary to develop a certain
+amount of acid which acts upon the casein. This acidity is measured by
+the development of the lactic-acid bacteria that normally abound in the
+milk; or, as the cheese-maker expresses it, the milk is "ripened" to the
+proper point. The action of the rennet, which is added to precipitate
+the casein of the milk, is markedly affected by the amount of acid
+present, as well as the temperature. Hence it is desirable to have a
+standard amount of acidity as well as a standard temperature for
+coagulation, so as to unify conditions. It frequently happens that the
+milk is abnormal with reference to its bacterial content, on account of
+the absence of the proper lactic bacteria, or the presence of forms
+capable of producing fermentative changes of an undesirable character.
+In such cases the maker attempts to overcome the effect of the unwelcome
+bacteria by adding a "starter;" or he must vary his method of
+manufacture to some extent to meet these new conditions.
+
+~Use of starters.~ A starter may be employed to hasten the ripening of
+milk that is extremely sweet, so as to curtail the time necessary to get
+the cheese to press; or it may be used to overcome the effect of
+abnormal conditions.
+
+The starter that is employed is generally one of domestic origin, and is
+usually taken from skim milk that has been allowed to ferment and sour
+under carefully controlled conditions. Of course much depends upon the
+quality of the starter, and in a natural starter there is always the
+possibility that it may not be perfectly pure.
+
+Within recent years the attempt has been made to control the effect of
+the starter more thoroughly by using pure cultures of some desirable
+lactic-acid form.[178] This has rendered the making of cheese not only
+more uniform, but has aided in repressing abnormal fermentations
+particularly those that are characterized by the production of gas.
+
+Recently, pure cultures of Adametz's _B. nobilis_, a digesting organism
+that is claimed to be the cause of the breaking down of the casein and
+also of the peculiar aroma of Emmenthaler cheese, has been placed on the
+market under the name _Tyrogen_. It is claimed that the use of this
+starter, which is added directly to the milk and also rubbed on the
+surface of the cheese, results in the improvement of the curds, assists
+in the development of the proper holes, imparts a favorable aroma and
+hastens ripening.[179]
+
+Campbell[180] states that the discoloration of cheese in England, which
+is due to the formation of white spots that are produced by the
+bleaching of the coloring matter in the cheese, may be overcome by the
+use of lactic-acid starters.
+
+The use of stringy or slimy whey has been advocated in Holland for some
+years as a means of overcoming the tendency toward gas formation in Edam
+cheese which is made from practically sweet milk. This fermentation, the
+essential feature of which is produced by a culture of _Streptococcus
+Hollandicus_,[181] develops acid in a marked degree, thereby inhibiting
+the production of gas.
+
+The use of masses of moldy bread in directing the fermentation of
+Roquefort cheese is another illustration of the empirical development of
+starters, although in this instance it is added after the curds have
+been prepared for the press.
+
+~Pasteurizing milk for cheese-making.~ If it were possible to use properly
+pasteurized milk in cheese-making, then practically all abnormal
+conditions could be controlled by the use of properly selected starters.
+Numerous attempts have been made to perfect this system with reference
+to cheddar cheese, but so far they have been attended with imperfect
+success. The reason for this is that in pasteurizing milk, the soluble
+lime salts are precipitated by the action of heat, and under these
+conditions rennet extract does not curdle the casein in a normal manner.
+This condition can be restored, in part at least, by the addition of
+soluble lime salts, such as calcium chlorid; but in our experience,
+desirable results were not obtained where heated milks to which this
+calcium solution had been added were made into cheddar cheese.
+Considerable experience has been gained in the use of heated milks in
+the manufacture of certain types of foreign cheese. Klein[182] finds
+that Brick cheese can be successfully made even where the milk is heated
+as high as 185° F. An increased weight is secured by the addition of the
+coagulated albumin and also increased moisture.
+
+~Bacteria in rennet.~ In the use of natural rennets, such as are
+frequently employed in the making of Swiss cheese, considerable numbers
+of bacteria are added to the milk. Although these rennets are preserved
+in salt, alcohol or boric acid, they are never free from bacteria.
+Adametz[183] found ten different species and from 640,000 to 900,000
+bacteria per cc. in natural rennets. Freudenreich has shown that rennet
+extract solutions can be used in Swiss cheese-making quite as well as
+natural rennets; but to secure the best results, a small quantity of
+pure lactic ferment must be added to simulate the conditions that
+prevail when natural rennets are soaked in whey, which, it must be
+remembered, is a fluid rich in bacterial life.
+
+Where rennet extract or tablets are used, as is generally the case in
+cheddar making, the number of bacteria added is so infinitesimal as to
+be negligible.
+
+~Development of acid.~ In the manufacture of cheddar cheese, the
+development of acid exerts an important influence on the character of
+the product. This is brought about by holding the curds at temperatures
+favorable to the growth of the bacteria in the same. Under these
+conditions the lactic-acid organisms, which usually predominate, develop
+very rapidly, producing thereby considerable quantities of acid which
+change materially the texture of the curds. The lactic acid acts upon
+the casein in solutions containing salt, causing it to dissolve to some
+extent, thus forming the initial compounds of digestion.[184] This
+solution of the casein is expressed physically by the "stringing" of the
+curds on a hot iron. This causes the curds to mat, producing a close,
+solid body, free from mechanical holes. Still further, the development
+of this acid is necessary for the digestive activity of the pepsin in
+the rennet extract.
+
+In some varieties of cheese, as the Swiss, acid is not developed and the
+character of the cheese is much different from that of cheddar. In all
+such varieties, a great deal more trouble is experienced from the
+production of "gassy" curds, because the development of the
+gas-producing bacteria is held in check by the rapid growth of the
+lactic acid-producing species.
+
+~Bacteria in green cheese.~ The conditions under which cheese is made
+permit of the development of bacteria throughout the entire process. The
+cooking or heating of curds to expel the excessive moisture is never so
+high as to be fatal to germ life; on the contrary, the acidity of the
+curd and whey is continually increased by the development of bacteria in
+the same.
+
+The body of green cheese fresh from the press is, to a considerable
+extent, dependent upon the acid produced in the curds. If the curds are
+put to press in a relatively sweet condition the texture is open and
+porous. The curd particles do not mat closely together and "mechanical
+holes," rough and irregular in outline, occur. Very often, at relatively
+high temperatures, such cheese begin to "huff," soon after being taken
+from the press, a condition due to the development of gas, produced by
+gas-generating bacteria acting on the sugar in the curd. This gas finds
+its way readily into these ragged holes, greatly distending them, as in
+Fig. 30.
+
+[Illustration: FIG. 30. _L_, a sweet curd cheese direct from the press.
+"Mechanical" holes due to lack of acid development; _P_, same cheese
+four days later, mechanical holes distended by development of gas.]
+
+~Physical changes in ripening cheese.~ When a green cheese is taken from
+the press, the curd is tough, firm, but elastic. It has no value as a
+food product for immediate use, because it lacks a desirable flavor and
+is not readily digestible. It is nothing but precipitated casein and
+fat. In a short time, a deep-seated change occurs. Physically this
+change is demonstrated in the modification that the curd undergoes.
+Gradually it breaks down and becomes plastic, the elastic, tough curd
+being changed into a softened mass. This change in texture of the cheese
+is also accompanied by a marked change in flavor. The green cheese has
+no distinctively cheese flavor, but in course of time, with the gradual
+change of texture, the peculiar flavor incident to ripe cheese is
+developed.
+
+The characteristic texture and flavor are susceptible of considerable
+modification that is induced not only by variation in methods of
+manufacture, but by the conditions under which the cheese are cured. The
+amount of moisture incorporated with the curd materially affects the
+physical appearance of the cheese, and the rate of change in the same.
+The ripening temperature, likewise the moisture content of the
+surrounding air, also exerts a marked influence on the physical
+properties of the cheese. To some extent the action of these forces is
+purely physical, as in the gradual loss by drying, but in other respects
+they are associated with chemical transformations.
+
+~Chemical changes in ripening cheese.~ Coincident with the physical
+breaking down of the curd comes a change in the chemical nature of the
+casein. The hitherto insoluble casein is gradually transformed into
+soluble nitrogenous substances (_caseone_ of Duclaux, or _caseogluten_
+of Weigmann). This chemical phenomenon is a breaking-down process that
+is analogous to the peptonization of proteids, although in addition to
+the peptones and albumoses characteristic of peptic digestion,
+amido-acids and ammonia are to be found. The quantity of these lower
+products increases with the age of the cheese.
+
+The chemical reaction of cheese is normally acid to phenolphthalein,
+although there is generally no free acid, as shown by Congo red, the
+lactic acid being converted into salts as fast as formed. In very old
+cheese, undergoing putrefactive changes, especially on the outside, an
+alkaline reaction may be present, due to the formation of free ammonia.
+
+The changes that occur in a ripening cheese are for the most part
+confined to the proteids. According to most investigators the fat
+remains practically unchanged, although the researches of Weigmann and
+Backe[185] show that fatty acids are formed from the fat. In the green
+cheese considerable milk-sugar is present, but, as a result of the
+fermentation that occurs, this is rapidly converted into acid products.
+
+~Bacterial flora of cheese.~ It might naturally be expected that the green
+cheese, fresh from the press, would contain practically the same kind of
+bacteria that are in the milk, but a study of cheese shows a peculiar
+change in the character of the flora. In the first place, fresh cottage
+cheese, made by the coagulation of the casein through the action of
+acid, has a more diversified flora than cheese made with rennet, for the
+reason, as given by Lafar,[186] that the fermentative process is farther
+advanced.
+
+When different varieties of cheese are made from milk in the same
+locality, the germ content of even the ripened product has a marked
+similarity, as is illustrated by Adametz's work[187] on Emmenthaler or
+Swiss hard cheese, and Schweitzer Hauskäse, a soft variety. Of the nine
+species of bacilli and cocci found in mature Emmenthaler, eight of them
+were also present in ripened Hauskäse.
+
+Different investigators have studied the bacterial flora of various
+kinds of cheese, but as yet little comparative systematic work has been
+done. Freudenreich[188] has determined the character and number of
+bacteria in Emmenthaler cheese, and Russell[189] the same for cheddar
+cheese. The same general law has also been noted in Canadian[190] and
+English[191] cheese. At first a marked decrease in numbers is usually
+noted, lasting for a day or two. This is followed by an enormous
+increase, caused by the rapid growth of the lactic-acid type. The
+development may reach scores of millions and often over a hundred
+million organisms per gram. Synchronous with this increase, the
+peptonizing and gas-producing bacteria gradually disappear. This rapid
+development, which lasts only for a few weeks, is followed by a general
+decline.
+
+In the ripening of cheese a question arises as to whether the process
+goes on throughout the entire mass of cheese, or whether it is more
+active at or near the surface. In the case of many of the soft cheese,
+such as Brie and limburger, bacterial and mold development is
+exceedingly active on the exterior, and the enzyms secreted by these
+organisms diffuse toward the interior. That such a condition occurs in
+the hard type of cheese made with rennet is extremely improbable. Most
+observers agree that in this type of cheese the ripening progresses
+throughout the entire mass, although Adametz opposes this view and
+considers that in Emmenthaler cheese the development of the specific
+aroma-producing organism occurs in the superficial layers. Jensen has
+shown, however, that the greatest amount of soluble nitrogenous products
+are to be found in the innermost part of the cheese, a condition that is
+not reconcilable with the view that the most active ripening is on the
+exterior.[192]
+
+The course of development of bacteria in cheddar cheese is materially
+influenced by the ripening temperature. In cheese ripened at relatively
+low temperatures (50°-55° F.),[193] a high germ content is maintained
+for a much longer period of time than at higher temperatures. Under
+these conditions the lactic-acid type continues in the ascendancy as
+usual. In cheese cured at high temperatures (80°-86° F.) the number of
+organisms is greatly diminished, and they fail to persist in appreciable
+numbers for as long a time as in cheese cured at temperatures more
+frequently employed.
+
+~Influence of temperature on curing.~ Temperature exerts a most potent
+influence on the quality of the cheese, as determined not only by the
+rate of ripening but the nature of the process itself. Much of the poor
+quality of cheese is attributable to the effect of improper curing
+conditions. Probably in the initial stage of this industry cheese were
+allowed to ripen without any sort of control, with the inevitable result
+that during the summer months the temperature generally fluctuated so
+much as to impair seriously the quality. The effect of high temperatures
+(70° F. and above) is to produce a rapid curing, and, therefore, a short
+lived cheese; also a sharp, strong flavor, and generally a more or less
+open texture. Unless the cheese is made from the best quality of milk,
+it is very apt to undergo abnormal fermentations, more especially those
+of a gassy character.
+
+[Illustration: FIG. 31. Influence of curing temperature on texture of
+cheese. Upper row ripened eight months at 60° F.; lower row at 40° F.]
+
+Where cheese is ripened at low temperatures, ranging from 50° F. down to
+nearly the freezing temperatures, it is found that the quality is
+greatly improved.[194] Such cheese are thoroughly broken down from a
+physical point of view even though they may not show such a high per
+cent of soluble nitrogenous products. They have an excellent texture,
+generally solid and firm, free from all tendency to openness; and,
+moreover, their flavor is clean and entirely devoid of the sharp,
+undesirable tang that so frequently appears in old cheese. The keeping
+quality of such cheese is much superior to the ordinary product. The
+introduction of this new system of cheese-curing promises much from a
+practical point of view, and undoubtedly a more complete study of the
+subject from a scientific point of view will aid materially in
+unraveling some of the problems as to flavor production.
+
+~Theories of cheese curing.~ Within the last few years considerable study
+has been given the subject of cheese curing or ripening, in order to
+explain how this physical and chemical transformation is brought about.
+
+Much of the misconception that has arisen relative to the cause of
+cheese ripening comes from a confusion of terms. In the ordinary use of
+the word, ripening or curing of cheese is intended to signify the sum
+total of all the changes that result in converting the green product as
+it comes from the press into the edible substance that is known as cured
+cheese. As previously shown, the most marked chemical transformation
+that occurs is that which has to do with the peptonization or breaking
+down of the casein. It is true that under ordinary conditions this
+decomposition process is also accompanied with the formation of certain
+flavor-producing substances, more or less aromatic in character; but it
+by no means follows that these two processes are necessarily due to the
+same cause. The majority of investigators have failed to consider these
+two questions of casein decomposition and flavor as independent, or at
+least as not necessarily related. They are undoubtedly closely bound
+together, but it will be shown later that the problems are quite
+different and possibly susceptible of more thorough understanding when
+considered separately.
+
+In the earlier theories of cheese ripening it was thought to be purely a
+chemical change, but, with the growth of bacteriological science,
+evidence was forthcoming that seemed to indicate that the activity of
+organisms entered into the problem. Schaffer[195] showed that if milk
+was boiled and made into cheese, the casein failed to break down.
+Adametz[196] added to green cheese various disinfectants, as creolin and
+thymol, and found that this practically stopped the curing process. From
+these experiments he drew the conclusion that bacteria must be the cause
+of the change, because these organisms were killed; but when it is
+considered that such treatment would also destroy the activity of enzyms
+as well as vital ferments, it is evident that these experiments were
+quite indecisive.
+
+A determination of the nature of the by-products found in maturing
+cheese indicates that the general character of the ripening change is a
+peptonization or digestion of the casein.
+
+Until recently the most widely accepted views relating to the cause of
+this change have been those which ascribed the transformation to the
+activity of micro-organisms, although concerning the nature of these
+organisms there has been no unanimity of opinion. The overwhelming
+development of bacteria in all cheeses naturally gave support to this
+view; and such experiments as detailed above strengthened the idea that
+the casein transformation could not occur where these ferment organisms
+were destroyed.
+
+The very nature of the changes produced in the casein signified that to
+take part in this process any organism must possess the property of
+dissolving the proteid molecule, casein, and forming therefrom
+by-products that are most generally found in other digestive or
+peptonizing changes of this class.
+
+~Digestive bacterial theory.~ The first theory propounded was that of
+Duclaux,[197] who in 1887 advanced the idea that this change was due to
+that type of bacteria which is able to liquefy gelatin, peptonize milk,
+and cause a hydrolytic change in proteids. To this widely-spread group
+that he found in cheese, he gave the generic name _Tyrothrix_ (cheese
+hairs). According to him, these organisms do not function directly as
+ripening agents, but they secrete an enzym or unorganized ferment to
+which he applies the name _casease_. This ferment acts upon the casein
+of milk, converting it into a soluble product known as _caseone_. These
+organisms are found in normal milk, and if they function as casein
+transformers, one would naturally expect them to be present, at least
+frequently, if not predominating in the ripening cheese; but such is not
+the case. In typical cheddar or Swiss cheese, they rapidly disappear (p.
+168), although in the moister, softer varieties, they persist for
+considerable periods of time. According to Freudenreich, even where
+these organisms are added in large numbers to the curd, they soon
+perish, an observation that is not regarded as correct by the later
+adherents to the digestive bacterial theory, as Adametz and Winkler.
+
+Duclaux's experiments were made with liquid media for isolation
+purposes, and his work, therefore, cannot be regarded as satisfactory as
+that carried out with more modern technical methods. Recently this
+theory has been revived by Adametz,[198] who claims to have found in
+Emmenthaler cheese a digesting species, one of the Tyrothrix type, which
+is capable of peptonizing the casein and at the same time producing the
+characteristic flavor of this class of cheese. This organism, called by
+him _Bacillus nobilis_, the Edelpilz of Emmenthaler cheese, has been
+subjected to comparative experiments, and in the cheese made with pure
+cultures of this germ better results are claimed to have been secured.
+Sufficient experiments have not as yet been reported by other
+investigators to warrant the acceptance of the claims made relative to
+the effect of this organism.
+
+~Lactic-acid bacterial theory.~ It has already been shown that the
+lactic-acid bacteria seems to find in the green cheese the optimum
+conditions of development; that they increase enormously in numbers for
+a short period, and then finally decline. This marked development,
+coincident with the breaking down of the casein, has led to the view
+which has been so ably expounded by Freudenreich[199] that this type of
+bacterial action is concerned in the ripening of cheese. This group of
+bacteria is, under ordinary conditions, unable to liquefy gelatin, or
+digest milk, or, in fact, to exert, under ordinary conditions, any
+proteolytic or peptonizing properties. This has been the stumbling-block
+to the acceptance of this hypothesis, as an explanation of the breaking
+down of the casein. Freudenreich has recently carried on experiments
+which he believes solve the problem. By growing cultures of these
+organisms in milk, to which sterile, freshly precipitated chalk had been
+added, he was able to prolong the development of bacteria for a
+considerable period of time, and as a result finds that an appreciable
+part of the casein is digested; but this action is so slow compared with
+what normally occurs in a cheese, that exception may well be taken to
+this type of experiment alone. Weigmann[200] inclines to the view that
+the lactic-acid bacteria are not the true cause of the peptonizing
+process, but that their development prepares the soil, as it were, for
+those forms that are more directly concerned in the peptonizing process.
+This they do by developing an acid substratum that renders possible the
+more luxuriant growth of the aroma-producing species. According to
+Gorini,[201] certain of the Tyrothrix forms function at high
+temperatures as lactic acid producing bacteria, while at lower
+temperatures they act as peptonizers. On this basis he seeks to
+reconcile the discrepancies that appear in the experiments of other
+investigators.
+
+~Digestive milk enzym theory.~ In 1897 Babcock and the writer[202] showed
+that milk underwent digestive changes spontaneously when bacterial
+activity was suspended by the addition of such anaesthetics as ether,
+chloroform and benzol. The chemical nature of the by-products produced
+by this auto-digestion of milk resembles quite closely those found in
+ripened cheese, except that ammonia is not produced as is the case in
+old cheese. The cause of the decomposition of the casein, they found to
+be due to the action of a milk enzym which is inherent to the milk
+itself. This digestive ferment may be separated from fresh milk by
+concentrating centrifuge slime extracts by the usual physiological
+reagents. This ferment, called by them _galactase_, on account of its
+origin in milk, is a proteolytic enzym of the tryptic type. Its activity
+is destroyed by strong chemicals such as formaldehyde, corrosive
+sublimate, also when heated to 175° F. or above. When such extracts are
+added to boiled milk, the digestive process is started anew, and the
+by-products produced are very similar to those noted in a normal cheese.
+
+Jensen[203] has also shown that the addition of pancreatic extracts to
+cheese accelerated the formation of soluble nitrogenous products.
+
+The action of galactase in milk and cheese has been confirmed by
+Freudenreich[204] and Jensen,[205] as well as by American investigators,
+and this enzym is now generally accepted as one of the factors concerned
+in the decomposition of the casein. Freudenreich believes it is able to
+change casein into albumose and peptones, but that the lactic-acid
+bacteria are chiefly responsible for the further decomposition of the
+nitrogen to amid form.
+
+Failure before to recognize the presence of galactase in milk is
+attributable to the fact that all attempts to secure sterile milk had
+been made by heating the same, in which case galactase was necessarily
+destroyed. A brief exposure at 176° F. is sufficient to destroy its
+activity, and even an exposure at lower temperatures weakens its action
+considerably, especially if the reaction of the medium is acid. This
+undoubtedly explains the contradictory results obtained in the ripening
+of cheese from pasteurized milk, such cheese occasionally breaking down
+in an abnormal manner.
+
+The results mentioned on page 172, in which cheese failed to ripen when
+treated with disinfectants,--experiments which were supposed at that
+time to be the foundation of the bacterial theory of casein
+digestion--are now explicable on an entirely different basis. In these
+cases the casein was not peptonized, because these strong disinfectants
+destroyed the activity of the enzyms as well as the bacteria.
+
+Another important factor in the breaking down of the casein is the
+_pepsin_ in the rennet extract. The digestive influence of this agent
+was first demonstrated for cheddar cheese by Babcock, Russell and
+Vivian,[206] and simultaneously, although independently, by Jensen[207]
+in Emmenthaler cheese. In this digestive action, only albumoses and
+higher peptones are produced. The activity of pepsin does not become
+manifest until there is about 0.3 per cent. acid which is approximately
+the amount developed in the cheddar process. These two factors
+undoubtedly account for by far the larger proportion of the changes in
+the casein; and yet, the formation of ammonia in well ripened cheese is
+not accounted for by these factors. This by-product is the main end
+product of proteid digestion by the liquefying bacteria but their
+apparent infrequency in cheese makes it difficult to understand how they
+can function prominently in the change, unless the small quantity of
+digestive enzyms excreted by them in their growth in milk is capable of
+continuing its action until a cumulative effect is obtained. Although
+much light has been thrown on this question by the researches of the
+last few years, the matter is far from being satisfactorily settled at
+the present time and the subject needs much more critical work. If
+liquefying bacteria abound in the milk, doubtless they exert some
+action, but the rôle of bacteria is doubtless much greater in the
+production of flavor than in the decomposition of the curd.
+
+~Conditions determining quality.~ In determining the quality of cheese,
+several factors are to be taken into consideration. First and foremost
+is the flavor, which determines more than anything else the value of the
+product. This should be mild and pleasant, although with age the
+intensity of the same generally increases but at no time should it have
+any bitter, sour, or otherwise undesirable taste or aroma. Texture
+registers more accurately the physical nature of the ripening. The
+cheese should not be curdy and harsh, but should yield quite readily to
+pressure under the thumb, becoming on manipulation waxy and plastic
+instead of crumbly or mealy. Body refers to the openness or closeness of
+the curd particles, a close, compact mass being most desirable. The
+color of cheese should be even, not wavy, streaked or bleached.
+
+For a cheese to possess all of these characteristics in an optimum
+degree is to be perfect in every respect--a condition that is rarely
+reached.
+
+So many factors influence this condition that the problem of making a
+perfect cheese becomes exceedingly difficult. Not only must the quality
+of the milk--the raw material to be used in the manufacture--be
+perfectly satisfactory, but the factory management while the curds are
+in the vat demands great skill and careful attention; and finally, the
+long period of curing in which variation in temperature or moisture
+conditions may seriously affect the quality,--all of these stages, more
+or less critical, must be successfully gone through, before the product
+reaches its highest state of development.
+
+It is of course true that many phases of this complex series of
+processes have no direct relation to bacteria, yet it frequently happens
+that the result attained is influenced at some preceding stage by the
+action of bacteria in one way or another. Thus the influence of the
+acidity developed in the curds is felt throughout the whole life of the
+cheese, an over-development of lactic-acid bacteria producing a sour
+condition that leaves its impress not only on flavor but texture. An
+insufficient development of acid fails to soften the curd-particles so
+as to permit of close matting, the consequence being that the body of
+the cheese remains loose and open, a condition favorable to the
+development of gas-generating organisms.
+
+~Production of flavor.~ The importance of flavor as determining the
+quality of cheese makes it imperative that the nature of the substances
+that confer on cheese its peculiar aromatic qualities and taste be
+thoroughly understood. It is to be regretted that the results obtained
+so far are not more satisfactory, for improvement in technique is hardly
+to be expected until the reason for the process is thoroughly
+understood.
+
+The view that is most generally accepted is that this most important
+phase of cheese curing is dependent upon bacterial activity, but the
+organisms that are concerned in this process have not as yet been
+satisfactorily determined. In a number of cases, different species of
+bacteria have been separated from milk and cheese that have the power of
+producing aromatic compounds that resemble, in some cases, the peculiar
+flavors and odors that characterize some of the foreign kinds of cheese;
+but an introduction of these into curd has not resulted in the
+production of the peculiar variety, even though the methods of
+manufacture and curing were closely followed. The similarity in germ
+content in different varieties of cheese made in the same locality has
+perhaps a bearing on this question of flavor as related to bacteria. Of
+the nine different species of bacteria found in Emmenthaler cheese by
+Adametz, eight of them were also present in ripened Hauskäse. If
+specific flavors are solely the result of specific bacterial action, it
+might naturally be expected that the character of the flora would
+differ.
+
+Some suggestive experiments were made by Babcock and Russell on the
+question of flavor as related to bacterial growth, by changing the
+nature of the environment in cheese by washing the curds on the racks
+with warm water. In this way the sugar and most of the ash were removed.
+Under such conditions the character of the bacterial flora was
+materially modified. While the liquefying type of bacteria was very
+sparse in normal cheddar, they developed luxuriantly in the washed
+cheese. The flavor at the same time was markedly affected. The control
+cheddar was of good quality, while that made from the washed curds was
+decidedly off, and in the course of ripening became vile. It may be
+these two results are simply coincidences, but other data[208] bear out
+the view that the flavor was to some extent related to the nature of the
+bacteria developing in the cheese. This was strengthened materially by
+adding different sugars to washed curds, in which case it was found that
+the flavor was much improved, while the more normal lactic-acid type of
+bacteria again became predominant.
+
+~Ripening of moldy cheese.~ In a number of foreign cheeses, the peculiar
+flavor obtained is in part due to the action of various fungi which grow
+in the cheese, and there produce certain by-products that flavor the
+cheese. Among the most important of these are the Roquefort cheese of
+France, Stilton of England, and Gorgonzola of Italy.
+
+Roquefort cheese is made from goat's or cow's milk, and in order to
+introduce the desired mold, which is the ordinary bread-mold,
+_Penicillium glaucum_, carefully-prepared moldy bread-crumbs are added
+to the curd.
+
+At ordinary temperatures this organism develops too rapidly, so that the
+cheese to ripen properly must be kept at a low temperature. The town of
+Roquefort is situated in a limestone country, in a region full of
+caves, and it is in these natural caves that most of the ripening is
+done. These caverns are always very moist and have a temperature ranging
+from 35° to 44° F., so that the growth of the fungus is retarded
+considerably. The spread of the mold throughout the ripening mass is
+also assisted in a mechanical way. The partially-matured cheese are run
+through a machine that pricks them full of small holes. These slender
+canals allow the mold organism to penetrate the whole mass more
+thoroughly, the moldy straw matting upon which the ripening cheese are
+placed helping to furnish an abundant seeding of the desired germ.
+
+When new factories are constructed it is of advantage to introduce this
+necessary germ in quantities, and the practice is sometimes followed of
+rubbing the walls and cellars of the new location with material taken
+from the old established factory. In this custom, developed in purely an
+empirical manner, is to be seen a striking illustration of a
+bacteriological process crudely carried out.
+
+In the Stilton cheese, one of the highly prized moldy cheeses of
+England, the desired mold fungus is introduced into the green cheese by
+exchanging plugs taken with a cheese trier from a ripe Stilton.
+
+~Ripening of soft cheese.~ The type of ripening which takes place in the
+soft cheeses is materially different from that which occurs in the hard
+type. The peptonizing action does not go on uniformly throughout the
+cheese, but is hastened by the development of molds and bacteria on the
+outside that exert a solvent action on the casein. For this reason, soft
+cheeses are usually made up in small sizes, so that this action may be
+hastened. The organisms that take part in this process are those that
+are able to form enzyms (similar in their action to trypsin, galactase,
+etc.), and these soluble ferments gradually diffuse from the outside
+through the cheese.
+
+Most of these peptonizing bacteria are hindered in their growth by the
+presence of lactic acid, so that in many cases the appearance of the
+digesting organisms on the surface is delayed until the acidity of the
+mass is reduced to the proper point by the development of other
+organisms, principally molds, which prefer an acid substratum for their
+growth.
+
+In Brie cheese a blue coating of mold develops on the surface. In the
+course of a few weeks, a white felting appears which later changes to
+red. This slimy coat below the mold layer is made up of diverse species
+of bacteria and fungi that are able to grow after the acid is reduced by
+the blue mold. The organisms in the red slimy coat act upon the casein,
+producing an alkaline reaction that is unfavorable to the growth of the
+blue mold. Two sets of organisms are, therefore essential in the
+ripening process, one preparing the soil for the ferment that later
+produces the requisite ripening changes. As ordinarily carried on, the
+process is an empirical one, and if the red coat does not develop as
+expected, the maker resorts to all kinds of devices to bring out the
+desired ferment. The appearance of the right form is dependent, however,
+upon the proper reaction of the cheese, and if this is not suitable, the
+wished-for growth will not appear.
+
+
+INFLUENCE OF BACTERIA IN ABNORMAL CHEESE PROCESSES.
+
+The reason why cheese is more subject to abnormal fermentation than
+butter is because its high nitrogen content favors the continued
+development of bacteria for some time after it is made. It must be
+borne in mind, in considering the more important of these changes, that
+not all defective conditions in cheese are attributable to the influence
+of living organisms. Troubles frequently arise from errors in
+manufacturing details, as too prolonged cooking of curds, too high
+heating, or the development of insufficient or too much acid. Then
+again, the production of undesirable flavors or impairment in texture
+may arise from imperfect curing conditions.
+
+Our knowledge regarding the exact nature of these indefinite faults is
+as yet too inadequate to enable many of these undesirable conditions to
+be traced to their proper source; but in many cases the taints observed
+in a factory are due to the abnormal development of certain bacteria,
+capable of evolving unpleasant or even putrid odors. Most of them are
+seeded in the milk before it comes to the factory and are due to
+careless manipulation of the milk while it is still on the farm. Others
+gain access to the milk in the factory, owing to unclean conditions of
+one sort or another. Sometimes the cheese-maker is able to overcome
+these taints by vigorous treatment, but often they pass on into the
+cheese, only to detract from the market value of the product. Most
+frequently these "off" flavors appear in cheese that are cured at too
+high temperatures, say above 65° F.
+
+~"Gassy" fermentations in cheese.~ One of the worst and at the same time
+most common troubles in cheese-making is where the cheese undergoes a
+fermentation marked by the evolution of gas. The presence of gas is
+recognized by the appearance either of spherical or lens-shaped holes of
+various sizes in the green cheese; often they appear in the curd before
+it is put to press. Usually in this condition the curds look as if they
+had been punctured with a pin, and are known as "pin holey" curds. Where
+the gas holes are larger, they are known as "Swiss holes" from their
+resemblance to the normal holes in the Swiss product. If the development
+of gas is abundant, these holes are restricted in size. Often the
+formation of gas may be so intense as to cause the curds to float on the
+surface of the whey before they are removed. Such curds are known as
+"floaters" or "bloaters."
+
+If "gassy" curds are put to press, the abnormal fermentation may
+continue. The further production of gas causes the green cheese to
+"huff" or swell, until it may be considerably distorted as in Fig. 33.
+In such cases the texture of the cheese is greatly injured, and the
+flavor is generally impaired.
+
+[Illustration: FIG. 33. Cheese made from gassy milk.]
+
+Such abnormal changes may occur at any season of the year, but the
+trouble is most common in summer, especially in the latter part.
+
+This defect is less likely to occur in cheese that is well cheddared
+than in sweet curd cheese. When acidity is produced, these gassy
+fermentations are checked, and in good cheddar the body is so close and
+firm as not readily to permit of gaseous changes.
+
+In Swiss cheese, which is essentially a sweet curd cheese, these
+fermentations are very troublesome. Where large holes are formed in
+abundance (blähen), the trouble reaches its maximum. If the gas holes
+are very numerous and therefore small it is called a "nissler."
+Sometimes the normal "eyes" are even wanting when it is said to be
+"blind" or a "gläsler."
+
+[Illustration: FIG. 34. Block Swiss cheese showing "gassy"
+fermentation.]
+
+One method of procedure which is likely to cause trouble in Swiss
+factories is often produced by the use of sour, fermented whey in which
+to soak the natural rennets. Freudenreich and Steinegger[209] have shown
+that a much more uniform quality of cheese can be made with rennet
+extract if it is prepared with a starter made from a pure lactic
+ferment.
+
+The cause of the difficulty has long been charged to various sources,
+such as a lack of aeration, improper feeding, retention of animal gases,
+etc., but in all these cases it was nothing more than a surmise. Very
+often the milk does not betray any visible symptom of fermentation when
+received, and the trouble is not to be recognized until the process of
+cheese-making is well advanced.
+
+Studies from a biological standpoint have, however, thrown much light on
+this troublesome problem; and it is now known that the formation of gas,
+either in the curd or after it has been put to press, is due entirely to
+the breaking down of certain elements, such as the sugar of milk, due to
+the influence of various living germs. This trouble is, then, a type
+fermentation, and is, therefore, much more widely distributed than it
+would be if it was caused by a single specific organism. These
+gas-producing organisms are to be found, sparingly at least, in almost
+all milks, but are normally held in check by the ordinary lactic
+species. Among them are a large number of the bacteria, although yeasts
+and allied germs are often present and are likewise able to set up
+fermentative changes of this sort. In these cases the milk-sugar is
+decomposed in such a way as to give off CO_{2} and H, and in some cases,
+alcohol. Russell and Hastings[210] found a lactose-splitting yeast in a
+severe outbreak of gassy cheese in a Swiss factory. In this case the gas
+did not develop until the cheese were a few weeks old. In severe cases
+the cheese actually cracked to pieces.
+
+According to Guillebeau, a close relation exists between those germs
+that are able to produce an infectious inflammation (mastitis) in the
+udder of the cow and some forms capable of gas evolution.
+
+If pure cultures of these gas-producing bacteria are added to perfectly
+sweet milk, it is possible to artificially produce the conditions in
+cheese that so frequently appear in practice.
+
+~Treatment of "pin-holey" curds.~ When this type of fermentation appears
+during the manufacture of the cheese, the maker can control it in part
+within certain limits. These methods of treatment are, as a rule, purely
+mechanical, as when the curds are piled and turned, and subsequently
+ground in a curd mill. After the gas has been forced out, the curds are
+then put to press and the whole mats into a compact mass.
+
+Another method of treatment based upon bacteriological principles is the
+addition of a starter to induce the formation of acid. Where acid is
+developed as a result of the growth of the lactic-acid bacteria, the
+gas-producing species do not readily thrive. Another reason why acid
+aids in repressing the development of gas is that the curd particles are
+partially softened or digested by the action of the acid. This causes
+them to mat together more closely, and there is not left in the cheese
+the irregular mechanical openings in which the developing gas may find
+lodgment.
+
+Another method that is also useful with these curds is to employ salt.
+This represses gaseous fermentations, and the use of more salt than
+usual in making the cheese will very often restrain the production of
+gas. Tendency to form gas in Edam cheese is controlled by the addition
+of a starter prepared from slimy whey (lange wei) which is caused by the
+development of an acid-forming organism.
+
+Some have recommended the custom of washing the curds to remove the whey
+and the gas-producing bacteria contained therein. Care must be taken not
+to carry this too far, for the removal of the sugar permits
+taint-producing organisms to thrive.[211]
+
+The temperature at which the cheese is cured also materially affects the
+development of gas. At high curing temperatures, gas-producing organisms
+develop rapidly; therefore more trouble is experienced in summer than at
+other seasons.
+
+If milks which are prone to undergo "gassy" development are excluded
+from the general supply, it would be possible to eliminate the source of
+the entire trouble. To aid in the early recognition of such milks that
+are not apparently affected when brought to the factory, fermentation or
+curd tests (p. 76) are of great value. The use of this test in the hands
+of the factory operator often enables him to detect the exact source of
+the trouble, which may frequently be confined to the milk delivered by a
+single patron.
+
+~"Fruity" or "sweet" flavor.~ Not infrequently the product of a factory
+may acquire during the process of ripening what is known as a "sweet" or
+"fruity" flavor. This flavor resembles the odor of fermented fruit or
+the bouquet of certain kinds of wine. It has been noted in widely
+different sections of the country and its presence bears no relation to
+the other qualities of the cheese. The cause of this trouble has
+recently been traced[212] to the presence of various kinds of yeasts.
+Ordinarily yeasts are rarely present in good cheese, but in cheese
+affected with this trouble they abound. The addition of starters made
+from yeast cultures resulted in the production of the undesirable
+condition.
+
+~Mottled cheese.~ The color of cheese is sometimes cut to that extent that
+the cheese presents a wavy or mottled appearance. This condition is apt
+to appear if the ripening temperature is somewhat high, or larger
+quantities of rennet used than usual. The cause of the defect is
+obscure, but it has been demonstrated that the same is communicable if a
+starter is made by grating some of this mottled cheese into milk. The
+bacteriology of the trouble has not yet been worked out, but the defect
+is undoubtedly due to an organism that is able to grow in the ripening
+cheese. It has been claimed that the use of a pure lactic ferment as a
+starter enables one to overcome this defect.
+
+~Bitter cheese.~ Bitter flavors are sometimes developed in cheese
+especially where the ripening process is carried on at a low temperature
+in the presence of an excess of moisture for a considerable length of
+time.
+
+Guillebeau[213] isolated several forms from Emmenthaler cheese which he
+connected with udder inflammation that were able to produce a bitter
+substance in cheese.
+
+Von Freudenreich[214] has described a new form _Micrococcus casei amari_
+(micrococcus of bitter cheese) that was found in a sample of bitter
+cheese. This germ is closely related to Conn's micrococcus of bitter
+milk. It develops lactic acid rapidly, coagulating the milk and
+producing an intensely bitter taste in the course of one to three days.
+When milk infected with this organism is made into cheese, there is
+formed in a few days a decomposition product that imparts a marked
+bitter flavor to the cheese.
+
+Harrison[215] has recently found a yeast that grows in the milk and also
+in the cheese which produces an undesirable bitter change.
+
+It is peculiar that some of the organisms that are able to produce
+bitter products in milk do not retain this property when the milk is
+worked up into cheese.
+
+~Putrid or rotten cheese.~ Sometimes cheese undergoes a putrefactive
+decomposition in which the texture is profoundly modified and various
+foul smelling gases are evolved. These often begin on the exterior as
+small circumscribed spots that slowly extend into the cheese, changing
+the casein into a soft slimy mass. Then, again, the interior of the
+cheese undergoes this slimy decomposition. The soft varieties are more
+prone toward this fermentation than the hard, although the firm cheeses
+are by no means exempt from the trouble. The "Verlaufen" or "running" of
+limburger cheese is a fermentation allied to this. It is where the
+inside of the cheese breaks down into a soft semi-fluid mass. In severe
+cases, the rind may even be ruptured, in which case the whole interior
+of the cheese flows out as a thick slimy mass, having sometimes a putrid
+odor. The conditions favoring this putrid decomposition are usually
+associated with an excess of moisture, and an abnormally low ripening
+temperature.
+
+~Rusty spot.~ This name is applied to the development of small
+yellowish-red or orange spots that are formed sometimes throughout the
+whole mass of cheddar cheese. A close inspection shows the colored
+points to be located along the edges of the curd particles. According to
+Harding,[216] this trouble is most common in spring and fall. The cause
+of the difficulty has been traced by Connell[217] to the development of
+a chromogenic bacterium, _Bacillus rudensis_. The organism can be most
+readily isolated on a potato surface rather than with the usual
+isolating media, agar or gelatin.
+
+~Other pigment changes.~ Occasionally, with the hard type of cheese, but
+more frequently with the softer foreign varieties, various abnormal
+conditions arise that are marked by the production of different pigments
+in or on the cheese. More frequently these are merely superficial and
+affect only the outer layers of the cheese. Generally they are
+attributable to the development of certain chromogenic organisms
+(bacteria, molds and yeasts), although occasionally due to other causes,
+as in the case of a blue discoloration sometimes noted in foreign cheese
+made in copper kettles.[218]
+
+De Vries[219] has described a blue condition that is found in Edam
+cheese. It appears first as a small blue spot on the inside, increasing
+rapidly in size until the whole mass is affected. This defect he was
+able to show was produced by a pigment-forming organism, _B.
+cyaneo-fuscus_. By the use of slimy whey (lange wei) this abnormal
+change was controlled.
+
+~Moldy cheese.~ With many varieties of cheese, especially some of the
+foreign types, the presence of mold on the exterior is not regarded as
+detrimental; in fact a limited development is much desired. In hard
+rennet cheese as cheddar or Swiss, the market demands a product free
+from mold, although it should be said that this condition is imposed by
+the desire to secure a good-looking cheese rather than any injury in
+flavor that the mold causes. Mold spores are so widely distributed that,
+if proper temperature and moisture conditions prevail, these spores will
+always develop. At temperatures in the neighborhood of 40° F. and
+below, mold growth is exceedingly slow, and often fructification does
+not occur, the only evidence of the mold being the white, felt-like
+covering that is made up of the vegetating filaments. The use of
+paraffin has been suggested as a means of overcoming this growth, the
+cheese being dipped at an early stage into melted paraffin. Recent
+experiments have shown that "off" flavors sometimes develop where cheese
+are paraffined directly from the press. If paraffin is too hard, it has
+a tendency to crack and separate from the rind, thus allowing molds to
+develop beneath the paraffin coat, where the conditions are ideal as to
+moisture, for evaporation is excluded and the air consequently
+saturated. The use of formalin (2% solution) has been suggested as a
+wash for the outside of the cheese. This substance or sulfur is also
+applied in a gaseous form. Double bandaging is also resorted to as a
+means of making the cheese more presentable through the removal of the
+outer bandage.
+
+The nature of these molds has not been thoroughly studied as yet. The
+ordinary blue-green bread mold, _Penicillium glaucum_, is most
+frequently found, but there are numerous other forms that appear,
+especially at low temperatures.
+
+~Poisonous cheese.~ Cases of acute poisoning arising from the ingestion of
+cheese are reported from time to time. Vaughan has succeeded in showing
+that this condition is due to the formation of a highly poisonous
+alkaloid which he has isolated, and which he calls _tyrotoxicon_.[220]
+This poisonous ptomaine has also been demonstrated in milk and other
+milk products, and is undoubtedly due to the development of various
+putrefactive bacteria that find their way into the milk. It seems quite
+probable that the development of these toxic organisms can also go on
+in the cheese after it is taken from the press.
+
+~Prevention or cheese defects.~ The defective conditions previously
+referred to can rarely be overcome in cheese so as to improve the
+affected product, for they only become manifest in most cases during the
+later stages of the curing process. The only remedy against future loss
+is to recognize the conditions that are apt to prevail during the
+occurrence of an outbreak and see that the cheese are handled in such a
+way as to prevent a recurrence of the difficulty.
+
+Many abnormal and undesirable results are incident to the manufacture of
+the product, such as "sour" or "mealy" cheese, conditions due to the
+development of too much acid in the milk or too high a "cook." These are
+under the direct control of the maker and for them he alone is
+responsible. The development of taints due to the growth of unwelcome
+bacteria that have gained access to the milk while it is yet on the farm
+are generally beyond the control of the cheese maker, unless they are so
+pronounced as to appear during the handling of the curds. If this does
+occur he is sometimes able, through the intervention of a starter or by
+varying some detail in making, to handle the milk in such a way as to
+minimize the trouble, but rarely is he able to eliminate it entirely.
+
+One of the most strenuous duties which the maker must perform at all
+times is to point out to his patrons the absolute necessity of their
+handling the milk in such a way as to prevent the introduction of
+organisms of a baleful type.
+
+FOOTNOTES:
+
+[178] Russell, 13 Rept. Wis. Expt. Stat., 1896, p. 112; Campbell, Trans.
+High. & Agr. Soc. Scotland, 5 ser., 1898, 10:181.
+
+[179] Winkler, Milch Zeit. (Hildesheim), Nov. 24, 1900.
+
+[180] Campbell, No. Brit., Agric., May 12, 1897.
+
+[181] Weigmann, Milch Zeit., No. 50, 1889.
+
+[182] Klein, Milch Zeit. (Hildesheim), No. 17, 1900.
+
+[183] Adametz, Landw. Jahr., 18:256.
+
+[184] Van Slyke and Hart, Bull. 214, N. Y. Expt. Stat., July 1902.
+
+[185] Milch Zeit., 1898, No. 49.
+
+[186] Lafar, Technical Mycology, p. 216.
+
+[187] Adametz, Landw. Jahr., 18:228.
+
+[188] Freudenreich, Landw. Jahr. d. Schweiz, 4:17; 5:16.
+
+[189] Russell, 13 Rept. Wis. Expt. Stat., 1896, p. 95.
+
+[190] Harrison and Connell, Rev. gen. du Lait, Nos. 4, 5, 6, 7 and 8,
+1903-04.
+
+[191] Lloyd, Bath and West of Eng. Soc. Rept., 1892, 2:180.
+
+[192] Freudenreich, Landw. Jahr. d. Schweiz, 1900; Adametz, Oest. Molk.
+Zeit., 1899, No. 7.
+
+[193] Russell, 14 Wis. Expt. Stat., 1897, p. 203. Harrison and Connell,
+Rev. gen. du Lait Nos. 4, etc., 1903-04.
+
+[194] Babcock and Russell, 18 Rept. Wis. Expt. Stat., 1901. Dean,
+Harrison and Harcourt, Bull. 121, Ont. Agr'l. Coll., June 1902.
+
+[195] Schaffer, Milch Zeit., 1889, p. 146.
+
+[196] Adametz, Landw. Jahr., 18:261.
+
+[197] Duclaux, Le Lait, p. 213.
+
+[198] Adametz, Oest. Molk. Zeit., 1900, Nos. 16-18.
+
+[199] Freudenreich, Landw. Jahr. d. Schweiz, 1897, p. 85.
+
+[200] Weigmann, Cent. f. Bakt., II Abt., 1898, 4:593; also 1899, 5:630.
+
+[201] Gorini, Abs. in Expt. Stat. Rec., 11:388.
+
+[202] Babcock and Russell, 14 Rept. Wis. Expt. Stat., 1897, p. 161.
+
+[203] Jensen, Cent. f. Bakt., II Abt., 3:752.
+
+[204] Freudenreich, Cent. f. Bakt., II Abt., 1900, 6:332.
+
+[205] Jensen, Ibid., 1900, 6:734.
+
+[206] 17 Rept. Wis. Expt. Stat., 1900, p. 102.
+
+[207] Jensen, Landw. Jahr. d. Schweiz, 1900.
+
+[208] Babcock and Russell, 18 Rept. Wis. Expt. Stat., 1901.
+
+[209] Cent. f. Bakt. 1899, p. 14.
+
+[210] Bull. 128, Wis. Expt. Stat., Sept. 1905.
+
+[211] Babcock and Russell, 18 Rept. Wis. Expt. Stat., 1901.
+
+[212] Harding, Rogers and Smith, Bull. 183, N. Y. (Geneva) Expt. Stat.,
+Dec., 1900.
+
+[213] Guillebeau, Landw. Jahr., 1890, p. 27.
+
+[214] Freudenreich, Füehl. Landw. Ztg., 43:361.
+
+[215] Harrison, Bull. 123 Ont. Agr'l. Coll., May, 1902.
+
+[216] Bull. 183, N. Y. (Geneva) Expt. Stat., Dec. 1900.
+
+[217] Connell, Bull. Canadian Dept. of Agr., 1897.
+
+[218] Schmöger, Milch Zeit., 1883, p. 483.
+
+[219] De Vries, Milch Zeit., 1888, pp. 861, 885.
+
+[220] Zeit. f. physiol. Chemie, 10:146.
+
+
+
+
+INDEX.
+
+
+Acid, effect of, on churning, 137;
+  in butter-making, 138.
+
+Acid test, 52.
+
+Aeration of milk, 59.
+
+Aerobic bacteria, 7.
+
+Alcoholic fermentation in milk, 72.
+
+Anaerobic bacteria, 7.
+
+Animal, influence of, on milk infection, 34.
+
+Animal odor, 56.
+
+Anthrax, 94.
+
+Antiseptics, 9, 88.
+
+Aroma, of butter, 140.
+
+
+Bacillus: definition of, 2.
+  _acidi lactici_, 64;
+  _cyaneo-fuscus_, 188;
+  _cyanogenus_, 74;
+  _foetidus lactis_, 157;
+  _lactis aerogenes_, 65;
+  _lactis erythrogenes_, 74;
+  _lactis saponacei_, 67;
+  _lactis viscosus_, 71;
+  _nobilis_, 162, 174;
+  _prodigiosus_, 74;
+  _rudensis_, 188;
+  _synxanthus_, 75;
+  _tuberculosis_, 84.
+
+Bacteria:
+  on hairs, 35;
+  kinds in milk, 63;
+  in barn air, 42;
+  in milk pails, 27;
+  in butter, 154;
+  classification of, 4;
+  in cheese, 160;
+  culture of, 17;
+  in cream, 128;
+  discovery of, 1;
+  external conditions affecting, 8;
+  form of, 2;
+  in butter, 142;
+  in butter-making, 127;
+  in centrifuge slime, 39;
+  In fore milk, 28;
+  in rennet, 163;
+  In separator slime, 39;
+  manure, 37;
+  number of, in milk, 50.
+  Distribution of:
+    milk of American cities, 50;
+    European cities, 50;
+    in relation to cheese, 168.
+  Of disease:
+    anthrax, 94;
+    cholera, 98;
+    diphtheria, 99;
+    lockjaw, 94;
+    toxic, 100;
+    tuberculosis, 84;
+    typhoid fever, 98.
+  Methods of study of:
+    culture, 15;
+    culture media, 13;
+    isolation, 14.
+
+Bitter butter, 158;
+  cheese, 189;
+  milk, 72.
+
+Bloody milk, 74.
+
+Blue cheese, 191;
+  milk, 74.
+
+Bovine tuberculosis, 84.
+
+Brie cheese, 182.
+
+Butter:
+  bacteria in, 154;
+  bitter, 158;
+  "cowy," 157;
+  fishy, 159;
+  lardy, 157;
+  moldy, 158;
+  mottled, 156;
+  oily, 158;
+  putrid, 156;
+  rancid, 155;
+  tallowy, 157;
+  turnip flavor in, 157.
+  Making:
+    aroma, 140;
+    flavor in, 140;
+    pure culture, 143;
+    in ripening of cream, 136.
+
+Butyric acid fermentation, 69.
+
+By-products of factory, methods of preserving, 25.
+
+
+Casease, 68.
+
+Caseone, 68.
+
+Centrifugal force, cleaning milk by, 38.
+
+Cheese:
+  bacterial flora of, 168;
+  bitter, 189;
+  blue, 187;
+  Brie, 182;
+  Edam, 72, 162;
+  Emmenthaler, 185;
+  flavor of, 179;
+  gassy fermentations in, 183;
+  Gorgonzola, 180;
+  molds on, 191;
+  mottled, 189;
+  "nissler," 185;
+  poisonous, 192;
+  putrid, 190;
+  ripening of moldy, 180;
+  ripening of soft, 181;
+  Roquefort, 180;
+  rusty spot in, 188;
+  Stilton, 180;
+  Swiss, 185.
+  Making and curing:
+    chemical changes in curing, 166;
+    influence of temperature on curing, 169;
+    influence of rennet, 177;
+    physical changes in curing, 165;
+    prevention of defects, 193;
+    starters in, 161;
+    temperature in relation to bacterial influence, 169.
+  Theories of curing:
+    digestive, 173;
+    galactase, 175, 177;
+    lactic acid, 174.
+
+Chemical changes in cheese-ripening, 166.
+
+Chemical disinfectants in milk:
+  bleaching powder, 81;
+  corrosive sublimate, 81;
+  formalin, 80;
+  sulfur, 80;
+  whitewash, 81;
+  vitriol, 81.
+
+Chemical preservatives, 80.
+
+Children, milk for, 45.
+
+Cholera in milk, 98.
+
+Classification by separator, 38.
+
+Coccus, definition of, 2.
+
+Cold, influence on bacteria, 8, 48.
+
+Contamination of milk through disease germs, 95, 191.
+
+Covered milk pails, 41.
+
+Cream, bacterial changes in, 135;
+  mechanical causes for bacteria in, 135;
+  pasteurized, 113;
+  restoration of consistency of pasteurized, 132.
+  Ripening of, 136;
+    advantage of pure cultures in, 144;
+    by natural starters, 142;
+    characteristics of pure cultures in, 145;
+    objections to pure cultures in, 146;
+    principles of pure cultures in, 143;
+    propagation of pure cultures, 151;
+    purity of commercial starters, 150;
+    home-made starters in, 146.
+
+Creaming methods, 134.
+
+Curd test, 76.
+
+
+Dairy utensils a source of contamination, 21.
+
+Diarrhoeal diseases, 100.
+
+Digesting bacteria, 67.
+
+Digestibility of heated milk, 111.
+
+Diphtheria, 99.
+
+Dirt in milk, 34.
+
+Dirt, exclusion of, 36.
+
+Disease germs in milk, 95;
+  effect of heat on, 91;
+  origin of, 83.
+
+Disinfectants, 9:
+  carbolic acid, 81;
+  chloride of lime, 81;
+  corrosive sublimate, 81;
+  formalin, 80;
+  sulfur, 80;
+  vitriol salts, 81;
+  whitewash, 79.
+
+Disinfectants in milk:
+  alkaline salts, 106;
+  boracic acid, 106;
+  formalin, 106;
+  preservaline, 107;
+  salicylic acid, 106.
+
+Domestic pasteurizing apparatus, 119.
+
+Drugs, taints in milk due to, 56.
+
+Drying, effect of, 8.
+
+
+Edam cheese, 72, 162.
+
+Emmenthaler cheese, 185.
+
+Endospores, 3.
+
+Enzyms, 10.
+
+
+Factory by-products, 22;
+  treatment of, 25.
+
+Farrington alkaline tablet, 52.
+
+Fecal bacteria, effect of, on butter, 35.
+
+Fermentation:
+  In cheese: gassy, 183.
+  In milk:
+    alcoholic, 72;
+    bitter, 72;
+    blue, 74;
+    butyric, 69;
+    digesting, 67;
+    gassy, 66;
+    kephir, 72;
+    koumiss, 72;
+    lactic acid, 63;
+    lange-wei, 72;
+    red, 74;
+    ropy, 69;
+    slimy, 69;
+    soapy, 73;
+    souring, 63;
+    sweet curdling, 67;
+    treatment of, 75.
+  Tests, 76;
+    Gerber's, 76;
+    Walther's, 76;
+    Wisconsin curd, 76.
+
+Filtration of milk, 38.
+
+Fishy butter, 159.
+
+Flavor:
+  of butter, 140;
+  of cheese, 179.
+
+Foot and mouth disease, 93.
+
+Fore milk, 28.
+
+Formaldehyde, 80.
+
+Formalin, 80.
+
+Fruity flavor in cheese, 188.
+
+
+Galactase in cheese, 175.
+
+Gassy fermentations:
+  in cheese, 183;
+  in milk, 67;
+  in Swiss cheese, 167.
+
+Gläsler, 185.
+
+Gorgonzola cheese, 180.
+
+Growth of bacteria, essential conditions for, 4;
+  in milk, 46.
+
+
+Hair, bacteria on, 35.
+
+Heat, influence on bacterial growth, 8.
+
+Heated milk:
+  characteristics of, 109;
+  action toward rennet, 112;
+  body, 110;
+  digestibility, 111;
+  fermentative changes, 111;
+  flavor, 110;
+  hydrogen peroxid test in, 23;
+  Storch's test, 23.
+
+Hygienic milk, bacteria in, 45.
+
+
+Infection of milk:
+  animal, 34;
+  dairy utensils, 21;
+  fore milk, 28;
+  milker, 36.
+
+Isolation of bacteria, methods of, 14.
+
+
+Kephir, 72.
+
+Koumiss, 72.
+
+
+Lactic acid:
+  fermentation in milk, 63;
+  theory in cheese-curing, 174.
+
+Lange-wei, 72.
+
+Lardy butter, 157.
+
+Light, action on bacteria, 9.
+
+
+Manure, bacteria in, 33.
+
+Methods:
+  of isolation, 14;
+  culture, 15.
+
+_Micrococcus casei amari_, 189.
+
+Microscope, use of, 17.
+
+Milk:
+  a bacterial food medium, 19;
+  bacteria in, 48.
+  Disease organisms in:
+    anthrax, 94;
+    cholera, 98;
+    diphtheria, 99;
+    foot and mouth disease, 93;
+    poisonous, 101;
+    ptomaines, 101;
+    scarlet fever, 99;
+    tuberculosis, 84;
+    typhoid fever, 98.
+  Contamination, 20:
+    from air, 42;
+    from animal odors, 55;
+    dirt, 34;
+    distinction between bacterial and non-bacterial, 57;
+    fore milk, 28;
+    infection in factory, 59;
+    milker, 36;
+    relative importance of various kinds, 43;
+    utensils, 21.
+
+Milk fermentations:
+  alcoholic, 72;
+  bitter, 72;
+  bloody, 74;
+  blue, 74;
+  butyric acid, 69;
+  gassy, 66, 167;
+  kephir, 72;
+  koumiss, 72;
+  lactic acid, 63;
+  red, 72;
+  ropy, 69;
+  slimy, 69;
+  soapy, 74;
+  souring, 63;
+  sweet curdling, 67;
+  tests for, 76;
+  treatment of, 75;
+  yellow, 75.
+
+Milk, heated:
+  action towards rennet, 112;
+  digestibility, 111;
+  flavor of, 110;
+  fermentative changes in, 111;
+  hydrogen peroxid test, 110.
+
+Milking machines, influence of, on germ content, 37.
+
+Milk preservation:
+  chemical agents in, 106;
+  condensation, 107;
+  freezing, 108;
+  heat, 108;
+  pasteurization, 113;
+  sterilization, 112.
+
+Milk-sugar as bacterial food, 19.
+
+Mold, in butter, 158;
+  in cheese, 191.
+
+Mottled cheese, 189.
+
+
+"Nissler" cheese, 185.
+
+
+Odors, direct absorption of, in milk, 55.
+
+_Oidium lactis_, 159.
+
+Oily butter, 158.
+
+
+Pasteurization of milk;
+  acid test in, 128;
+  bacteriological study of, 124, 126, 149;
+  for butter, 147;
+  for cheese, 162;
+  for direct use, 113;
+  of skim milk, 25;
+  details of, 128;
+  temperature and time limit in, 118.
+
+Pasteurizing apparatus:
+  continuous flow, 122;
+  coolers, 131;
+  Danish, 123;
+  domestic, 119;
+  Farrington, 122;
+  intermittant flow, 121;
+  Miller, 122;
+  Potts, 121;
+  regenerator, 122;
+  Reid, 126;
+  Russell, 121;
+  testing rate of flow, 124.
+
+_Penicillium glaucum_, 159, 180, 190.
+
+Pepsin, 10.
+
+Physical changes in cheese-ripening, 165.
+
+Poisonous bacteria:
+  in cheese, 192;
+  in milk, 100, 101.
+
+Preservaline, 167.
+
+Preservation of milk:
+  by exclusion, 103;
+  chemical agents, 106;
+  condensing, 107;
+  filtration, 38;
+  freezing, 108;
+  pasteurization, 112;
+  physical agents, 107;
+  sterilization, 112.
+
+Ptomaine poisoning, 101.
+
+Pure cultures, 15.
+
+Pure culture starters:
+  advantages of, 144;
+  characteristics of, 145;
+  home-made cultures compared with, 146;
+  propagation of, 151.
+
+Putrid cheese, 190;
+  butter, 156.
+
+
+Rancidity in butter, 155.
+
+Red milk, 74.
+
+Rennet:
+  action in heated milk, 112;
+  bacteria in, 163;
+  influence of, on cheese-ripening, 177.
+
+Restoration of consistency in pasteurized cream, 132.
+
+Ripening of cheese:
+  moldy cheese, 180;
+  soft cheese, 181.
+  Of cream, 136;
+    artificial starters, 143;
+    natural starters, 142;
+    principles of pure culture starters in, 143.
+
+Ropy milk, 69.
+
+Roquefort cheese, 180.
+
+Rusty spot in cheese, 190.
+
+Rusty cans: effect of, on acidity, 53.
+
+
+Sanitary milk, 45, 104.
+
+Sanitary pails, 41.
+
+Scarlet fever in milk, 99.
+
+Separator slime:
+  bacteria in, 39;
+  tubercle bacillus in, 93.
+
+Scalded layer, resistance of bacteria in, 91.
+
+Skim-milk, a distributor of disease, 96.
+
+Slimy milk, 69.
+
+Soapy milk, 74.
+
+Soft cheese, ripening of, 186.
+
+Sources of contamination in milk:
+  barn air, 42;
+  dairy utensils, 21;
+  dirt from animals, 34;
+  factory cans, 25;
+  fore-milk, 28;
+  milker, 36.
+
+Souring of milk, 63.
+
+Spirillum, definition of, 2.
+
+Spores, 3.
+
+Starters:
+  in cheese-making, 161;
+  in butter-making, 142;
+  propagation of, 151;
+  pure cultures in cream-ripening, 143.
+
+Sterilization of milk, 112.
+
+_Streptococcus Hollandicus_, 72, 162.
+
+Stilton cheese, 181.
+
+Storch's test, 23.
+
+Sulfur as a disinfectant, 81.
+
+Sweet curdling milk, 68.
+
+Sweet flavor in cheese, 188.
+
+Swiss cheese, 177;
+  gassy fermentations in, 24, 185.
+
+
+Taints, absorption of, 55.
+
+Taints, bacterial vs. physical, 58.
+
+Taints in milk, absorption of, 55.
+
+Taints, use of starters in overcoming, 79.
+
+Taints in butter:
+  putrid, 156;
+  rancidity, 155;
+  turnip flavor, 157.
+
+Tallowy butter, 157.
+
+Temperature:
+  effect on bacterial development, 6, 48;
+  effect of low, 108;
+  effect of high, 108;
+  and time limit in milk pasteurization, 113.
+
+Tests for milk:
+  fermentation, 76;
+  Storch's, 23;
+  acid, 52.
+
+Theories in cheese-curing:
+  digestive, 171;
+  galactase, 175, 177;
+  lactic acid, 174.
+
+Trypsin, 10.
+
+Tubercle bacillus:
+  in milk, 88;
+  in separator slime, 93;
+  thermal death limits, 117.
+
+Tuberculin test, 86.
+
+Tuberculosis, bovine, 84.
+
+Turnip flavor in butter, 157.
+
+Typhoid fever, 98.
+
+Tyrogen, 162.
+
+Tyrotoxicon, 101, 190.
+
+
+Udder:
+  artificial introduction of bacteria into, 32;
+  milk germ-free in, 19;
+  infection of, 28;
+  washing, 89;
+  tuberculosis in, 87.
+
+
+Viscogen, 132.
+
+
+Water: as a source of infection, 61.
+
+Whey, pollution of vats, 23;
+  method of preserving, 25;
+  treatment of, in vats, 25.
+
+Whitewash, 81.
+
+Wisconsin curd test, 76.
+
+
+Yeasts:
+  alcoholic ferments in milk, 73;
+  fruity flavor in cheese, 186;
+  gassy due to yeasts, 186;
+  in bitter cheese, 189;
+  in canned butter, 159;
+  kephir, 72.
+
+
+
+
+
+
+End of the Project Gutenberg EBook of Outlines of Dairy Bacteriology, 8th
+edition, by H. L. Russell
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+
+The Project Gutenberg EBook of Outlines of Dairy Bacteriology, 8th edition, by 
+H. L. Russell
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
+
+Title: Outlines of Dairy Bacteriology, 8th edition
+       A Concise Manual for the Use of Students in Dairying
+
+Author: H. L. Russell
+
+Release Date: January 11, 2009 [EBook #27778]
+
+Language: English
+
+Character set encoding: ISO-8859-1
+
+*** START OF THIS PROJECT GUTENBERG EBOOK DAIRY BACTERIOLOGY, 8TH EDITION ***
+
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+
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+and the Online Distributed Proofreading Team at
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+</pre>
+
+
+
+<h3>OUTLINES</h3>
+
+<h5>OF</h5>
+
+<h1>DAIRY BACTERIOLOGY</h1>
+
+<h2>A CONCISE MANUAL FOR THE USE OF STUDENTS IN DAIRYING</h2>
+
+<h3>BY</h3>
+
+<h2>H. L. RUSSELL</h2>
+
+<h4><span class="smcap">Dean of the College of Agriculture, University of Wisconsin</span></h4>
+
+<p class="center">
+EIGHTH EDITION<br />
+<span class="smcap">Thoroughly Revised</span><br />
+<br />
+MADISON, WISCONSIN<br />
+H. L. RUSSELL<br />
+1907<br />
+</p>
+
+
+<p class="center">
+<span class="smcap">Copyrighted</span> 1905<br />
+<span class="smcap">BY</span><br />
+H. L. RUSSELL<br />
+<br />
+<br />
+STATE JOURNAL PRINTING COMPANY,<br />
+<span class="smcap">Printers And Stereotypers,<br />
+Madison, Wis.</span><br />
+</p>
+
+<p class="notes">Transcriber's note: Minor typos have been corrected.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_iii" id="Page_iii">[Pg iii]</a></span></p>
+<h2>PREFACE.</h2>
+
+
+<p>Knowledge in dairying, like all other technical industries, has grown
+mainly out of experience. Many facts have been learned by observation,
+but the <i>why</i> of each is frequently shrouded in mystery.</p>
+
+<p>Modern dairying is attempting to build its more accurate knowledge upon
+a broader and surer foundation, and in doing this is seeking to
+ascertain the cause of well-established processes. In this, bacteriology
+is playing an important r&ocirc;le. Indeed, it may be safely predicted that
+future progress in dairying will, to a large extent, depend upon
+bacteriological research. As Fleischmann, the eminent German dairy
+scientist, says: "The gradual abolition of uncertainty surrounding dairy
+manufacture is the present important duty which lies before us, and its
+solution can only be effected by bacteriology."</p>
+
+<p>It is therefore natural that the subject of Dairy Bacteriology has come
+to occupy an important place in the curriculum of almost every Dairy
+School. An exposition of its principles is now recognized as an integral
+part of dairy science, for modern dairy practice is rapidly adopting the
+methods that have been developed as the result of bacteriological study.
+The rapid development of the subject has necessitated a frequent
+revision of this work, and it is gratifying to the writer that the
+attempt which has been made to keep these Outlines abreast of
+bacteriological advance has been appreciated by students of dairying.</p>
+
+<p>While the text is prepared more especially for the practical<span class='pagenum'><a name="Page_iv" id="Page_iv">[Pg iv]</a></span> dairy
+operator who wishes to understand the principles and reasons underlying
+his art, numerous references to original investigations have been added
+to aid the dairy investigator who wishes to work up the subject more
+thoroughly.</p>
+
+<p>My acknowledgments are due to the following for the loan of
+illustrations: Wisconsin Agricultural Experiment Station; Creamery
+Package Mfg. Co., Chicago, Ill.; and A. H. Reid, Philadelphia, Pa.</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0"><span class="smcap">H. L. Russell.</span><br /></span>
+</div><div class="stanza">
+<span class="i4"><span class="smcap">University of Wisconsin.</span><br /></span>
+</div></div>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_v" id="Page_v">[Pg v]</a></span></p>
+<h2>CONTENTS.</h2>
+
+<p>
+<span class="smcap">Chapter</span> I. Structure of the bacteria and conditions governing their development and distribution     <span class="tocnum"><a href='#Page_1'>1</a></span> <br />
+<br />
+<span class="smcap">Chapter</span> II. Methods of studying bacteria     <span class="tocnum"><a href='#Page_13'>13</a></span> <br />
+<br />
+<span class="smcap">Chapter</span> III. Contamination of milk     <span class="tocnum"><a href='#Page_19'>19</a></span> <br />
+<br />
+<span class="smcap">Chapter</span> IV. Fermentations in milk and their treatment     <span class="tocnum"><a href='#Page_62'>62</a></span> <br />
+<br />
+<span class="smcap">Chapter</span> V. Relation of disease-bacteria to milk     <span class="tocnum"><a href='#Page_82'>82</a></span> <br />
+<br />
+<span style="margin-left: 1em;">Diseases transmissible from animal to man through diseased milk   <span class="tocnum"><a href='#Page_84'>84</a></span> </span><br />
+<br />
+<span style="margin-left: 1em;">Diseases transmissible to man through infection of milk after withdrawal    <span class="tocnum"><a href='#Page_94'>94</a></span> </span><br />
+<br />
+<span class="smcap">Chapter</span> VI. Preservation of milk for commercial purposes     <span class="tocnum"><a href='#Page_102'>102</a></span> <br />
+<br />
+<span class="smcap">Chapter</span> VII. Bacteria and butter making     <span class="tocnum"><a href='#Page_134'>134</a></span> <br />
+<br />
+Bacterial defects in butter     <span class="tocnum"><a href='#Page_156'>156</a></span> <br />
+<br />
+<span class="smcap">Chapter</span> VIII. Bacteria in cheese     <span class="tocnum"><a href='#Page_160'>160</a></span> <br />
+<br />
+<span style="margin-left: 1em;">Influence Of bacteria in normal cheese processes   <span class="tocnum"><a href='#Page_160'>160</a></span> </span><br />
+<br />
+<span style="margin-left: 1em;">Influence of bacteria in abnormal cheese processes   <span class="tocnum"><a href='#Page_182'>182</a></span> </span><br />
+</p>
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_1" id="Page_1">[Pg 1]</a></span></p>
+<h2>CHAPTER I.</h2>
+
+<h3>STRUCTURE OF THE BACTERIA AND CONDITIONS GOVERNING THEIR DEVELOPMENT AND DISTRIBUTION.</h3>
+
+
+<p>Before one can gain any intelligent conception of the manner in which
+bacteria affect dairying, it is first necessary to know something of the
+life history of these organisms in general, how they live, move and
+react toward their environment.</p>
+
+<p><b>Nature of Bacteria.</b> Toadstools, smuts, rusts and mildews are known to
+even the casual observer, because they are of evident size. Their
+plant-like nature can be more readily understood from their general
+structure and habits of life. The bacteria, however, are so small, that
+under ordinary conditions, they only become evident to our unaided
+senses by the by-products of their activity.</p>
+
+<p>When Leeuwenhoek (pronounced Lave-en-hake) in 1675 first discovered
+these tiny, rapidly-moving organisms he thought they were animals.
+Indeed, under a microscope, many of them bear a close resemblance to
+those minute worms found in vinegar that are known as "vinegar-eels."
+The idea that they belonged to the animal kingdom continued to hold
+ground until after the middle of the nineteenth century; but with the
+improvement in microscopes, a more thorough study of these tiny
+structures was made possible, and their vegetable nature demonstrated.
+The bacteria as a class are separated from the fungi mainly by their
+method of growth; from the lower algae by the absence of chlorophyll,
+the green coloring matter of vegetable organisms.<span class='pagenum'><a name="Page_2" id="Page_2">[Pg 2]</a></span></p>
+
+<p><b>Structure of bacteria.</b> So far as structure is concerned the bacteria
+stand on the lowest plane of vegetable life. The single individual is
+composed of but a single cell, the structure of which does not differ
+essentially from that of many of the higher types of plant life. It is
+composed of a protoplasmic body which is surrounded by a thin membrane
+that separates it from neighboring cells that are alike in form and
+size.</p>
+
+<p><b>Form and size.</b> When a plant is composed of a single cell but little
+difference in form is to be expected. While there are intermediate
+stages that grade insensibly into each other, the bacteria may be
+grouped into three main types, so far as form is concerned. These are
+spherical, elongated, and spiral, and to these different types are given
+the names, respectively, <i>coccus</i>, <i>bacillus</i> and <i>spirillum</i> (plural,
+<i>cocci</i>, <i>bacilli</i>, <i>spirilla</i>) (fig. 1). A ball, a short rod, and a
+corkscrew serve as convenient models to illustrate these different
+forms.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/fig1.jpg" width="500" height="280" alt="Fig. 1." title="" />
+<span class="caption">Fig. 1.</span>
+</div>
+
+<div class="blockquot"><p>Different forms of bacteria. <i>a</i>, <i>b</i>, <i>c</i>,
+represent different types as to form: <i>a</i>, coccus, <i>b</i>, bacillus, <i>c</i>,
+spirillum; <i>d</i>, diplococcus or twin coccus; <i>e</i>, staphylococcus or
+cluster coccus; <i>f</i> and <i>g</i>, different forms of bacilli, <i>g</i> shows
+internal endospores within cell; <i>h</i> and <i>i</i>, bacilli with motile organs
+(cilia).</p></div>
+
+<p>In size, the bacteria are the smallest organisms that are known to
+exist. Relatively there is considerable difference in<span class='pagenum'><a name="Page_3" id="Page_3">[Pg 3]</a></span> size between the
+different species, yet in absolute amount this is so slight as to
+require the highest powers of the microscope to detect it. As an average
+diameter, one thirty-thousandth of an inch may be taken. It is difficult
+to comprehend such minute measurements, but if a hundred individual
+germs could be placed side by side, their total thickness would not
+equal that of a single sheet of paper upon which this page is printed.</p>
+
+<p><b>Manner of Growth.</b> As the cell increases in size as a result of growth,
+it elongates in one direction, and finally a new cell wall is formed,
+dividing the so-called mother-cell into two, equal-sized daughter-cells.
+This process of cell division, known as <i>fission</i>, is continued until
+growth ceases and is especially characteristic of bacteria.</p>
+
+<p><b>Cell Arrangement.</b> If fission goes on in the same plane continually, it
+results in the formation of a cell-row. A coccus forming such a chain of
+cells is called <i>strepto-coccus</i> (chain-coccus). If only two cells
+cohere, it is called a <i>diplo-coccus</i> (twin-coccus). If the second cell
+division plane is formed at right angles to the first, a <i>cell surface</i>
+or <i>tetrad</i> is formed. If growth takes place in three dimensions of
+space, a <i>cell mass</i> or <i>sarcina</i> is produced. Frequently, these cell
+aggregates cohere so tenaciously that this arrangement is of value in
+distinguishing different species.</p>
+
+<p><b>Spores.</b> Some bacteria possess the property of forming <i>spores</i> within
+the mother cell (called <i>endospores</i>, fig. 1g) that are analogous in
+function to the seeds of higher plants and spores of fungi. By means of
+these structures which are endowed with greater powers of resistance
+than the vegetating cell, the organism is able to protect itself from
+the effect of an unfavorable environment. Many of the bacilli form
+endospores but the cocci do not. It is these<span class='pagenum'><a name="Page_4" id="Page_4">[Pg 4]</a></span> spore forms that make it
+so difficult to thoroughly sterilize milk.</p>
+
+<p><b>Movement.</b> Many bacteria are unable to move from place to place. They
+have, however, a vibrating movement known as the <i>Brownian</i> motion that
+is purely physical. Many other kinds are endowed with powers of
+locomotion. Motion is produced by means of fine thread-like processes of
+protoplasm known as <i>cilia</i> (sing. <i>cilium</i>) that are developed on the
+outer surface of the cell. By means of the rapid vibration of these
+organs, the cell is propelled through the medium. Nearly all cocci are
+immotile, while the bacilli may or may not be. These cilia are so
+delicate that it requires special treatment to demonstrate their
+presence.</p>
+
+<p><b>Classification.</b> In classifying or arranging the different members of any
+group of living objects, certain similarities and dissimilarities must
+be considered. These are usually those that pertain to the structure and
+form, as such are regarded as most constant. With the bacteria these
+differences are so slight that they alone do not suffice to distinguish
+distinctly one species from another. As far as these characters can be
+used, they are taken, but in addition, many characteristics of a
+physiological nature are added. The way that the organism grows in
+different kinds of cultures, the by-products produced in different
+media, and effect on the animal body when injected into the same are
+also used as data in distinguishing one species from another.</p>
+
+<p><b>Conditions favoring bacterial growth.</b> The bacteria, in common with all
+other living organisms are affected by external conditions, either
+favorably or unfavorably. Certain conditions must prevail before
+development can occur. Thus, the organism must be supplied with an
+adequate<span class='pagenum'><a name="Page_5" id="Page_5">[Pg 5]</a></span> and suitable food supply and with moisture. The temperature
+must also range between certain limits, and finally, the oxygen
+requirements of the organism must be considered.</p>
+
+<p><b>Food supply.</b> Most bacteria are capable of living on dead, inert, organic
+matter, such as meats, milk and vegetable material, in which case, they
+are known as <i>saprophytes</i>. In contradistinction to this class is a
+smaller group known as <i>parasites</i>, which derive their nourishment from
+the living tissues of animals or plants. The first group comprise by far
+the larger number of known organisms which are concerned for the most
+part in the decomposition of organic matter. The parasitic group
+includes those which are the cause of various communicable diseases.
+Between these two groups there is no sharp line of division, and in some
+cases, certain species possess the faculty of growing either as
+parasites or saprophytes, in which case they are known as <i>facultative</i>
+parasites or saprophytes.</p>
+
+<p>The great majority of bacteria of interest in dairying belong to the
+saprophytic class; only those species capable of infecting milk through
+the development of disease in the animal are parasites in the strict
+sense of the term. Most disease-producing species, as diphtheria or
+typhoid fever, while parasitic in man lead a saprophytic method of life
+so far as their relation to milk is concerned.</p>
+
+<p>Bacteria require for their growth, nitrogen, hydrogen, carbon, oxygen,
+together with a limited amount of mineral matter. The nitrogen and
+carbon are most available in the form of organic compounds, such as
+albuminous material. Carbon in the form of carbohydrates, as sugar or
+starch, is most readily attacked by bacteria.</p>
+
+<p>Inasmuch as the bacteria are plant-cells, they must imbibe<span class='pagenum'><a name="Page_6" id="Page_6">[Pg 6]</a></span> their food
+from material in solution. They are capable of living on solid
+substances, but in such cases, the food elements must be rendered
+soluble, before they can be appropriated. If nutritive liquids are too
+highly concentrated, as in the case of syrups and condensed milk,
+bacteria cannot grow therein, although all the necessary ingredients may
+be present. Generally, bacteria prefer a neutral or slightly alkaline
+medium, rather than one of acid reaction; but there are numerous
+exceptions to this general rule, especially among the bacteria found in
+milk.</p>
+
+<p><b>Temperature.</b> Growth of bacteria can only occur within certain
+temperature limits, the extremes of which are designated as the
+<i>minimum</i> and <i>maximum</i>. Below and above these respective limits, life
+may be retained in the cell for a time, but actual cell-multiplication
+is stopped. Somewhere between these two cardinal temperature points, and
+generally nearer the maximum limit is the most favorable temperature for
+growth, known as the <i>optimum</i>. The temperature zone of most dairy
+bacteria in which growth occurs ranges from 40&deg;-45&deg; F. to somewhat above
+blood-heat, 105&deg;-110&deg; F., the optimum being from 80&deg;-95&deg; F. Many
+parasitic species, because of their adaptation to the bodies of
+warm-blooded animals, generally have a narrower range, and a higher
+optimum, usually approximating the blood heat (98&deg;-99&deg; F). The broader
+growth limits of bacteria in comparison with other kinds of life explain
+why these organisms are so widely distributed in nature.</p>
+
+<p><b>Air supply.</b> Most bacteria require as do the green plants and animal
+life, the free oxygen of the air for their respiration. These are called
+<i>aerobic</i>. Some species, however, and some yeasts as well possess the
+peculiar property of taking the oxygen which they need from organic
+compounds<span class='pagenum'><a name="Page_7" id="Page_7">[Pg 7]</a></span> such as sugar, etc., and are therefore able to live and grow
+under conditions where the atmospheric air is excluded. These are known
+as <i>anaerobic</i>. While some species grow strictly under one condition or
+the other, and hence are <i>obligate</i> aerobes or anaerobes, others possess
+the ability of growing under either condition and are known as
+<i>facultative</i> or optional forms. The great majority of milk bacteria are
+either obligate or facultative aerobes.</p>
+
+<p><b>Rate of growth.</b> The rate of bacterial development is naturally very much
+affected by external conditions, food supply and temperature exerting
+the most influence. In the neighborhood of the freezing point but little
+growth occurs. The rate increases with a rise in temperature until at
+the <i>optimum</i> point, which is generally near the blood heat or slightly
+below (90&deg;-98&deg; F.), a single cell will form two cells in 20 to 30
+minutes. If temperature rises much above blood heat rate of growth is
+lessened and finally ceases. Under ideal conditions, rapidity of growth
+is astounding, but this initially rapid rate of development cannot be
+maintained indefinitely, for growth is soon limited by the accumulation
+of by-products of cell activity. Thus, milk sours rapidly at ordinary
+temperatures until the accumulation of acid checks its development.</p>
+
+<p><b>Detrimental effect of external conditions.</b> Environmental influences of a
+detrimental character are constantly at work on bacteria, tending to
+repress their development or destroy them. These act much more readily
+on the vegetating cell than on the more resistant spore. A thorough
+knowledge of the effect of these antagonistic forces is essential, for
+it is often by their means that undesirable bacteria may be killed out.<span class='pagenum'><a name="Page_8" id="Page_8">[Pg 8]</a></span></p>
+
+<p><b>Effect of cold.</b> While it is true that chilling largely prevents
+fermentative action, and actual freezing stops all growth processes,
+still it does not follow that exposure to low temperatures will
+effectually destroy the vitality of bacteria, even in the vegetative
+condition. Numerous non-spore-bearing species remain alive in ice for a
+prolonged period, and recent experiments with liquid air show that even
+a temperature of -310&deg; F. for hours does not effectually kill all
+exposed cells.</p>
+
+<p><b>Effect of heat.</b> High temperatures, on the other hand, will destroy any
+form of life, whether in the vegetative or latent stage. The temperature
+at which the vitality of the cell is lost is known as the <i>thermal death
+point</i>. This limit is not only dependent upon the nature of the
+organism, but varies with the time of exposure and the condition in
+which the heat is applied. In a moist atmosphere the penetrating power
+of heat is great; consequently cell-death occurs at a lower temperature
+than in a dry atmosphere. An increase in time of exposure lowers the
+temperature point at which death occurs.</p>
+
+<p>For vegetating forms the thermal death point of most bacteria ranges
+from 130&deg;-140&deg; F. where the exposure is made for ten minutes which is
+the standard arbitrarily selected. In the spore stage resistance is
+greatly increased, some forms being able to withstand steam at 210&deg;-212&deg;
+F. from one to three hours. If dry heat is employed, 260&deg;-300&deg; F. for an
+hour is necessary to kill spores. Where steam is confined under
+pressure, a temperature of 230&deg;-240&deg; F. for 15-20 minutes suffices to
+kill all spores.</p>
+
+<p><b>Drying.</b> Spore-bearing bacteria like anthrax withstand drying with
+impunity; even tuberculous material, although not possessing spores
+retains its infectious properties for<span class='pagenum'><a name="Page_9" id="Page_9">[Pg 9]</a></span> many months. Most of the dairy
+bacteria do not produce spores, and yet in a dry condition, they retain
+their vitality unimpaired for considerable periods, if they are not
+subjected to other detrimental influences.</p>
+
+<p><b>Light.</b> Bright sunlight exerts on many species a powerful disinfecting
+action, a few hours being sufficient to destroy all cells that are
+reached by the sun's rays. Even diffused light has a similar effect,
+although naturally less marked. The active rays in this disinfecting
+action are those of the chemical or violet end of the spectrum, and not
+the heat or red rays.</p>
+
+<p><b>Influence of chemical substances.</b> A great many chemical substances exert
+a more or less powerful toxic action of various kinds of life. Many of
+these are of great service in destroying or holding bacterial growth in
+check. Those that are toxic and result in the death of the cell are
+known as <i>disinfectants</i>; those that merely inhibit, or retard growth
+are known as <i>antiseptics</i>. All disinfectants must of necessity be
+antiseptic in their action, but not all antiseptics are disinfectants
+even when used in strong doses. Disinfectants have no place in dairy
+work, except to destroy disease bacteria, or preserve milk for
+analytical purposes. Corrosive sublimate or potassium bichromate are
+most frequently used for these purposes. The so-called chemical
+preservatives used to "keep" milk depend for their effect on the
+inhibition of bacterial growth. With a substance so violently toxic as
+formaldehyde (known as formalin, freezene) antiseptic doses are likely
+to be exceeded. In this country most states prohibit the use of these
+substances in milk. Their only function in the dairy should be to check
+fermentative or putrefactive processes outside of milk and so keep the
+air free from taints.<span class='pagenum'><a name="Page_10" id="Page_10">[Pg 10]</a></span></p>
+
+<p><b>Products of growth.</b> All bacteria in their development form certain more
+or less characteristic by-products. With most dairy bacteria, these
+products are formed from the decomposition of the medium in which the
+bacteria may happen to live. Such changes are known, collectively, as
+fermentations, and are characterised by the production of a large amount
+of by-products, as a result of the development of a relatively small
+amount of cell-life. The souring of milk, the formation of butyric acid,
+the making of vinegar from cider, are all examples of fermentative
+changes.</p>
+
+<p>With many bacteria, especially those that affect proteid matter,
+foul-smelling gases are formed. These are known as putrefactive changes.
+All organic matter, under the action of various organisms, sooner or
+later undergoes decay, and in different stages of these processes,
+acids, alkalies, gases and numerous other products are formed. Many of
+these changes in organic matter occur only when such material is brought
+in direct contact with the living bacterial cell.</p>
+
+<p>In other instances, soluble, non-vital ferments known as <i>enzyms</i> are
+produced by the living cell, which are able to act on organic matter, in
+a medium free from live cells, or under conditions where the activity of
+the cell is wholly suspended. These enzyms are not confined to bacteria
+but are found throughout the animal and plant world, especially in those
+processes that are concerned in digestion. Among the better known of
+these non-vital ferments are rennet, the milk-curdling enzym; diastase
+or ptyalin of the saliva, the starch-converting enzym; pepsin and
+trypsin, the digestive ferments of the animal body.</p>
+
+<p>Enzyms of these types are frequently found among the bacteria and yeasts
+and it is by virtue of this characteristic<span class='pagenum'><a name="Page_11" id="Page_11">[Pg 11]</a></span> that these organisms are
+able to break down such enormous quantities of organic matter. Most of
+these enzyms react toward heat, cold and chemical poisons in a manner
+quite similar to the living cells. In one respect they are readily
+differentiated, and that is, that practically all of them are capable of
+producing their characteristic chemical transformations under
+anaesthetic conditions, as in a saturated ether or chloroform
+atmosphere.</p>
+
+<p><b>Distribution of bacteria.</b> As bacteria possess greater powers of
+resistance than most other forms of life, they are to be found more
+widely distributed than any other type. At the surface of the earth,
+where conditions permit of their growth, they are found everywhere,
+except in the healthy tissues of animals and plants. In the superficial
+soil layers, they exist in myriads, as here they have abundance of
+nourishment. At the depth of several feet however, they diminish rapidly
+in numbers, and in the deeper soil layers, from six to ten feet or more,
+they are not present, because of the unsuitable growth conditions.</p>
+
+<p>The bacteria are found in the air because of their development in the
+soil below. They are unable to grow even in a moist atmosphere, but are
+so readily dislodged by wind currents that over land areas the lower
+strata of the air always contain them. They are more numerous in summer
+than in winter; city air contains larger numbers than country air.
+Wherever dried fecal matter is present, as in barns, the air contains
+many forms.</p>
+
+<p>Water contains generally enough organic matter in solution, so that
+certain types of bacterial life find favorable growth conditions. Water
+in contact with the soil surface takes up many impurities, and is of
+necessity rich in microbes. As the rain water percolates into the soil,
+it loses<span class='pagenum'><a name="Page_12" id="Page_12">[Pg 12]</a></span> its germ content, so that the normal ground water, like the
+deeper soil layers, contains practically no bacterial life. Springs
+therefore are relatively deficient in germ life, except as they become
+infected with soil organisms, as the water issues from the soil. Water
+may serve to disseminate certain infectious diseases as typhoid fever
+and cholera among human beings, and a number of animal maladies.</p>
+
+<p>While the inner tissues of healthy animals are free from bacteria, the
+natural passages as the respiratory and digestive tracts, being in more
+direct contact with the exterior, become more readily infected. This is
+particularly true with reference to the intestinal tract, for in the
+undigested residue, bacterial activity is at a maximum. The result is
+that fecal matter contains enormous numbers of organisms so that the
+possibility of pollution of any food medium such as milk with such
+material is sure to introduce elements that seriously affect the quality
+of the product.</p>
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_13" id="Page_13">[Pg 13]</a></span></p>
+<h2>CHAPTER II.</h2>
+
+<h3>METHODS OF STUDYING BACTERIA.</h3>
+
+
+<p><b>Necessity of bacterial masses for study.</b> The bacteria are so extremely
+small that it is impossible to study individual germs separately without
+the aid of first-class microscopes. For this reason, but little advance
+was made in the knowledge of these lower forms of plant life, until the
+introduction of culture methods, whereby a single organism could be
+cultivated and the progeny of this cell increased to such an extent in a
+short course of time, that they would be visible to the unaided eye.</p>
+
+<p>This is done by growing the bacteria in masses on various kinds of food
+media that are prepared for the purpose, but inasmuch as bacteria are so
+universally distributed, it becomes an impossibility to cultivate any
+special form, unless the medium in which they are grown is first freed
+from all pre-existing forms of germ life. To accomplish this, it is
+necessary to subject the nutrient medium to some method of
+sterilization, such as heat or filtration, whereby all life is
+completely destroyed or eliminated. Such material after it has been
+rendered germ-free is kept in sterilized glass tubes and flasks, and is
+protected from infection by cotton stoppers.</p>
+
+<p><b>Culture media.</b> For culture media, many different substances are
+employed. In fact, bacteria will grow on almost any organic substance
+whether it is solid or fluid, provided the other essential conditions of
+growth are furnished. The food substances that are used for culture
+purposes are divided into two classes; solids and liquids.<span class='pagenum'><a name="Page_14" id="Page_14">[Pg 14]</a></span></p>
+
+<p>Solid media may be either permanently solid like potatoes, or they may
+retain their solid properties only at certain temperatures like gelatin
+or agar. The latter two are of utmost importance in bacteriological
+research, for their use, which was introduced by Koch, permits the
+separation of the different forms that may happen to be in any mixture.
+Gelatin is used advantageously because the majority of bacteria present
+wider differences due to growth upon this medium than upon any other. It
+remains solid at ordinary temperatures, becoming liquid at about 70&deg; F.
+Agar, a gelatinous product derived from a Japanese sea-weed, has a much
+higher melting point, and can be successfully used, especially with
+those organisms whose optimum growth point is above the melting point of
+gelatin.</p>
+
+<p>Besides these solid media, different liquid substances are extensively
+used, such as beef broth, milk, and infusions of various vegetable and
+animal tissues. Skim-milk is of especial value in studying the milk
+bacteria and may be used in its natural condition, or a few drops of
+litmus solution may be added in order to detect any change in its
+chemical reaction due to the bacteria.</p>
+
+<div class="figcenter" style="width: 350px;">
+<img src="images/fig2.jpg" width="350" height="305" alt="Fig. 2." title="" />
+<span class="caption">Fig. 2. A gelatin plate culture showing appearance of
+different organisms in a sample of milk. Each mass represents a
+bacterial growth (colony) derived from a single cell. Different forms
+react differently toward the gelatin, some liquefying the same, others
+growing in a restricted mass. a, represents a colony of the ordinary
+bread mold; b, a liquefying bacterium; c, and d, solid forms.</span>
+</div>
+
+<p><b>Methods of isolation.</b> Suppose for instance one wishes to isolate the
+different varieties of bacteria found in milk. The method of procedure
+is as follows: Sterile gelatin in glass tubes is melted and cooled down
+so as to be barely warm. To this gelatin which is germ-free a drop of
+milk is added. The gelatin is then gently shaken so as to thoroughly
+distribute the milk particles, and poured out into a sterile flat glass
+dish and quickly covered. This is allowed to stand on a cool surface
+until the gelatin hardens. After the culture plate has been left for
+twenty-four to thirty-six<span class='pagenum'><a name="Page_15" id="Page_15">[Pg 15]</a></span> hours at the proper temperature, tiny spots
+will begin to appear on the surface, or in the depth of the culture
+medium. These patches are called <i>colonies</i> and are composed of an
+almost infinite number of individual germs, the result of the continued
+growth of a single organism that was in the drop of milk which was
+firmly held in place when the gelatin solidified. The number of these
+colonies represents approximately the number of germs that were present
+in the milk drop. If the plate is not too thickly sown with these germs,
+the colonies will continue to grow and increase in size, and as they do,
+minute differences will begin to appear. These differences may be in the
+color, the contour and the texture of the colony, or<span class='pagenum'><a name="Page_16" id="Page_16">[Pg 16]</a></span> the manner in
+which it acts toward gelatin. In order to make sure that the seeding in
+not too copious so as to interfere with continued study, an
+<i>attenuation</i> is usually made. This consists in taking a drop of the
+infected gelatin in the first tube, and transferring it to another tube
+of sterile media. Usually this operation is repeated again so that these
+culture plates are made with different amounts of seed with the
+expectation that in at least one plate the seeding will not be so thick
+as to prevent further study. For transferring the culture a loop made of
+platinum wire is used. By passing this through a gas flame, it can be
+sufficiently sterilized.</p>
+
+<div class="figcenter" style="width: 400px;">
+<img src="images/fig3.jpg" width="400" height="145" alt="Fig. 3." title="" />
+<span class="caption">Fig. 3.</span>
+</div>
+
+<div class="blockquot"><p>Profile view of gelatin plate culture; <i>b</i>, a
+liquefying form that dissolves the gelatin; <i>c</i> and <i>d</i>, surface
+colonies that do not liquefy the gelatin.</p></div>
+
+<p>To further study the peculiarities of different germs, the separate
+colonies are transferred to other sterile tubes of culture material and
+thus <i>pure cultures</i> of the various germs are secured. These cultures
+then serve as a basis for continued study and must be planted and grown
+upon all the different kinds of media that are obtainable. In this way
+the slight variations in the growth of different forms are detected and
+the peculiar characteristics are determined, so that the student is able
+to recognize this form when he meets it again.</p>
+
+<p>These culture methods are of essential importance in bacteriology, as it
+is the only way in which it is possible to secure a quantity of germs of
+the same kind.<span class='pagenum'><a name="Page_17" id="Page_17">[Pg 17]</a></span></p>
+
+<p><b>The microscope in bacterial investigation.</b> In order to verify the purity
+of the cultures, the microscope is in constant demand throughout all the
+different stages of the isolating process. For this purpose, it is
+essential that the instrument used shall be one of strong magnifying
+powers (600-800 diameters), combined with sharp definition.</p>
+
+<div class="figcenter" style="width: 488px;">
+<img src="images/fig4.jpg" width="488" height="450" alt="Fig. 4." title="" />
+<span class="caption">Fig. 4.</span>
+</div>
+
+<div class="blockquot"><p>Pure cultures of different kinds of bacteria in
+gelatin tubes. <i>a</i>, growth slight in this medium; <i>b</i>, growth copious at
+and near surface. Fine parallel filaments growing out into medium
+liquefying at surface; <i>c</i>, a rapid liquefying form; <i>d</i>, a
+gas-producing form that grows equally well in lower part of tube as at
+surface (facultative anaerobe); <i>e</i>, an obligate anaerobe, that develops
+only in absence of air.</p></div>
+
+<p>The microscopical examination of any germ is quite as<span class='pagenum'><a name="Page_18" id="Page_18">[Pg 18]</a></span> essential as the
+determination of culture characteristics; in fact, the two must go hand
+in hand. The examination reveals not only the form and size of the
+individual germs, but the manner in which they are united with each
+other, as well as any peculiarities of movement that they may possess.</p>
+
+<p>In carrying out the microscopical part of the work, not only is the
+organism examined in a living condition, but preparations are made by
+using solutions of anilin dyes as staining agents. These are of great
+service in bringing out almost imperceptible differences. The art of
+staining has been carried to the highest degree of perfection in
+bacteriology, especially in the detection of germs that are found in
+diseased tissues in the animal or human body.</p>
+
+<p>In studying the peculiarities of any special organism, not only is it
+necessary that these cultural and microscopical characters should be
+closely observed, but special experiments must be carried out along
+different lines, in order to determine any special properties that the
+germ may possess. Thus, the ability of any form to act as a fermentative
+organism can be tested by fermentation experiments; the property of
+causing disease, studied by the inoculation of pure cultures into
+animals. A great many different methods have been devised for the
+purpose of studying special characteristics of different bacteria, but a
+full description of these would necessarily be so lengthy that in a work
+of this character they must be omitted. For details of this nature
+consult standard reference books on bacteriological technique.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_19" id="Page_19">[Pg 19]</a></span></p>
+<h2>CHAPTER III.</h2>
+
+<h3>CONTAMINATION OF MILK.</h3>
+
+
+<p>No more important lesson is to be learned than that which relates to the
+ways in which milk is contaminated with germ life of various kinds; for
+if these sources of infection are thoroughly recognized they can in
+large measure be prevented, and so the troubles which they engender
+overcome. Various organisms find in milk a congenial field for
+development. Yeasts and some fungi are capable of growth, but more
+particularly the bacteria.</p>
+
+<p><b>Milk a suitable bacterial food.</b> The readiness with which milk undergoes
+fermentative changes indicates that it is well adapted to nourish
+bacterial life. Not only does it contain all the necessary nutritive
+substances but they are diluted in proper proportions so as to render
+them available for bacterial as well as mammalian life.</p>
+
+<p>Of the nitrogenous compounds, the albumen is in readily assimilable
+form. The casein, being insoluble, is not directly available, until it
+is acted upon by proteid-dissolving enzyms like trypsin which may be
+secreted by bacteria. The fat is relatively resistant to change,
+although a few forms are capable of decomposing it. Milk sugar, however,
+is an admirable food for many species, acids and sometimes gases being
+generally produced.</p>
+
+<p><b>Condition when secreted.</b> When examined under normal conditions milk
+always reveals bacterial life, yet in the secreting cells of the udder
+of a healthy cow germ life is not found. Only when the gland is diseased
+are bacteria<span class='pagenum'><a name="Page_20" id="Page_20">[Pg 20]</a></span> found in any abundance. In the passage of the milk from
+the secreting cells to the outside it receives its first infection, so
+that when drawn from the animal it generally contains a considerable
+number of organisms.</p>
+
+<div class="figcenter" style="width: 350px;">
+<img src="images/fig5.jpg" width="350" height="356" alt="Fig. 5. Microscopic appearance of milk showing relative
+size of fat globules and bacteria." title="" />
+<span class="caption">Fig. 5. Microscopic appearance of milk showing relative
+size of fat globules and bacteria.</span>
+</div>
+
+<p><b>Contamination of milk.</b> From this time until it is consumed in one form
+or another, it is continually subjected to contamination. The major part
+of this infection occurs while the milk is on the farm and the degree of
+care which is exercised while the product is in the hands of the milk
+producer is the determining factor in the course of bacterial changes
+involved. This of course does not exclude the possibility of
+contamination in the factory, but usually milk is so thoroughly seeded
+by the time it reaches the factory that the infection which occurs here
+plays a relatively minor r&ocirc;le to that which happens earlier. The great
+majority of the organisms in milk are in no wise dangerous to health,
+but many species are capable of producing various fermentative changes
+that injure the quality of the product for butter or cheese. To be able
+to control abnormal changes of an undesirable character one must know
+the sources of infection which permit of the introduction of these
+unwelcome intruders.<span class='pagenum'><a name="Page_21" id="Page_21">[Pg 21]</a></span></p>
+
+<p><b>Sources of infection.</b> The bacterial life that finds its way into milk
+while it is yet on the farm may be traced to several sources, which may
+be grouped under the following heads: unclean dairy utensils, fore milk,
+coat of animal, and general atmospheric surroundings. The relative
+importance of these various factors fluctuates in each individual
+instance.</p>
+
+<p><b>Dairy utensils.</b> Of first importance are the vessels that are used during
+milking, and also all storage cans and other dairy utensils that come in
+contact with the milk after it is drawn. By unclean utensils, actually
+<i>visible</i> dirt need not always be considered, although such material is
+often present in cracks and angles of pails and cans. Unless cleansed
+with especial care, these are apt to be filled with foul and decomposing
+material that suffices to seed thoroughly the milk. Tin utensils are
+best. Where made with joints, they should be well flushed with solder so
+as to be easily cleaned (see Fig. 6). In much of the cheaper tin ware on
+the market, the soldering of joints and seams is very imperfect,
+affording a place of refuge for bacteria and dirt.</p>
+
+<p>Cans are often used when they are in a condition wholly unsuitable for
+sanitary handling of milk. When the tin coating becomes broken and the
+can is rusty, the quality of the milk is often profoundly affected.
+Olson<a name="FNanchor_1_1" id="FNanchor_1_1"></a><a href="#Footnote_1_1" class="fnanchor">[1]</a> of the Wisconsin Station has shown that the action of rennet is
+greatly impaired where milk comes in contact with a rusty iron surface.</p>
+
+<div class="figcenter" style="width: 168px;">
+<img src="images/fig6.jpg" width="168" height="150" alt="Fig. 6." title="" />
+<span class="caption">Fig. 6.</span>
+</div>
+
+<p>With the introduction of the form or hand separator a new milk utensil
+has been added to those previously in use and one which is very
+frequently not well cleaned. Where<span class='pagenum'><a name="Page_22" id="Page_22">[Pg 22]</a></span> water is run through the machine to
+rinse out the milk particles, gross bacterial contamination occurs and
+the use of the machine much increases the germ content of the milk.
+Every time the separator is used it should be taken apart and thoroughly
+cleaned and dried before reassembling.<a name="FNanchor_2_2" id="FNanchor_2_2"></a><a href="#Footnote_2_2" class="fnanchor">[2]</a></p>
+
+<p><b>Use of milk-cans for transporting factory by-products.</b> The general
+custom of using the milk-cans to carry back to the farm the factory
+by-products (skim-milk or whey) has much in it that is to be deprecated.
+These by-products are generally rich in bacterial life, more especially
+where the closest scrutiny is not given to the daily cleaning of the
+vats and tanks. Too frequently the cans are not cleaned immediately upon
+arrival at the farm, so that the conditions are favorable for rapid
+fermentation. Many of the taints that bother factories are directly
+traceable to such a cause. A few dirty patrons will thus seriously
+infect the whole supply. The responsibility for this defect should,
+however, not be laid entirely upon the shoulders of the producer. The
+factory operator should see that the refuse material does not accumulate
+in the waste vats from day to day and is not transformed into a more or
+less putrid mass. A dirty whey tank is not an especially good object
+lesson to the patron to keep his cans clean.</p>
+
+<p>It is possible that abnormal fermentations or even contagious diseases
+may thus be disseminated.</p>
+
+<p>Suppose there appears in a dairy an infectious milk trouble, such as
+bitter milk. This milk is taken to the factory and passes unnoticed into
+the general milk-supply. The skim-milk from the separator is of course
+infected with the germ, and if conditions favor its growth, the whole
+lot soon becomes tainted. If this waste product is returned to the
+different patrons in the same cans that are used for the fresh milk, the
+probabilities are strongly in<span class='pagenum'><a name="Page_23" id="Page_23">[Pg 23]</a></span> favor of some of the cans being
+contaminated and thus infecting the milk supply of the patrons. If the
+organism is endowed with spores so that it can withstand unfavorable
+conditions, this taint may be spread from patron to patron simply
+through the infection of the vessels that are used in the transportation
+of the by-products. Connell has reported just such a case in a Canadian
+cheese factory where an outbreak of slimy milk was traced to infected
+whey vats. Typhoid fever among people, foot and mouth disease and
+tuberculosis among stock are not infrequently spread in this way. In
+Denmark, portions of Germany and some states in America, compulsory
+heating of factory by-products is practiced to eliminate this danger.<a name="FNanchor_3_3" id="FNanchor_3_3"></a><a href="#Footnote_3_3" class="fnanchor">[3]</a></p>
+
+<p><b>Pollution of cans from whey tanks.</b> The danger is greater in cheese
+factories than in creameries, for whey usually represents a more
+advanced stage of fermentation than skim-milk. The higher temperature at
+which it is drawn facilitates more rapid bacterial growth, and the
+conditions under which it is stored in many factories contribute to the
+ease with which fermentative changes can go on in it. Often this
+by-product is stored in wooden cisterns or tanks, situated below ground,
+where it becomes impossible to clean them out thoroughly. A custom that
+is almost universally followed in the Swiss cheese factories, here in
+this country, as in Switzerland, is fully as reprehensible as any<span class='pagenum'><a name="Page_24" id="Page_24">[Pg 24]</a></span> dairy
+custom could well be. In Fig. 7 the arrangement in vogue for the
+disposal of the whey is shown. The hot whey is run out through the
+trough from the factory into the large trough that is placed over the
+row of barrels, as seen in the foreground. Each patron thus has allotted
+to him in his individual barrel his portion of the whey, which he is
+supposed to remove day by day. No attempt is made to clean out these
+receptacles, and the inevitable result is that they become filled with a
+foul, malodorous liquid, especially in summer. When such material is
+taken home in the same set of cans that is used to bring the fresh milk
+(twice a day as is the usual custom in Swiss factories), it is no wonder
+that this industry is seriously handicapped by "gassy" fermentations
+that injure materially the quality of the product. Not only is the above
+danger a very<span class='pagenum'><a name="Page_25" id="Page_25">[Pg 25]</a></span> considerable one, but the quality of the factory
+by-product for feeding purposes, whether it is skim-milk or whey, is
+impaired through the development of fermentative changes.</p>
+
+<div class="figcenter" style="width: 450px;">
+<img src="images/fig7.jpg" width="450" height="338" alt="Fig. 7." title="" />
+<span class="caption">Fig. 7.</span>
+</div>
+
+<div class="blockquot"><p>Swiss cheese factory (Wisconsin), showing
+careless way in which whey is handled. Each patron's share is placed in
+a barrel, from which it is removed by him. No attempt is made to cleanse
+these receptacles.</p></div>
+
+<p><b>Improved methods of disposal of by-products.</b> The difficulties which
+attend the distribution of these factory by-products have led to
+different methods of solution. One is to use another separate set of
+receptacles to carry back these products to the farm. This method has
+been tried, and while it is deemed impracticable by many to handle two
+sets of vessels, yet some of the most progressive factories report
+excellent results where this method is in use.</p>
+
+<p>Large barrels could be used for this purpose to economize in wagon
+space.</p>
+
+<p>Another method that has met with wider acceptance, especially in
+creameries, is the custom of pasteurizing or scalding the skim-milk
+immediately after it is separated, so that it is returned to the farmer
+in a hot condition. In factories where the whole milk is pasteurized,
+further treatment of the by-product is not necessary. In most factories
+steam, generally exhaust, is used directly in the milk, and experience
+has shown that such milk, without any cooling, will keep sweet for a
+considerable number of hours longer than the untreated product. It is
+noteworthy that the most advanced and progressive factories are the ones
+that appreciate the value of this work, and although it involves some
+time and expense, experience has shown the utility of the process in
+that a better grade of milk is furnished by the patrons of factories
+which follow this practice.<a name="FNanchor_4_4" id="FNanchor_4_4"></a><a href="#Footnote_4_4" class="fnanchor">[4]</a> The exclusion of all danger of animal or
+human disease is also possible in this way.</p>
+<p><span class='pagenum'><a name="Page_26" id="Page_26">[Pg 26]</a></span></p>
+<p><b>Cleaning dairy utensils.</b> The thorough cleaning of all dairy apparatus
+that in any way comes in contact with the milk is one of the most
+fundamental and important problems in dairying. All such apparatus
+should be so constructed as to permit of easy cleaning. Tinware,
+preferably of the pressed variety, gives the best surface for this
+purpose and is best suited for the handling of milk.</p>
+
+<p>Milk vessels should never be allowed to become dry when dirty, for dried
+particles of milk residue are extremely difficult to remove. In cleaning
+dairy utensils they should first be rinsed in lukewarm instead of hot
+water, so as to remove organic matter without coagulating the milk. Then
+wash thoroughly in hot water, using a good washing powder. The best
+washing powders possess considerable disinfecting action.<a name="FNanchor_5_5" id="FNanchor_5_5"></a><a href="#Footnote_5_5" class="fnanchor">[5]</a> Strong
+alkalies should not be used. After washing rinse thoroughly in clean hot
+water. If steam is available, as it always is in creameries, cans and
+pails should be turned over jet for a few moments. While a momentary
+exposure will not suffice to completely sterilize such a vessel, yet
+many bacteria are killed in even a short exposure, and the cans dry more
+thoroughly and quickly when heated by steam.</p>
+
+<p>Not only should the greatest care be paid to the condition of the cans
+and milk-pails, but all dippers, strainers, and other utensils that come
+in contact with the milk must be kept equally clean. Cloth strainers,
+unless attended to, are objectionable, for the fine mesh of the cloth
+retains so much moisture that they become a veritable hot-bed of
+bacterial life, unless they are daily boiled or steamed.</p>
+
+<p>The inability to thoroughly render vessels bacteria-free with the
+conveniences which are generally to be found on the farm has led in some
+cases to the custom of washing and sterilizing the milk cans at the
+factory.</p>
+<p><span class='pagenum'><a name="Page_27" id="Page_27">[Pg 27]</a></span></p>
+<p><b>Germ content of milk utensils.</b> Naturally the number of bacteria found in
+different milk utensils after they have received their regular cleaning
+will be subject to great fluctuations; but, nevertheless, such
+determinations are of value as giving a scientific foundation for
+practical methods of improvement. The following studies may serve to
+indicate the relative importance of the utensils as a factor in milk
+contamination.</p>
+
+<p>Two cans were taken, one of which had been cleaned in the ordinary way,
+while the other was sterilized by steaming. Before milking, the udder
+was thoroughly cleaned and special precautions taken to avoid raising of
+dust; the fore milk was rejected. Milk drawn into these two cans showed
+the following germ content:</p>
+
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="">
+<tr><td align='left'>&nbsp;</td><td align='left'>No. bacteria per cc.</td><td align='left'>Hours before souring.</td></tr>
+<tr><td align='left'>Steamed pail</td><td align='right'>165</td><td align='right'>28-1/2</td></tr>
+<tr><td align='left'>Ordinary pail</td><td align='right'>426</td><td align='right'>523</td></tr>
+</table></div>
+
+
+<p>Harrison<a name="FNanchor_6_6" id="FNanchor_6_6"></a><a href="#Footnote_6_6" class="fnanchor">[6]</a> has shown how great a variation is in the bacterial content
+in milk cans. The utensils were rinsed with 100 cc. of sterile water and
+numerical determinations of this rinsing water made. In poorly cleaned
+cans, the average germ content was 442,000; in cans washed in tepid
+water and then scalded&mdash;the best farm practice&mdash;54,000, and in cans
+carefully washed and then steamed for five minutes, 880.</p>
+
+<p>Another method used by the writer is to wash the utensil with 100 cc.
+sterile wash water, using a sterile swab to remove dirt. Then repeat the
+process twice or more with fresh rinsing waters, making plate cultures
+from each. The following data were obtained from three such
+determinations:<span class='pagenum'><a name="Page_28" id="Page_28">[Pg 28]</a></span></p>
+
+
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="">
+<tr><td colspan="4">No. bacteria in different washings.</td></tr>
+<tr><td align='center'>I.</td><td align='center'>II.</td><td align='center'>III.</td><td align='center'>Total No. bacteria.</td></tr>
+<tr><td align='right'>7,800,000</td><td align='right'>1,450,000</td><td align='right'>49,000</td><td align='right'>9,299,000</td></tr>
+<tr><td align='right'>283,000</td><td align='right'>43,400</td><td align='right'>35,000</td><td align='right'>361,400</td></tr>
+<tr><td align='right'>1,685,000</td><td align='right'>105,000</td><td align='right'>61,400</td><td align='right'>1,851,400</td></tr>
+</table></div>
+
+<p><b>Infection of milk in udder cavity.</b> A frequently neglected but
+considerable factor of infection is that which is attributable to the
+bacteria which are present in the udder and which are removed in large
+numbers during the milking process. An examination of the fore milk, i.
+e., the first few streams from each teat, and that which is subsequently
+withdrawn, generally reveals a very much larger number of organisms in
+the fore milk.<a name="FNanchor_7_7" id="FNanchor_7_7"></a><a href="#Footnote_7_7" class="fnanchor">[7]</a> Not infrequently will this part of the milk when drawn
+under as careful conditions as possible, contain several score thousand
+organisms per cc. If successive bacterial determinations are made at
+different periods of the milking, as shown in the following experiment,
+a marked diminution is to be noted after the first portion of the milk
+is removed:</p>
+
+
+<h4><i>Bacterial content at different periods of milking.</i></h4>
+
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="">
+<tr><td align='left'></td><td align='center'>Fore milk.</td><td align='center'>200th cc.</td><td align='center'>2000th cc.</td><td align='center'>4300th cc.</td><td align='center'>6500th cc.</td><td align='center'>Strippings.</td></tr>
+<tr><td align='left'>Expt. 1</td><td align='right'>6,500</td><td align='right'>1,700</td><td align='right'>475</td><td align='right'>220</td><td align='right'>75</td><td align='right'>5</td></tr>
+<tr><td align='left'>Expt. 2</td><td align='right'>8,100</td><td align='right'>1,650</td><td align='right'>400</td><td align='right'>240</td><td align='right'>50</td><td align='right'>10</td></tr>
+</table></div>
+
+
+<p>By some observers it has been claimed that it is possible to secure
+absolutely sterile milk in the strippings but this is rarely so. It is
+quite probable that such reported results are due to the fact that too
+small quantities of milk were used in the examinations and so erroneous
+conclusions were drawn. This marked diminution in numbers indicates that
+the larger proportion of the organisms found in<span class='pagenum'><a name="Page_29" id="Page_29">[Pg 29]</a></span> the fore milk are
+present in the lower portion of the udder and milk ducts. When
+consideration is given to the structure of the udder, it is readily
+apparent that infection will be greater here than above.</p>
+
+<div class="figcenter" style="width: 400px;">
+<img src="images/fig8.jpg" width="400" height="746" alt="Fig. 8." title="" />
+<span class="caption">Fig. 8.</span>
+</div>
+
+<div class="blockquot"><p>Sectional view of udder showing teat with milk
+duct connecting exterior with the milk cistern. Milk sinuses are mostly
+shown in cross section interspersed and below the secreting tissue
+(Moore and Ward).]</p></div>
+
+<p>The udder is composed of secreting tissue (<i>gland cells</i>) held in place
+by fibrous connective tissue. Ramifying throughout this glandular
+structure are numerous channels (<i>milk sinuses</i>) that serve to convey
+the milk from the cells where it is produced into the <i>milk cistern</i>, a
+common receptacle just above the teats. This cavity is connected with
+the exterior through the milk duct in the teat, which is more or less
+tightly closed by the circular sphincter muscles, thus preventing the
+milk from flowing out. The mucous membranes of the milk duct and cistern
+are naturally moist. The habits of the animal render it impossible to
+prevent infection of the<span class='pagenum'><a name="Page_30" id="Page_30">[Pg 30]</a></span> external opening at the end of the teat and
+there is no mechanical reason why bacteria cannot readily find their way
+along the moist lining membrane for some distance. If organisms are
+adapted to this kind of an environment, ideal conditions exist for their
+multiplication, as moisture, warmth and suitable food supply are
+present. The question arises how far up into this organ is penetration
+possible? Within late years numerous observations have been made on the
+presence of organisms in the upper portion of the udder in contact with
+the secreting tissue.<a name="FNanchor_8_8" id="FNanchor_8_8"></a><a href="#Footnote_8_8" class="fnanchor">[8]</a></p>
+
+<p>These investigations prove that bacteria are distributed throughout the
+whole of the udder, although numerically they are much less abundant in
+the region of the milk-secreting tissue than in the lower portion.
+Ward's conclusions are "that milk when secreted by the glands of a
+healthy udder is sterile. It may however, immediately become
+contaminated by the bacteria which are normally present in the smaller
+milk ducts of the udder."</p>
+
+<p><b>Nature of bacteria in fore milk.</b> Generally speaking the number of
+different species found in the fore milk is not large, and of those
+which do appear many occur at only occasional intervals. Moore<a name="FNanchor_9_9" id="FNanchor_9_9"></a><a href="#Footnote_9_9" class="fnanchor">[9]</a> in the
+examination of 9 udders found 20 different forms, and of these only 3
+species predominated, all of which proved to be micrococci. Streptococci
+have also been quite frequently reported. Freudenreich<a name="FNanchor_10_10" id="FNanchor_10_10"></a><a href="#Footnote_10_10" class="fnanchor">[10]</a> found the
+most common types to be cocci, belonging to both the liquefying and
+non-liquefying class.</p>
+
+<p><span class='pagenum'><a name="Page_31" id="Page_31">[Pg 31]</a></span></p><p>Peptonizing<a name="FNanchor_11_11" id="FNanchor_11_11"></a><a href="#Footnote_11_11" class="fnanchor">[11]</a> and spore-bearing<a name="FNanchor_12_12" id="FNanchor_12_12"></a><a href="#Footnote_12_12" class="fnanchor">[12]</a> species have also been reported as
+well as gas-producing<a name="FNanchor_13_13" id="FNanchor_13_13"></a><a href="#Footnote_13_13" class="fnanchor">[13]</a> forms allied to the colon bacillus. Such
+findings are, however, due in all probability to accidental invasion.
+Most investigators report the absence of the distinctively lactic-acid
+group of organisms.<a name="FNanchor_14_14" id="FNanchor_14_14"></a><a href="#Footnote_14_14" class="fnanchor">[14]</a></p>
+
+<p><b>Origin of bacteria in udder.</b> There is no question but that many of the
+types of bacteria found in the udder gain access from the outside. Those
+belonging to the spore-bearing, digesting and intestinal types have such
+a favorable opportunity for introduction from outside and are so
+unlikely to have come directly from the body of the animal, that the
+external source of infection is much more probable. Whether this
+explanation answers the origin of the cocci that are so generally found
+in the upper portion of the udder is questionable. The statement is
+ordinarily made that the inner tissues of healthy organs are
+bacteria-free, but the studies of Ford<a name="FNanchor_15_15" id="FNanchor_15_15"></a><a href="#Footnote_15_15" class="fnanchor">[15]</a> seem to indicate that 70 per
+cent. of such organs, removed under aseptic conditions from guinea pigs,
+rabbits, dogs and cats contained living organisms. Others have reported
+similar results in which cocci have been found<a name="FNanchor_16_16" id="FNanchor_16_16"></a><a href="#Footnote_16_16" class="fnanchor">[16]</a> very similar to those
+occurring in the udder. These findings increase the probability that the
+origin of this type is from the blood. The persistence<span class='pagenum'><a name="Page_32" id="Page_32">[Pg 32]</a></span> of certain
+species in the udder for months as noted by Ward indicates possibility
+of growth of some forms at least. Stocking<a name="FNanchor_17_17" id="FNanchor_17_17"></a><a href="#Footnote_17_17" class="fnanchor">[17]</a> has shown where cows are
+not milked clean that the germ content of succeeding milkings is greatly
+increased.</p>
+
+<p><b>Artificial introduction of bacteria into udder.</b> If bacteria are capable
+of actually developing in the udder proper, it ought to be possible to
+easily demonstrate this by the artificial introduction of cultures. In a
+number of cases<a name="FNanchor_18_18" id="FNanchor_18_18"></a><a href="#Footnote_18_18" class="fnanchor">[18]</a> such experiments have been made with various
+saprophytic forms, such as <i>B. prodigiosus</i>, lactic acid bacilli and
+others. In no case has it appeared evident that actual growth has
+occurred, although the introduced organism has been demonstrated in
+diminishing numbers for 5-6 days. Even the common lactic acid germ and a
+yellow liquefying coccus isolated from the fore milk failed to persist
+for more than a few days when thus artificially introduced. This failure
+to colonize is indeed curious and needs explanation. Is it due to
+unsuitable environmental conditions or attributable to the germicidal
+influence of the milk?</p>
+
+<p>Various body fluids are known to possess the property of destroying
+bacteria and it is claimed by Fokker<a name="FNanchor_19_19" id="FNanchor_19_19"></a><a href="#Footnote_19_19" class="fnanchor">[19]</a> that this same property was
+found in freshly drawn milk. This peculiarity has also been investigated
+by Freudenreich,<a name="FNanchor_20_20" id="FNanchor_20_20"></a><a href="#Footnote_20_20" class="fnanchor">[20]</a> and Hunziker<a name="FNanchor_21_21" id="FNanchor_21_21"></a><a href="#Footnote_21_21" class="fnanchor">[21]</a> who find a similar property.</p>
+
+<p>No material increase in germ content takes place in milk for several
+hours when chilled to 40&deg;-70&deg; F.; on the other<span class='pagenum'><a name="Page_33" id="Page_33">[Pg 33]</a></span> hand an actual, but
+usually not a marked decrease is observed for about 6 hours. This
+phenomenon varies with the milk of different cows. Nothing is known as
+to the cause of this apparent germicidal action. The question is yet by
+no means satisfactorily settled, although the facts on which the
+hypothesis is based are not in controversy. If such a peculiarity
+belongs to milk, it is not at all improbable that it may serve to keep
+down the germ content in the udder. Freudenreich<a name="FNanchor_22_22" id="FNanchor_22_22"></a><a href="#Footnote_22_22" class="fnanchor">[22]</a> found that udders
+which were not examined for some time after death showed abundant
+growth, which fact he attributed to the loss of this germicidal
+property.</p>
+
+<p>The infection of the whole milk can be materially reduced by rejecting
+the fore milk, but it is questionable whether such rejection is worth
+while, except in the case of "sanitary" dairies where milk is produced
+with as low a germ content as possible. The intrinsic loss in butter fat
+in the fore milk is inconsiderable as the first few streams contain only
+about one-fifth the normal fat content.</p>
+
+<p><b>Infection of milk after withdrawal from animal.</b> The germ content of the
+milk, when it is being drawn from the animal is immediately increased
+upon contact with the atmosphere. These organisms are derived from the
+surrounding air and the utensils in which the milk is received and
+stored. The number of organisms which find their way into the milk
+depends largely upon the character of the surroundings. Bacteria are so
+intimately associated with dirt, dust and filth of all kinds that
+wherever the latter are found, the former are sure to be present in
+abundance.</p>
+
+<p>The most important factors in the infection of the milk after withdrawal
+are the pollution which is directly traceable<span class='pagenum'><a name="Page_34" id="Page_34">[Pg 34]</a></span> to the animal herself and
+the condition of the milk utensils. Fortunately both of these sources of
+contamination are capable of being greatly minimized by more careful
+methods of handling.</p>
+
+<p><b>Infection directly from the cow.</b> It is a popular belief that the
+organisms found in milk are derived from the feed and water which the
+cow consumes, the same passing directly from the intestinal tract to the
+milk by the way of the blood circulation. Such a view has no foundation
+in fact as bacteria absorbed into the circulation are practically all
+destroyed in the tissues by the action of the body fluids and cells.<a name="FNanchor_23_23" id="FNanchor_23_23"></a><a href="#Footnote_23_23" class="fnanchor">[23]</a>
+While organisms cannot pass readily from the intestine to the udder, yet
+this must not be interpreted as indicating that no attention should be
+given to the bacterial character of the material consumed. The water
+supply given should be pure and wholesome and no decomposed or spoiled
+food should be used.</p>
+
+<p>The infection traceable directly to the cow is modified materially by
+the conditions under which the animal is kept and the character of the
+feed consumed. The nature of the fecal matter is in part dependent upon
+the character of the food. The more nitrogenous rations with which
+animals are now fed leads to the production of softer fecal discharges
+which is more likely to soil the coat of the animal unless care is
+taken. The same is true with animals kept on pasture in comparison with
+those fed dry fodder.</p>
+
+<p>Stall-fed animals, however, are more likely to have their flanks fouled,
+unless special attention is paid to the removal of the manure. All dairy
+stalls should be provided<span class='pagenum'><a name="Page_35" id="Page_35">[Pg 35]</a></span> with a manure drop which should be cleaned as
+frequently as circumstances will permit.</p>
+
+<div class="figcenter" style="width: 450px;">
+<img src="images/fig9.jpg" width="450" height="432" alt="Fig. 9." title="" />
+<span class="caption">Fig. 9.</span>
+</div>
+
+<div class="blockquot"><p>Showing the bacterial contamination arising from
+hair. These hairs were allowed to fall on a sterile gelatin surface. The
+adherent bacteria developed readily in this medium, and the number of
+bacteria thus introduced into the milk from these hairs can be estimated
+by the number of developing colonies.]</p></div>
+
+<p>The animal herself contributes materially to the quota of germ life
+finding its way into the milk through the dislodgment of dust and filth
+particles adhering to its hairy coat. The nature of this coat is such as
+to favor the retention of these particles. Unless care is taken the
+flanks and udder become polluted with fecal matter, which upon drying is
+displaced with every movement of the animal. Every hair or dirt particle
+so dislodged and finding its way into the milk-pail adds its quota of
+organisms to the liquid. This can be readily demonstrated by placing
+cow's hairs collected with care on the moist surface of gelatin culture
+plates. Almost invariably, bacteria will be found in considerable
+numbers adhering to such hairs as is indicated in Fig. 9. Dirt particles
+are even richer in germ life. Not only is there the dislodgment of
+hairs, epithelial scales and masses of dirt and filth, but during<span class='pagenum'><a name="Page_36" id="Page_36">[Pg 36]</a></span> the
+milking process, as at all other times, every motion of the animal is
+accompanied by a shower of <i>invisible</i> particles more or less teeming
+with bacterial life.</p>
+
+<p>The amount of actual impurities found in milk is often considerable and
+when it is remembered that about one-half of fresh manure dissolves in
+milk,<a name="FNanchor_24_24" id="FNanchor_24_24"></a><a href="#Footnote_24_24" class="fnanchor">[24]</a> and thus does not appear as sediment, the presence of this
+undissolved residue bespeaks filthy conditions as to milking. From
+actual tests made, it is computed that the city of Berlin, Germany
+consumes about 300 pounds of such dirt and filth daily. Renk has laid
+down the following rule with reference to this insoluble dirt: If a
+sample of milk shows any evidence of impurity settling on a transparent
+bottom within two hours, it should be regarded as too dirty for use.</p>
+
+<p>While the number of organisms here introduced is at all times large, the
+character of the species is of even greater import. Derived primarily
+from dirt and fecal matter, it is no wonder that such forms are able to
+produce very undesirable fermentative changes.</p>
+
+<p><b>Influence of milker.</b> The condition of the milker is not to be ignored in
+determining all possible factors of infection, for when clothed in the
+dust-laden garments that have been worn all day, a favorable opportunity
+for direct contamination is possible. The filthy practice of wetting the
+hands with milk just before milking is to be condemned. The milker's
+hands should be washed immediately before milking in clean water and
+dried. A pinch of vaseline on hands is sometimes used to obtain a firmer
+grasp and prevents the ready dislodgment of scales.<a name="FNanchor_25_25" id="FNanchor_25_25"></a><a href="#Footnote_25_25" class="fnanchor">[25]</a> It must also be<span class='pagenum'><a name="Page_37" id="Page_37">[Pg 37]</a></span>
+borne in mind that the milker may spread disease through the milk. In
+typhoid fever and diphtheria, the germs often remain in the system for
+weeks and thus make infection possible. Stocking<a name="FNanchor_26_26" id="FNanchor_26_26"></a><a href="#Footnote_26_26" class="fnanchor">[26]</a> has shown that the
+individual milker exerts a potent influence on the total germ content of
+milk, even where the procedure is quite the same. In sanitary dairies
+milkers are usually clad in white duck suits.</p>
+
+<p><b>Milking by machinery.</b> Several mechanical devices have been invented for
+milking, some of which have been tested bacteriologically as to their
+efficiency. Harrison<a name="FNanchor_27_27" id="FNanchor_27_27"></a><a href="#Footnote_27_27" class="fnanchor">[27]</a> has examined the "Thistle" machine but found a
+much higher germ content than with hand-drawn milk. The recent
+introduction of the Burrel-Lawrence-Kennedy machine has led to numerous
+tests in which very satisfactory results have been obtained. If the
+rubber parts of the milker are thoroughly cleaned and kept in lime water
+solution, they remain nearly sterile. When milk is properly handled, the
+germ content may be greatly reduced.</p>
+
+<p><b>Reduction in dirt and adherent bacteria.</b> No factor of contamination is
+so susceptible of improvement as that which relates to the reduction in
+filth and dirt which gains access during and immediately subsequent to
+the milking. The care which is taken to keep the coat of the animal
+clean by grooming lessens very much the grosser portion of such
+contamination, but with a dry, hairy coat, fine scales and dust
+particles must of necessity be dislodged.<a name="FNanchor_28_28" id="FNanchor_28_28"></a><a href="#Footnote_28_28" class="fnanchor">[28]</a> Ordinarily the patron
+thinks all evidence of such dirt is removed if the milk is strained, but
+this process only lessens the difficulty; it does not overcome it.
+Various methods are in use, the effectiveness of which is subject to<span class='pagenum'><a name="Page_38" id="Page_38">[Pg 38]</a></span>
+considerable variation. Some of these look to the elimination of the
+bacteria after they are once introduced into the milk; others to the
+prevention of infection in the first place.</p>
+
+<p><i>1. Straining the milk.</i> Most of the visible, solid particles of filth,
+such as hairs, dirt particles, etc., can be removed by simple straining,
+the time-honored process of purification. As ordinarily carried out,
+this process often contributes to instead of diminishing the germ life
+in milk. The strainer cloths unless washed and thoroughly sterilized by
+boiling harbor multitudes of organisms from day to day and may thus
+actually add to the organisms present. Various methods have been
+suggested for this simple process, but the most practical and efficient
+strainer is that made of fine wire gauze to which is added 3-4 layers of
+cheese cloth, the whole to set over the storage milk can.</p>
+
+<p><i>2. Filtration.</i> In Europe especially, the system of cleaning milk by
+filtration through sand, gravel and other substances has been quite
+extensively used. These filters are built in sections and the milk
+passes from below upward. The filtering substance is washed in hot water
+immediately after use and then steamed and finally baked. While it is
+possible to remove the solid impurities in this way, the germ content
+cannot be greatly reduced.<a name="FNanchor_29_29" id="FNanchor_29_29"></a><a href="#Footnote_29_29" class="fnanchor">[29]</a> Cellulose filters have also been
+suggested<a name="FNanchor_30_30" id="FNanchor_30_30"></a><a href="#Footnote_30_30" class="fnanchor">[30]</a> as an improvement over the sand filters. Methods of
+filtration of this character have not been used under commercial
+conditions here in this country.</p>
+
+<p><i>3. Clarification in separator.</i> Within recent years the custom has
+grown of clarifying milk or removing the visible<span class='pagenum'><a name="Page_39" id="Page_39">[Pg 39]</a></span> dirt by passing the
+milk through a centrifugal separator the cream and skim milk being
+remixed after separation. This process naturally removes the solid
+impurities as dirt, hairs, epithelial scales and cells, also some of the
+casein, making what is known as centrifuge slime. This conglomerate mass
+is incomparably rich in germ life and the natural inference would be
+that the bacterial content of the milk would be greatly reduced by this
+procedure. Eckles and Barnes<a name="FNanchor_31_31" id="FNanchor_31_31"></a><a href="#Footnote_31_31" class="fnanchor">[31]</a> noted a reduction of 37 to 56 per cent.
+of the bacteria but others have failed to observe such reductions.<a name="FNanchor_32_32" id="FNanchor_32_32"></a><a href="#Footnote_32_32" class="fnanchor">[32]</a>
+This condition is explained by the more thorough breaking up of the
+bacterial masses in the process, thus apparently not reducing them in
+numbers.</p>
+
+<p>It is somewhat surprising that in spite of the elimination of much
+organic matter and bacteria, such clarified milk sours as rapidly as the
+untreated product.<a name="FNanchor_33_33" id="FNanchor_33_33"></a><a href="#Footnote_33_33" class="fnanchor">[33]</a></p>
+
+<p>The mechanical shock of separation ruptures the clusters of fat globules
+and so delays creaming and also lessens the consistency of cream derived
+from such milk. This practical disadvantage together with the increased
+expense of the operation and the failure to materially enhance the
+keeping quality of the product outweigh the advantage which might come
+from removal of solid impurities which can be largely accomplished on
+the farm by efficient straining.</p>
+
+<p><i>4. Washing the udder.</i> If a surface is moist, bacteria adherent to it
+cannot be dislodged by ordinary movements. Thus the air over
+snow-covered mountains or oceans is<span class='pagenum'><a name="Page_40" id="Page_40">[Pg 40]</a></span> relatively devoid of germ life. The
+method of moistening the udder is applied with success to the hairy coat
+of the animal thus subserving the double purpose of cleaning the animal
+and preventing in large measure the continual dislodgment of dust
+particles. After these parts have been well carded to remove loose hairs
+and dirt particles, the skin should be thoroughly moistened with clean
+water and then wiped. It has been urged that this procedure lessens the
+yield of milk but Eckles<a name="FNanchor_34_34" id="FNanchor_34_34"></a><a href="#Footnote_34_34" class="fnanchor">[34]</a> concludes from experiments that when the
+animal becomes accustomed to this treatment, no noticeable change in
+amount of milk or butter-fat is produced.</p>
+
+<p>The effectiveness of this method in reducing the actual amount of dirt
+and filth introduced into the milk as well as the great diminution in
+germ life is shown by the instructive experiments of Fraser<a name="FNanchor_35_35" id="FNanchor_35_35"></a><a href="#Footnote_35_35" class="fnanchor">[35]</a> who
+found that the actual amount of dirt dislodged from udders of apparently
+clean animals during the milking process was three and one-half times as
+much as when the cow's udders were washed. From udders visibly polluted
+one ounce of such filth was removed in 275 pounds of milk, while from
+cows whose udders had been washed, the same amount of dirt was
+distributed through 24,030 pounds.</p>
+
+<p>Fraser observed as a result of 420 examinations that the average germ
+content of 4-inch culture dishes under clean but unwashed udders was
+578, while under washed animals it was reduced to 192. From numerous
+tests made in the writer's laboratory, it is evident that the germ
+content of the milk in the pail is increased from 20,000-40,000 bacteria
+<i>per minute</i> during the milking period. By far the<span class='pagenum'><a name="Page_41" id="Page_41">[Pg 41]</a></span> larger part of this
+pollution can be easily prevented by cleaning and dampening the udder.</p>
+
+<p><i>5. Diminishing exposed surface of pail.</i> The entrance of organisms into
+the milk can be greatly reduced by lessening the area of the milk pail
+directly exposed to the dust shower. A number of so-called sanitary or
+hygienic milk pails have been devised for this purpose. In one case the
+pail is smaller at the top than bottom, but in most of them the common
+form is kept and the exposed area is lessened by means of a cover, the
+milk being received through a narrower opening. In some cases, strainers
+are also interposed so as to remove more effectually the coarse
+particles. It is necessary to have these covers and strainers
+constructed in such a way so they can be easily removed and cleaned.</p>
+
+<div class="figcenter" style="width: 400px;">
+<img src="images/fig10.jpg" width="400" height="241" alt="Fig. 10. Sanitary milk pails designed to diminish the
+introduction of hairs, scales, dirt, etc., into milk." title="" />
+<span class="caption">Fig. 10. Sanitary milk pails designed to diminish the
+introduction of hairs, scales, dirt, etc., into milk.</span>
+</div>
+
+<p>Stocking tested one of these pails (A, Fig. 10) and found that 63 per
+cent of the dirt and 29 per cent. of the bacteria were prevented from
+passing into the milk. Eckles examined one in which the germ content was
+found to be<span class='pagenum'><a name="Page_42" id="Page_42">[Pg 42]</a></span> 3200 per cc. as against 43200 per cc. in a common open
+pail. This milk did not sour until it was 64 hours old in the first case
+while in the latter it curdled in 43 hours.</p>
+
+<p><b>Air in barn.</b> The atmosphere of the barn where the milking is done may
+frequently contribute considerable infection. Germ life is incapable of
+development in the air, but in a dried condition, organisms may retain
+their vitality for long periods. Anything which contributes to the
+production of dust in the stable and aids in the stirring up of the same
+increases the number of organisms to be found in the air (Fig. 11).
+Thus, the feeding of dry fodder and the bedding of animals with straw
+adds greatly to the germ life floating in the air. Dust may vary much in
+its germ content depending upon its origin. Fraser found the<span class='pagenum'><a name="Page_43" id="Page_43">[Pg 43]</a></span> dust from
+corn meal to contain only about one-sixth to one-eighth as much germ
+life as that from hay or bran.<a name="FNanchor_36_36" id="FNanchor_36_36"></a><a href="#Footnote_36_36" class="fnanchor">[36]</a> In time most of these dust particles
+settle to the floor, but where the herd is kept in the barn, the
+constant movement of the animals keeps these particles more or less in
+motion. Much can be done by forethought to lessen the germ content of
+stables. Feeding dry feed should not be done until after milking.<a name="FNanchor_37_37" id="FNanchor_37_37"></a><a href="#Footnote_37_37" class="fnanchor">[37]</a> In
+some of the better sanitary dairies, it is customary to have a special
+milking room that is arranged with special reference to the elimination
+of all dust. In this way this source of infection may be quite obviated
+as the air of a clean, still room is relatively free from bacteria,
+especially if the floor is moistened. It has often been noted that the
+milk of stall-fed animals does not keep as well as that milked out of
+doors, a condition in part attributable to the lessened contamination.</p>
+
+<div class="figcenter" style="width: 450px;">
+<img src="images/fig11.jpg" width="450" height="429" alt="Fig. 11." title="" />
+<span class="caption">Fig. 11.</span>
+</div>
+
+<div class="blockquot"><p>Effect of contaminated air. The number of spots
+indicate the colonies that have developed from the bacteria which fell
+in 30 seconds on the surface of the gelatin plate (3 inches in
+diameter). This exposure was made at time the cows were fed.</p></div>
+
+<p><b>Relative importance of different sources of infection.</b> It is impossible
+to measure accurately the influence of the different sources of
+infection as these are continually subject to modification in each and
+every case. As a general rule, however, where milk is drawn and handled
+without any special care, the utensils and the animal contribute the
+larger proportion of dirt and bacteria that find their way into milk.
+Where the manner of milking and handling is designed to exclude the
+largest number of organisms possible, the bacteria appearing in the fore
+milk make up the major portion remaining. By putting into practice the
+various suggestions that have been made with reference to diminishing
+the bacterial content of milk, it is possible to greatly reduce the
+number of organisms found therein, and at the same time materially
+improve the keeping<span class='pagenum'><a name="Page_44" id="Page_44">[Pg 44]</a></span> quality of the milk. Backhaus<a name="FNanchor_38_38" id="FNanchor_38_38"></a><a href="#Footnote_38_38" class="fnanchor">[38]</a> estimates that
+the germ life in milk can be easily reduced to one-two thousandth of its
+original number by using care in milking. He reports a series of
+experiments covering two years in which milk was secured that averaged
+less than 10,000 bacteria per cc., while that secured under ordinary
+conditions averaged over 500,000.</p>
+
+<div class="figcenter" style="width: 450px;">
+<img src="images/fig12.jpg" width="450" height="434" alt="Fig. 12." title="" />
+<span class="caption">Fig. 12.</span>
+</div><div class="blockquot"><p> Bacterial content of milk handled in ordinary
+way. Each spot represents a colony growing on gelatin plate. Compare
+with Fig. 13, where same quantity of milk is used in making culture.
+Over 15,000 bacteria per cc. in this milk.</p></div>
+
+<p>Fig. 13 gives an illustration as to what care in milking will do in the
+way of eliminating bacteria. Fig. 12 shows a gelatin plate seeded with
+the same quantity of milk that was used in making the culture indicated
+by Fig. 13. The first plate was inoculated with milk drawn under good
+conditions, the germ content of which was found to be<span class='pagenum'><a name="Page_45" id="Page_45">[Pg 45]</a></span> 15,500 bacteria
+per cc., while the sample secured under as nearly aseptic conditions as
+possible (Fig. 13) contained only 330 organisms in the same volume.</p>
+
+<div class="figcenter" style="width: 450px;">
+<img src="images/fig13.jpg" width="450" height="449" alt="Fig. 13." title="" />
+<span class="caption">Fig. 13.</span>
+</div><div class="blockquot"><p>Bacterial content of milk drawn with care.
+Diminished germ content is shown by smaller number of colonies (330
+bacteria per cc.). Compare this culture with that shown in Fig. 12.</p></div>
+
+<p><b>"Sanitary" or "certified" milk.</b> Within recent years there has been more
+or less generally introduced into many cities, the custom of supplying
+high grade milk that has been handled in a way so as to diminish its
+germ content as much as possible. Milk of this character is frequently
+known as "sanitary," "hygienic" or "certified," the last term being used
+in connection with a certification from veterinary authorities or boards
+of health as to the freedom of animals from contagious disease.
+Frequently a numerical bacterial standard is exacted as a pre-requisite<span class='pagenum'><a name="Page_46" id="Page_46">[Pg 46]</a></span>
+to the recommendation of the board of examining physicians. Thus, the
+Pediatric Society of Philadelphia requires all children's milk that
+receives its recommendation to have not more than 10,000 bacteria per
+cc. Such a standard has its value in the scrupulous cleanliness that
+must prevail in order to secure these results. This in itself is
+practically a guarantee of the absence of those bacteria liable to
+produce trouble in children. The number of organisms found in such milks
+is surprisingly low when compared with ordinary milk. Naturally, there
+is considerable fluctuation from day to day, and occasionally the germ
+content is increased to a high figure without any apparent reason. The
+average results though, show a greatly reduced number of organisms. De
+Schweinitz<a name="FNanchor_39_39" id="FNanchor_39_39"></a><a href="#Footnote_39_39" class="fnanchor">[39]</a> found in a Washington dairy in 113 examinations extending
+throughout a year, an average of 6,485 bacteria per cc. The daily
+analyses made of the Walker-Gordon supply sold in Philadelphia for an
+entire year, showed that the milk almost always contained less than
+5,000 bacteria per cc. and on 120 days out of the year the germ content
+was 1,000 organisms per cc. or less.</p>
+
+<p>From a practical point of view, the improvement in quality of sanitary
+milk, in comparison with the ordinary product is seen in the enhanced
+keeping quality. During the Paris Exposition in 1900, milk and cream
+from several such dairies in the United States were shipped to Paris,
+arriving in good condition after 15 to 18 days transit. When milk has
+been handled in such a way, it is evident that it is much better suited
+to serve as a food supply than where it has undergone the fermentative
+changes incident to the development of myriads of organisms.<span class='pagenum'><a name="Page_47" id="Page_47">[Pg 47]</a></span></p>
+
+<p><b>Application of foregoing precautions to all milk producers.</b> Milk is so
+susceptible to bacterial changes that it is necessary to protect it from
+invasion, if its original purity is to be maintained, and yet, from a
+practical point of view, the use to which it is destined has much to do
+with the care necessary to take in handling. The effect of the bacterial
+contamination of milk depends largely upon the way in which the product
+is used. To the milk-man engaged in the distribution of milk for direct
+consumption, all bacterial life is more or less of a detriment, while to
+the butter-maker and cheese-maker some forms are a direct necessity. It
+is unnecessary and impracticable to require the same degree of care in
+handling milk destined to be worked up into factory products as is done,
+for instance, in sanitary milk supplies, but this fact should not be
+interpreted to mean that the care of milk for factories is a matter of
+small consequence. In fact no more important dairy problem exists, and
+the purer and better the quality of the raw material the better the
+product will be. Particularly is this true with reference to
+cheese-making.</p>
+
+<p>Dairymen have learned many lessons in the severe school of experience,
+but it is earnestly to be hoped that future conditions will not be
+summed up in the words of the eminent German dairy scientist, Prof.
+Fleischmann, when he says that "all the results of scientific
+investigation which have found such great practical application in the
+treatment of disease, in disinfection, and in the preservation of
+various products, are almost entirely ignored in milking."</p>
+
+<p><b>Growth of bacteria in milk.</b> Milk is so well suited as a medium for the
+development of germ life that it might be<span class='pagenum'><a name="Page_48" id="Page_48">[Pg 48]</a></span> expected that all
+micro&ouml;rganisms would develop rapidly therein, and yet, as a matter of
+fact, growth does not begin at once, even though the milk may be richly
+seeded. At ordinary temperatures, such as 70&deg; F., no appreciable
+increase is to be noted for a period of 6-9 hours; at lower temperatures
+(54&deg; F.) this period is prolonged to 30-40 hours or even longer. After
+this period has elapsed, active growth begins and continues more or less
+rapidly until after curdling.</p>
+
+<p>The cause of this suspended development is attributed to the germicidal
+properties inherent to the milk.<a name="FNanchor_40_40" id="FNanchor_40_40"></a><a href="#Footnote_40_40" class="fnanchor">[40]</a></p>
+
+<p>Milk is of course seeded with a considerable variety of organisms at
+first. The liquefying and inert species are the most abundant, the
+distinctively lactic acid class occurring sparsely, if at all. As milk
+increases in age, germ growth begins to occur. More or less development
+of all types go on, but soon the lactic species gain the ascendency,
+owing to their being better suited to this environment; they soon
+outstrip all other species, with the result that normal curdling
+generally supervenes. The growth of this type is largely conditioned by
+the presence of the milk sugar. If the sugar is removed from milk by
+dialysis, the liquid undergoes putrefactive changes due to the fact that
+the putrefactive bacteria are able to grow if no acid is produced.</p>
+
+<p><b>Relation of temperature to growth.</b> When growth does once begin in milk,
+the temperature at which it is stored exerts the most profound effect on
+the rate of development. When milk is not artificially cooled, it
+retains its heat for some hours, and consequently the conditions become
+very<span class='pagenum'><a name="Page_49" id="Page_49">[Pg 49]</a></span> favorable for the rapid multiplication of the contained organisms,
+as is shown in following results obtained by Freudenreich<a name="FNanchor_41_41" id="FNanchor_41_41"></a><a href="#Footnote_41_41" class="fnanchor">[41]</a>:</p>
+
+
+<h4><i>No. of bacteria per cc. in milk kept at different temperatures.</i></h4>
+
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="">
+<tr><td align='left'>&nbsp;</td><td align='left'>&nbsp;</td><td align='left'>&nbsp;</td><td align='left'>&nbsp;</td><td align='center'>77&deg; F.</td><td align='center'>95&deg; F.</td></tr>
+<tr><td align='left'>5</td><td align='left'>hrs.</td><td align='left'>after</td><td align='left'>milking</td><td align='right'>10,000</td><td align='right'>30,000</td></tr>
+<tr><td align='left'>8</td><td align='center'>"</td><td align='center'>"</td><td align='center'>"</td><td align='right'>25,000</td><td align='right'>12,000,000</td></tr>
+<tr><td align='left'>12</td><td align='center'>"</td><td align='center'>"</td><td align='center'>"</td><td align='right'>46,000</td><td align='right'>35,280,000</td></tr>
+<tr><td align='left'>26</td><td align='center'>"</td><td align='center'>"</td><td align='center'>"</td><td align='right'>5,700,000</td><td align='right'>50,000,000</td></tr>
+</table></div>
+<p><br /><br /><br /></p>
+
+
+<div class="figcenter" style="width: 400px;">
+<img src="images/fig14.jpg" width="400" height="196" alt="Fig. 14. Effect of cooling milk on the growth of
+bacteria." title="" />
+<span class="caption">Fig. 14. Effect of cooling milk on the growth of
+bacteria.</span>
+</div>
+
+<p>Conn<a name="FNanchor_42_42" id="FNanchor_42_42"></a><a href="#Footnote_42_42" class="fnanchor">[42]</a> is inclined to regard temperature of more significance in
+determining the keeping quality than the original infection of the milk
+itself. Milk which curdled in 18 hours at 98&deg; F., did not curdle in 48
+hours at 70&deg;, and often did not change in two weeks, if the temperature
+was kept at 50&deg; F.</p>
+
+<p>Where kept for a considerable period at this low temperature, the milk
+becomes filled with bacteria of the undesirable putrefactive type, the
+lactic group being unable to form acid in any appreciable amounts.
+Running well<span class='pagenum'><a name="Page_50" id="Page_50">[Pg 50]</a></span> water can be used for cooling, if it is possible to secure
+it at a temperature of 48&deg;-50&deg; F. The use of ice, of course, gives
+better results, and in summer is greatly to be desired. The influence of
+these lowered temperatures makes it possible to ship milk long
+distances<a name="FNanchor_43_43" id="FNanchor_43_43"></a><a href="#Footnote_43_43" class="fnanchor">[43]</a> by rail for city supplies, if the temperature is kept low
+during transit.</p>
+
+<p><b>Mixing night and morning milk.</b> Not infrequently it happens when old milk
+is mixed with new, that the course of the fermentative changes is more
+rapid than would have been the case if the two milks had been kept
+apart. Thus, adding the cooled night milk to the warm morning milk
+sometimes produces more rapid changes in both. The explanation for this
+often imperfectly understood phenomenon is that germ growth may have
+gone on in the cooled milk, and when this material is added to the
+warmer, but bacteria-poor, fresh milk, the temperature of the whole mass
+is raised to a point suitable for the more rapid growth of all bacteria
+than would have occurred if the older milk had been kept chilled.</p>
+
+<p><b>Number of bacteria in milk.</b> The number of organisms found in milk
+depends upon (1) the original amount of contamination, (2) the age of
+the milk, and (3) the temperature at which it has been held. These
+factors all fluctuate greatly in different cases; consequently, the germ
+life is subject to exceedingly wide variations. Here in America, milk
+reaches the consumer with less bacteria than in Europe, although it may
+often be older. This is due largely to the more wide-spread use of ice
+for chilling the milk <i>en route</i> to market. Examinations have been made
+of various supplies with the following results: Sedgwick and Batchelder<span class='pagenum'><a name="Page_51" id="Page_51">[Pg 51]</a></span>
+found in 57 tests of Boston milk from 30,000-4,220,000 per cc. Jordan
+and Heineman found 30% of samples of Chicago milk to range from 100,000
+to 1,000,000 while nearly one half were from 1-20,000,000 per cc. The
+germ content of city milks increase rapidly in the summer months.
+Park<a name="FNanchor_44_44" id="FNanchor_44_44"></a><a href="#Footnote_44_44" class="fnanchor">[44]</a> found 250,000 organisms per cc. in winter, about 1,000,000 in
+cool weather and 5,000,000 per cc. in hot summer weather. Knox and
+Bassett in Baltimore report 1,500,000 in spring and nearly 4,500,000 in
+summer. Eckles<a name="FNanchor_45_45" id="FNanchor_45_45"></a><a href="#Footnote_45_45" class="fnanchor">[45]</a> studied milk under factory conditions. He finds from
+1,000,000 to 5,000,000 per cc. in winter, and in summer from 15-30
+millions.</p>
+
+<p><b>Bacterial standards for city supplies.</b> It would be very desirable to
+have a hygienic standard for city milk supplies, as there is a butter
+fat and milk-solid test, but the wide spread variation in germ content
+and the impracticability of utilizing ordinary bacterial determinations
+(on account of time required) makes the selection of such a standard
+difficult. Some hold, as Park, that such a standard is feasible. The New
+York City Milk commission has set a standard of 30,000 bacteria per cc.
+for their certified milk and 100,000 per cc. for inspected milk.
+Rochester, N. Y. has attempted the enforcement of such a standard
+(limit, 100,000 per cc.) with good results it is claimed while Boston
+has placed the legal limit at 500,000 per cc. Quantitative standards
+would seem more applicable to "certified" or sanitary supplies than to
+general city supplies, where the wide range in conditions lead to such
+enormous variations that the bacterial standard seems too refined a
+method for practical routine inspection.<span class='pagenum'><a name="Page_52" id="Page_52">[Pg 52]</a></span></p>
+
+<p><b>Other tests.</b> Any test to be of much service must be capable of being
+quickly applied. The writer believes for city milk inspectors that the
+acid test would serve a very useful purpose. This test measures the
+acidity of the milk. There is, of course, no close and direct
+relationship between the development of acidity and the growth of
+bacteria, yet in a general way one follows the other at normal
+temperatures. Where the temperature is kept rather low, bacterial growth
+might go on without much acid development, but in the great majority of
+cases a high degree of acidity means either old milk, in which there has
+been a long period of incubation, or high temperature, where rapid
+bacterial growth has been possible. Either of these conditions
+encourages germ growth and thus impairs the quality of the milk.</p>
+
+<p>The rapid determination of acidity may be made in an approximate manner
+so as to serve as a test at the weigh-can or intake. The test is best
+made by the use of the well known alkaline tablet which is composed of a
+solid alkali, and the indicator, phenolphthalein. The tablets are
+dissolved in water, one to each ounce used. A number of white cups are
+filled with the proper quantity of the solution necessary to neutralize
+say, 0.2 per cent. lactic acid. Then, as the milk is delivered, the
+proper quantity is taken from each can to which is added the tablet
+solution. A retention of the pink color shows that there is not
+sufficient acid in the milk to neutralize the alkali used; a
+disappearance of color indicates an excess of acid. The standard
+selected is of course arbitrarily chosen. In our experience, 0.2 per
+cent. acidity (figured as lactic), has proven a satisfactory point. With
+carefully handled milk the acidity ought to be reduced to about 0.15 per
+cent.<span class='pagenum'><a name="Page_53" id="Page_53">[Pg 53]</a></span> The acidity of the milk may be abnormally reduced if milk is kept
+in rusty cans, owing to action of acid on the metal.</p>
+
+<div class="figcenter" style="width: 400px;">
+<img src="images/fig15.jpg" width="400" height="318" alt="Fig. 15." title="" />
+<span class="caption">Fig. 15.</span>
+</div><div class="blockquot"><p> Apparatus used in making rapid acid test. A
+definite quantity of the alkali solution and indicator is placed in the
+white tea cup. To this is added the quantity of milk by means of the
+cartridge measure which would just be neutralized if the acidity was 0.2
+per cent. A retention of the pink color shows a low acid milk; its
+disappearance, a high acid milk.</p></div>
+
+<p><b>Kinds of bacteria in milk.</b> The number of bacteria in milk is not of so
+much consequence as the kinds present. With reference to the number of
+different species, the more dirt and foreign matter the milk contains,
+the larger the number of varieties found in the same. While milk may
+contain forms that are injurious to man, still the great majority of
+them have no apparent effect on human health.<span class='pagenum'><a name="Page_54" id="Page_54">[Pg 54]</a></span> In their effect on milk,
+the case is much different. Depending upon their action in milk, they
+may be grouped into three classes:</p>
+
+<p>1. Inert group, those producing no visible change in the milk.</p>
+
+<p>2. Sour milk forms, those breaking up the milk sugar with or without the
+formation of gas.</p>
+
+<p>3. Digesting or peptonizing group, those capable of rendering the casein
+of milk soluble and more or less completely digested.</p>
+
+<p>A surprisingly large number of bacteria that are found in milk belong to
+the first class. Undoubtedly they affect the chemical characteristics of
+the milk somewhat, but not to the extent that it becomes physically
+perceptible. Eckles<a name="FNanchor_46_46" id="FNanchor_46_46"></a><a href="#Footnote_46_46" class="fnanchor">[46]</a> reports in a creamery supply from 20 to 55 per
+cent. of entire flora as included in this class.</p>
+
+<p>By far the most important group is that embraced under the second head.
+It includes not only the true lactic acid types in which no gas is
+formed, but those species capable of producing gases and various kinds
+of acids. These organisms are the distinctively milk bacteria, although
+they do not predominate when the milk is first drawn. Their adaptation
+to this medium is normally shown, however, by this extremely rapid
+growth, in which they soon gain the ascendency over all other species
+present. It is to this lactic acid class that the favorable
+flavor-producing organisms belong which are concerned in butter-making.
+They are also indispensable in cheese-making.</p>
+
+<p>The third class represents those capable of producing a liquefied or
+digested condition on gelatin or in milk. They<span class='pagenum'><a name="Page_55" id="Page_55">[Pg 55]</a></span> are usually the
+spore-bearing species which gain access from filth and dirt. Their high
+powers of resistance due to spores makes it difficult to eradicate this
+type, although they are materially held in subjection by the lactic
+bacteria. The number of different kinds that have been found in milk is
+quite considerable, something over 200 species having been described
+more or less thoroughly. In all probability, however, many of these
+forms will be found to be identical when they are subjected to a more
+critical study.</p>
+
+<p><b>Direct absorption of taints.</b> A tainted condition in milk may result from
+the development of bacteria, acting upon various constituents of the
+milk, and transforming these in such a way as to produce by-products
+that impair the flavor or appearance of the liquid; or it may be
+produced by the milk being brought in contact with any odoriferous or
+aromatic substance, under conditions that permit of the direct
+absorption of such odors.</p>
+
+<p>This latter class of taints is entirely independent of bacterial action,
+and is largely attributable to the physical property which milk
+possesses of being able to absorb volatile odors, the fat in particular,
+having a great affinity for many of these substances. This direct
+absorption may occur before the milk is withdrawn from the animal, or
+afterwards if exposed to strong odors.</p>
+
+<p>It is not uncommon for the milk of animals advanced in lactation to have
+a more or less strongly marked odor and taste; sometimes this is apt to
+be bitter, at other times salty to the taste. It is a defect that is
+peculiar to individual animals and is liable to recur at approximately
+the same period in lactation.<span class='pagenum'><a name="Page_56" id="Page_56">[Pg 56]</a></span></p>
+
+<p>The peculiar "cowy" or "animal odor" of fresh milk is an inherent
+peculiarity that is due to the direct absorption of volatile elements
+from the animal herself. This condition is very much exaggerated when
+the animal consumes strong-flavored substances as garlic, leeks, turnips
+and cabbage. The volatile substances that give to these vegetables their
+characteristic odor are quickly diffused through the system, and if such
+foods are consumed some few hours before milking, the odor in the milk
+will be most pronounced. The intensity of such taints is diminished
+greatly and often wholly disappears, if the milking is not done for some
+hours (8-12) after such foods are consumed.</p>
+
+<p>This same principle applies in lesser degree to many green fodders that
+are more suitable as feed for animals, as silage, green rye, rape, etc.
+Not infrequently, such fodders as these produce so strong a taint in
+milk as to render it useless for human use. Troubles from such sources
+could be entirely obviated by feeding limited quantities of such
+material immediately after milking. Under such conditions the taint
+produced is usually eliminated before the next milking. The milk of
+swill-fed cows is said to possess a peculiar taste, and the urine of
+animals fed on this food is said to be abnormally acid. Brewers' grains
+and distillery slops when fed in excess also induce a similar condition
+in the milk.</p>
+
+<p>Milk may also acquire other than volatile substances directly from the
+animal, as in cases where drugs, as belladonna, castor oil, sulfur,
+turpentine, jalap, croton oil, and many others have been used as
+medicine. Such mineral poisons as arsenic have been known to appear
+eight hours after ingestion, and persist for a period of three weeks
+before being eliminated.<span class='pagenum'><a name="Page_57" id="Page_57">[Pg 57]</a></span></p>
+
+<p><b>Absorption of odors after milking.</b> If milk is brought in contact with
+strong odors after being drawn from the animal, it will absorb them
+readily, as in the barn, where frequently it is exposed to the odor of
+manure and other fermenting organic matter.</p>
+
+<p>It has long been a popular belief that milk evolves odors and cannot
+absorb them so long as it is warmer than the surrounding air, but from
+experimental evidence, the writer<a name="FNanchor_47_47" id="FNanchor_47_47"></a><a href="#Footnote_47_47" class="fnanchor">[47]</a> has definitely shown that the
+direct absorption of odors takes place much more rapidly when the milk
+is warm than when cold, although under either condition, it absorbs
+volatile substances with considerable avidity. In this test fresh milk
+was exposed to an atmosphere impregnated with odors of various essential
+oils and other odor-bearing substances. Under these conditions, the
+cooler milk was tainted very much less than the milk at body temperature
+even where the period of exposure was brief. It is therefore evident
+that an exposure in the cow barn where the volatile emanations from the
+animals themselves and their excreta taint the air will often result in
+the absorption of these odors by the milk to such an extent as to
+seriously affect the flavor.</p>
+
+<p>The custom of straining the milk in the barn has long been deprecated as
+inconsistent with proper dairy practice, and in the light of the above
+experiments, an additional reason is evident why this should not be
+done.</p>
+
+<p>Even after milk is thoroughly cooled, it may absorb odors as seen where
+the same is stored in a refrigerator with certain fruits, meats, fish,
+etc.</p>
+
+<p><b>Distinguishing bacterial from non-bacterial taints.</b> In perfectly fresh
+milk, it is relatively easy to distinguish between<span class='pagenum'><a name="Page_58" id="Page_58">[Pg 58]</a></span> taints caused by the
+growth of bacteria and those attributable to direct absorption.</p>
+
+<p>If the taint is evident at time of milking, it is in all probability due
+to character of feed consumed, or possibly to medicines. If, however,
+the intensity of the taint grows more pronounced as the milk becomes
+older, then it is probably due to living organisms, which require a
+certain period of incubation before their fermentative properties are
+most evident.</p>
+
+<p>Moreover, if the difficulty is of bacterial origin, it can be frequently
+transferred to another lot of milk (heated or sterilized is preferable)
+by inoculating same with some of the original milk. Not all abnormal
+fermentations are able though to compete with the lactic acid bacteria,
+and hence outbreaks of this sort soon die out by the re-establishment of
+more normal conditions.</p>
+
+<p><b>Treatment of directly absorbed taints.</b> Much can be done to overcome
+taints of this nature by exercising greater care in regard to the feed
+of animals, and especially as to the time of feeding and milking. But
+with milk already tainted, it is often possible to materially improve
+its condition. Thorough aeration has been frequently recommended, but
+most satisfactory results have been obtained where a combined process of
+aeration and pasteurization was resorted to. Where the milk is used in
+making butter, the difficulty has been successfully met by washing the
+cream with twice its volume of hot water in which a little saltpeter has
+been dissolved (one teaspoonful per gallon), and then separating it
+again.<a name="FNanchor_48_48" id="FNanchor_48_48"></a><a href="#Footnote_48_48" class="fnanchor">[48]</a></p>
+
+<p>The treatment of abnormal conditions due to bacteria has been given
+already under the respective sources of infection,<span class='pagenum'><a name="Page_59" id="Page_59">[Pg 59]</a></span> and is also still
+further amplified in following chapter.</p>
+
+<p><b>Aeration.</b> It is a common belief that aeration is of great aid in
+improving the quality of milk, yet, when closely studied, no material
+improvement can be determined, either where the milk is made into butter
+or sold as milk. Dean in Canada and Storch in Denmark have both
+experimented on the influence of aeration in butter making, but with
+negative results. Marshall and Doane failed to observe any material
+improvement in keeping quality, but it is true that odors are eliminated
+from the milk during aeration. The infection of the milk during aeration
+often more than counterbalances the reputed advantage. Especially is
+this so if the aeration is carried out in an atmosphere that is not
+perfectly clean and pure.</p>
+
+<p>In practice aeration differs greatly. In some cases, air is forced into
+the milk; in others, the milk is allowed to distribute itself in a thin
+sheet over a broad surface and fall some distance so that it is brought
+intimately in contact with the air. This latter process is certainly
+much more effective if carried out under conditions which preclude
+infection. It must be remembered that aeration is frequently combined
+with cooling, in which case the reputed advantages may not be entirely
+attributable to the process of aeration.</p>
+
+<p><b>Infection of milk in the factory.</b> The problem of proper handling of milk
+is not entirely solved when the milk is delivered to the factory or
+creamery, although it might be said that the danger of infection is much
+greater while the milk is on the farm.</p>
+
+<p>In the factory, infection can be minimized because effective measures of
+cleanliness can be more easily applied.<span class='pagenum'><a name="Page_60" id="Page_60">[Pg 60]</a></span> Steam is available in most
+cases, so that all vats, cans, churns and pails can be thoroughly
+scalded. Special emphasis should be given to the matter of cleaning
+pumps and pipes. The difficulty of keeping these utensils clean often
+leads to neglect and subsequent infection. In Swiss cheese factories,
+the custom of using home-made rennet solutions is responsible for
+considerable factory infection. Natural rennets are soaked in whey which
+is kept warm in order to extract the rennet ferment. This solution when
+used for curdling the milk often adds undesirable yeasts and other
+gas-generating organisms, which are later the cause of abnormal ferment
+action in the cheese (See page 186).</p>
+
+<p>The influence of the air on the germ content of the milk is, as a rule,
+overestimated. If the air is quiet, and free from dust, the amount of
+germ life in the same is not relatively large. In a creamery or factory,
+infection from this source ought to be much reduced, for the reason that
+the floors and wall are, as a rule, quite damp, and hence germ life
+cannot easily be dislodged. The majority of organisms found under such
+conditions come from the person of the operators and attendants. Any
+infection can easily be prevented by having the ripening cream-vats
+covered with a canvas cloth. The clothing of the operator should be
+different from the ordinary wearing-apparel. If made of white duck, the
+presence of dirt is more quickly recognized, and greater care will
+therefore be taken than if ordinary clothes are worn.</p>
+
+<p>The surroundings of the factory have much to do with the danger of germ
+infection. Many factories are poorly constructed and the drainage is
+poor, so that filth and slime collect about and especially under the
+factory. The emanations from these give the peculiar "factory odor" that
+indicates fermenting matter. Not only are these odors<span class='pagenum'><a name="Page_61" id="Page_61">[Pg 61]</a></span> absorbed
+directly, but germ life from the same is apt to find its way into the
+milk. Connell<a name="FNanchor_49_49" id="FNanchor_49_49"></a><a href="#Footnote_49_49" class="fnanchor">[49]</a> has recently reported a serious defect in cheese that
+was traced to germ infection from defective factory drains.</p>
+
+<p>The water supply of a factory is also a question of prime importance.
+When taken from a shallow well, especially if surface drainage from the
+factory is possible, the water may be contaminated to such an extent as
+to introduce undesirable bacteria in such numbers that the normal course
+of fermentation may be changed. The quality of the water, aside from
+flavor, can be best determined by making a curd test (p. 76) which is
+done by adding some of the water to boiled milk and incubating the same.
+If "gassy" fermentations occur, it signifies an abnormal condition. In
+deep wells, pumped as thoroughly as is generally the case with factory
+wells, the germ content should be very low, ranging from a few score to
+a few hundred bacteria per cc. at most.</p>
+
+<p>Harrison<a name="FNanchor_50_50" id="FNanchor_50_50"></a><a href="#Footnote_50_50" class="fnanchor">[50]</a> has recently traced an off-flavor in cheese in a Canadian
+factory to an infection arising from the water-supply. He found the same
+germ in both water and cheese and by inoculating a culture into
+pasteurized milk succeeded in producing the undesirable flavor. The
+danger from ice is much less, for the reason that good dairy practice
+does not sanction using ice directly in contact with milk or cream.
+Then, too, ice is largely purified in the process of freezing, although
+if secured from a polluted source, reliance should not be placed in the
+method of purification; for even freezing does not destroy all
+vegetating bacteria.</p>
+
+<div class="footnotes"><h3>FOOTNOTES:</h3>
+
+<div class="footnote"><p><a name="Footnote_1_1" id="Footnote_1_1"></a><a href="#FNanchor_1_1"><span class="label">[1]</span></a> Olson. 24 Rept. Wis. Expt. Stat., 1907.</p></div>
+
+<div class="footnote"><p><a name="Footnote_2_2" id="Footnote_2_2"></a><a href="#FNanchor_2_2"><span class="label">[2]</span></a> Erf and Melick Bull. 131, Kan. Expt. Stat., Apr. 1905.</p></div>
+
+<div class="footnote"><p><a name="Footnote_3_3" id="Footnote_3_3"></a><a href="#FNanchor_3_3"><span class="label">[3]</span></a> Storch (40 Rept. Danish Expt. Stat., Copenhagen, 1898) has
+devised a test whereby it can be determined whether this treatment has
+been carried out or not: Milk contains a soluble enzym known as
+galactase which has the property of decomposing hydrogen peroxid. If
+milk is heated to 176&deg; F. (80&deg; C.) or above, this enzym is destroyed so
+that the above reaction no longer takes place. If potassium iodid and
+starch are added to unheated milk and the same treated with hydrogen
+peroxid, the decomposition of the latter agent releases oxygen which
+acts on the potassium salt, which in turn gives off free iodine that
+turns the starch blue.</p></div>
+
+<div class="footnote"><p><a name="Footnote_4_4" id="Footnote_4_4"></a><a href="#FNanchor_4_4"><span class="label">[4]</span></a> McKay, N. Y. Prod. Rev., Mch. 22, 1899.</p></div>
+
+<div class="footnote"><p><a name="Footnote_5_5" id="Footnote_5_5"></a><a href="#FNanchor_5_5"><span class="label">[5]</span></a> Doane, Bull. 79, Md. Expt. Stat., Jan. 1902.</p></div>
+
+<div class="footnote"><p><a name="Footnote_6_6" id="Footnote_6_6"></a><a href="#FNanchor_6_6"><span class="label">[6]</span></a> Harrison, 22 Rept. Ont. Agr'l Coll., 1896, p. 113.</p></div>
+
+<div class="footnote"><p><a name="Footnote_7_7" id="Footnote_7_7"></a><a href="#FNanchor_7_7"><span class="label">[7]</span></a> Moore and Ward, Bull. 158, Cornell Expt. Stat., Jan. 1899;
+Ward, Bull. 178, Cornell Expt. Stat., Jan. 1900.</p></div>
+
+<div class="footnote"><p><a name="Footnote_8_8" id="Footnote_8_8"></a><a href="#FNanchor_8_8"><span class="label">[8]</span></a> Harrison, 22 Rept. Ont. Agr. Coll., 1896, p. 108; Moore, 12
+Rept. Bur. Animal Ind., U. S. Dept. Ag., 1895-6, p. 261.</p></div>
+
+<div class="footnote"><p><a name="Footnote_9_9" id="Footnote_9_9"></a><a href="#FNanchor_9_9"><span class="label">[9]</span></a> Moore, Bacteria in Milk, N. Y. Dept. Ag., 1902.</p></div>
+
+<div class="footnote"><p><a name="Footnote_10_10" id="Footnote_10_10"></a><a href="#FNanchor_10_10"><span class="label">[10]</span></a> Freudenreich, Cent. f. Bakt., II Abt., 10: 418, 1903.</p></div>
+
+<div class="footnote"><p><a name="Footnote_11_11" id="Footnote_11_11"></a><a href="#FNanchor_11_11"><span class="label">[11]</span></a> Harrison, 22 Rept. Ont. Agr. Coll., 1896, p. 108.</p></div>
+
+<div class="footnote"><p><a name="Footnote_12_12" id="Footnote_12_12"></a><a href="#FNanchor_12_12"><span class="label">[12]</span></a> Marshall, Bull. 147, Mich. Expt. Stat., p. 42.</p></div>
+
+<div class="footnote"><p><a name="Footnote_13_13" id="Footnote_13_13"></a><a href="#FNanchor_13_13"><span class="label">[13]</span></a> Moore and Ward, Bull. 158, Cornell Expt. Stat., Jan.
+1899.</p></div>
+
+<div class="footnote"><p><a name="Footnote_14_14" id="Footnote_14_14"></a><a href="#FNanchor_14_14"><span class="label">[14]</span></a> Burr, R. H. Cent. f. Bakt., II Abt., 8: 236, 1902.
+Freudenreich, l. c. p. 418. Ward, Bull. 178, Cornell Expt. Stat., p.
+277. Bolley (Cent. f. Bakt., II Abt., 1: 795, 1895), in 30 experiments
+found 12 out of 16 species to belong to lactic class. Harrison (Trans.
+Can. Inst., 7: 474, 1902-3) records the lactic type as most commonly
+present.</p></div>
+
+<div class="footnote"><p><a name="Footnote_15_15" id="Footnote_15_15"></a><a href="#FNanchor_15_15"><span class="label">[15]</span></a> Ford, Journ. of Hyg., 1901, 1: 277.</p></div>
+
+<div class="footnote"><p><a name="Footnote_16_16" id="Footnote_16_16"></a><a href="#FNanchor_16_16"><span class="label">[16]</span></a> Freudenreich, l. c. p. 421.</p></div>
+
+<div class="footnote"><p><a name="Footnote_17_17" id="Footnote_17_17"></a><a href="#FNanchor_17_17"><span class="label">[17]</span></a> Stocking, Bull. 42, Storrs Expt. Stat., June, 1906.</p></div>
+
+<div class="footnote"><p><a name="Footnote_18_18" id="Footnote_18_18"></a><a href="#FNanchor_18_18"><span class="label">[18]</span></a> Dinwiddie, Bull, 45 Ark. Expt. Stat., p. 57. Ward, Journ.
+Appld. Mic. 1: 205, 1898. Appel, Milch Zeit., No. 17, 1900. Harrison and
+Cumming, Journ. Appld. Mic. 5: 2087. Russell and Hastings, 21 Rept. Wis.
+Expt. Stat., 158, 1904.</p></div>
+
+<div class="footnote"><p><a name="Footnote_19_19" id="Footnote_19_19"></a><a href="#FNanchor_19_19"><span class="label">[19]</span></a> Fokker, Zeit. f. Hyg., 9: 41, 1890.</p></div>
+
+<div class="footnote"><p><a name="Footnote_20_20" id="Footnote_20_20"></a><a href="#FNanchor_20_20"><span class="label">[20]</span></a> Freudenreich, Ann. de Microg., 3: 118, 1891.</p></div>
+
+<div class="footnote"><p><a name="Footnote_21_21" id="Footnote_21_21"></a><a href="#FNanchor_21_21"><span class="label">[21]</span></a> Hunziker, Bull. 197, Cornell Expt. Stat., Dec. 1901.</p></div>
+
+<div class="footnote"><p><a name="Footnote_22_22" id="Footnote_22_22"></a><a href="#FNanchor_22_22"><span class="label">[22]</span></a> Freudenreich, Cent. f. Bakt., II Abt., 10: 417, 1903.</p></div>
+
+<div class="footnote"><p><a name="Footnote_23_23" id="Footnote_23_23"></a><a href="#FNanchor_23_23"><span class="label">[23]</span></a> This general statement is in the main correct, although
+Ford (Journ. of Hyg., 1: 277, 1901) claims to have found organisms
+sparingly present in healthy tissues.</p></div>
+
+<div class="footnote"><p><a name="Footnote_24_24" id="Footnote_24_24"></a><a href="#FNanchor_24_24"><span class="label">[24]</span></a> Backhaus, Milch Zeit., 26: 357, 1897.</p></div>
+
+<div class="footnote"><p><a name="Footnote_25_25" id="Footnote_25_25"></a><a href="#FNanchor_25_25"><span class="label">[25]</span></a> Freudenreich, Die Bakteriologie, p. 30.</p></div>
+
+<div class="footnote"><p><a name="Footnote_26_26" id="Footnote_26_26"></a><a href="#FNanchor_26_26"><span class="label">[26]</span></a> Stocking, Bull. 42, Storrs Expt. Stat., June 1906.</p></div>
+
+<div class="footnote"><p><a name="Footnote_27_27" id="Footnote_27_27"></a><a href="#FNanchor_27_27"><span class="label">[27]</span></a> Harrison, Cent. f. Bakt., II Abt., 5: 183, 1899.</p></div>
+
+<div class="footnote"><p><a name="Footnote_28_28" id="Footnote_28_28"></a><a href="#FNanchor_28_28"><span class="label">[28]</span></a> Drysdale, Trans. High. and Agr. Soc. Scotland. 5 Series,
+10: 166, 1898.</p></div>
+
+<div class="footnote"><p><a name="Footnote_29_29" id="Footnote_29_29"></a><a href="#FNanchor_29_29"><span class="label">[29]</span></a> Schuppan, (Cent. f. Bakt., 13: 155, 1893) claims to have
+found a reduction of 48 per cent. in the Copenhagen filters while in the
+more extended work of Dunbar and Kister (Milch Zeit., pp. 753, 787,
+1899) the bacterial content was higher in the filtered milk in 17 cases
+out of 22.</p></div>
+
+<div class="footnote"><p><a name="Footnote_30_30" id="Footnote_30_30"></a><a href="#FNanchor_30_30"><span class="label">[30]</span></a> Backhaus and Cronheim, Journ. f. Landw., 45: 222, 1897.</p></div>
+
+<div class="footnote"><p><a name="Footnote_31_31" id="Footnote_31_31"></a><a href="#FNanchor_31_31"><span class="label">[31]</span></a> Eckles and Barnes, Bull. 159 Iowa Expt. Stat., Aug. 1901.</p></div>
+
+<div class="footnote"><p><a name="Footnote_32_32" id="Footnote_32_32"></a><a href="#FNanchor_32_32"><span class="label">[32]</span></a> Dunbar and Kister, Milch Zeit., p. 753, 1899. Harrison and
+Streit, Trans. Can. Inst., 7: 488, 1902-3.</p></div>
+
+<div class="footnote"><p><a name="Footnote_33_33" id="Footnote_33_33"></a><a href="#FNanchor_33_33"><span class="label">[33]</span></a> Doane, Bull. 88 Md. Expt. Stat., May 1903.</p></div>
+
+<div class="footnote"><p><a name="Footnote_34_34" id="Footnote_34_34"></a><a href="#FNanchor_34_34"><span class="label">[34]</span></a> Eckles, Hoard's Dairyman, July 8, 1898.</p></div>
+
+<div class="footnote"><p><a name="Footnote_35_35" id="Footnote_35_35"></a><a href="#FNanchor_35_35"><span class="label">[35]</span></a> Fraser, Bull. 91, Ill. Expt. Stat.</p></div>
+
+<div class="footnote"><p><a name="Footnote_36_36" id="Footnote_36_36"></a><a href="#FNanchor_36_36"><span class="label">[36]</span></a> Fraser, Bull. 91, Ill. Expt. Stat., Dec. 1903.</p></div>
+
+<div class="footnote"><p><a name="Footnote_37_37" id="Footnote_37_37"></a><a href="#FNanchor_37_37"><span class="label">[37]</span></a> Stocking, Bull. 42, Storrs Expt. Stat., June, 1906.</p></div>
+
+<div class="footnote"><p><a name="Footnote_38_38" id="Footnote_38_38"></a><a href="#FNanchor_38_38"><span class="label">[38]</span></a> Backhaus. Ber. Landw. Inst. Univ. K&ouml;nigsberg 2: 12, 1897.</p></div>
+
+<div class="footnote"><p><a name="Footnote_39_39" id="Footnote_39_39"></a><a href="#FNanchor_39_39"><span class="label">[39]</span></a> De Schweinitz, Nat. Med. Rev., April, 1899.</p></div>
+
+<div class="footnote"><p><a name="Footnote_40_40" id="Footnote_40_40"></a><a href="#FNanchor_40_40"><span class="label">[40]</span></a> Conn, Proc. Soc. Amer. Bacteriologists, 1902.</p></div>
+
+<div class="footnote"><p><a name="Footnote_41_41" id="Footnote_41_41"></a><a href="#FNanchor_41_41"><span class="label">[41]</span></a> Freudenreich, Ann. de Microg., 2:115, 1890.</p></div>
+
+<div class="footnote"><p><a name="Footnote_42_42" id="Footnote_42_42"></a><a href="#FNanchor_42_42"><span class="label">[42]</span></a> Conn, Bull. 26, Storrs Expt. Stat.</p></div>
+
+<div class="footnote"><p><a name="Footnote_43_43" id="Footnote_43_43"></a><a href="#FNanchor_43_43"><span class="label">[43]</span></a> New York City is supplied with milk that is shipped 350
+miles.</p></div>
+
+<div class="footnote"><p><a name="Footnote_44_44" id="Footnote_44_44"></a><a href="#FNanchor_44_44"><span class="label">[44]</span></a> Park, N. Y. Univ. Bull., 1: 85, 1901.</p></div>
+
+<div class="footnote"><p><a name="Footnote_45_45" id="Footnote_45_45"></a><a href="#FNanchor_45_45"><span class="label">[45]</span></a> Eckles, Bull. 59, Iowa Expt. Stat., Aug. 1901.</p></div>
+
+<div class="footnote"><p><a name="Footnote_46_46" id="Footnote_46_46"></a><a href="#FNanchor_46_46"><span class="label">[46]</span></a> Eckles, Bull. 59, Iowa Expt. Stat., Aug. 1901.</p></div>
+
+<div class="footnote"><p><a name="Footnote_47_47" id="Footnote_47_47"></a><a href="#FNanchor_47_47"><span class="label">[47]</span></a> Russell, 15 Rept. Wis. Expt. Stat. 1898, p. 104.</p></div>
+
+<div class="footnote"><p><a name="Footnote_48_48" id="Footnote_48_48"></a><a href="#FNanchor_48_48"><span class="label">[48]</span></a> Alvord, Circ. No. 9, U. S. Dept. Agric. (Div. of Bot.).</p></div>
+
+<div class="footnote"><p><a name="Footnote_49_49" id="Footnote_49_49"></a><a href="#FNanchor_49_49"><span class="label">[49]</span></a> Connell, Rept. of Commissioner of Agr., Canada, 1897, part
+XVI, p. 15.</p></div>
+
+<div class="footnote"><p><a name="Footnote_50_50" id="Footnote_50_50"></a><a href="#FNanchor_50_50"><span class="label">[50]</span></a> Harrison, Hoard's Dairyman, March 4, 1898.</p></div>
+</div>
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_62" id="Page_62">[Pg 62]</a></span></p>
+<h2>CHAPTER IV.</h2>
+
+<h3>FERMENTATIONS IN MILK AND THEIR TREATMENT.</h3>
+
+
+<p>Under the conditions in which milk is drawn, it is practically
+impossible to secure the same without bacterial contamination. The
+result of the introduction of these organisms often changes its
+character materially as most bacteria cause the production of more or
+less pronounced fermentative processes. Under normal conditions, milk
+sours, i. e., develops lactic acid, but at times this more common
+fermentation may be replaced by other changes which are marked by the
+production of some other more or less undesirable flavor, odor or change
+in appearance.</p>
+
+<p>In referring to these changes, it is usually customary to designate them
+after the most prominent by-product formed, but it must be kept in mind
+that generally some other decomposition products are usually produced.
+Whether the organisms producing this or that series of changes prevail
+or not depends upon the initial seeding, and the conditions under which
+the milk is kept. Ordinarily, the lactic acid organisms grow so
+luxuriantly in the milk that they overpower all competitors and so
+determine the nature of the fermentation; but occasionally the milk
+becomes infected with other types of bacteria in relatively large
+numbers and the conditions may be especially suitable to the development
+of these forms, thereby modifying the course of the normal changes that
+occur.<span class='pagenum'><a name="Page_63" id="Page_63">[Pg 63]</a></span></p>
+
+<p>The kinds of bacteria that find it possible to develop in milk may be
+included under two heads:</p>
+
+<p>1. Those which cause no appreciable change in the milk, either in taste,
+odor or appearance. While these are frequently designated as the inert
+bacteria, it must not be supposed that they have absolutely no effect on
+milk. It is probably true in most cases that slight changes of a
+chemical nature are produced, but the nature of the changes do not
+permit of ready recognition.</p>
+
+<p>2. This class embraces all those organisms which, as a result of their
+growth, are capable of producing evident changes. These transformations
+may be such as to affect the taste, as in the sour milk or in the bitter
+fermentations, or the odor, as in some of the fetid changes, or the
+appearance of the milk, as in the slimy and color changes later
+described.</p>
+
+<p><b>Souring of milk.</b> Ordinarily if milk is allowed to stand for several days
+at ordinary temperatures it turns sour. This is due to the formation of
+lactic acid, which is produced by the decomposition of the milk-sugar.
+While this change is well nigh universal, it does not occur without a
+pre-existing cause, and that is the presence of certain living bacterial
+forms. These organisms develop in milk with great rapidity, and the
+decomposition changes that are noted in souring are due to the
+by-products of their development.</p>
+
+<p>The milk-sugar undergoes fermentation, the chief product being lactic
+acid, although various other by-products, as other organic acids
+(acetic, formic and succinic), different alcohols and gaseous products,
+as CO<sub>2</sub>, H, N and methane (CH<sub>4</sub>) are produced in small amounts.</p>
+
+<p>In this fermentation, the acidity begins to be evident to the taste when
+it reaches about 0.3 per cent., calculated as lactic acid. As the
+formation of acid goes on, the casein<span class='pagenum'><a name="Page_64" id="Page_64">[Pg 64]</a></span> is precipitated and incipient
+curdling or lobbering of the milk occurs. This begins to be apparent
+when the acidity is about 0.4 per cent., but the curd becomes more solid
+with increasing acidity. The rapidity of curdling is also dependent upon
+the temperature of the milk. Thus milk which at ordinary temperatures
+might remain fluid often curdles when heated. The growth of the bacteria
+is continued until about 0.8 to 1.0 per cent. acid is formed, although
+the maximum amount fluctuates considerably with different lactic acid
+species. Further formation then ceases even though all of the milk-sugar
+is not used up, because of the inability of the lactic bacteria to
+continue their growth in such acid solutions.</p>
+
+<p>As this acidity is really in the milk serum, cream never develops so
+much acid as milk, because a larger proportion of its volume is made up
+of butter-fat globules. This fact must be considered in the ripening of
+cream in butter-making where the per cent. of fat is subject to wide
+fluctuations.</p>
+
+<p>The formation of lactic acid is a characteristic that is possessed by a
+large number of bacteria, micrococci as well as bacilli being numerously
+represented. Still the preponderance of evidence is in favor of the view
+that a few types are responsible for most of these changes. The most
+common type found in spontaneously soured milk changes the milk-sugar
+into lactic acid without the production of any gas. This type has been
+described by various workers on European as well as American milks, and
+is designated by Conn as the <i>Bact. lactis acidi</i> type.<a name="FNanchor_51_51" id="FNanchor_51_51"></a><a href="#Footnote_51_51" class="fnanchor">[51]</a> It is
+subject to considerable variation under different conditions.</p>
+<p><span class='pagenum'><a name="Page_65" id="Page_65">[Pg 65]</a></span></p>
+<p>Curiously enough if milk which has been drawn with special care is
+examined immediately after milking, the lactic organisms are not usually
+found. They are incapable of development in the udder itself, as shown
+by injections into the milk cistern. They abound, however, on hay, in
+dust, in the barn air, on the hairy coat of the animal, and from these
+sources easily gain access to the milk. In this medium they find an
+exceptionally favorable environment and soon begin a very rapid growth,
+so that by the time milk is consumed, either in the form of milk or milk
+products, they make up numerically the larger portion of the bacteria
+present.</p>
+
+<p>Another widely disseminated, although numerically less prevalent, type
+is <i>B. lactis aerogenes</i>. This type forms gas in milk so that the soured
+milk is torn by the presence of gas bubbles. It also grows more
+luxuriantly in contact with the air.</p>
+
+<p>Other types occur more or less sporadically, some of which are capable
+of liquefying the casein of milk while at the same time they also
+develop lactic acid. Conn and Aikman refer to the fact that over one
+hundred species capable of producing variable quantities of lactic acid
+are already known. It is fair to presume, however, that a careful
+comparative study of these would show that simply racial differences
+exist in many cases, and therefore, that they are not distinct species.</p>
+
+<p>As a group these bacteria are characterized by their inability to
+liquefy gelatin or develop spores. On account of this latter
+characteristic they are easily destroyed when milk is pasteurized. They
+live under aerobic or anaerobic conditions, many of them being able to
+grow in either environment, although, according to McDonnell,<a name="FNanchor_52_52" id="FNanchor_52_52"></a><a href="#Footnote_52_52" class="fnanchor">[52]</a> they
+are more virulent when air is not excluded.</p>
+<p><span class='pagenum'><a name="Page_66" id="Page_66">[Pg 66]</a></span></p>
+<p>While growth of these lactic forms may go on in milk throughout a
+relatively wide range in temperature, appreciable quantities of acid are
+not produced except very slowly at temperatures below 50&deg; F.<a name="FNanchor_53_53" id="FNanchor_53_53"></a><a href="#Footnote_53_53" class="fnanchor">[53]</a></p>
+
+<p>From the standpoint of frequency the most common abnormal changes that
+occur in milk are those in which gases of varying character are
+developed in connection with acids, from the milk sugar. Other volatile
+products imparting bad flavors usually accompany gas production. These
+fermentations are of most serious import in the cheese industry, as they
+are especially prone to develop in the manufacture of milk into certain
+types of cheese. Not often is their development so rapid that they
+appear in the milk while it is yet in the hands of the milk producer,
+but almost invariably the introduction of the causal organisms takes
+place while the milk is on the farm. Numerous varieties of bacteria
+possess this property of producing gas (H and CO<sub>2</sub> are most common
+although N and methane (CH<sub>4</sub>) are sometimes produced). The more common
+forms are those represented by <i>B. lactis aerogenes</i> and the common
+fecal type, <i>B. coli commune</i>. The ordinary habitat of this type is dirt
+and intestinal filth. Hence careless methods of milk handling invite
+this type of abnormal change in milk.</p>
+
+<p>It is a wide-spread belief that thunder storms cause milk to sour
+prematurely, but this idea has no scientific foundation. Experiments<a name="FNanchor_54_54" id="FNanchor_54_54"></a><a href="#Footnote_54_54" class="fnanchor">[54]</a>
+with the electric spark, ozone and loud detonations show no effect on
+acid development, but the atmospheric conditions usually incident to a
+thunder storm are such as permit of a more rapid growth of organisms.
+There is no reason to believe but that the phenomenon<span class='pagenum'><a name="Page_67" id="Page_67">[Pg 67]</a></span> of souring is
+wholly related to the development of bacteria. Sterile milks are never
+affected by the action of electric storms.</p>
+
+<p><b>"Gassy" milks.</b> Where these gas bacteria abound, the amount of lactic
+acid is generally reduced, due to the splitting up of some of the sugar
+into gaseous products. This type of germ life does not seem to be able
+to develop well in the presence of the typical lactic acid non
+gas-forming bacteria.</p>
+
+<div class="figcenter" style="width: 450px;">
+<img src="images/fig16.jpg" width="450" height="368" alt="Fig. 16. Cheese made from &quot;gassy&quot; milk." title="" />
+<span class="caption">Fig. 16. Cheese made from &quot;gassy&quot; milk.</span>
+</div>
+
+<p><b>"Sweet curdling" and digesting fermentations.</b> Not infrequently milk,
+instead of undergoing spontaneous souring, curdles in a weakly acid or
+neutral condition, in which state it is said to have undergone "sweet
+curdling." The coagulation of the milk is caused by the action of enzyms
+of a rennet type that are formed by the growth of various species of
+bacteria. Later the whey separates more or less perfectly from the curd,
+producing a "wheyed off" condition. Generally the coagulum in these
+cases is soft and<span class='pagenum'><a name="Page_68" id="Page_68">[Pg 68]</a></span> somewhat slimy. The curd usually diminishes in bulk,
+due to the gradual digestion or peptonization of the casein by
+proteid-dissolving enzyms (tryptic type) that are also produced by the
+bacteria causing the change.</p>
+
+<p>A large number of bacteria possess the property of affecting milk in
+this way. So far as known they are able to liquefy gelatin (also a
+peptonizing process) and form spores. The Tyrothrix type of bacteria (so
+named by Duclaux on account of the supposed relation to cheese ripening)
+belongs to this class. The hay and potato forms are also digesters.
+Organisms of this type are generally associated with filth and manure,
+and find their way into the milk from the accumulations on the coat of
+the animal.</p>
+
+<p>Conn<a name="FNanchor_55_55" id="FNanchor_55_55"></a><a href="#Footnote_55_55" class="fnanchor">[55]</a> has separated the rennet enzym from bacterial cultures in a
+relatively pure condition, while Fermi<a name="FNanchor_56_56" id="FNanchor_56_56"></a><a href="#Footnote_56_56" class="fnanchor">[56]</a> has isolated the digestive
+ferment from several species.</p>
+
+<p>Duclaux<a name="FNanchor_57_57" id="FNanchor_57_57"></a><a href="#Footnote_57_57" class="fnanchor">[57]</a> has given to this digesting enzym the name <i>casease</i> or
+cheese ferment. These isolated ferments when added to fresh milk possess
+the power of causing the characteristic curdling and subsequent
+digestion quite independent of cell development. The quantity of ferment
+produced by different species differs materially in some cases. In these
+digestive fermentations, the chemical transformations are profound, the
+complex proteid molecule being broken down into albumoses, peptones,
+amido-acids (tyrosin and leucin) and ammonia as well as fatty acids.</p>
+
+<p>Not infrequently these fermentations gain the ascendency over the normal
+souring change, but under ordinary conditions they are held in abeyance,
+although this type of bacteria is always present to some extent in milk.
+When<span class='pagenum'><a name="Page_69" id="Page_69">[Pg 69]</a></span> the lactic acid bacteria are destroyed, as in boiled, sterilized
+or pasteurized milk, these rennet-producing, digesting species develop.</p>
+
+<p><b>Butyric acid fermentations.</b> The formation of butyric acid in milk which
+may be recognized by the "rancid butter" odor is not infrequently seen
+in old, sour milk, and for a long time was thought to be a continuation
+of the lactic fermentation, but it is now believed that these organisms
+find more favorable conditions for growth, not so much on account of the
+lactic acid formed as in the absence of dissolved oxygen in the milk
+which is consumed by the sour-milk organisms.</p>
+
+<p>Most of the butyric class of bacteria are spore-bearing, and hence they
+are frequently present in boiled or sterilized milk. The by-products
+formed in this series of changes are quite numerous. In most cases,
+butyric acid is prominent, but in addition to this, other organic acids,
+as lactic, succinic, and acetic, are produced, likewise different
+alcohols. Concerning the chemical origin of butyric acid there is yet
+some doubt. Duclaux<a name="FNanchor_58_58" id="FNanchor_58_58"></a><a href="#Footnote_58_58" class="fnanchor">[58]</a> affirms that the fat, sugar and casein are all
+decomposed by various forms. In some cases, the reaction of the milk is
+alkaline, with other species it may be neutral or acid. This type of
+fermentation has not received the study it deserves.</p>
+
+<p>In milk these organisms are not of great importance, as this
+fermentation does not readily gain the ascendency over the lactic
+bacteria.</p>
+
+<p><b>Ropy or slimy milk.</b> The viscosity of milk is often markedly increased
+over that which it normally possesses. The intensity of this abnormal
+condition may vary much; in some cases the milk becoming viscous or
+slimy; in others<span class='pagenum'><a name="Page_70" id="Page_70">[Pg 70]</a></span> stringing out into long threads, several feet in
+length, as in Fig. 17. Two sets of conditions are responsible for these
+ropy or slimy milks. The most common is where the milk is clotted or
+stringy when drawn, as in some forms of garget. This is generally due to
+the presence of viscid pus, and is often accompanied by a bloody
+discharge, such a condition representing an inflamed state of the udder.
+Ropiness of this character is not usually communicable from one lot of
+milk to another.</p>
+
+<div class="figcenter" style="width: 125px;">
+<img src="images/fig17.jpg" width="125" height="500" alt="Fig. 17. Ropy milk." title="" />
+<span class="caption">Fig. 17. Ropy milk.</span>
+</div>
+
+<p>The communicable form of ropy milk only appears after the milk has been
+drawn from the udder for a day or so, and is caused by the development
+of various species of bacteria which find their way into the milk after
+it is drawn. These defects are liable to occur at any season of the
+year. Their presence in a dairy is a source of much trouble, as the
+unsightly appearance of the milk precludes its use as food, although
+there is no evidence that these ropy fermentations are dangerous to
+health.</p>
+
+<p>There are undoubtedly a number of different species of bacteria that are
+capable of producing these viscid changes,<a name="FNanchor_59_59" id="FNanchor_59_59"></a><a href="#Footnote_59_59" class="fnanchor">[59]</a> but it is quite probable
+that they are not of equal importance in infecting milk under natural
+conditions.</p>
+
+<p>In the majority of cases studied in this country,<a name="FNanchor_60_60" id="FNanchor_60_60"></a><a href="#Footnote_60_60" class="fnanchor">[60]</a> the<span class='pagenum'><a name="Page_71" id="Page_71">[Pg 71]</a></span> causal
+organism seems to be <i>B. lactis viscosus</i>, a form first found by Adametz
+in surface waters.<a name="FNanchor_61_61" id="FNanchor_61_61"></a><a href="#Footnote_61_61" class="fnanchor">[61]</a> This organism possesses the property of
+developing at low temperatures (45&deg;-50&deg; F.), and consequently it is
+often able in winter to supplant the lactic-acid forms. Ward has found
+this germ repeatedly in water tanks where milk cans are cooled; and
+under these conditions it is easy to see how infection of the milk might
+occur. Marshall<a name="FNanchor_62_62" id="FNanchor_62_62"></a><a href="#Footnote_62_62" class="fnanchor">[62]</a> reports an outbreak which he traced to an external
+infection of the udder; in another case, the slime-forming organism was
+abundant in the barn dust. A defect of this character is often
+perpetuated in a dairy for some time, and may therefore become
+exceedingly troublesome. In one instance in the writer's experience, a
+milk dealer lost over $150 a month for several months from ropy cream.
+Failure to properly sterilize cans, and particularly strainer cloths, is
+frequently responsible for a continuance of trouble of this sort.</p>
+
+<p>The slimy substance formed in milk comes from various constituents of
+the milk, and the chemical character of the slime produced also varies
+with different germs. In some cases the slimy material is merely the
+swollen outer cell membrane of the bacteria themselves as in the case of
+<i>B. lactis viscosus</i>; in others it is due to the decomposition of the
+proteids, but often the chief decomposition product appears to come from
+a viscous fermentation of the milk-sugar.</p>
+
+<p>An interesting case of a fermentation of this class being utilized in
+dairying is seen in the use of "lange wei" (long or stringy whey) which
+is employed as a starter in Holland to control the gassy fermentations
+in Edam cheese.<span class='pagenum'><a name="Page_72" id="Page_72">[Pg 72]</a></span> This slimy change is due to the growth of
+<i>Streptococcus Hollandicus</i>.<a name="FNanchor_63_63" id="FNanchor_63_63"></a><a href="#Footnote_63_63" class="fnanchor">[63]</a></p>
+
+<p><b>Alcoholic fermentations.</b> Although glucose or cane-sugar solutions are
+extremely prone to undergo alcoholic fermentation, milk sugar does not
+readily undergo this change. Where such changes are produced it is due
+to yeasts. Several outbreaks attributable to such a cause have been
+reported.<a name="FNanchor_64_64" id="FNanchor_64_64"></a><a href="#Footnote_64_64" class="fnanchor">[64]</a> Russell and Hastings<a name="FNanchor_65_65" id="FNanchor_65_65"></a><a href="#Footnote_65_65" class="fnanchor">[65]</a> have found these milk-sugar
+splitting yeasts particularly abundant in regions where Swiss cheese is
+made, a condition made possible by the use of whey-soaked rennets in
+making such cheese.</p>
+
+<p>Kephir and Koumiss are liquors much used in the Orient which are made
+from milk that has undergone alcoholic fermentation. Koumiss was
+originally made from mare's milk but is now often made from cows' milk
+by adding cane sugar and yeast. In addition to the CO<sub>2</sub> developed,
+alcohol, lactic acid, and casein-dissolving ferments are formed. Kephir
+is made by adding to milk Kephir grains, which are a mass of yeast and
+bacterial cells. The yeasts produce alcohol and CO<sub>2</sub> while the
+bacteria change the casein of milk, rendering it more digestible. These
+beverages are frequently recommended to persons who seem to be unable to
+digest raw milk readily. The exact nature of the changes produced are
+not yet well understood.<a name="FNanchor_66_66" id="FNanchor_66_66"></a><a href="#Footnote_66_66" class="fnanchor">[66]</a></p>
+
+<p><b>Bitter milk.</b> The presence of bitter substances in milk may be ascribed
+to a variety of causes. A number of plants, such as lupines, ragweed and
+chicory, possess the property of affecting milk when the same are
+consumed by animals.<span class='pagenum'><a name="Page_73" id="Page_73">[Pg 73]</a></span> At certain stages in lactation, a bitter salty
+taste is occasionally to be noted that is peculiar to individual
+animals.</p>
+
+<p>A considerable number of cases of bitter milk have, however, been traced
+to bacterial origin. For a number of years the bitter fermentation of
+milk was thought to be associated with the butyric fermentation, but
+Weigmann<a name="FNanchor_67_67" id="FNanchor_67_67"></a><a href="#Footnote_67_67" class="fnanchor">[67]</a> showed that the two conditions were not dependent upon each
+other. He found that the organism which produced the bitter taste acted
+upon the casein.</p>
+
+<p>Conn<a name="FNanchor_68_68" id="FNanchor_68_68"></a><a href="#Footnote_68_68" class="fnanchor">[68]</a> observed a coccus form in bitter cream that was able to impart
+a bitter flavor to milk. Sometimes a bitter condition does not develop
+in the milk, but may appear later in the milk products, as in the case
+of a micrococcus which Freudenreich<a name="FNanchor_69_69" id="FNanchor_69_69"></a><a href="#Footnote_69_69" class="fnanchor">[69]</a> found in cheese.</p>
+
+<p>Harrison<a name="FNanchor_70_70" id="FNanchor_70_70"></a><a href="#Footnote_70_70" class="fnanchor">[70]</a> has traced a common bitter condition in Canadian milk to a
+milk-sugar splitting yeast, <i>Torula amara</i> which not only grows rapidly
+in milk but produces an undesirable bitterness in cheddar cheese.</p>
+
+<p>Cream ripened at low temperatures not infrequently develops a bitter
+flavor, showing that the optimum temperature for this type of
+fermentation is below the typical lactic acid change.</p>
+
+<p>Milk that has been heated often develops a bitter condition. The
+explanation of this is that the bacteria producing the bitter substances
+usually possess endospores, and that while the boiling or sterilizing of
+milk easily kills the lactic acid germs, these forms on account of their
+greater resisting powers are not destroyed by the heat.</p>
+
+<p><span class='pagenum'><a name="Page_74" id="Page_74">[Pg 74]</a></span></p><p><b>Soapy milk:</b> A soapy flavor in milk was traced by Weigmann and Zirn<a name="FNanchor_71_71" id="FNanchor_71_71"></a><a href="#Footnote_71_71" class="fnanchor">[71]</a>
+to a specific bacillus, <i>B. lactis saponacei</i>, that they found gained
+access to the milk in one case from the bedding and in another instance
+from hay. A similar outbreak has been reported in this country,<a name="FNanchor_72_72" id="FNanchor_72_72"></a><a href="#Footnote_72_72" class="fnanchor">[72]</a> due
+to a germ acting on the casein and albumen.</p>
+
+<p><b>Red milk.</b> The most common trouble of this nature in milk is due to
+presence of blood, which is most frequently caused by some wound in the
+udder. The ingestion of certain plants as sedges and scouring rushes is
+also said to cause a bloody condition; madders impart a reddish tinge
+due to coloring matter absorbed. Defects of this class can be readily
+distinguished from those due to germ growth because they are apparent at
+time of milking. Where blood is actually present, the corpuscles settle
+out in a short time if left undisturbed.</p>
+
+<p>There are a number of chromogenic or color-producing bacteria that are
+able to grow in milk, but their action is so slow that generally they
+are not of much consequence. Moreover their development is usually
+confined to the surface of the milk as it stands in a vessel. The most
+important is the well-known <i>B. prodigiosus</i>. Another form found at
+times in milk possessing low acidity<a name="FNanchor_73_73" id="FNanchor_73_73"></a><a href="#Footnote_73_73" class="fnanchor">[73]</a> is <i>B. lactis erythrogenes</i>.
+This species only develops the red color in the dark. In the light, it
+forms a yellow pigment. Various other organisms have been reported at
+different times.<a name="FNanchor_74_74" id="FNanchor_74_74"></a><a href="#Footnote_74_74" class="fnanchor">[74]</a></p>
+
+<p><b>Blue milk.</b> Blue milk has been known for many years, its communicable
+nature being established as long ago as 1838. It appears on the surface
+of milk first as isolated<span class='pagenum'><a name="Page_75" id="Page_75">[Pg 75]</a></span> particles of bluish or grey color, which
+later become confluent, the blue color increasing in intensity as the
+acidity increases. The causal organism, <i>B. cyanogenes</i>, is very
+resistant toward drying,<a name="FNanchor_75_75" id="FNanchor_75_75"></a><a href="#Footnote_75_75" class="fnanchor">[75]</a> thus accounting for its persistence. In
+Mecklenberg an outbreak of this sort once continued for several years.
+It has frequently been observed in Europe in the past, but is not now so
+often reported. Occasional outbreaks have been reported in this country.</p>
+
+<p><b>Other kinds of colored milk.</b> Two or three chromogenic forms producing
+still other colors have occasionally been found in milk. Adametz<a name="FNanchor_76_76" id="FNanchor_76_76"></a><a href="#Footnote_76_76" class="fnanchor">[76]</a>
+discovered in a sample of cooked milk a peculiar form (<i>Bacillus
+synxanthus</i>) that produced a citron-yellow appearance which precipitated
+and finally rendered soluble the casein. Adametz, Conn, and List have
+described other species that confer tints of yellow on milk. Some of
+these are bright lemon, others orange, and some amber in color.</p>
+
+<p>Still other color-producing bacteria, such as those that produce violet
+or green changes in the milk, have been observed. In fact, almost any of
+the chromogenic bacteria are able to produce their color changes in milk
+as it is such an excellent food medium. Under ordinary conditions, these
+do not gain access to milk in sufficient numbers so that they modify the
+appearance of it except in occasional instances.</p>
+
+<p><b>Treatment of abnormal fermentations.</b> If the taint is recognized as of
+bacterial origin (see p. 57) and is found in the mixed milk of the herd,
+it is necessary to ascertain, first, whether it is a general trouble, or
+restricted to one or more animals. This can sometimes be done by
+separating<span class='pagenum'><a name="Page_76" id="Page_76">[Pg 76]</a></span> the milk of the different cows and noting whether any
+abnormal condition develops in the respective samples.</p>
+
+<p><b>Fermentation tests.</b> The most satisfactory way to detect the presence of
+the taints more often present is to make a fermentation test of one kind
+or another. These tests are most frequently used at the factory, to
+enable the maker to detect the presence of milk that is likely to prove
+unfit for use, especially in cheese making. They are based upon the
+principle that if milk is held at a moderately high temperature, the
+bacteria will develop rapidly. A number of different methods have been
+devised for this purpose. In Walther's lacto-fermentator samples of milk
+are simply allowed to stand in bottles or glass jars until they sour.
+They are examined at intervals of several hours. If the curdled milk is
+homogeneous and has a pure acid smell, the milk is regarded as all
+right. If it floats in a turbid serum, is full of gas or ragged holes,
+it is abnormal. As generally carried out, no attempt is made to have
+these vessels sterile. Gerber's test is a similar test that has been
+extensively employed in Switzerland. Sometimes a few drops of rennet are
+added to the milk so as to curdle the same, and thus permit of the more
+ready detection of the gas that is evolved.</p>
+
+<p><b>Wisconsin curd test.</b> The method of testing milk described below was
+devised at the Wisconsin Experiment Station in 1895 by Babcock, Russell
+and Decker.<a name="FNanchor_77_77" id="FNanchor_77_77"></a><a href="#Footnote_77_77" class="fnanchor">[77]</a> It was used first in connection with experimental work
+on the influence of gas-generating bacteria in cheese making, but its
+applicability to the detection of all taints in milk produced by
+bacteria makes it a valuable test for abnormal fermentations in general.</p>
+
+<p>In the curd test a small pat of curd is made in a glass<span class='pagenum'><a name="Page_77" id="Page_77">[Pg 77]</a></span> jar from each
+sample of milk. These tests may be made in any receptacle that has been
+cleaned in boiling water, and to keep the temperature more nearly
+uniform these jars should be immersed in warm water, as in a wash tub or
+some other receptacle. When the milk is about 95&deg; F., about ten drops of
+rennet extract are added to each sample and mixed thoroughly with the
+milk. The jars should then remain undisturbed until the milk is
+completely curdled; then the curd is cut into small pieces with a case
+knife and stirred to expel the whey. The whey should be poured off at
+frequent intervals until the curd mats. If the sample be kept at blood
+heat (98&deg; F.) for six to eight hours, it will be ready to examine.</p>
+
+<div class="figcenter" style="width: 450px;">
+<img src="images/fig18.jpg" width="450" height="266" alt="Fig. 18." title="" />
+<span class="caption">Fig. 18.</span>
+</div><div class="blockquot"><p> Improved bottles for making curd test. <i>A</i>, test
+bottle complete; <i>B</i>, bottle showing construction of cover; <i>S</i>, sieve
+to hold back the curd when bottle is inverted; <i>C</i>, outer cover with <i>(D
+H)</i> drain holes to permit of removal of whey.]</p></div>
+
+<p>More convenient types of this test than the improvised apparatus just
+alluded to have been devised by different dairy manufacturers.
+Generally, they consist of a special bottle having a full-sized top,
+thus permitting the easy removal of the curd. The one shown in Fig. 18
+is provided<span class='pagenum'><a name="Page_78" id="Page_78">[Pg 78]</a></span> with a sieve of such construction that the bottles will
+drain thoroughly if inclined in an inverted position.</p>
+
+<p><b>Interpretation of results of test.</b> The curd from a good milk has a firm,
+solid texture, and should contain at most only a few small pin holes. It
+may have some large, irregular, "mechanical" holes where the curd
+particles have failed to cement, as is seen in Fig. 19. If gas-producing
+bacteria are very prevalent in the milk, the conditions under which the
+test is made cause such a rapid growth of the same that the evidence of
+the abnormal fermentation may be readily seen in the spongy texture of
+the curd (Fig. 20). If the undesirable organisms are not very abundant
+and the conditions not especially suited to their growth, the "pin
+holes" will be less frequent.</p>
+
+<div class="figcenter" style="width: 450px;">
+<img src="images/fig19.jpg" width="450" height="245" alt="Fig. 19. Curd from a good milk. The large irregular holes
+are mechanical." title="" />
+<span class="caption">Fig. 19. Curd from a good milk. The large irregular holes
+are mechanical.</span>
+</div>
+
+<p>Sometimes the curds show no evidence of gas, but their abnormal
+condition can be recognized by the "mushy" texture and the presence of
+"off" flavors that are rendered more apparent by keeping them in closed
+bottles. This condition is abnormal and is apt to produce quite as
+serious results as if gas was formed.<span class='pagenum'><a name="Page_79" id="Page_79">[Pg 79]</a></span></p>
+
+<p><b>Overcoming taints by use of starters.</b> Another method of combatting
+abnormal fermentations that is often fruitful, is that which rests upon
+the inability of one kind of bacteria to grow in the same medium in
+competition with certain other species.</p>
+
+<p>Some of the undesirable taints in factories can be controlled in large
+part by the introduction of starters made from certain organisms that
+are able to obtain the ascendency over the taint-producing germ. Such a
+method is commonly followed when a lactic ferment, either a commercial
+pure culture, or a home-made starter, is added to milk to overcome the
+effect of gas-generating bacteria.</p>
+
+<div class="figcenter" style="width: 450px;">
+<img src="images/fig20.jpg" width="450" height="248" alt="Fig. 20. Curd from a badly tainted milk. Large ragged
+holes are mechanical; numerous small holes due to gas. This curd was a
+&quot;floater.&quot;" title="" />
+<span class="caption">Fig. 20. Curd from a badly tainted milk. Large ragged
+holes are mechanical; numerous small holes due to gas. This curd was a
+&quot;floater.&quot;</span>
+</div>
+
+<p>A similar illustration is seen in the case of the "lange wei" (slimy
+whey), that is used in the manufacture of Edam cheese to control the
+character of the fermentation of the milk.</p>
+
+<p>This same method is sometimes applied in dealing with certain abnormal
+fermentations that are apt to occur on the farm. It is particularly
+useful with those tainted milks known as "sweet curdling." The ferment
+organisms concerned<span class='pagenum'><a name="Page_80" id="Page_80">[Pg 80]</a></span> in this change are unable to develop in the
+presence of lactic acid bacteria, so the addition of a clean sour milk
+as a starter restores the normal conditions by giving the ordinary milk
+bacteria the ascendency.</p>
+
+<p><b>Chemical disinfection.</b> In exceptional instances it may be necessary to
+employ chemical disinfectants to restore the normal conditions. Of
+course with such diseases as tuberculosis, very stringent measures are
+required, as they are such a direct menace to human life, but with these
+abnormal or taint-producing fermentations, care and cleanliness, well
+directed, will usually overcome the trouble.</p>
+
+<p>If it becomes necessary to employ chemical substances as disinfecting
+agents, their use should always be preceded by a thorough cleansing with
+hot water so that the germicide may come in direct contact with the
+surface to be disinfected.</p>
+
+<p>It must be borne in mind that many chemicals act as deodorants, <i>i.e.</i>,
+destroy the offensive odor, without destroying the cause of the trouble.</p>
+
+<p><i>Sulfur</i> is often recommended as a disinfecting agent, but its use
+should be carefully controlled, otherwise the vapors have but little
+germicidal power. The common practice of burning a small quantity in a
+room or any closed space for a few moments has little or no effect upon
+germ life. The effect of sulfur vapor (SO<sub>2</sub>) alone upon germ life is
+relatively slight, but if this gas is produced in the presence of
+moisture, sulfurous acid (H<sub>2</sub>SO<sub>3</sub>) is formed, which is much more
+efficient. To use this agent effectively, it must be burned in large
+quantities in a moist atmosphere (three lbs. to every 1,000 cubic feet
+of space), for at least twelve hours. After this operation, the space
+should be thoroughly aired.</p>
+
+<p><i>Formalin</i>, a watery solution of a gas known as formaldehyde, is a new
+disinfectant that recent experience has<span class='pagenum'><a name="Page_81" id="Page_81">[Pg 81]</a></span> demonstrated to be very useful.
+It may be used as a gas where rooms are to be disinfected, or applied as
+a liquid where desired. It is much more powerful in its action than
+sulfur, and it has a great advantage over mercury and other strong
+disinfectants, as it is not so poisonous to man as it is to the lower
+forms of life.</p>
+
+<p><i>Bleaching powder or chloride of lime</i> is often recommended where a
+chemical can be advantageously used. This substance is a good
+disinfectant as well as a deodorant, and if applied as a wash, in the
+proportion of four to six ounces of the powder to one gallon of water,
+it will destroy most forms of life. In many cases this agent is
+inapplicable on account of its odor.</p>
+
+<p><i>Corrosive sublimate</i> (HgCl<sub>2</sub>) for most purposes is a good
+disinfectant, but it is such an intense poison that its use is dangerous
+in places that are at all accessible to stock.</p>
+
+<p>For the disinfection of walls in stables and barns, common thin <i>white
+wash</i> Ca(OH)<sub>2</sub> is admirably adapted if made from freshly-burned quick
+lime. It possesses strong germicidal powers, increases the amount of
+light in the barn, is a good absorbent of odors, and is exceedingly
+cheap.</p>
+
+<p>Carbolic acid, creosote, and such products, while excellent
+disinfectants, cannot well be used on account of their odor, especially
+in factories.</p>
+
+<p>For gutters, drains, and waste pipes in factories, <i>vitriol salts</i>
+(sulfates of copper, iron and zinc) are sometimes used. These are
+deodorants as well as disinfectants, and are not so objectionable to use
+on account of their odor.</p>
+
+<p>These suggestions as to the use of chemicals, however, only apply to
+extreme cases and should not be brought into requisition until a
+thorough application of hot water, soap, a little soda, and the
+scrubbing brush have failed to do their work.</p>
+
+<div class="footnotes"><h3>FOOTNOTES:</h3>
+
+<div class="footnote"><p><a name="Footnote_51_51" id="Footnote_51_51"></a><a href="#FNanchor_51_51"><span class="label">[51]</span></a> G&uuml;nther and Thierfelder, Arch. f. Hyg., 25:164, 1895;
+Leichmann, Cent. f. Bakt., 2:281, 1896; Esten, 9 Rept. Storrs Expt.
+Stat., p. 44, 1896; Dinwiddie, Bull. 45, Ark. Expt. Stat., May, 1897;
+Kozai, Zeit. f. Hyg., 38:386, 1901; Weigmann, Hyg. Milk Congress,
+Hamburg, 1903, p. 375.</p></div>
+
+<div class="footnote"><p><a name="Footnote_52_52" id="Footnote_52_52"></a><a href="#FNanchor_52_52"><span class="label">[52]</span></a> McDonnell, Inaug. Diss., Kiel. 1899, p. 39.</p></div>
+
+<div class="footnote"><p><a name="Footnote_53_53" id="Footnote_53_53"></a><a href="#FNanchor_53_53"><span class="label">[53]</span></a> Kayser, Cent. f. Bakt. II. Abt. 1:436.</p></div>
+
+<div class="footnote"><p><a name="Footnote_54_54" id="Footnote_54_54"></a><a href="#FNanchor_54_54"><span class="label">[54]</span></a> Treadwell, Science, 1894, 17:178.</p></div>
+
+<div class="footnote"><p><a name="Footnote_55_55" id="Footnote_55_55"></a><a href="#FNanchor_55_55"><span class="label">[55]</span></a> Conn, 5 Rept. Storrs Expt. Stat., 1892, p. 396.</p></div>
+
+<div class="footnote"><p><a name="Footnote_56_56" id="Footnote_56_56"></a><a href="#FNanchor_56_56"><span class="label">[56]</span></a> Fermi, Arch. f. Hyg., 1892, 14:1.</p></div>
+
+<div class="footnote"><p><a name="Footnote_57_57" id="Footnote_57_57"></a><a href="#FNanchor_57_57"><span class="label">[57]</span></a> Duclaux, Le Lait, p. 121.</p></div>
+
+<div class="footnote"><p><a name="Footnote_58_58" id="Footnote_58_58"></a><a href="#FNanchor_58_58"><span class="label">[58]</span></a> Duclaux, Principes de Laiterie, p. 67.</p></div>
+
+<div class="footnote"><p><a name="Footnote_59_59" id="Footnote_59_59"></a><a href="#FNanchor_59_59"><span class="label">[59]</span></a> Guillebeau (Milch Zeit., 1892, p. 808) has studied over a
+dozen different forms that possess this property.</p></div>
+
+<div class="footnote"><p><a name="Footnote_60_60" id="Footnote_60_60"></a><a href="#FNanchor_60_60"><span class="label">[60]</span></a> Ward, Bull. 165, Cornell Expt. Stat., Mch., 1899; also
+Bull. 195, Ibid., Nov., 1901.</p></div>
+
+<div class="footnote"><p><a name="Footnote_61_61" id="Footnote_61_61"></a><a href="#FNanchor_61_61"><span class="label">[61]</span></a> Adametz, Landw. Jahr., 1891, p. 185.</p></div>
+
+<div class="footnote"><p><a name="Footnote_62_62" id="Footnote_62_62"></a><a href="#FNanchor_62_62"><span class="label">[62]</span></a> Marshall, Mich. Expt. Stat., Bull. 140.</p></div>
+
+<div class="footnote"><p><a name="Footnote_63_63" id="Footnote_63_63"></a><a href="#FNanchor_63_63"><span class="label">[63]</span></a> Milch Zeit., 1899, p. 982.</p></div>
+
+<div class="footnote"><p><a name="Footnote_64_64" id="Footnote_64_64"></a><a href="#FNanchor_64_64"><span class="label">[64]</span></a> Duclaux, Principes de Laiterie, p. 60. Heinze and Cohn,
+Zeit. f. Hyg., 46: 286, 1904.</p></div>
+
+<div class="footnote"><p><a name="Footnote_65_65" id="Footnote_65_65"></a><a href="#FNanchor_65_65"><span class="label">[65]</span></a> Bull. 128, Wis. Expt. Stat., Sept. 1905.</p></div>
+
+<div class="footnote"><p><a name="Footnote_66_66" id="Footnote_66_66"></a><a href="#FNanchor_66_66"><span class="label">[66]</span></a> Freudenreich, Landw. Jahr. d. Schweiz, 1896, 10; 1.</p></div>
+
+<div class="footnote"><p><a name="Footnote_67_67" id="Footnote_67_67"></a><a href="#FNanchor_67_67"><span class="label">[67]</span></a> Weigmann, Milch Zeit., 1890, p. 881.</p></div>
+
+<div class="footnote"><p><a name="Footnote_68_68" id="Footnote_68_68"></a><a href="#FNanchor_68_68"><span class="label">[68]</span></a> Conn, 3 Rept. Storrs Expt. Stat., 1890, p. 158.</p></div>
+
+<div class="footnote"><p><a name="Footnote_69_69" id="Footnote_69_69"></a><a href="#FNanchor_69_69"><span class="label">[69]</span></a> Freudenreich, F&uuml;hl. Landw. Ztg. 43: 361.</p></div>
+
+<div class="footnote"><p><a name="Footnote_70_70" id="Footnote_70_70"></a><a href="#FNanchor_70_70"><span class="label">[70]</span></a> Harrison, Bull. 120 Ont. Agr'l. Coll., May, 1902.</p></div>
+
+<div class="footnote"><p><a name="Footnote_71_71" id="Footnote_71_71"></a><a href="#FNanchor_71_71"><span class="label">[71]</span></a> Milch Zeit. 22:569.</p></div>
+
+<div class="footnote"><p><a name="Footnote_72_72" id="Footnote_72_72"></a><a href="#FNanchor_72_72"><span class="label">[72]</span></a> Marshall, Bull. 146, Mich. Expt. Stat., p. 16.</p></div>
+
+<div class="footnote"><p><a name="Footnote_73_73" id="Footnote_73_73"></a><a href="#FNanchor_73_73"><span class="label">[73]</span></a> Grotenfelt, Milch Zeit., 1889, p. 263.</p></div>
+
+<div class="footnote"><p><a name="Footnote_74_74" id="Footnote_74_74"></a><a href="#FNanchor_74_74"><span class="label">[74]</span></a> Menge, Cent. f. Bakt., 6:596; Keferstein, Cent. f. Bakt.,
+21:177.</p></div>
+
+<div class="footnote"><p><a name="Footnote_75_75" id="Footnote_75_75"></a><a href="#FNanchor_75_75"><span class="label">[75]</span></a> Heim, Arb. a. d. Kais. Gesundheitsamte, 5:578.</p></div>
+
+<div class="footnote"><p><a name="Footnote_76_76" id="Footnote_76_76"></a><a href="#FNanchor_76_76"><span class="label">[76]</span></a> Adametz, Milch Zeit., 1890, p. 225.</p></div>
+
+<div class="footnote"><p><a name="Footnote_77_77" id="Footnote_77_77"></a><a href="#FNanchor_77_77"><span class="label">[77]</span></a> 12 Rept. Wis. Expt. Stat., 1895, p. 148; also Bull. 67,
+Ibid., June, 1898.</p></div>
+</div>
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_82" id="Page_82">[Pg 82]</a></span></p>
+<h2>CHAPTER V.</h2>
+
+<h3>RELATION OF DISEASE-BACTERIA TO MILK.</h3>
+
+
+<p>Practical experience with epidemic disease has abundantly demonstrated
+the fact that milk not infrequently serves as a vehicle for the
+dissemination of contagion. Attention has been prominently called to
+this relation by Ernest Hart,<a name="FNanchor_78_78" id="FNanchor_78_78"></a><a href="#Footnote_78_78" class="fnanchor">[78]</a> who in 1880 compiled statistical
+evidence showing the numerous outbreaks of various contagious diseases
+that had been associated with milk infection up to that time. Since
+then, further compilations have been made by Freeman,<a name="FNanchor_79_79" id="FNanchor_79_79"></a><a href="#Footnote_79_79" class="fnanchor">[79]</a> and also by
+Busey and Kober,<a name="FNanchor_80_80" id="FNanchor_80_80"></a><a href="#Footnote_80_80" class="fnanchor">[80]</a> who have collected the data with reference to
+outbreaks from 1880 to 1899.</p>
+
+<p>These statistics indicate the relative importance of milk as a factor in
+the dissemination of disease.</p>
+
+<p>The danger from this source is much intensified for the reason that
+milk, generally speaking, is consumed in a raw state; and also because a
+considerable number of disease-producing bacteria are able, not merely
+to exist, but actually thrive and grow in milk, even though the normal
+milk bacteria are also present. Moreover the recognition of the presence
+of such pathogenic forms is complicated by the fact that often they do
+not alter the appearance of<span class='pagenum'><a name="Page_83" id="Page_83">[Pg 83]</a></span> the milk sufficiently so that their
+presence can be detected by a physical examination. These facts which
+have been experimentally determined, coupled with the numerous clinical
+cases on record, make a strong case against milk serving as an agent in
+the dissemination of disease.</p>
+
+<p><b>Origin of pathogenic bacteria in milk.</b> Disease-producing bacteria may be
+grouped with reference to their relation toward milk into two classes,
+depending upon the manner in which infection occurs:</p>
+
+<p>Class I. Disease-producing bacteria capable of being transmitted
+directly from a diseased animal to man through the medium of infected
+milk.</p>
+
+<p>Class II. Bacteria pathogenic for man but not for cattle which are
+capable of thriving in milk after it is drawn from the animal.</p>
+
+<p>In the first group the disease produced by the specific organism must be
+common to both cattle and man. The organism must live a parasitic life
+in the animal, developing in the udder, and so infect the milk supply.
+It may, of course, happen that diseases toward which domestic animals
+alone are susceptible may be spread from one animal to another in this
+way without affecting human beings.</p>
+
+<p>In the second group, the bacterial species lives a saprophytic
+existence, growing in milk, if it happens to find its way therein. In
+such cases milk indirectly serves as an agent in the dissemination of
+disease, by giving conditions favorable to the growth of the disease
+germ.</p>
+
+<p>By far the most important of diseases that may be transmitted directly
+from animal to man through a diseased milk supply is tuberculosis, but
+in addition to this, foot and mouth disease (aphthous fever in
+children), anthrax and acute enteric troubles have also been traced to a
+similar source of infection.<span class='pagenum'><a name="Page_84" id="Page_84">[Pg 84]</a></span></p>
+
+<p>The most important specific diseases that have been disseminated through
+subsequent pollution of the milk are typhoid fever, diphtheria, scarlet
+fever and cholera, but, of course, the possibility exists that any
+disease germ capable of living and thriving in milk may be spread in
+this way. In addition to these diseases that are caused by the
+introduction of specific organisms (the causal organism of scarlet fever
+has not yet been definitely determined), there are a large number of
+more or less illy-defined troubles of an intestinal character that occur
+especially in infants and young children that are undoubtedly
+attributable to the activity of micro&ouml;rganisms that gain access to milk
+during and subsequent to the milking, and which produce changes in milk
+before or after its ingestion that result in the formation of toxic
+products.</p>
+
+
+<h4>DISEASES TRANSMISSIBLE FROM ANIMAL TO MAN THROUGH DISEASED MILK.</h4>
+
+<p><b>Tuberculosis.</b> In view of the wide-spread distribution of this disease in
+both the human and the bovine race, the relation of the same to milk
+supplies is a question of great importance. It is now generally admitted
+that the different types of tubercular disease found in different kinds
+of animals and man are attributable to the development of the same
+organism, <i>Bacillus tuberculosis</i>, although there are varieties of this
+organism found in different species of animals that are sufficiently
+distinct to permit of recognition.</p>
+
+<p>The question of prime importance is, whether the bovine type is
+transmissible to the human or not. Artificial inoculation of cattle with
+tuberculous human sputum as well as pure cultures of this variety show
+that the human type<span class='pagenum'><a name="Page_85" id="Page_85">[Pg 85]</a></span> is able to make but slight headway in cattle. This
+would indicate that the danger of cattle acquiring the infection from
+man would in all probability be very slight, but these experiments offer
+no answer as to the possibility of transmission from the bovine to the
+human. Manifestly it is impossible to solve this problem by direct
+experiment upon man except by artificial inoculation, but comparative
+experiments upon animals throw some light on the question.</p>
+
+<p>Theo. Smith<a name="FNanchor_81_81" id="FNanchor_81_81"></a><a href="#Footnote_81_81" class="fnanchor">[81]</a> and others<a name="FNanchor_82_82" id="FNanchor_82_82"></a><a href="#Footnote_82_82" class="fnanchor">[82]</a> have made parallel experiments with
+animals such as guinea pigs, rabbits and pigeons, inoculated with both
+bovine and human cultures of this organism. The results obtained in the
+case of all animals tested show that the virulence of the two types was
+much different, but that the bovine cultures were much more severe.
+While of course this does not prove that transmission from bovine to
+human is possible, still the importance of the fact must not be
+overlooked.</p>
+
+<p>In a number of cases record of accidental infection from cattle to man
+has been noted.<a name="FNanchor_83_83" id="FNanchor_83_83"></a><a href="#Footnote_83_83" class="fnanchor">[83]</a> These have occurred with persons engaged in making
+post-mortem examinations on tuberculous animals, and the tubercular
+nature of the wound was proven in some cases by excision and
+inoculation.</p>
+
+<p>In addition to data of this sort that is practically experimental in
+character, there are also strong clinical reasons for considering that
+infection of human beings may occur through the medium of milk.
+Naturally such infection should produce intestinal tuberculosis, and it
+is noteworthy that this phase of the disease is quite common in
+children<span class='pagenum'><a name="Page_86" id="Page_86">[Pg 86]</a></span> especially between the ages of two and five.<a name="FNanchor_84_84" id="FNanchor_84_84"></a><a href="#Footnote_84_84" class="fnanchor">[84]</a> It is
+difficult to determine, though, whether primary infection occurred
+through the intestine, for, usually, other organs also become involved.
+In a considerable number of cases in which tubercular infection by the
+most common channel, inhalation, seems to be excluded, the evidence is
+strong that the disease was contracted through the medium of the milk,
+but it is always very difficult to exclude the possibility of pulmonary
+infection.</p>
+
+<p>Tuberculosis as a bovine disease has increased rapidly during recent
+decades throughout many portions of the world. This has been most marked
+in dairy regions. Its extremely insidious nature does not permit of an
+early recognition by physical means, and it was not until the
+introduction of the tuberculin test<a name="FNanchor_85_85" id="FNanchor_85_85"></a><a href="#Footnote_85_85" class="fnanchor">[85]</a> in 1892, as a diagnostic aid
+that accurate knowledge of its distribution was possible. The quite
+general introduction of this test in many regions has revealed an
+alarmingly large percentage of animals as affected. In Denmark in 1894
+over forty per cent were diagnosed as tubercular. In some parts of
+Germany almost as bad a condition has been revealed. Slaughter-house
+statistics also show that the disease has increased rapidly since 1890.
+In this country the disease on the average is much less than in Europe
+and is also very irregularly distributed. In herds where it gained a
+foothold some years ago, often the majority of animals are frequently
+infected; many<span class='pagenum'><a name="Page_87" id="Page_87">[Pg 87]</a></span> herds, in fact the great majority, are wholly free from
+all taint. The disease has undoubtedly been most frequently introduced
+through the purchase of apparently healthy but incipiently affected
+animals. Consequently in the older dairy regions where stock has been
+improved the most by breeding, more of the disease exists than among the
+western and southern cattle.</p>
+
+<div class="figcenter" style="width: 450px;">
+<img src="images/fig21.jpg" width="450" height="392" alt="Fig. 21: Front view of a tuberculous udder, showing extent
+of swelling in single quarter." title="" />
+<span class="caption">Fig. 21: Front view of a tuberculous udder, showing extent
+of swelling in single quarter.</span>
+</div>
+
+<p><b>Infectiousness of milk of reacting animals.</b> Where the disease appears in
+the udder the milk almost invariably contains the tubercle organism.
+Under such conditions the appearance of the milk is not materially
+altered at first, but as the disease progresses the percentage of fat
+generally diminishes, and at times in the more advanced stages where the
+physical condition of the udder is changed (Fig. 21),<span class='pagenum'><a name="Page_88" id="Page_88">[Pg 88]</a></span> the milk may
+become "watery"; but the percentage of animals showing such udder
+lesions is not large, usually not more than a few per cent. (4 per cent.
+according to Ostertag.)</p>
+
+<p>On the other hand, in the earlier phases of the disease, where its
+presence has been recognized solely by the aid of the tuberculin test,
+before there are any recognizable physical symptoms in any part of the
+animal, the milk is generally unaffected. Between these extremes,
+however, is found a large proportion of cases, concerning which so
+definite data are not available. The results of investigators on this
+point are conflicting and further information is much desired. Some have
+asserted so long as the udder itself shows no lesions that no tubercle
+bacilli would be present,<a name="FNanchor_86_86" id="FNanchor_86_86"></a><a href="#Footnote_86_86" class="fnanchor">[86]</a> but the findings of a considerable number
+of investigators<a name="FNanchor_87_87" id="FNanchor_87_87"></a><a href="#Footnote_87_87" class="fnanchor">[87]</a> indicate that even when the udder is apparently not
+diseased the milk may contain the specific organism as revealed by
+inoculation experiments upon animals. In some cases, however, it has
+been demonstrated by post-mortem examination that discoverable udder
+lesions existed that were not recognizable before autopsy was made. In
+the experimental evidence collected, a varying percentage of reacting
+animals were found that gave positive results; and this number was
+generally sufficient to indicate that the danger of using milk from
+reacting animals was considerable, even though apparently no disease
+could be found in the udder.</p>
+
+<p>The infectiousness of milk can also be proven by the frequent
+contraction of the disease in other animals, such as calves and pigs
+which may be fed on the skim milk. The very rapid increase of the
+disease among the swine of Germany<span class='pagenum'><a name="Page_89" id="Page_89">[Pg 89]</a></span> and Denmark,<a name="FNanchor_88_88" id="FNanchor_88_88"></a><a href="#Footnote_88_88" class="fnanchor">[88]</a> and the frequently
+reported cases of intestinal infection of young stock also attest the
+presence of the organism in milk.</p>
+
+<p>The tubercle bacillus is so markedly parasitic in its habits, that,
+under ordinary conditions, it is incapable of growing at normal air
+temperatures. There is, therefore, no danger of the germ developing in
+milk after it is drawn from the animal, unless the same is kept at
+practically blood heat.</p>
+
+<p>Even though the milk of some reacting animals may not contain the
+dangerous organism at the time of making the test, it is quite
+impossible to foretell how long it will remain free. As the disease
+becomes more generalized, or if tuberculous lesions should develop in
+the udder, the milk may pass from a healthy to an infectious state.</p>
+
+<p>This fact makes it advisable to exclude from milk supplies intended for
+human use, all milk of animals that respond to the tuberculin test; or
+at least to treat it in a manner so as to render it safe. Whether it is
+necessary to do this or not if the milk is made into butter or cheese is
+a somewhat different question. Exclusion or treatment is rendered more
+imperative in milk supplies, because the danger is greater with children
+with whom milk is often a prominent constituent of their diet, and also
+for the reason that the child is more susceptible to intestinal
+infection than the adult.</p>
+
+<p>The danger of infection is much lessened in butter or cheese, because
+the processes of manufacture tend to diminish the number of organisms
+originally present in the milk, and inasmuch as no growth can ordinarily
+take place in these products the danger is minimized. Moreover, the fact
+that<span class='pagenum'><a name="Page_90" id="Page_90">[Pg 90]</a></span> these foods are consumed by the individual in smaller amounts than
+is generally the case where milk is used, and also to a greater extent
+by adults, lessens still further the danger of infection.</p>
+
+<p>Notwithstanding this, numerous observers<a name="FNanchor_89_89" id="FNanchor_89_89"></a><a href="#Footnote_89_89" class="fnanchor">[89]</a> especially in Germany have
+succeeded in finding the tubercle bacillus in market butter, but this
+fact is not so surprising when it is remembered that a very large
+fraction of their cattle show the presence of the disease as indicated
+by the tuberculin test, a condition that does not obtain in any large
+section in this country.</p>
+
+<p>The observations on the presence of the tubercle bacillus in butter have
+been questioned somewhat of late[2] by the determination of the fact
+that butter may contain an organism that possesses the property of being
+stained in the same way as the tubercle organism. Differentiation
+between the two forms is rendered more difficult by the fact that this
+tubercle-like organism is also capable of producing in animals lesions
+that stimulate those of tuberculosis, although a careful examination
+reveals definite differences. Petri<a name="FNanchor_90_90" id="FNanchor_90_90"></a><a href="#Footnote_90_90" class="fnanchor">[90]</a> has recently determined that
+both the true tubercle and the acid-resisting butter organism may be
+readily found in market butter.</p>
+
+<p>In the various milk products it has been experimentally determined that
+the true tubercle bacillus is able to retain its vitality in butter for
+a number of months and in cheese for nearly a year.</p>
+
+<p><b>Treatment of milk from tuberculosis cows.</b> While it has been shown that
+it is practically impossible to foretell whether the milk of any
+reacting animal actually contains<span class='pagenum'><a name="Page_91" id="Page_91">[Pg 91]</a></span> tubercle bacilli or not, still the
+interests of public health demand that no milk from such stock be used
+for human food until it has been rendered safe by some satisfactory
+treatment.</p>
+
+<p><i>1. Heating.</i> By far the best treatment that can be given such milk is
+to heat it. The temperature at which this should be done depends upon
+the thermal death point of the tubercle bacillus, a question concerning
+which there has been considerable difference of opinion until very
+recently. According to the work of some of the earlier investigators,
+the tubercle bacillus in its vegetative stage is endowed with powers of
+resistance greater than those possessed by any other pathogenic
+organism. This work has not been substantiated by the most recent
+investigations on this subject. In determining the thermal death point
+of this organism, as of any other, not only must the temperature be
+considered, but the period of exposure as well, and where that exposure
+is made in milk, another factor must be considered, viz., the presence
+of conditions permitting of the formation of a "scalded layer," for as
+Smith<a name="FNanchor_91_91" id="FNanchor_91_91"></a><a href="#Footnote_91_91" class="fnanchor">[91]</a> first pointed out, the resistance of the tubercle organism
+toward heat is greatly increased under these conditions. If tuberculous
+milk is heated in a closed receptacle where this scalded membrane cannot
+be produced, the tubercle bacillus is killed at 140&deg; F. in 15 to 20
+minutes. These results which were first determined by Smith, under
+laboratory conditions, and confirmed by Russell and Hastings,<a name="FNanchor_92_92" id="FNanchor_92_92"></a><a href="#Footnote_92_92" class="fnanchor">[92]</a> where
+tuberculous milk was heated in commercial pasteurizers, have also been
+verified by Hesse.<a name="FNanchor_93_93" id="FNanchor_93_93"></a><a href="#Footnote_93_93" class="fnanchor">[93]</a> A great practical advantage which accrues from
+the treatment of milk at<span class='pagenum'><a name="Page_92" id="Page_92">[Pg 92]</a></span> 140&deg; F. is that the natural creaming is
+practically unaffected. Of course, where a higher temperature is
+employed, the period of exposure may be materially lessened. If milk is
+momentarily heated to 176&deg; F., it is certainly sufficient to destroy the
+tubercle bacillus. This is the plan practiced in Denmark where all skim
+milk and whey must be heated to this temperature before it can be taken
+back to the farm, a plan which is designed to prevent the dissemination
+of tuberculosis and foot and mouth disease by means of the mixed
+creamery by-products. This course renders it possible to utilize with
+perfect safety, for milk supplies, the milk of herds reacting to the
+tuberculin test, and as butter of the best quality can be made from
+cream or milk heated to even high temperatures,<a name="FNanchor_94_94" id="FNanchor_94_94"></a><a href="#Footnote_94_94" class="fnanchor">[94]</a> it thus becomes
+possible to prevent with slight expense what would otherwise entail a
+large loss.</p>
+
+<p><i>2. Dilution.</i> Another method that has been suggested for the treatment
+of this suspected milk is dilution with a relatively large volume of
+perfectly healthy milk. It is a well known fact that to produce
+infection, it requires the simultaneous introduction of a number of
+organisms, and in the case of tuberculosis, especially that produced by
+ingestion, this number is thought to be considerable. Gebhardt<a name="FNanchor_95_95" id="FNanchor_95_95"></a><a href="#Footnote_95_95" class="fnanchor">[95]</a> found
+that the milk of tuberculous cows, which was virulent when injected by
+itself into animals, was innocuous when diluted with 40 to 100 times its
+volume of healthy milk. This fact is hardly to be relied upon in
+practice, unless the proportion of reacting to healthy cows is
+positively known.</p>
+
+<p>It has also been claimed in the centrifugal separation of<span class='pagenum'><a name="Page_93" id="Page_93">[Pg 93]</a></span> cream from
+milk<a name="FNanchor_96_96" id="FNanchor_96_96"></a><a href="#Footnote_96_96" class="fnanchor">[96]</a> that by far the larger number of tubercle bacilli were thrown
+out with the separator slime. Moore<a name="FNanchor_97_97" id="FNanchor_97_97"></a><a href="#Footnote_97_97" class="fnanchor">[97]</a> has shown that the tubercle
+bacilli in an artificially infected milk might be reduced in this way,
+so as to be no longer microscopically demonstrable, yet the purification
+was not complete enough to prevent the infection of animals inoculated
+with the milk.</p>
+
+<p>Another way to exclude all possibility of tubercular infection in milk
+supplies is to reject all milk from reacting animals. This method is
+often followed where pasteurization or sterilization is not desired. In
+dairies where the keeping quality is dependent upon the exclusion of
+bacteria by stringent conditions as to milking and handling ("sanitary"
+or "hygienic" milk), the tuberculin test is frequently used as a basis
+to insure healthy milk.</p>
+
+<p><b>Foot and mouth disease.</b> The wide-spread extension of this disease
+throughout Europe in recent years has given abundant opportunity to show
+that while it is distinctively an animal malady, it is also
+transmissible to man, although the disease is rarely fatal. The causal
+organism has not been determined with certainty, but it has been shown
+that the milk of affected animals possesses infectious properties<a name="FNanchor_98_98" id="FNanchor_98_98"></a><a href="#Footnote_98_98" class="fnanchor">[98]</a>
+although appearing unchanged in earlier phases of the disease.</p>
+
+<p>Hertwig showed the direct transmissibility of the disease to man by
+experiments made on himself and others. By ingesting milk from an
+affected animal, he was able to produce the symptoms of the disease, the
+mucous membrane of the mouth being covered with the small vesicles that
+characterize the malady. It has also been shown that<span class='pagenum'><a name="Page_94" id="Page_94">[Pg 94]</a></span> the virus of the
+disease may be conveyed in butter.<a name="FNanchor_99_99" id="FNanchor_99_99"></a><a href="#Footnote_99_99" class="fnanchor">[99]</a> This disease is practically
+unknown in this country, although widely spread in Europe.</p>
+
+<p>There are a number of other bovine diseases such as anthrax,<a name="FNanchor_100_100" id="FNanchor_100_100"></a><a href="#Footnote_100_100" class="fnanchor">[100]</a>
+lockjaw,<a name="FNanchor_101_101" id="FNanchor_101_101"></a><a href="#Footnote_101_101" class="fnanchor">[101]</a> and hydrophobia<a name="FNanchor_102_102" id="FNanchor_102_102"></a><a href="#Footnote_102_102" class="fnanchor">[102]</a> in which it has been shown that the
+virus of the disease is at times to be found in the milk supply, but
+often the milk becomes visibly affected, so that the danger of using the
+same is greatly minimized.</p>
+
+<p>There are also a number of inflammatory udder troubles known as garget
+or mammitis. In most of these, the physical appearance of the milk is so
+changed, and often pus is present to such a degree as to give a very
+disagreeable appearance to the milk. Pus-forming bacteria (staphylococci
+and streptococci) are to be found associated with such troubles. A
+number of cases of gastric and intestinal catarrh have been reported as
+caused by such milks.<a name="FNanchor_103_103" id="FNanchor_103_103"></a><a href="#Footnote_103_103" class="fnanchor">[103]</a></p>
+
+
+<h4>DISEASES TRANSMISSIBLE TO MAN THROUGH INFECTION OF MILK AFTER
+WITHDRAWAL.</h4>
+
+<p>Milk is so well adapted to the development of bacteria in general, that
+it is not surprising to find it a suitable medium for the growth of many
+pathogenic species even at ordinary temperatures. Not infrequently,
+disease-producing bacteria are able to grow in raw milk in competition
+with the normal milk bacteria, so that even a slight contamination may
+suffice to produce infection.</p>
+
+<p>The diseases that are most frequently disseminated in<span class='pagenum'><a name="Page_95" id="Page_95">[Pg 95]</a></span> this way are
+typhoid fever, diphtheria, scarlet fever and cholera, together with the
+various illy-defined intestinal troubles of a toxic character that occur
+in children, especially under the name of cholera infantum, summer
+complaint, etc.</p>
+
+<p>Diseases of this class are not derived directly from animals because
+cattle are not susceptible to the same.</p>
+
+<p><b>Modes of infection.</b> In a variety of ways, however, the milk may be
+subject to contaminating influences after it is drawn from the animal,
+and so give opportunity for the development of disease-producing
+bacteria. The more important methods of infection are as follows:</p>
+
+<p><i>1. Infection directly from a pre-existing case of disease on premises.</i>
+Quite frequently a person in the early stage of a diseased condition may
+continue at his usual vocation as helper in the barn or dairy, and so
+give opportunity for direct infection to occur. In the so-called cases
+of "walking typhoid," this danger is emphasized. It is noteworthy in
+typhoid fever that the bacilli frequently persist in the urine and in
+diphtheria they often remain in the throat until after convalescence. In
+some cases infection has been traced to storage of the milk in rooms in
+the house where it became polluted directly by the emanations of the
+patient.<a name="FNanchor_104_104" id="FNanchor_104_104"></a><a href="#Footnote_104_104" class="fnanchor">[104]</a> Among the dwellings of the lower classes where a single
+room has to be used in common this source of infection has been most
+frequently observed.</p>
+
+<p><i>2. Infection through the medium of another person.</i> Not infrequently
+another individual may serve in the capacity of nurse or attendant to a
+sick person, and also assist in the handling of the milk, either in
+milking the animals or<span class='pagenum'><a name="Page_96" id="Page_96">[Pg 96]</a></span> caring for the milk after it has been drawn.
+Busey and Kober report twenty-one outbreaks of typhoid fever in which
+dairy employees also acted in the capacity of nurses.</p>
+
+<p><i>3. Pollution of milk utensils.</i> The most frequent method of infection
+of cans, pails, etc., is in cleaning them with water that may be
+polluted with disease organisms. Often wells may be contaminated with
+diseased matter of intestinal origin, as in typhoid fever, and the use
+of water at normal temperatures, or even in a lukewarm condition, give
+conditions permitting of infection. Intentional adulteration of milk
+with water inadvertently taken from polluted sources has caused quite a
+number of typhoid outbreaks.<a name="FNanchor_105_105" id="FNanchor_105_105"></a><a href="#Footnote_105_105" class="fnanchor">[105]</a> Sedgwick and Chapin<a name="FNanchor_106_106" id="FNanchor_106_106"></a><a href="#Footnote_106_106" class="fnanchor">[106]</a> found in the
+Springfield, Mass., epidemic of typhoid that the milk cans were placed
+in a well to cool the milk, and it was subsequently shown that the well
+was polluted with typhoid fecal matter.</p>
+
+<p><i>4. Pollution of udder</i> of animal <i>by wading in infected water</i>, or by
+washing same with contaminated water. This method of infection would
+only be likely to occur in case of typhoid. An outbreak at the
+University of Virginia in 1893<a name="FNanchor_107_107" id="FNanchor_107_107"></a><a href="#Footnote_107_107" class="fnanchor">[107]</a> was ascribed to the latter cause.</p>
+
+<p><i>5. Pollution of creamery by-products, skim-milk, etc.</i> Where the milk
+supply of one patron becomes infected with pathogenic bacteria, it is
+possible that disease may be disseminated through the medium of the
+creamery, the infective agent remaining in the skim milk after
+separation and so polluting the mixed supply. This condition is more
+likely to prevail with typhoid because of the greater tolerance of this
+organism for acids such as would be found in raw<span class='pagenum'><a name="Page_97" id="Page_97">[Pg 97]</a></span> milk. The outbreaks at
+Brandon,<a name="FNanchor_108_108" id="FNanchor_108_108"></a><a href="#Footnote_108_108" class="fnanchor">[108]</a> England, in 1893, Castle Island,<a name="FNanchor_109_109" id="FNanchor_109_109"></a><a href="#Footnote_109_109" class="fnanchor">[109]</a> Ireland, and
+Marlboro,<a name="FNanchor_110_110" id="FNanchor_110_110"></a><a href="#Footnote_110_110" class="fnanchor">[110]</a> Mass., in 1894, were traced to such an origin.</p>
+
+<p>While most outbreaks of disease associated with a polluted milk supply
+originate in the use of the milk itself, yet infected milk may serve to
+cause disease even when used in other ways. Several outbreaks of typhoid
+fever have been traced to the use of ice cream where there were strong
+reasons for believing that the milk used in the manufacture of the
+product was polluted.<a name="FNanchor_111_111" id="FNanchor_111_111"></a><a href="#Footnote_111_111" class="fnanchor">[111]</a> Hankin<a name="FNanchor_112_112" id="FNanchor_112_112"></a><a href="#Footnote_112_112" class="fnanchor">[112]</a> details a case of an Indian
+confection made largely from milk that caused a typhoid outbreak in a
+British regiment.</p>
+
+<p>Although the evidence that milk may not infrequently serve as an agent
+in spreading disease is conclusive enough to satisfactorily prove the
+proposition, yet it should be borne in mind that the organism of any
+specific disease in question has rarely ever been found. The reasons for
+this are quite the same as those that govern the situation in the case
+of polluted waters, except that the difficulties of the problem are much
+greater in the case of milk than with water. The inability to readily
+separate the typhoid germ, for instance, from the colon bacillus, an
+organism frequently found in milk, presents technical difficulties not
+easily overcome. The most potent reason of failure to find disease
+bacteria is the fact that infection in any case must occur sometime
+previous to the appearance of the outbreak. Not only is there the usual
+period of incubation, but it rarely happens that an outbreak is
+investigated until a number of<span class='pagenum'><a name="Page_98" id="Page_98">[Pg 98]</a></span> cases have occurred. In this interim the
+original cause of infection may have ceased to be operative.</p>
+
+<p><b>Typhoid fever.</b> With reference to the diseases likely to to be
+disseminated through the medium of milk, infected after being drawn from
+the animal, typhoid fever is the most important. The reason for this is
+due (1) to the wide spread distribution of the disease; (2) to the fact
+that the typhoid bacillus is one that is capable of withstanding
+considerable amounts of acid, and consequently finds even in raw milk
+containing the normal lactic acid bacteria conditions favorable for its
+growth.<a name="FNanchor_113_113" id="FNanchor_113_113"></a><a href="#Footnote_113_113" class="fnanchor">[113]</a> Ability to grow under these conditions can be shown not
+only experimentally, but there is abundant clinical evidence that even a
+slight infection often causes extensive outbreaks, as in the Stamford,
+Conn., outbreak in 1895 where 386 cases developed in a few weeks, 97 per
+cent. of which occurred on the route of one milk-man. In this case the
+milk cans were thoroughly and properly cleaned, but were rinsed out with
+<i>cold</i> water from a shallow well that was found to be polluted.</p>
+
+<p>The most common mode of pollution of milk with typhoid organisms is
+where the milk utensils are infected in one way or another.<a name="FNanchor_114_114" id="FNanchor_114_114"></a><a href="#Footnote_114_114" class="fnanchor">[114]</a> Second
+in importance is the carrying of infection by persons serving in the
+dual capacity of nurse and dairy attendant.</p>
+
+<p><b>Cholera.</b> This germ does not find milk so favorable a nutrient medium as
+the typhoid organism, because it is much more sensitive toward the
+action of acids. Kitasato<span class='pagenum'><a name="Page_99" id="Page_99">[Pg 99]</a></span><a name="FNanchor_115_115" id="FNanchor_115_115"></a><a href="#Footnote_115_115" class="fnanchor">[115]</a> found, however, that it could live in
+raw milk from one to four days, depending upon the amount of acid
+present. In boiled or sterilized milk it grows more freely, as the
+acid-producing forms are thereby eliminated. In butter it dies out in a
+few days (4 to 5).</p>
+
+<p>On account of the above relation not a large number of cholera outbreaks
+have been traced to milk, but Simpson<a name="FNanchor_116_116" id="FNanchor_116_116"></a><a href="#Footnote_116_116" class="fnanchor">[116]</a> records a very striking case
+in India where a number of sailors, upon reaching port, secured a
+quantity of milk. Of the crew which consumed this, every one was taken
+ill, and four out of ten died, while those who did not partake escaped
+without any disease. It was later shown that the milk was adulterated
+with water taken from an open pool in a cholera infected district.</p>
+
+<p><b>Diphtheria.</b> Milk occasionally, though not often, serves as a medium for
+the dissemination of diphtheria. Swithinbank and Newman<a name="FNanchor_117_117" id="FNanchor_117_117"></a><a href="#Footnote_117_117" class="fnanchor">[117]</a> cites four
+cases in which the causal organism has been isolated from milk. It has
+been observed that growth occurs more rapidly in raw than in sterilized
+milk.<a name="FNanchor_118_118" id="FNanchor_118_118"></a><a href="#Footnote_118_118" class="fnanchor">[118]</a></p>
+
+<p>Infection in this disease is more frequently attributable to direct
+infection from patient on account of the long persistence of this germ
+in the throat, or indirectly through the medium of an attendant.</p>
+
+<p><b>Scarlet fever.</b> Although it is more difficult to study the relation of
+this disease to contaminated milk supplies, because the causal germ of
+scarlet fever is not yet known, yet the origin of a considerable number
+of epidemics has<span class='pagenum'><a name="Page_100" id="Page_100">[Pg 100]</a></span> been traced to polluted milk supplies. Milk doubtless
+is infected most frequently from persons in the earlier stages of the
+disease when the infectivity of the disease is greater.</p>
+
+<p><b>Diarrhoeal diseases.</b> Milk not infrequently acquires the property of
+producing diseases of the digestive tract by reason of the development
+of various bacteria that form more or less poisonous by-products. These
+troubles occur most frequently during the summer months, especially with
+infants and children, as in cholera infantum and summer complaint. The
+higher mortality of bottle-fed infants<a name="FNanchor_119_119" id="FNanchor_119_119"></a><a href="#Footnote_119_119" class="fnanchor">[119]</a> in comparison with those
+that are nursed directly is explicable on the theory that cows' milk is
+the carrier of the infection, because in many cases it is not consumed
+until there has been ample time for the development of organisms in it.
+Where milk is pasteurized or boiled it is found that the mortality among
+children is greatly reduced. As a cause of sickness and death these
+diseases exceed in importance all other specific diseases previously
+referred to. These troubles have generally been explained as produced by
+bacteria of the putrefactive class which find their way into the milk
+through the introduction of filth and dirt at time of milking.<a name="FNanchor_120_120" id="FNanchor_120_120"></a><a href="#Footnote_120_120" class="fnanchor">[120]</a>
+Fl&uuml;gge<a name="FNanchor_121_121" id="FNanchor_121_121"></a><a href="#Footnote_121_121" class="fnanchor">[121]</a> has demonstrated that certain peptonizing species possess
+toxic properties for animals. Recent experimental inquiry<a name="FNanchor_122_122" id="FNanchor_122_122"></a><a href="#Footnote_122_122" class="fnanchor">[122]</a> has
+demonstrated that the dysentery bacillus (Shiga) probably bears a causal
+relation to some of these summer complaints.</p>
+
+<p><span class='pagenum'><a name="Page_101" id="Page_101">[Pg 101]</a></span></p><p><b>Ptomaine poisoning.</b> Many cases of poisoning from food products are also
+reported with adults. These are due to the formation of various toxic
+products, generally ptomaines, that are produced as a result of
+infection of foods by different bacteria. One of these substances,
+<i>tyrotoxicon</i>, was isolated by Vaughan<a name="FNanchor_123_123" id="FNanchor_123_123"></a><a href="#Footnote_123_123" class="fnanchor">[123]</a> from cheese and various
+other products of milk, and found to possess the property of producing
+symptoms of poisoning similar to those that are noted in such cases. He
+attributes the production of this toxic effect to the decomposition of
+the elements in the milk induced by putrefactive forms of bacteria that
+develop where milk is improperly kept.<a name="FNanchor_124_124" id="FNanchor_124_124"></a><a href="#Footnote_124_124" class="fnanchor">[124]</a> Often outbreaks of this
+character<a name="FNanchor_125_125" id="FNanchor_125_125"></a><a href="#Footnote_125_125" class="fnanchor">[125]</a> assume the proportions of an epidemic, where a large
+number of persons use the tainted food.</p>
+
+<div class="footnotes"><h3>FOOTNOTES:</h3>
+
+<div class="footnote"><p><a name="Footnote_78_78" id="Footnote_78_78"></a><a href="#FNanchor_78_78"><span class="label">[78]</span></a> Hart, Trans. Int. Med. Cong., London, 1881, 4:491-544.</p></div>
+
+<div class="footnote"><p><a name="Footnote_79_79" id="Footnote_79_79"></a><a href="#FNanchor_79_79"><span class="label">[79]</span></a> Freeman, Med. Rec., March 28, 1896.</p></div>
+
+<div class="footnote"><p><a name="Footnote_80_80" id="Footnote_80_80"></a><a href="#FNanchor_80_80"><span class="label">[80]</span></a> Busey and Kober, Rept. Health Off. of Dist. of Col.,
+Washington, D. C., 1895, p. 299. These authors present in this report an
+elaborate article on morbific and infectious milk, giving a very
+complete bibliography of 180 numbers. They append to Hart's list (which
+is published in full) additional outbreaks which have occurred since,
+together with full data as to extent of epidemic, circumstances
+governing the outbreak, as well as name of original reporter and
+reference.</p></div>
+
+<div class="footnote"><p><a name="Footnote_81_81" id="Footnote_81_81"></a><a href="#FNanchor_81_81"><span class="label">[81]</span></a> Smith, Theo., Journ. of Expt. Med., 1898, 3:451.</p></div>
+
+<div class="footnote"><p><a name="Footnote_82_82" id="Footnote_82_82"></a><a href="#FNanchor_82_82"><span class="label">[82]</span></a> Dinwiddie, Bull. 57, Ark. Expt. Stat., June, 1899;
+Ravenel, Univ. of Penn. Med. Bull., Sept. 1901.</p></div>
+
+<div class="footnote"><p><a name="Footnote_83_83" id="Footnote_83_83"></a><a href="#FNanchor_83_83"><span class="label">[83]</span></a> Ravenel, Journ. of Comp. Med. &amp; Vet. Arch., Dec. 1897;
+Hartzell, Journ. Amer. Med. Ass'n, April 16, 1898.</p></div>
+
+<div class="footnote"><p><a name="Footnote_84_84" id="Footnote_84_84"></a><a href="#FNanchor_84_84"><span class="label">[84]</span></a> Stille, Brit. Med. Journ., Aug. 19, 1899.</p></div>
+
+<div class="footnote"><p><a name="Footnote_85_85" id="Footnote_85_85"></a><a href="#FNanchor_85_85"><span class="label">[85]</span></a> This test is made by injecting into the animal a small
+quantity of tuberculin, which is a sterilized glycerin extract of
+cultures of the tubercle bacillus. In a tuberculous animal, even in the
+very earliest phases of the disease, tuberculin causes a temporary fever
+that lasts for a few hours. By taking the temperature a number of times
+before and after injection it is possible to readily recognize any
+febrile condition. A positive diagnosis is made where the temperature
+after inoculation is at least 2.0&deg; F. above the average normal, and
+where the reaction fever is continued for a period of some hours.</p></div>
+
+<div class="footnote"><p><a name="Footnote_86_86" id="Footnote_86_86"></a><a href="#FNanchor_86_86"><span class="label">[86]</span></a> Martin, Brit. Med. Journ. 1895, 1:937; Nocard, Les
+Tuberculoses animales, 1895.</p></div>
+
+<div class="footnote"><p><a name="Footnote_87_87" id="Footnote_87_87"></a><a href="#FNanchor_87_87"><span class="label">[87]</span></a> C. O. Jensen, Milch Kunde und Milch hygiene, p. 69.</p></div>
+
+<div class="footnote"><p><a name="Footnote_88_88" id="Footnote_88_88"></a><a href="#FNanchor_88_88"><span class="label">[88]</span></a> Ostertag, Milch Zeit., 22:672.</p></div>
+
+<div class="footnote"><p><a name="Footnote_89_89" id="Footnote_89_89"></a><a href="#FNanchor_89_89"><span class="label">[89]</span></a> Oberm&uuml;ller, Hyg. Rund., 1897, p. 712; Petri, Arb. a. d.
+Kais. Ges. Amte, 1898, 14: 1; Hormann und Morgenroth, Hyg. Rund., 1898,
+p. 217.</p></div>
+
+<div class="footnote"><p><a name="Footnote_90_90" id="Footnote_90_90"></a><a href="#FNanchor_90_90"><span class="label">[90]</span></a> Rabinowitsch, Zeit. f. Hyg., 1897, 26: 90.</p></div>
+
+<div class="footnote"><p><a name="Footnote_91_91" id="Footnote_91_91"></a><a href="#FNanchor_91_91"><span class="label">[91]</span></a> Th. Smith. Journ. of Expt. Med., 1899, 4:217.</p></div>
+
+<div class="footnote"><p><a name="Footnote_92_92" id="Footnote_92_92"></a><a href="#FNanchor_92_92"><span class="label">[92]</span></a> Russell and Hastings, 18 Rept. Wis. Expt. Stat., 1901.</p></div>
+
+<div class="footnote"><p><a name="Footnote_93_93" id="Footnote_93_93"></a><a href="#FNanchor_93_93"><span class="label">[93]</span></a> Hesse, Zeit. f. Hyg., 1900, 34:346.</p></div>
+
+<div class="footnote"><p><a name="Footnote_94_94" id="Footnote_94_94"></a><a href="#FNanchor_94_94"><span class="label">[94]</span></a> Practically all of the finest butter made in Denmark is
+made from cream that has been pasteurized at temperatures varying from
+160&deg;-185&deg; F.</p></div>
+
+<div class="footnote"><p><a name="Footnote_95_95" id="Footnote_95_95"></a><a href="#FNanchor_95_95"><span class="label">[95]</span></a> Gebhardt, Virch. Arch., 1890, 119:12.</p></div>
+
+<div class="footnote"><p><a name="Footnote_96_96" id="Footnote_96_96"></a><a href="#FNanchor_96_96"><span class="label">[96]</span></a> Scheurlen, Arb. a. d. k. Ges. Amte, 1891, 7:269; Bang,
+Milch Zeit., 1893, p. 672.</p></div>
+
+<div class="footnote"><p><a name="Footnote_97_97" id="Footnote_97_97"></a><a href="#FNanchor_97_97"><span class="label">[97]</span></a> Moore, Year Book of U. S. Dept. Agr., 1895, p. 432.</p></div>
+
+<div class="footnote"><p><a name="Footnote_98_98" id="Footnote_98_98"></a><a href="#FNanchor_98_98"><span class="label">[98]</span></a> Weigel and Noack, Jahres. d. Ges. Med., 1890, p. 642;
+Weissenberg, Allg. med. Cent. Zeit., 1890, p. 1; Baum, Arch. f.
+Thierheilkunde, 1892, 18:16.</p></div>
+
+<div class="footnote"><p><a name="Footnote_99_99" id="Footnote_99_99"></a><a href="#FNanchor_99_99"><span class="label">[99]</span></a> Schneider, M&uuml;nch, med. Wochenschr., 1893, No. 27; Fr&ouml;hner,
+Zeit f. Fleisch u. Milchhygiene, 1891, p. 55.</p></div>
+
+<div class="footnote"><p><a name="Footnote_100_100" id="Footnote_100_100"></a><a href="#FNanchor_100_100"><span class="label">[100]</span></a> Feser, Deutsche Zeit. f. Thiermed., 1880, 6:166.</p></div>
+
+<div class="footnote"><p><a name="Footnote_101_101" id="Footnote_101_101"></a><a href="#FNanchor_101_101"><span class="label">[101]</span></a> Nocard, Bull. G&eacute;n., 1885, p. 54.</p></div>
+
+<div class="footnote"><p><a name="Footnote_102_102" id="Footnote_102_102"></a><a href="#FNanchor_102_102"><span class="label">[102]</span></a> Deutsche Viertelsjahr. f. offentl. Gesundheitspflege,
+1890, 20:444.</p></div>
+
+<div class="footnote"><p><a name="Footnote_103_103" id="Footnote_103_103"></a><a href="#FNanchor_103_103"><span class="label">[103]</span></a> Zeit. f. Fleisch und Milch hygiene, 11:114.</p></div>
+
+<div class="footnote"><p><a name="Footnote_104_104" id="Footnote_104_104"></a><a href="#FNanchor_104_104"><span class="label">[104]</span></a> E. Roth, Deutsche Vierteljahresschr. f. offentl.
+Gesundheitspfl., 1890, 22:238</p></div>
+
+<div class="footnote"><p><a name="Footnote_105_105" id="Footnote_105_105"></a><a href="#FNanchor_105_105"><span class="label">[105]</span></a> S. W. North, London Practitioner, 1889, 43:393.</p></div>
+
+<div class="footnote"><p><a name="Footnote_106_106" id="Footnote_106_106"></a><a href="#FNanchor_106_106"><span class="label">[106]</span></a> Sedgwick and Chapin, Boston Med. &amp; Surg. Journ., 1893,
+129:485.</p></div>
+
+<div class="footnote"><p><a name="Footnote_107_107" id="Footnote_107_107"></a><a href="#FNanchor_107_107"><span class="label">[107]</span></a> Dabney, Phila. Med. News, 1893, 63:630.</p></div>
+
+<div class="footnote"><p><a name="Footnote_108_108" id="Footnote_108_108"></a><a href="#FNanchor_108_108"><span class="label">[108]</span></a> Welphy, London Lancet, 1894, 2:1085.</p></div>
+
+<div class="footnote"><p><a name="Footnote_109_109" id="Footnote_109_109"></a><a href="#FNanchor_109_109"><span class="label">[109]</span></a> Brit. Med. Journ., 1894, 1:815.</p></div>
+
+<div class="footnote"><p><a name="Footnote_110_110" id="Footnote_110_110"></a><a href="#FNanchor_110_110"><span class="label">[110]</span></a> Mass. Bd. Health Rept., 1894, p. 765.</p></div>
+
+<div class="footnote"><p><a name="Footnote_111_111" id="Footnote_111_111"></a><a href="#FNanchor_111_111"><span class="label">[111]</span></a> Turner, London Practitioner, 1892, 49:141; Munro, Brit.
+Med. Journ., 1894, 2:829.</p></div>
+
+<div class="footnote"><p><a name="Footnote_112_112" id="Footnote_112_112"></a><a href="#FNanchor_112_112"><span class="label">[112]</span></a> Hankin, Brit. Med. Journ., 1894, 2:613.</p></div>
+
+<div class="footnote"><p><a name="Footnote_113_113" id="Footnote_113_113"></a><a href="#FNanchor_113_113"><span class="label">[113]</span></a> Heim (Arb. a. d. Kais. Gesundheitsamte, 1889, 5:303)
+finds it capable of living from 20-30 days in milk.</p></div>
+
+<div class="footnote"><p><a name="Footnote_114_114" id="Footnote_114_114"></a><a href="#FNanchor_114_114"><span class="label">[114]</span></a> Sch&uuml;der (Zeit. f. Hyg., 1902, 38:34) examined the
+statistics of 638 typhoid epidemics. He found 71 per cent. due to
+infected drinking water, 17 per cent. to infected milk, and 3.5 per
+cent. caused by other forms of food.</p></div>
+
+<div class="footnote"><p><a name="Footnote_115_115" id="Footnote_115_115"></a><a href="#FNanchor_115_115"><span class="label">[115]</span></a> Kitasato. Arb. a. d. Kais. Gesundheitsamte, 1:470.</p></div>
+
+<div class="footnote"><p><a name="Footnote_116_116" id="Footnote_116_116"></a><a href="#FNanchor_116_116"><span class="label">[116]</span></a> Simpson, London Practitioner, 1887, 39:144.</p></div>
+
+<div class="footnote"><p><a name="Footnote_117_117" id="Footnote_117_117"></a><a href="#FNanchor_117_117"><span class="label">[117]</span></a> Swithinbank and Newman, Bacteriology of Milk, p. 341.</p></div>
+
+<div class="footnote"><p><a name="Footnote_118_118" id="Footnote_118_118"></a><a href="#FNanchor_118_118"><span class="label">[118]</span></a> Schottelius and Ellerhorst. Milch Zeit., 1897, pp. 40 and
+73.</p></div>
+
+<div class="footnote"><p><a name="Footnote_119_119" id="Footnote_119_119"></a><a href="#FNanchor_119_119"><span class="label">[119]</span></a> Baginsky, Hyg. Rund., 1895, p. 176.</p></div>
+
+<div class="footnote"><p><a name="Footnote_120_120" id="Footnote_120_120"></a><a href="#FNanchor_120_120"><span class="label">[120]</span></a> Gaffky, Deutsch. med. Wochen., 18:14.</p></div>
+
+<div class="footnote"><p><a name="Footnote_121_121" id="Footnote_121_121"></a><a href="#FNanchor_121_121"><span class="label">[121]</span></a> Fl&uuml;gge. Zeit., f. Hyg., 17:272, 1894.</p></div>
+
+<div class="footnote"><p><a name="Footnote_122_122" id="Footnote_122_122"></a><a href="#FNanchor_122_122"><span class="label">[122]</span></a> Duval and Bassett, Studies from the Rockefeller Inst. for
+Med. Research, 2:7, 1904.</p></div>
+
+<div class="footnote"><p><a name="Footnote_123_123" id="Footnote_123_123"></a><a href="#FNanchor_123_123"><span class="label">[123]</span></a> Zeit. f. physiol. Chemie, 10:146; 9 Intern. Hyg. Cong.
+(London), 1891, p. 118.</p></div>
+
+<div class="footnote"><p><a name="Footnote_124_124" id="Footnote_124_124"></a><a href="#FNanchor_124_124"><span class="label">[124]</span></a> Vaughan and Perkins, Arch. f. Hyg., 27:308.</p></div>
+
+<div class="footnote"><p><a name="Footnote_125_125" id="Footnote_125_125"></a><a href="#FNanchor_125_125"><span class="label">[125]</span></a> Newton and Wallace (Phila. Med. News, 1887, 50:570)
+report three outbreaks at Long Branch, N. J., two of which occurred in
+summer hotels.</p></div>
+</div>
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_102" id="Page_102">[Pg 102]</a></span></p>
+<h2>CHAPTER VI.</h2>
+
+<h3>BACTERIA AND MILK SUPPLIES WITH ESPECIAL REFERENCE TO METHODS OF PRESERVATION.</h3>
+
+
+<p>To the milk dealer or distributor, bacteria are more or less of a
+detriment. None of the organisms that find their way into milk, nor the
+by-products formed by their growth, improve the quality of milk
+supplies. It is therefore especially desirable from the milk-dealer's
+point of view that these changes should be held in abeyance as much as
+possible. Then too, the possibility that milk may serve as a medium for
+the dissemination of disease-breeding bacteria makes it advisable to
+protect this food supply from all possible infection from suspicious
+sources.</p>
+
+<p>In considering, therefore, the relation of bacteria to general milk
+supplies, the <i>economic</i> and the <i>hygienic</i> standpoints must be taken
+into consideration. Ordinarily much more emphasis is laid upon the first
+requirement. If the supply presents no abnormal feature as to taste,
+odor and appearance, unfortunately but little attention is paid to the
+possibility of infection by disease germs. The methods of control which
+are applicable to general milk supplies are based on the following
+foundations: (1) the exclusion of all bacterial life, as far as
+practicable, at the time the milk is drawn, and the subsequent storage
+of the same at temperatures unfavorable for the growth of the organisms
+that do gain access; (2) the removal of the bacteria, wholly or in part,
+after they have once gained access.<span class='pagenum'><a name="Page_103" id="Page_103">[Pg 103]</a></span></p>
+
+<p>Until within comparatively recent years, practically no attention was
+given to the character of milk supplies, except possibly as to the
+percentage of butter fat, and sometimes the milk solids which it
+contained. So long as the product could be placed in the hands of the
+consumer in such shape as not to be rejected by him as unfit for food,
+no further attention was likely to be given to its character. At
+present, however, much more emphasis is being given to the quality of
+milk, especially as to its germ content; and the milk dealer is
+beginning to recognize the necessity of a greater degree of control.
+This control must not merely concern the handling of the product after
+it reaches him, but should go back to the milk producer on the farm.
+Here especially, it is necessary to inculcate those methods of
+cleanliness which will prevent in large measure the wholesale infection
+that ordinarily occurs.</p>
+
+<p>The two watch words which are of the utmost importance to the milk
+dealer are <i>cleanliness</i> and <i>cold</i>. If the milk is properly drawn from
+the animal in a clean manner and is immediately and thoroughly chilled,
+the dealer has little to fear as to his product. Whenever serious
+difficulties do arise, attributable to bacterial changes, it is because
+negligence has been permitted in one or both directions. The influence
+of cleanliness in diminishing the bacterial life in milk and that of low
+temperatures in repressing the growth of those forms which inevitably
+gain access has been fully dealt with in preceding chapters. It is of
+course not practicable to take all of these precautions to which
+reference has been made in the securing of large supplies of market milk
+for city use, but great improvement over existing conditions could be
+secured if the public would demand a better supervision of this
+important food article. Boards of health in<span class='pagenum'><a name="Page_104" id="Page_104">[Pg 104]</a></span> our larger cities are
+awakening to the importance of this question and are becoming
+increasingly active in the matter of better regulations and the
+enforcement of the same.</p>
+
+<p>New York City Board of Health has taken an advanced position in
+requiring that all milk sold in the city shall be chilled down to 45&deg; F.
+immediately after milking and shall be transported to the city in
+refrigerator cars.</p>
+
+<p>Reference has already been made to the application of the acid test
+(page 52) in the inspection of city milk supplies, and it is the opinion
+of the writer that the curd test (see page 76) could also be used with
+advantage in determining the sanitary character of milk. This test
+reveals the presence of bacteria usually associated with dirt and
+permits of the recognition of milks that have been carelessly handled.
+From personal knowledge of examinations made of the milk supplies in a
+number of Wisconsin cities it appears that this test could be utilized
+with evident advantage.</p>
+
+<p><b>"Sanitary" or "certified" milk supplies.</b> In a number of the larger
+cities, the attempt has been made to improve the quality of the milk
+supplies by the installation of dairies in which is produced an
+especially high grade of milk. Frequently the inspection of the dairy as
+well as the examination of the milk at stated intervals is under the
+control of milk commissions or medical societies and as it is customary
+to distribute the certificate of the examining board with the product,
+such milks are frequently known as "certified." In such dairies the
+tuberculin test is used at regular intervals, and the herd inspected
+frequently by competent veterinarians. The methods of control
+inaugurated as to clean milking and subsequent handling are such as to
+insure the diminution of the bacteria to the lowest possible point. The
+bacterial limit set by the Pediatric<span class='pagenum'><a name="Page_105" id="Page_105">[Pg 105]</a></span> Society of Philadelphia is 10,000
+organisms per cc. Often it is possible to improve very materially on
+this standard and not infrequently is the supply produced where it
+contains only a few thousand organisms per cc. Where such a degree of
+care is exercised, naturally a considerably higher price must be paid
+for the product,<a name="FNanchor_126_126" id="FNanchor_126_126"></a><a href="#Footnote_126_126" class="fnanchor">[126]</a> and it should be remembered that the development
+of such a system is only possible in relatively large centers where the
+dealer can cater to a selected high-class trade. Moreover, it should
+also be borne in mind that such a method of control is only feasible in
+dairies that are under individual control. The impossibility of
+exercising adequate control with reference to the milking process and
+the care which should be given the milk immediately thereafter, when the
+same is produced on different farms under various auspices is evident.</p>
+
+
+<h4>PRESERVATION OF MILK SUPPLIES.</h4>
+
+<p>While much can be done to improve the quality of milk supplies by
+excluding a large proportion of the bacteria which normally gain access
+to the milk, and preventing the rapid growth of those that do find their
+way therein, yet for general municipal purposes, any practical method of
+preservation<a name="FNanchor_127_127" id="FNanchor_127_127"></a><a href="#Footnote_127_127" class="fnanchor">[127]</a> that is applicable on a commercial scale must rest
+largely upon the destruction of bacteria that are present in the milk.</p>
+
+<p>The two possible methods by which bacteria can be destroyed after they
+have once gained access is (1) by the use of chemical preservatives; (2)
+by the aid of physical methods.<span class='pagenum'><a name="Page_106" id="Page_106">[Pg 106]</a></span></p>
+
+<p><b>Chemical preservatives.</b> Numerous attempts have been made to find some
+chemical substance that could be added to milk which would preserve it
+without interfering with its nutritive properties, but as a general rule
+a substance that is toxic enough to destroy or inhibit the growth of
+bacterial life exerts a prejudicial effect on the tissues of the body.
+The use of chemicals, such as carbolic acid, mercury salts and mineral
+acids, that are able to entirely destroy all life, is of course
+excluded, except when milk is preserved for analytical purposes; but a
+number of milder substances are more or less extensively employed,
+although the statutes of practically all states forbid their use.</p>
+
+<p>The substances so used may be grouped in two classes:</p>
+
+<p>1. Those that unite chemically with certain by-products of bacterial
+growth to form inert substances. Thus bicarbonate of soda neutralizes
+the acid in souring milk, although it does not destroy the lactic acid
+bacteria.</p>
+
+<p>2. Those that act directly upon the bacteria in milk, restraining or
+inhibiting their development. The substances most frequently utilized
+are salicylic acid, formaldehyde and boracic acid. These are nearly
+always sold to the milk handler, under some proprietary name, at prices
+greatly in excess of what the crude chemicals could be bought for in the
+open market. Formaldehyde has been widely advertised of late, but its
+use is fraught with the greatest danger, for it practically renders
+insoluble all albuminous matter and its toxic effect is greatly
+increased in larger doses.</p>
+
+<p>These substances are generally used by milk handlers who know nothing of
+their poisonous action, and although it may be possible for adults to
+withstand their use in dilute form, without serious results, yet their
+addition to<span class='pagenum'><a name="Page_107" id="Page_107">[Pg 107]</a></span> general milk supplies that may be used by children is
+little short of criminal. The sale of these preparations for use in milk
+finds its only outlet with those dairymen who are anxious to escape the
+exactions that must be met by all who attempt to handle milk in the best
+possible manner. Farrington has suggested a simple means for the
+detection of preservalin (boracic acid).<a name="FNanchor_128_128" id="FNanchor_128_128"></a><a href="#Footnote_128_128" class="fnanchor">[128]</a> When this substance is
+added to fresh milk, it increases the acidity of milk without affecting
+its taste. As normal milk tastes sour when it contains about 0.3 per
+cent lactic acid, a milk that tests as much or more than this without
+tasting sour has been probably treated with this antiseptic agent.</p>
+
+<p><b>Physical methods of preservation.</b> Methods based upon the application of
+physical forces are less likely to injure the nutritive value of milk,
+and are consequently more effective, if of any value whatever. A number
+of methods have been tried more or less thoroughly in an experimental
+way that have not yet been reduced to a practical basis, as electricity,
+use of a vacuum, and increased pressure.<a name="FNanchor_129_129" id="FNanchor_129_129"></a><a href="#Footnote_129_129" class="fnanchor">[129]</a> Condensation has long been
+used with great success, but in this process the nature of the milk is
+materially changed. The keeping quality in condensed milk often depends
+upon the action of another principle, viz., the inhibition of bacterial
+growth by reason of the concentration of the medium. This condition is
+reached either by adding sugar and so increasing the soluble solids, or
+by driving off the water by evaporation, preferably in a vacuum pan.
+Temperature changes are, however, of the most value in preserving milk,
+for by a variation in temperature all bacterial growth can be brought to
+a standstill, and under proper conditions thoroughly destroyed.</p>
+
+<p><span class='pagenum'><a name="Page_108" id="Page_108">[Pg 108]</a></span></p><p><b>Use of low temperatures.</b> The effect of chilling or rapid cooling on the
+keeping quality of milk is well known. When the temperature of milk is
+lowered to the neighborhood of 45&deg; F., the development of bacterial life
+is so slow as to materially increase the period that milk remains sweet.
+Within recent years, attempts have been made to preserve milk so that it
+could be shipped long distances by freezing the product, which in the
+form of milk-ice could be held for an indefinite period without
+change.<a name="FNanchor_130_130" id="FNanchor_130_130"></a><a href="#Footnote_130_130" class="fnanchor">[130]</a> A modification of this process known as Casse's system has
+been in use more or less extensively in Copenhagen and in several places
+in Germany. This consists of adding a small block of milk-ice (frozen
+milk) to large cans of milk (one part to about fifty of milk) which may
+or may not be pasteurized.<a name="FNanchor_131_131" id="FNanchor_131_131"></a><a href="#Footnote_131_131" class="fnanchor">[131]</a> This reduces the temperature so that the
+milk remains sweet considerably longer. Such a process might permit of
+the shipment of milk for long distances with safety but as a matter of
+fact, the system has not met with especial favor.</p>
+
+<div class="figcenter" style="width: 450px;">
+<img src="images/fig22.jpg" width="450" height="461" alt="Fig. 22. Microscopic appearance of normal milk showing
+the fat-globules aggregated in clusters." title="" />
+<span class="caption">Fig. 22. Microscopic appearance of normal milk showing
+the fat-globules aggregated in clusters.</span>
+</div>
+
+<p><b>Use of high temperatures.</b> Heat has long been used as a preserving agent.
+Milk has been scalded or cooked to keep it from time immemorial. Heat
+may be used at different<span class='pagenum'><a name="Page_109" id="Page_109">[Pg 109]</a></span> temperatures, and when so applied exerts a
+varying effect, depending upon temperature employed. All methods of
+preservation by heat rest, however, upon the application of the heat
+under the following conditions:</p>
+
+<p>1. A temperature above the maximum growing-point (105&deg;-115&deg; F.) and
+below the thermal death-point (130&deg;-140&deg; F.) will prevent further
+growth, and consequently fermentative action.</p>
+
+<p>2. A temperature above the thermal death-point destroys bacteria, and
+thereby stops all changes. This temperature varies, however, with the
+condition of the bacteria, and for spores is much higher than for
+vegetative forms.</p>
+
+<p>Attempts have been made to employ the first principle in shipping milk
+by rail, viz., prolonged heating above growing temperature, but when
+milk is so heated, its physical appearance is changed.<a name="FNanchor_132_132" id="FNanchor_132_132"></a><a href="#Footnote_132_132" class="fnanchor">[132]</a> The methods
+of heating most satisfactorily used are known as sterilization and
+pasteurization, in which a degree of temperature is used approximating
+the boiling and scalding points respectively.</p>
+
+<div class="figcenter" style="width: 450px;">
+<img src="images/fig23.jpg" width="450" height="476" alt="Fig. 23." title="" />
+<span class="caption">Fig. 23.</span>
+</div><div class="blockquot"><p> Microscopic appearance of milk heated above 140&deg;
+F., showing the homogeneous distribution of fat-globules. The physical
+change noted in comparison with Fig. 22 causes the diminished
+consistency of pasteurized cream.]</p></div>
+
+<p><b>Effect of heat on milk.</b> When milk is subjected to the action of heat, a
+number of changes in its physical and chemical properties are to be
+noted.<span class='pagenum'><a name="Page_110" id="Page_110">[Pg 110]</a></span></p>
+
+<p><i>1. Diminished "body."</i> When milk, but more especially cream, is heated
+to 140&deg; F. or above, it becomes thinner in consistency or "body," a
+condition which is due to a change in the grouping of the fat globules.
+In normal milk, the butter fat for the most part is massed in
+microscopic clots as (Fig. 22). When exposed to 140&deg; F. or above for ten
+minutes these fat-globule clots break down, and the globules become
+homogeneously distributed (Fig. 23). A <i>momentary</i> exposure to heat as
+high as 158&deg;-160&deg; may be made without serious effect on the cream lime;
+but above this the cream rises so poorly and slowly that it gives the
+impression of thinner milk.</p>
+
+<p><i>2. Cooked Taste.</i> If milk is heated for some minutes to 160&deg; F., it
+acquires a cooked taste that becomes more pronounced as the temperature
+is further raised. Milk so heated develops on its surface a pellicle or
+"skin." The cause of this change in taste is not well known. Usually it
+has been explained as being produced by changes in the nitrogenous
+elements in the milk, particularly in the albumen. Thoerner<a name="FNanchor_133_133" id="FNanchor_133_133"></a><a href="#Footnote_133_133" class="fnanchor">[133]</a> has
+pointed out the coincidence that exists between the appearance of a
+cooked taste and the loss of certain gases that are expelled by heating.
+He finds that the milk heated in closed vessels from which the gas
+cannot escape has a much less pronounced cooked flavor than if heated in
+an open vessel. The so-called "skin" on the surface of heated milk is
+not formed when the milk is heated in a tightly-closed receptacle. By
+some<a name="FNanchor_134_134" id="FNanchor_134_134"></a><a href="#Footnote_134_134" class="fnanchor">[134]</a> it is asserted that this layer is composed of albumen, but
+there is<span class='pagenum'><a name="Page_111" id="Page_111">[Pg 111]</a></span> evidence to show that it is modified casein due to the rapid
+evaporation of the milk serum at the surface of the milk.</p>
+
+<p><i>3. Digestibility.</i> Considerable difference of opinion has existed in
+the minds of medical men as to the relative digestibility of raw and
+heated milks. A considerable amount of experimental work has been done
+by making artificial digestion experiments with enzyms, also digestion
+experiments with animals, and in a few cases with children. The results
+obtained by different investigators are quite contradictory, although
+the preponderance of evidence seems to be in favor of the view that
+heating does impair the digestibility of milk, especially if the
+temperature attains the sterilizing point.<a name="FNanchor_135_135" id="FNanchor_135_135"></a><a href="#Footnote_135_135" class="fnanchor">[135]</a> It has been observed
+that there is a noteworthy increase in amount of rickets,<a name="FNanchor_136_136" id="FNanchor_136_136"></a><a href="#Footnote_136_136" class="fnanchor">[136]</a> scurvy
+and marasmus in children where highly-heated milks are employed. These
+objections do not obtain with reference to milk heated to moderate
+temperatures, as in pasteurization, although even this lower temperature
+lessens slightly its digestibility. The successful use of pasteurized
+milks in children's hospitals is evidence of its usefulness.</p>
+
+<p><i>4. Fermentative changes.</i> The normal souring change in milk is due to
+the predominance of the lactic acid bacteria, but as these organisms as
+a class do not possess spores, they are readily killed when heated above
+the thermal death-point of the developing cell. The destruction of the
+lactic forms leaves the spore-bearing types possessors of the field, and
+consequently the fermentative changes in heated milk<span class='pagenum'><a name="Page_112" id="Page_112">[Pg 112]</a></span> are not those that
+usually occur, but are characterized by the curdling of the milk from
+the action of rennet enzyms.</p>
+
+<p><i>5. Action of rennet.</i> Heating milk causes the soluble lime salts to be
+precipitated, and as the curdling of milk by rennet (in cheese-making)
+is dependent upon the presence of these salts, their absence in heated
+milks greatly retards the action of rennet. This renders it difficult to
+utilize heated milks in cheese-making unless the soluble lime salts are
+restored, which can be done by adding solutions of calcium chlorid.</p>
+
+<p><b>Sterilization.</b> As ordinarily used in dairying, sterilization means the
+application of heat at temperatures approximating, if not exceeding,
+212&deg; F. It does not necessarily imply that milk so treated is sterile,
+i. e., germ-free; for, on account of the resistance of spores, it is
+practically impossible to destroy entirely <i>all</i> these hardy forms. If
+milk is heated at temperatures above the boiling point, as is done where
+steam pressure is utilized, it can be rendered practically germ-free.
+Such methods are employed where it is designed to keep milk sweet for a
+long period of time. The treatment of milk by sterilization has not met
+with any general favor in this country, although it has been more widely
+introduced abroad. In most cases the process is carried out after the
+milk is bottled; and considerable ingenuity has been exercised in the
+construction of devices which will permit of the closure of the bottles
+after the sterilizing process has been completed. Milks heated to so
+high a temperature have a more or less pronounced boiled or cooked
+taste, a condition that does not meet with general favor in this
+country. The apparatus suitable for this purpose must, of necessity, be
+so constructed as to withstand steam pressure, and consequently is
+considerably<span class='pagenum'><a name="Page_113" id="Page_113">[Pg 113]</a></span> more expensive than that required for the simpler
+pasteurizing process.</p>
+
+<p><b>Pasteurization.</b> In this method the degree of heat used ranges from 140&deg;
+to 185&deg; F. and the application is made for only a limited length of
+time. The process was first extensively used by Pasteur (from whom it
+derives its name) in combating various maladies of beer and wine. Its
+importance as a means of increasing the keeping quality of milk was not
+generally recognized until a few years ago; but the method is now
+growing rapidly in favor as a means of preserving milk for commercial
+purposes. The method does not destroy all germ-life in milk; it affects
+only those organisms that are in a growing, vegetative condition; but if
+the milk is quickly cooled, it enhances the keeping quality very
+materially. It is unfortunate that this same term is used in connection
+with the heating of cream as a preparatory step to the use of pure
+cultures in cream-ripening in butter-making. The objects to be
+accomplished vary materially and the details of the two processes are
+also quite different.</p>
+
+<p>While pasteurizing can be performed on a small scale by the individual,
+the process can also be adapted to the commercial treatment of large
+quantities of milk. The apparatus necessary for this purpose is not
+nearly so expensive as that used in sterilizing, a factor of importance
+when other advantages are considered. In this country pasteurization has
+made considerable headway, not only in supplying a milk that is designed
+to serve as children's food, but even for general purposes.</p>
+
+<p><b>Requirements essential in pasteurization.</b> While considerable latitude
+with reference to pasteurizing limits is<span class='pagenum'><a name="Page_114" id="Page_114">[Pg 114]</a></span> permitted, yet there are
+certain conditions which should be observed, and these, in a sense, fix
+the limits that should be employed. These may be designated as (1) the
+<i>physical</i>, and (2) the <i>biological</i> requirements.</p>
+
+<p><b>Physical requirements.</b> <i>1. Avoidance of scalded or cooked taste.</i> The
+English and American people are so averse to a scalded or cooked flavor
+in milk that it is practically impossible for a highly heated product to
+be sold in competition with ordinary raw milk. In pasteurization then,
+care must be taken not to exceed the temperature at which a permanently
+cooked flavor is developed. As previously observed, this point varies
+with the period of exposure. A momentary exposure to a temperature of
+about 170&deg; F. may be made without any material alteration, but if the
+heat is maintained for a few minutes (ten minutes or over), a
+temperature of 158&deg; to 160&deg; F. is about the maximum that can be employed
+with safety.</p>
+
+<p><i>2. Normal creaming of the milk.</i> It is especially desirable that a
+sharp and definite cream line be evident on the milk soon after
+pasteurization. If this fails to appear, the natural inference of the
+consumer is that the milk is skimmed. If the milk be heated to a
+temperature sufficiently high to cause the fat-globule clusters to
+disintegrate (see Figs. 22 and 23), the globules do not rise to the
+surface as readily as before and the cream line remains indistinct.
+Where the exposure is made for a considerable period of time (10 minutes
+or more), the maximum temperature which can be used without producing
+this change is about 140&deg; F.; if the exposure is made for a very brief
+time, a minute or less, the milk may be heated to 158&deg;-160 F.&deg; without
+injuring the creaming property.<span class='pagenum'><a name="Page_115" id="Page_115">[Pg 115]</a></span></p>
+
+<p><i>3. No diminution in cream "body."</i> Coincident with this change which
+takes place in the creaming of the milk is the change in body or
+consistency which is noted where cream is pasteurized at too high a
+temperature. For the same reason as given under (2) cream heated above
+these temperatures is reduced in apparent thickness and appears to
+contain less butter-fat. Of course the pasteurizing process does not
+change the fat content, but its "body" is apparently so affected. Thus a
+25 per cent. cream may seem to be no thicker or heavier than an 18 per
+cent. raw cream. This real reduction in consistency naturally affects
+the readiness with which the cream can be whipped.</p>
+
+<p><b>Biological requirements.</b> <i>1. Enhanced keeping quality.</i> In commercial
+practice the essential biological requirement is expressed in the
+enhanced keeping quality of the pasteurized milk. This expresses in a
+practical way the reduction in germ life accomplished by the
+pasteurizing process. The improvement in keeping quality depends upon
+the temperature and time of exposure, but fully as much also on the way
+in which the pasteurized product is handled after heating. The lowest
+temperature which can be used with success to kill the active,
+vegetative bacteria is about 140&deg; F., at which point it requires about
+ten minutes exposure. If this period is curtailed the temperature must
+be raised accordingly. An exposure to a temperature of 175&deg; F. for a
+minute has approximately the same effect as the lower degree of heat for
+the longer time.</p>
+
+<p>The following bacteriological studies as to the effect which a variation
+in temperature exerts on bacterial life in milk are of importance as
+indicating the foundation for the selection of the proper limits. In the
+following<span class='pagenum'><a name="Page_116" id="Page_116">[Pg 116]</a></span> table the exposures were made for a uniform period (20
+minutes):</p>
+
+
+<h4><i>The bacterial content of milk heated at different temperatures.</i></h4>
+
+<h5>Number of bacteria per cc. in milk.</h5>
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="">
+<tr><td align='left'>&nbsp;</td><td align='left'>&nbsp;</td><td align='center'>45&deg; C.</td><td align='center'>50&deg; C.</td><td align='center'>55&deg; C.</td><td align='center'>60&deg; C.</td><td align='center'>65&deg; C.</td><td align='center'>70&deg; C.</td></tr>
+<tr><td align='left'>&nbsp;</td><td align='center'> Unheated</td><td align='center'>113&deg; F.</td><td align='center'>122&deg; F.</td><td align='center'>131&deg; F.</td><td align='center'>140&deg; F.</td><td align='center'>149&deg; F.</td><td align='center'>158&deg; F.</td></tr>
+<tr><td align='left'>Series I.</td><td align='right'>2,895,000</td><td align='right'>&mdash;&mdash;</td><td align='right'>1,260,000</td><td align='right'>798,000</td><td align='right'>32,000</td><td align='right'>5,770</td><td align='right'>3,900</td></tr>
+<tr><td align='left'>Series II.</td><td align='right'>750,000</td><td align='right'>665,000</td><td align='right'>262,400</td><td align='right'>201,000</td><td align='right'>950</td><td align='right'>700</td><td align='right'>705</td></tr>
+<tr><td align='left'>Series III.</td><td align='right'>1,350,000</td><td align='right'>1,100,000</td><td align='right'>260,000</td><td align='right'>215,000</td><td align='right'>575</td><td align='right'>610</td><td align='right'>650</td></tr>
+<tr><td align='left'>Series IV.</td><td align='right'>1,750,000</td><td align='right'>&mdash;&mdash;</td><td align='right'>87,360</td><td align='right'>&mdash;&mdash;</td><td align='right'>4,000</td><td align='right'>3,500</td><td align='right'>3,600</td></tr>
+</table></div>
+
+
+<p>It appears from these results that the most marked decrease in
+temperature occurs at 140&deg; F. (60&deg; C.). It should also be observed that
+an increase in heat above this temperature did not materially diminish
+the number of organisms present, indicating that those forms remaining
+were in a spore or resistant condition. It was noted, however, that the
+developing colonies grew more slowly in the plates made from the highly
+heated milk, showing that their vitality was injured to a greater extent
+even though not killed.</p>
+
+<p><i>2. Destruction of disease bacteria.</i> While milk should be pasteurized
+so as to destroy all active, multiplying bacteria, it is particularly
+important to destroy any organisms of a disease nature that might find
+their way into the same. Fortunately most of the bacteria capable of
+thriving in milk before or after it is drawn from the animal are not
+able to form spores and hence succumb to proper pasteurization. Such is
+the case with the diphtheria, cholera and typhoid organisms.</p>
+
+<p>The organism that is invested with most interest in this connection is
+the tubercle bacillus. On account of its more or less frequent
+occurrence in milk and its reputed high<span class='pagenum'><a name="Page_117" id="Page_117">[Pg 117]</a></span> powers of resistance, it may
+well be taken as a standard in pasteurizing.</p>
+
+<p><b>Thermal death limits of tubercle bacillus.</b> Concerning the exact
+temperature at which this germ is destroyed there is considerable
+difference of opinion. Part of this arises from the inherent difficulty
+in determining exactly when the organism is killed (due to its failure
+to grow readily on artificial media), and part from the lack of uniform
+conditions of exposure. The standards that previously have been most
+generally accepted are those of De Man,<a name="FNanchor_137_137" id="FNanchor_137_137"></a><a href="#Footnote_137_137" class="fnanchor">[137]</a> who found that thirty
+minutes exposure at 149&deg; F., fifteen minutes at 155&deg; F., or ten minutes
+at 167&deg; F., sufficed to destroy this germ.</p>
+
+<p>More recently it has been demonstrated,<a name="FNanchor_138_138" id="FNanchor_138_138"></a><a href="#Footnote_138_138" class="fnanchor">[138]</a> and these results
+confirmed,<a name="FNanchor_139_139" id="FNanchor_139_139"></a><a href="#Footnote_139_139" class="fnanchor">[139]</a> that if tuberculous milk is heated in closed receptacles
+where the surface pellicle does not form, the vitality of this disease
+germ is destroyed at 140&deg; F. in 10-15 minutes, while an exposure at 160&deg;
+F. requires only about one minute.<a name="FNanchor_140_140" id="FNanchor_140_140"></a><a href="#Footnote_140_140" class="fnanchor">[140]</a> If the conditions of heating are
+such that the surface of the milk is exposed to the air, the resistance
+of bacteria is greatly increased. When heated in open vessels Smith
+found that the tubercle organism was not killed in some cases where the
+exposure was made for at least an hour. Russell and Hastings<a name="FNanchor_141_141" id="FNanchor_141_141"></a><a href="#Footnote_141_141" class="fnanchor">[141]</a> have
+shown an instance where the thermal death-point of a micrococcus
+isolated from pasteurized milk was increased 12.5&deg; F., by heating it
+under conditions that permitted of the formation of the scalded layer.
+It is therefore apparent that apparatus used for<span class='pagenum'><a name="Page_118" id="Page_118">[Pg 118]</a></span> pasteurization should
+be constructed so as to avoid this defect.</p>
+
+<p><b>Methods of treatment.</b> Two different systems of pasteurization have grown
+up in the treatment of milk. One of these has been developed from the
+hygienic or sanitary aspect of the problem and is used more particularly
+in the treatment of cream and relatively small milk supplies. The other
+system has been developed primarily from the commercial point of view
+where a large amount of milk must be treated in the minimum time. In the
+first method the milk is heated for a longer period of time, about
+fifteen minutes at a relatively low temperature from 140&deg;-155&deg; F.; in
+the other, the milk is exposed to the source of heat only while it is
+passing rapidly through the apparatus. Naturally, the exposure under
+such conditions must be made at a considerably higher temperature,
+usually in the neighborhood of 160&deg; F.</p>
+
+<p>The types of apparatus used in these respective processes naturally
+varies. Where the heating is prolonged, the apparatus employed is built
+on the principle of a <i>tank</i> or <i>reservoir</i> in which a given volume of
+milk may be held at any given temperature for any given period of time.</p>
+
+<p>When the heat is applied for a much shorter period of time, the milk is
+passed in a continuous stream through the machine. Naturally the
+capacity of a continuous-flow apparatus is much greater than a machine
+that operates on the intermittent principle; hence, for large supplies,
+as in city distribution, this system has a great advantage. The question
+as to relative efficiency is however one which should be given most
+careful consideration.<span class='pagenum'><a name="Page_119" id="Page_119">[Pg 119]</a></span></p>
+
+<p><b>Pasteurizing apparatus.</b> The problems to be solved in the pasteurization
+of milk and cream designed for direct consumption are so materially
+different from where the process is used in butter-making that the type
+of machinery for each purpose is quite different. The equipment
+necessary for the first purpose may be divided into two general classes:</p>
+
+<p>1. Apparatus of limited capacity designed for family use.</p>
+
+<p>2. Apparatus of sufficient capacity to pasteurize on a commercial scale.</p>
+
+<p><b>Domestic pasteurizers.</b> In pasteurizing milk for individual use, it is
+not desirable to treat at one time more than will be consumed in one
+day; hence an apparatus holding a few bottles will suffice. In this case
+the treatment can best be performed in the bottle itself, thereby
+lessening the danger of infection. Several different types of
+pasteurizers are on the market; but special apparatus is by no means
+necessary for the purpose. The process can be efficiently performed by
+any one with the addition of an ordinary dairy thermometer to the common
+utensils found in the kitchen. Fig. 24 indicates a simple contrivance
+that can be readily arranged for this purpose.</p>
+
+<p>The following suggestions indicate the different steps of the process:</p>
+
+<p>1. Use only fresh milk.</p>
+
+<p>2. Place milk in clean bottles or fruit cans, filling to a uniform
+level, closing bottles tightly with a cork or cover. If pint and quart
+cans are used at the same time, an inverted bowl will equalize the
+level. Set these in a flat-bottomed tin pail and fill with warm water to
+same level as milk. An inverted pie tin punched with holes will serve as
+a stand on which to place the bottles during the heating process.<span class='pagenum'><a name="Page_120" id="Page_120">[Pg 120]</a></span></p>
+
+<p>3. Heat water in pail until the temperature of same reaches 155&deg; to 160&deg;
+F.; then remove from source of direct heat, cover with a cloth or tin
+cover, and allow the whole to stand for half an hour. In the preparation
+of milk for children, it is not advisable to use the low-temperature
+treatment (140&deg; F.) that is recommended for commercial city delivery.</p>
+
+<div class="figcenter" style="width: 400px;">
+<img src="images/fig24.jpg" width="400" height="351" alt="Fig. 24. A home-made pasteurizer." title="" />
+<span class="caption">Fig. 24. A home-made pasteurizer.</span>
+</div>
+
+<p>4. Remove bottles of milk and cool them as rapidly as possible without
+danger to bottles and store in a refrigerator.</p>
+
+<p><b>Commercial pasteurizers.</b> The two methods of pasteurization practiced
+commercially for the preservation of milk and cream have been developed
+because of the two types of machinery now in use. Apparatus constructed
+on the reservoir or tank principle permits of the retention of the milk
+for any desired period of time. Therefore, a lower temperature can be
+employed in the treatment. In<span class='pagenum'><a name="Page_121" id="Page_121">[Pg 121]</a></span> those machines where the milk flows
+through the heater in a more or less continuous stream, the period of
+exposure is necessarily curtailed, thereby necessitating a higher
+temperature.</p>
+
+<p><b>Reservoir pasteurizers.</b> The simplest type of apparatus suitable for
+pasteurizing on this principle is where the milk is placed in shotgun
+cans and immersed in water heated by steam. Ordinary tanks surrounded
+with water spaces can also be used successfully. The Boyd cream ripening
+vat has also been tried. In this the milk is heated by a swinging coil
+immersed in the vat through which hot water circulates.</p>
+
+<p>In 1894 the writer<a name="FNanchor_142_142" id="FNanchor_142_142"></a><a href="#Footnote_142_142" class="fnanchor">[142]</a> constructed a tank pasteurizer which consisted
+of a long, narrow vat surrounded by a steam-heated water chamber. Both
+the milk and the water chambers were provided with mechanical agitators
+having a to-and-fro movement.</p>
+
+<div class="figcenter" style="width: 350px;">
+<img src="images/fig25.jpg" width="350" height="231" alt="Fig. 25. Pott&#39;s pasteurizer." title="" />
+<span class="caption">Fig. 25. Pott&#39;s pasteurizer.</span>
+</div>
+
+<p>Another machine which has been quite generally introduced is the Potts'
+rotating pasteurizer. This apparatus has a central milk chamber that is
+surrounded with an outer shell containing hot water. The whole machine
+revolves on a horizontal axis, and the cream or milk is thus thoroughly
+agitated during the heating process.</p>
+<p><span class='pagenum'><a name="Page_122" id="Page_122">[Pg 122]</a></span></p>
+<p><b>Continuous-flow pasteurizers.</b> The demand for greater capacity than can
+be secured in the reservoir machines has led to the perfection of
+several kinds of apparatus where the milk is heated momentarily as it
+flows through the apparatus. Most of these were primarily introduced for
+the treatment of cream for butter-making purposes, but they are
+frequently employed for the treatment of milk on a large scale in city
+milk trade. Many of them are of European origin although of late years
+several have been devised in this country.</p>
+
+<p>The general principle of construction is much the same in most of them.
+The milk is spread out in a thin sheet, and is treated by passing it
+over a surface, heated either with steam directly or preferably with hot
+water.</p>
+
+<p>Where steam is used directly, it is impossible to prevent the "scalding
+on" of the milk proteids to the heated surface.</p>
+
+<p>In some of these machines (Thiel, Kuehne, Lawrence, De Laval, and
+Hochmuth), a ribbed surface is employed over which the milk flows, while
+the opposite surface is heated with hot water or steam. Monrad, Lefeldt
+and Lentsch employ a centrifugal apparatus in which a thin layer of milk
+is heated in a revolving drum.</p>
+
+<p>In some types of apparatus, as in the Miller machine, an American
+pasteurizer, the milk is forced in a thin sheet between two heated
+surfaces, thereby facilitating the heating process. In the Farrington
+machine heated discs rotate in a reservoir through which the milk flows
+in a continuous stream.</p>
+
+<p>One of the most economical types of apparatus is the regenerator type (a
+German machine), in which the milk passes over the heating surface in a
+thin stream and then<span class='pagenum'><a name="Page_123" id="Page_123">[Pg 123]</a></span> is carried back over the incoming cold milk so
+that the heated liquid is partially cooled by the inflowing fresh milk.
+In machines of this class it requires very much less steam to heat up
+the milk than in those in which the cold milk is heated wholly by the
+hot water.</p>
+
+<p>A number of machines have been constructed on the principle of a
+reservoir which is fed by a constantly flowing stream. In some kinds of
+apparatus of this type no attempt is made to prevent the mixing of the
+recently introduced milk with that which has been partially heated. The
+pattern for this reservoir type is Fjord's heater, in which the milk is
+stirred by a stirrer. This apparatus was originally designed as a heater
+for milk before separation, but it has since been materially modified so
+that it is better adapted to the purposes of pasteurization. Reid was
+the first to introduce this type of machine into America.</p>
+
+<p><b>Objections to continuous flow pasteurizers.</b> In all continuous flow
+pasteurizers certain defects are more or less evident. While they
+fulfill the important requirement of large capacity, an absolute
+essential where large volumes of milk are being handled, it does not of
+necessity follow that they conform to all the hygienic and physical
+requirements that should be kept in mind. The greatest difficulty is the
+shortened period of exposure. The period which the milk is actually
+heated is often not more than a minute or so. Another serious defect is
+the inability to heat <i>all</i> of the milk for a uniform period of time. At
+best, the milk is exposed for an extremely short time, but even then
+portions pass through the machine much more quickly than do the
+remainder. Those portions in contact with the walls of the apparatus are
+retarded by friction and are materially delayed in their passage, while
+the particles in<span class='pagenum'><a name="Page_124" id="Page_124">[Pg 124]</a></span> the center of the stream, however thin, flow through
+in the least possible time.</p>
+
+<p>The following simple method enables the factory operator to test the
+period of exposure in the machine: Start the machine full of water, and
+after the same has become heated to the proper temperature, change the
+inflow to full-cream milk, continuing at the same rate. Note the exact
+time of change and also when first evidence of milkiness begins to
+appear at outflow. If samples are taken from first appearance of milky
+condition and thereafter at different intervals for several minutes, it
+is possible, by determining the amount of butter-fat in the same, to
+calculate with exactness how long it takes for the milk to entirely
+replace the water.</p>
+
+<p>Tests made by the writer<a name="FNanchor_143_143" id="FNanchor_143_143"></a><a href="#Footnote_143_143" class="fnanchor">[143]</a> on the Miller pasteurizer showed, when fed
+at the rate of 1,700 pounds per hour, the minimum period of exposure to
+be 15 seconds, and the maximum about 60-70 seconds, while about
+two-thirds of the milk passed the machine in 40-50 seconds. This
+manifest variation in the rate of flow of the milk through the machine
+is undoubtedly the reason why the results of this type of treatment are
+subject to so much variation. Naturally, even a fatal temperature to
+bacterial life can be reduced to a point where actual destruction of
+even vegetating cells does not occur.</p>
+
+<p><b>Bacterial efficiency of reservoir pasteurizers.</b> The bacterial content of
+pasteurized milk and cream will depend somewhat on the number of
+organisms originally present in the same. Naturally, if mixed milk
+brought to a creamery is pasteurized, the number of organisms remaining
+after treatment would be greater than if the raw material was fresh and
+produced on a single farm.</p>
+
+<p>An examination of milk and cream pasteurized on a commercial<span class='pagenum'><a name="Page_125" id="Page_125">[Pg 125]</a></span> scale in
+the Russell vat at the Wisconsin Dairy school showed that over 99.8 per
+cent of the bacterial life in raw milk or cream was destroyed by the
+heat employed, i. e., 155&deg; F. for twenty minutes duration.<a name="FNanchor_144_144" id="FNanchor_144_144"></a><a href="#Footnote_144_144" class="fnanchor">[144]</a> In
+nearly one-half of the samples of milk, the germ content in the
+pasteurized sample fell below 1,000 bacteria per cc., and the average of
+twenty-five samples contained 6,140 bacteria per cc. In cream the germ
+content was higher, averaging about 25,000 bacteria per cc. This milk
+was taken from the general creamery supply, which was high in organisms,
+containing on an average 3,675,000 bacteria per cc. De Schweinitz<a name="FNanchor_145_145" id="FNanchor_145_145"></a><a href="#Footnote_145_145" class="fnanchor">[145]</a>
+has reported the germ content of a supply furnished in Washington which
+was treated at 158&deg; to 160&deg; F. for fifteen minutes. This supply came
+from a single source. Figures reported were from 48-hour-old agar
+plates. Undoubtedly these would have been higher if a longer period of
+incubation had been maintained. The average of 82 samples, taken for the
+period of one year, showed 325 bacteria per cc.</p>
+
+<div class="figcenter" style="width: 450px;">
+<img src="images/fig26.jpg" width="450" height="449" alt="Fig. 26. Effect of pasteurizing on germ content of milk.
+Black square represents bacteria of raw milk; small white square, those
+remaining after pasteurization." title="" />
+<span class="caption">Fig. 26. Effect of pasteurizing on germ content of milk.
+Black square represents bacteria of raw milk; small white square, those
+remaining after pasteurization.</span>
+</div>
+
+<p><span class='pagenum'><a name="Page_126" id="Page_126">[Pg 126]</a></span></p><p><b>Bacterial efficiency of continuous-flow pasteurizers.</b> A quantitative
+determination of the bacteria found in milk and cream when treated in
+machinery of this class almost always shows a degree of variation in
+results that is not to be noted in the discontinuous apparatus.</p>
+
+<div class="figcenter" style="width: 450px;">
+<img src="images/fig27.jpg" width="450" height="517" alt="Fig. 27. Reid&#39;s Continuous Pasteurizer." title="" />
+<span class="caption">Fig. 27. Reid&#39;s Continuous Pasteurizer.</span>
+</div>
+
+<p>Harding and Rogers<a name="FNanchor_146_146" id="FNanchor_146_146"></a><a href="#Footnote_146_146" class="fnanchor">[146]</a> have tested the efficiency of one<span class='pagenum'><a name="Page_127" id="Page_127">[Pg 127]</a></span> of the Danish
+type of continuous pasteurizers. These experiments were made at 158&deg;,
+176&deg; and 185&deg; F. They found the efficiency of the machine not wholly
+satisfactory at the lower temperatures. At 158&deg; F. the average of
+fourteen tests gave 15,300 bacteria per cc., with a maximum to minimum
+range from 62,790 to 120. Twenty-five examinations at 176&deg; F. showed an
+average of only 117, with a range from 300 to 20. The results at 185&deg; F.
+showed practically the same results as noted at 176&deg; F. Considerable
+trouble was experienced with the "scalding on" of the milk to the walls
+of the machine when milk of high acidity was used.</p>
+
+<p>Jensen<a name="FNanchor_147_147" id="FNanchor_147_147"></a><a href="#Footnote_147_147" class="fnanchor">[147]</a> details the results of 139 tests in 1899, made by the
+Copenhagen Health Commission. In 66 samples from one hundred thousand to
+one million organisms per cc. were found, and in 22 cases from one to
+five millions. Nineteen tests showed less than 10,000 per cc.</p>
+
+<p>In a series of tests conducted by the writer<a name="FNanchor_148_148" id="FNanchor_148_148"></a><a href="#Footnote_148_148" class="fnanchor">[148]</a> on a Miller
+pasteurizer in commercial operation, an average of 21 tests showed
+12,350 bacteria remaining in the milk when the milk was pasteurized from
+156&deg;-164&deg; F. The raw milk in these tests ran from 115,000 to about one
+million organisms per cc.</p>
+
+<p>A recently devised machine of this type (Pasteur) has been tested by
+Lehmann, who found that it was necessary to heat the milk as high as
+176&deg; to 185&deg; F., in order to secure satisfactory results on the
+bacterial content of the cream.</p>
+
+<p>The writer tested Reid's pasteurizer at 155&deg; to 165&deg; F. with the
+following results: in some cases as many as 40 per cent. of the bacteria
+survived, which number in some cases exceeded 2,000,000 bacteria per
+cc.<span class='pagenum'><a name="Page_128" id="Page_128">[Pg 128]</a></span></p>
+
+<p><b>Pasteurizing details.</b> While the pasteurizing process is exceedingly
+simple, yet, in order to secure the best results, certain conditions
+must be rigidly observed in the treatment before and after the heating
+process.</p>
+
+<p>It is important to select the best possible milk for pasteurizing, for
+if the milk has not been milked under clean conditions, it is likely to
+be rich in the spore-bearing bacteria. Old milk, or milk that has not
+been kept at a low temperature, is much richer in germ-life than
+perfectly fresh or thoroughly chilled milk.</p>
+
+<p>The true standard for selecting milk for pasteurization should be to
+determine the actual number of bacterial <i>spores</i> that are able to
+resist the heating process, but this method is impracticable under
+commercial conditions.</p>
+
+<p>The following method, while only approximate in its results, will be
+found helpful: Assuming that the age or treatment of the milk bears a
+certain relation to the presence of spores, and that the acid increases
+in a general way with an increase in age or temperature, the amount of
+acid present may be taken as an approximate index of the suitability of
+the milk for pasteurizing purposes. Biological tests were carried out in
+the author's laboratory<a name="FNanchor_149_149" id="FNanchor_149_149"></a><a href="#Footnote_149_149" class="fnanchor">[149]</a> on milks having a high and low acid
+content, and it was shown that the milk with the least acid was, as a
+rule, the freest from spore-bearing bacteria.</p>
+
+<p>This acid determination can be made at the weigh-can by employing the
+Farrington alkaline tablet which is used in cream-ripening. Where milk
+is pasteurized under general creamery conditions, none should be used
+containing more than 0.2 per cent acidity. If only perfectly fresh milk
+is used, the amount of acid will generally be about 0.15 per cent with
+phenolphthalein as indicator.<span class='pagenum'><a name="Page_129" id="Page_129">[Pg 129]</a></span></p>
+
+<div class="figcenter" style="width: 301px;">
+<img src="images/fig28.jpg" width="301" height="600" alt="Fig. 28." title="" />
+<span class="caption">Fig. 28.</span>
+</div><div class="blockquot"><p> Diagram showing temperature changes in
+pasteurizing, and the relation of same to bacterial growth.</p>
+
+<p>Shaded zone represents limits of bacterial growth, 50&deg;-109&deg; F. (10&deg;-43&deg;
+C.), the intensity of shading indicating rapidity of development. The
+solid black line shows temperature of milk during the process. The
+necessity for rapid cooling is evident as the milk falls in temperature
+to that of growing zone.<span class='pagenum'><a name="Page_130" id="Page_130">[Pg 130]</a></span>]</p></div>
+
+<p>Emphasis has already been laid on the selection of a proper limit of
+pasteurizing (p. 114). It should be kept constantly in mind that the
+thermal death-point of any organism depends not alone on the temperature
+used, but on the period of exposure. With the lower limits given, 140&deg;
+F., it is necessary to expose the milk for not less than fifteen
+minutes. If a higher heat is employed (and the cooked flavor
+disregarded) the period of exposure may be curtailed.</p>
+
+<p><b>Chilling the milk.</b> It is very essential in pasteurizing that the heated
+milk be immediately chilled in order to prevent the germination of the
+resistant spores, for if germination once occurs, growth can go on at
+relatively low temperatures.</p>
+
+<p>The following experiments by Marshall<a name="FNanchor_150_150" id="FNanchor_150_150"></a><a href="#Footnote_150_150" class="fnanchor">[150]</a> are of interest as showing
+the influence of refrigeration on germination of spores:</p>
+
+<p>Cultures of organisms that had been isolated from pasteurized milk were
+inoculated into bouillon. One set was left to grow at room temperature,
+another was pasteurized and allowed to stand at same temperature, while
+another heated set was kept in a refrigerator. The unheated cultures at
+room temperature showed evidence of growth in thirty trials in an
+average of 26 hours; 29 heated cultures at room temperature all
+developed in an average of 50 hours, while the heated cultures kept in
+refrigerator showed no growth in 45 days with but four exceptions.</p>
+<p><span class='pagenum'><a name="Page_131" id="Page_131">[Pg 131]</a></span></p>
+<p>Practically all of the rapid-process machines are provided with
+especially constructed cooling devices. In some of them, as in the
+Miller and Farrington, the cooling is effected by passing the milk
+through two separate coolers that are constructed in the same general
+way as the heater. With the first cooler, cold running water is
+employed, the temperature often being lowered in this way to 58&deg; or 60&deg;
+F. Further lessening of the temperature is secured by an additional ice
+water or brine cooler which brings the temperature down to 40&deg;-50&deg; F.</p>
+
+<p>In the economical use of ice the ice itself should be applied as closely
+as possibly to the milk to be cooled, for the larger part of the
+chilling value of ice comes from the melting of the same. To convert a
+pound of ice at 32&deg; F. into a pound of water at the same temperature, if
+we disregard radiation, would require as much heat as would suffice to
+raise 142 pounds of water one degree F., or one pound of water 142&deg; F.
+The absorptive capacity of milk for heat (specific heat) is not quite
+the same as it is with water, being .847 for milk in comparison with 1.0
+for water.<a name="FNanchor_151_151" id="FNanchor_151_151"></a><a href="#Footnote_151_151" class="fnanchor">[151]</a> Hot milk would therefore require somewhat less ice to
+cool it than would be required by any equal volume of water at the same
+temperature.</p>
+
+<p><b>Bottling the product.</b> If the milk has been properly pasteurized, it
+should, of course, be dispensed in sterilized bottles. Glass bottles
+with plain pulp caps are best, and these should be thoroughly sterilized
+in steam before using. The bottling can best be done in a commercial
+bottling machine. Care must be taken to thoroughly clean this apparatus
+after use each day. Rubber valves in these machines suffer deterioration
+rapidly.</p>
+<p><span class='pagenum'><a name="Page_132" id="Page_132">[Pg 132]</a></span></p>
+<div class="figcenter" style="width: 450px;">
+<img src="images/fig29.jpg" width="450" height="442" alt="Fig. 29. Relative consistency of pasteurized cream before
+(A) and after (B) treatment with viscogen as shown by rate of flow down
+inclined glass plate." title="" />
+<span class="caption">Fig. 29. Relative consistency of pasteurized cream before
+(A) and after (B) treatment with viscogen as shown by rate of flow down
+inclined glass plate.</span>
+</div>
+
+<p><b>Restoration of "body" of pasteurized cream.</b> The action of heat causes
+the tiny groupings of fat globules in normal milk (Fig. 22) to break up,
+and with this change, which occurs in the neighborhood of 140&deg; F., where
+the milk is heated for about 15 minutes and at about 160-165&deg; F. where
+rapidly heated in a continuous stream, the consistency of the liquid is
+diminished, notwithstanding the fact that the fat-content remains
+unchanged. Babcock and the writer<a name="FNanchor_152_152" id="FNanchor_152_152"></a><a href="#Footnote_152_152" class="fnanchor">[152]</a> devised the following "cure" for
+this apparent defect. If a strong solution of cane sugar is added to<span class='pagenum'><a name="Page_133" id="Page_133">[Pg 133]</a></span>
+freshly slacked lime and the mixture allowed to stand, a clear fluid can
+be decanted off. The addition of this alkaline liquid, which is called
+"viscogen," to pasteurized cream in proportions of about one part of
+sugar-lime solution to 100 to 150 of cream, restores the consistency of
+the cream, as it causes the fat globules to cluster together in small
+groups.</p>
+
+<p>The relative viscosity of creams can easily be determined by the
+following method (Fig. 29):</p>
+
+<p>Take a perfectly clean piece of glass (plate or picture glass is
+preferable, as it is less liable to be wavy). Drop on one edge two or
+three drops of cream at intervals of an inch or so. Then incline piece
+of glass at such an angle as to cause the cream to flow down surface of
+glass. The cream, having the heavier body or viscosity, will move more
+slowly. If several samples of each cream are taken, then the aggregate
+lengths of the different cream paths may be taken, thereby eliminating
+slight differences due to condition of glass.</p>
+
+<div class="footnotes"><h3>FOOTNOTES:</h3>
+
+<div class="footnote"><p><a name="Footnote_126_126" id="Footnote_126_126"></a><a href="#FNanchor_126_126"><span class="label">[126]</span></a> From 10 to 16 cents per quart is usually paid for such
+milks.</p></div>
+
+<div class="footnote"><p><a name="Footnote_127_127" id="Footnote_127_127"></a><a href="#FNanchor_127_127"><span class="label">[127]</span></a> Much improvement in quality could be made by more careful
+control of milk during shipment, especially as to refrigeration; also as
+to the care taken on the farms. The use of the ordinary milking machine
+(see page 37), would go far to reduce the germ content of milk.</p></div>
+
+<div class="footnote"><p><a name="Footnote_128_128" id="Footnote_128_128"></a><a href="#FNanchor_128_128"><span class="label">[128]</span></a> Farrington, Journ. Amer. Chem. Soc., Sept., 1896.</p></div>
+
+<div class="footnote"><p><a name="Footnote_129_129" id="Footnote_129_129"></a><a href="#FNanchor_129_129"><span class="label">[129]</span></a> Hite, Bull. 58, West Va. Expt. Stat., 1899.</p></div>
+
+<div class="footnote"><p><a name="Footnote_130_130" id="Footnote_130_130"></a><a href="#FNanchor_130_130"><span class="label">[130]</span></a> Milch Zeit., 1895, No. 9.</p></div>
+
+<div class="footnote"><p><a name="Footnote_131_131" id="Footnote_131_131"></a><a href="#FNanchor_131_131"><span class="label">[131]</span></a> Ibid., 1897, No. 33.</p></div>
+
+<div class="footnote"><p><a name="Footnote_132_132" id="Footnote_132_132"></a><a href="#FNanchor_132_132"><span class="label">[132]</span></a> Bernstein, Milch Zeit., 1894, pp. 184, 200.</p></div>
+
+<div class="footnote"><p><a name="Footnote_133_133" id="Footnote_133_133"></a><a href="#FNanchor_133_133"><span class="label">[133]</span></a> Thoerner, Chem. Zeit., 18:845.</p></div>
+
+<div class="footnote"><p><a name="Footnote_134_134" id="Footnote_134_134"></a><a href="#FNanchor_134_134"><span class="label">[134]</span></a> Snyder, Chemistry of Dairying, p. 59.</p></div>
+
+<div class="footnote"><p><a name="Footnote_135_135" id="Footnote_135_135"></a><a href="#FNanchor_135_135"><span class="label">[135]</span></a> Doane and Price (Bull. 77, Md. Expt. Stat., Aug. 1901)
+give quite a full resum&eacute; of the work on this subject in connection with
+rather extensive experiments made by them on feeding animals with raw,
+pasteurized and sterilized milks.</p></div>
+
+<div class="footnote"><p><a name="Footnote_136_136" id="Footnote_136_136"></a><a href="#FNanchor_136_136"><span class="label">[136]</span></a> Rickets is a disease in which the bones lack sufficient
+mineral matter to give them proper firmness. Marasmus is a condition in
+which the ingested food seems to fail to nourish the body and gradual
+wasting away occurs.</p></div>
+
+<div class="footnote"><p><a name="Footnote_137_137" id="Footnote_137_137"></a><a href="#FNanchor_137_137"><span class="label">[137]</span></a> De Man, Arch. f. Hyg., 1893, 18:133.</p></div>
+
+<div class="footnote"><p><a name="Footnote_138_138" id="Footnote_138_138"></a><a href="#FNanchor_138_138"><span class="label">[138]</span></a> Th. Smith, Journ. of Expt. Med., 1899, 4:217.</p></div>
+
+<div class="footnote"><p><a name="Footnote_139_139" id="Footnote_139_139"></a><a href="#FNanchor_139_139"><span class="label">[139]</span></a> Russell and Hastings, 17 Rept. Wis. Expt. Stat., 1900, p.
+147.</p></div>
+
+<div class="footnote"><p><a name="Footnote_140_140" id="Footnote_140_140"></a><a href="#FNanchor_140_140"><span class="label">[140]</span></a> Russell and Hastings, 21 Rept. Ibid., 1904.</p></div>
+
+<div class="footnote"><p><a name="Footnote_141_141" id="Footnote_141_141"></a><a href="#FNanchor_141_141"><span class="label">[141]</span></a> Russell and Hastings, 18 Rept. Ibid., 1901.</p></div>
+
+<div class="footnote"><p><a name="Footnote_142_142" id="Footnote_142_142"></a><a href="#FNanchor_142_142"><span class="label">[142]</span></a> Russell, Bull. 44, Wis. Expt. Stat.</p></div>
+
+<div class="footnote"><p><a name="Footnote_143_143" id="Footnote_143_143"></a><a href="#FNanchor_143_143"><span class="label">[143]</span></a> Russell, 22 Wis. Expt. Stat. Rept., 1905, p. 232.</p></div>
+
+<div class="footnote"><p><a name="Footnote_144_144" id="Footnote_144_144"></a><a href="#FNanchor_144_144"><span class="label">[144]</span></a> Russell, 12 Wis. Expt. Stat. Rept., 1895, p. 160.</p></div>
+
+<div class="footnote"><p><a name="Footnote_145_145" id="Footnote_145_145"></a><a href="#FNanchor_145_145"><span class="label">[145]</span></a> De Schweinitz, Nat. Med. Rev., 1899, No. 11.</p></div>
+
+<div class="footnote"><p><a name="Footnote_146_146" id="Footnote_146_146"></a><a href="#FNanchor_146_146"><span class="label">[146]</span></a> Harding and Rogers. Bull. 182, N. Y. (Geneva) Expt.
+Stat., Dec., 1899.</p></div>
+
+<div class="footnote"><p><a name="Footnote_147_147" id="Footnote_147_147"></a><a href="#FNanchor_147_147"><span class="label">[147]</span></a> Jensen, Milchkunde und Milch Hygiene, p. 132.</p></div>
+
+<div class="footnote"><p><a name="Footnote_148_148" id="Footnote_148_148"></a><a href="#FNanchor_148_148"><span class="label">[148]</span></a> 22 Wis. Expt. Stat. Rept., 1905, p. 236.</p></div>
+
+<div class="footnote"><p><a name="Footnote_149_149" id="Footnote_149_149"></a><a href="#FNanchor_149_149"><span class="label">[149]</span></a> Shockley, Thesis, Univ. of Wis., 1896.</p></div>
+
+<div class="footnote"><p><a name="Footnote_150_150" id="Footnote_150_150"></a><a href="#FNanchor_150_150"><span class="label">[150]</span></a> Marshall, Mich. Expt. Stat., Bull. 147, p. 47.</p></div>
+
+<div class="footnote"><p><a name="Footnote_151_151" id="Footnote_151_151"></a><a href="#FNanchor_151_151"><span class="label">[151]</span></a> Fleischmann, Landw. Versuchts Stat., 17:251.</p></div>
+
+<div class="footnote"><p><a name="Footnote_152_152" id="Footnote_152_152"></a><a href="#FNanchor_152_152"><span class="label">[152]</span></a> Babcock and Russell, Bull. 54, Wis. Expt. Stat., Aug.
+1896.</p></div>
+</div>
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_134" id="Page_134">[Pg 134]</a></span></p>
+<h2>CHAPTER VII.</h2>
+
+<h3>BACTERIA AND BUTTER-MAKING.</h3>
+
+
+<p>In making butter from the butter fat in milk, it is necessary to
+concentrate the fat globules into cream, preliminary to the churning
+process. The cream may be raised by the gravity process or separated
+from the milk by centrifugal action. In either case the bacteria that
+are normally present in the milk differentiate themselves in varying
+numbers in the cream and the skim-milk. The cream always contains per
+cc. a great many more than the skim-milk, the reason for this being that
+the bacteria are caught and held in the masses of fat globules, which,
+on account of their lighter specific gravity, move toward the surface of
+the milk or toward the interior of the separator bowl. This filtering
+action of the fat globules is similar to what happens in muddy water
+upon standing. As the suspended particles fall to the bottom they carry
+with them a large number of the organisms that are in the liquid.</p>
+
+<p><b>Various creaming methods.</b> The creaming method has an important bearing
+on the kind as well as the number of the bacteria that are to be found
+in the cream. The difference in species is largely determined by the
+difference in ripening temperature, while the varying number is governed
+more by the age of the milk.</p>
+
+<p><i>1. Primitive gravity methods.</i> In the old shallow-pan process, the
+temperature of the milk is relatively high, as the milk is allowed to
+cool naturally. This comparatively high temperature favors especially
+the development of those forms whose optimum growing-point is near the
+air<span class='pagenum'><a name="Page_135" id="Page_135">[Pg 135]</a></span> temperature. By this method the cream layer is exposed to the air
+for a longer time than with any other, and consequently the
+contamination from this source is greater. Usually cream obtained by the
+shallow-pan process will contain a larger number of species and also
+have a higher acid content.</p>
+
+<p><i>2. Modern gravity methods.</i> In the Cooley process, or any of the modern
+gravity methods where cold water or ice is used to lower the
+temperature, the conditions do not favor the growth of a large variety
+of species. The number of bacteria in the cream will depend largely upon
+the manner in which the milk is handled previous to setting. If care is
+used in milking, and the milk is kept so as to exclude outside
+contamination, the cream will be freer from bacteria than if
+carelessness prevails in handling the milk. Only those forms will
+develop in abundance that are able to grow at the low temperature at
+which the milk is set. Cream raised by this method is less frequently
+infected with undesirable forms than that which is creamed at a higher
+temperature.</p>
+
+<p><i>3. Centrifugal method.</i> Separator cream should contain less germ-life
+than that which is secured in the old way. It should contain only those
+forms that have found their way into the milk during and subsequent to
+the milking, for the cream is ordinarily separated so soon that there is
+but little opportunity of infection, if care is taken in the handling.
+As a consequence, the number of species found therein is smaller.</p>
+
+<p>Where milk is separated, it is always prudent to cool the cream so as to
+check growth, as the milk is generally heated before separating in order
+to skim efficiently.</p>
+
+<p>Although cream is numerically much richer in bacteria<span class='pagenum'><a name="Page_136" id="Page_136">[Pg 136]</a></span> than milk, yet
+the changes due to bacterial action are slower; hence milk sours more
+rapidly than cream. For this same reason, cream will sour sooner when it
+remains on the milk than it will if it is separated as soon as possible.
+This fact indicates the necessity of early creaming, so as to increase
+the keeping quality of the product, and is another argument in favor of
+the separator process.</p>
+
+<p><b>Ripening of cream.</b> If cream is allowed to remain at ordinary
+temperatures, it undergoes a series of fermentation changes that are
+exceedingly complex in character, the result of which is to produce in
+butter made from the same the characteristic flavor and aroma that are
+so well known in this article. We are so accustomed to the development
+of these flavors in butter that they are not generally recognized as
+being intimately associated with bacterial activity unless compared with
+butter made from perfectly fresh cream. Sweet-cream butter lacks the
+aromatic principle that is prominent in the ripened product, and while
+the flavor is delicate, it is relatively unpronounced.</p>
+
+<p>In the primitive method of butter-making, where the butter was made on
+the farm, the ripening of cream became a necessity in order that
+sufficient material might be accumulated to make a churning. The
+ripening change occurred spontaneously without the exercise of any
+especial control. With the development of the creamery system came the
+necessity of exercising a control of this process, and therefore the
+modern butter-maker must understand the principles which are involved in
+this series of complex changes that largely give to his product its
+commercial value.</p>
+
+<p>In these ripening changes three different factors are to be taken into
+consideration: the development of acid, flavor and aroma. Much confusion
+in the past has arisen from a<span class='pagenum'><a name="Page_137" id="Page_137">[Pg 137]</a></span> failure to discriminate between these
+qualities. While all three are produced simultaneously in ordinary
+ripening, it does not necessarily follow that they are produced by the
+same cause. If the ripening changes are allowed to go too far,
+undesirable rather than beneficial decomposition products are produced.
+These greatly impair the value of butter, so that it becomes necessary
+to know just to what extent this process should be carried.</p>
+
+<p>In cream ripening there is a very marked bacterial growth, the extent of
+which is determined mainly by the temperature of the cream. Conn and
+Esten<a name="FNanchor_153_153" id="FNanchor_153_153"></a><a href="#Footnote_153_153" class="fnanchor">[153]</a> find that the number of organisms may vary widely in
+unripened cream, but that the germ content of the ripened product is
+more uniform. When cream is ready for the churn, it often contains
+500,000,000 organisms per cc., and frequently even a higher number. This
+represents a germ content that has no parallel in any natural material.</p>
+
+<p>The larger proportion of bacteria in cream as it is found in the
+creamery belong to the acid-producing class, but in the process of
+ripening, these forms seem to thrive still better, so that when it is
+ready for churning the germ content of the cream is practically made up
+of this type.</p>
+
+<p><b>Effect on churning.</b> In fresh cream the fat globules which are suspended
+in the milk serum are surrounded by a film of albuminous material which
+prevents them from coalescing readily. During the ripening changes, this
+enveloping substance is modified, probably by partial solution, so that
+the globules cohere when agitated, as in churning. The result is that
+ripened cream churns more easily, and as it is possible to cause a
+larger number of the smaller fat-globules to cohere to the butter
+granules, the yield is<span class='pagenum'><a name="Page_138" id="Page_138">[Pg 138]</a></span> slightly larger&mdash;a point of considerable
+economic importance where large quantities of butter are made.</p>
+
+<p><b>Development of acid.</b> The result of this enormous bacterial
+multiplication is that acid is produced in cream, lactic being the
+principal acid so formed.</p>
+
+<p>Other organic acids are undoubtedly formed as well as certain aromatic
+products. While the production of acid as a result of fermentative
+activity is usually accompanied with a development of flavor, the flavor
+is not directly produced by the formation of acid. If cream is treated
+in proper proportions with a commercial acid, as hydrochloric,<a name="FNanchor_154_154" id="FNanchor_154_154"></a><a href="#Footnote_154_154" class="fnanchor">[154]</a> it
+assumes the same churning properties as found in normally ripened cream,
+but is devoid of the desired aromatic qualities. Lactic acid<a name="FNanchor_155_155" id="FNanchor_155_155"></a><a href="#Footnote_155_155" class="fnanchor">[155]</a> has
+also been used in a similar way but with no better results.</p>
+
+<p>The amount of acidity that should be developed under natural conditions
+so as to secure the optimum quality as to flavor and aroma is the most
+important question in cream ripening. Concerning this there have been
+two somewhat divergent views as to what is best in practice, some
+holding that better results were obtained with cream ripened to a high
+degree of acidity than where a less amount was developed.<a name="FNanchor_156_156" id="FNanchor_156_156"></a><a href="#Footnote_156_156" class="fnanchor">[156]</a> The
+present tendency seems to be to develop somewhat more than formerly, as
+it is thought that this secures more of the "high, quick" flavor wanted
+in the market. On the average, cream is ripened to about 0.5 to 0.65 per
+cent. acidity, a higher percentage than this giving a strong-flavored
+butter. In the determination of acidity, the most convenient method is
+to employ the Farrington<span class='pagenum'><a name="Page_139" id="Page_139">[Pg 139]</a></span> alkaline tablet, which permits of an accurate
+and rapid estimation of the acidity in the ripening cream. The amount of
+acidity to be produced must of necessity be governed by the amount of
+butter-fat present, for the formation of acid is confined to the serum
+of the cream; consequently, a rich cream would show less acid by
+titration than a thinner cream, and still contain really as much acid as
+the other. The importance of this factor is evident in gathered-cream
+factories.</p>
+
+<p>The rate of ripening is dependent upon the conditions that affect the
+rate of growth of bacterial life, such as time and temperature, number
+of organisms in cream and also the per cent of butter fat in the cream.
+Some years ago it was customary to ripen cream at about 50&deg; to 60&deg; F.,
+but more recently better results have been obtained, it is claimed,
+where the ripening temperature is increased and the period of ripening
+lessened. As high a temperature as 70&deg; to 75&deg; F. has been recommended.
+It should be said that this variation in practice may have a valid
+scientific foundation, for the temperature of the ripening cream is
+undoubtedly the most potent factor in determining what kind of bacteria
+will develop most luxuriantly. It is well known that those forms that
+are capable of producing bitter flavors are able to thrive better at a
+lower temperature than some of the desirable ripening species.</p>
+
+<p>The importance of this factor would be lessened where a pure culture was
+used in pasteurized cream, because here practically the selected
+organism alone controls the field.</p>
+
+<p>It is frequently asserted that better results are obtained by stirring
+the cream and so exposing it to the air as much as possible. Experiments
+made at the Ontario Agricultural College, however, show practically no
+difference in<span class='pagenum'><a name="Page_140" id="Page_140">[Pg 140]</a></span> the quality of the butter made by these two methods. The
+great majority of the bacteria in the cream belong to the facultative
+class, and are able to grow under conditions where they are not in
+direct contact with the air.</p>
+
+<p><b>Flavor and aroma.</b> The basis for the peculiar flavor or taste which
+ripened cream-butter possesses is due, in large part, to the formation
+of certain decomposition products formed by various bacteria. Aroma is a
+quality often confounded with flavor, but this is produced by volatile
+products only, which appeal to the sense of smell rather than taste.
+Generally a good flavor is accompanied by a desirable aroma, but the
+origin of the two qualities is not necessarily dependent on the same
+organisms. The quality of flavor and aroma in butter is, of course, also
+affected by other conditions, as, for instance, the presence or absence
+of salt, as well as the inherent qualities of the milk, that are
+controlled, to some extent at least, by the character of the feed which
+is consumed by the animal. The exact source of these desirable but
+evanescent qualities in butter is not yet satisfactorily determined.
+According to Storch,<a name="FNanchor_157_157" id="FNanchor_157_157"></a><a href="#Footnote_157_157" class="fnanchor">[157]</a> flavors are produced by the decomposition of
+the milk sugar and the absorption of the volatile flavors by the butter
+fat. Conn<a name="FNanchor_158_158" id="FNanchor_158_158"></a><a href="#Footnote_158_158" class="fnanchor">[158]</a> holds that the nitrogenous elements in cream serve as
+food for bacteria, and in the decomposition of which the desired
+aromatic substance is produced. The change is unquestionably a complex
+one, and cannot be explained as a single fermentation.</p>
+
+<p>There is no longer much doubt but that both acid-forming and
+casein-digesting species can take part in the production of proper
+flavors as well as desirable aromas. The researches<span class='pagenum'><a name="Page_141" id="Page_141">[Pg 141]</a></span> of Conn,<a name="FNanchor_159_159" id="FNanchor_159_159"></a><a href="#Footnote_159_159" class="fnanchor">[159]</a> who
+has studied this question most exhaustively, indicate that both of these
+types of decomposition participate in the production of flavor and
+aroma. He has shown that both flavor and aroma production are
+independent of acid; that many good flavor-producing forms belong to
+that class which renders milk alkaline, or do not change the reaction at
+all. Some of these species liquefied gelatin and would therefore belong
+to the casein-dissolving class. Those species that produced bad flavors
+are also included in both fermentative types. Conn has found a number of
+organisms that are favorable flavor-producers; in fact they were much
+more numerous than desirable aroma-yielding species. None of the
+favorable aroma forms according to his investigations were lactic-acid
+species,&mdash;a view which is also shared by Weigmann.<a name="FNanchor_160_160" id="FNanchor_160_160"></a><a href="#Footnote_160_160" class="fnanchor">[160]</a></p>
+
+<p>McDonnell<a name="FNanchor_161_161" id="FNanchor_161_161"></a><a href="#Footnote_161_161" class="fnanchor">[161]</a> has found that the production of aroma in certain cases
+varies at different temperatures, the most pronounced being evolved near
+the optimum growing temperature, which, as a general rule, is too high
+for cream ripening.</p>
+
+<p>The majority of bacteria in ripening cream do not seem to exert any
+marked influence in butter. A considerable number of species are
+positively beneficial, inasmuch as they produce a good flavor or aroma.
+A more limited number are concerned in the production of undesirable
+ripening changes. This condition being true, it may seem strange that
+butter is as good as it is, because so frequently the requisite care is
+not given to the development of proper ripening. In all probability the
+chief reason why this is so is that those bacteria that find milk and
+cream pre-eminently<span class='pagenum'><a name="Page_142" id="Page_142">[Pg 142]</a></span> suited to their development, e. g. the lactic-acid
+class, are either neutral or beneficial in their effect on butter.</p>
+
+<p><b>Use of starters.</b> Experience has amply demonstrated that it is possible
+to control the nature of the fermentative changes that occur in ripening
+cream to such an extent as to materially improve the quality of the
+butter. This is frequently done by the addition of a "starter." While
+starters have been employed for many years for the purpose mentioned, it
+is only recently that their nature has been understood. A starter may be
+selected from widely divergent sources, but in all cases it is sure to
+contain a large number of bacteria, and the presumption is that they are
+of such a nature as to produce desirable fermentative changes in the
+cream.</p>
+
+<p>In the selection of these so-called natural starters, it follows that
+they must be chosen under such conditions as experience has shown to
+give favorable results. For this purpose, whole milk from a single
+animal is often used where the same is observed to sour with the
+production of no gas or other undesirable taint. A skim-milk starter
+from a mixed supply is recommended by many. Butter milk is frequently
+employed, but in the opinion of butter experts is not as suitable as the
+others mentioned.</p>
+
+<p>It not infrequently happens that the practical operator may be misled in
+selecting a starter that is not desirable, or by continuing its use
+after it has become contaminated.</p>
+
+<p>In 1890<a name="FNanchor_162_162" id="FNanchor_162_162"></a><a href="#Footnote_162_162" class="fnanchor">[162]</a> a new system of cream ripening was introduced in Denmark by
+Storch that possesses the merit of being a truly scientific and at the
+same time practical method. This consisted in the use of pure cultures
+of<span class='pagenum'><a name="Page_143" id="Page_143">[Pg 143]</a></span> specific organisms that were selected on account of their ability to
+produce a desirable ripening change in cream. The introduction of these
+so-called culture starters has become universal in Denmark, and in parts
+of Germany. Their use is also rapidly extending in this country,
+Australia and New Zealand.</p>
+
+<p><b>Principles of pure-culture cream-ripening.</b> In the proper use of pure
+cultures for ripening cream, it is necessary first to eliminate as far
+as possible the bacteria already present in cream before the culture
+starter is added. This result is accomplished by heating the cream to a
+temperature sufficiently high to destroy the vegetating organisms. The
+addition of a properly selected starter will then give the chosen
+organism such an impetus as will generally enable it to gain the
+ascendency over any other bacteria and so control the character of the
+ripening. The principle employed is quite like that practiced in raising
+grain. The farmer prepares his soil by plowing, in this way killing the
+weeds. Then he sows his selected grain, which is merely a pure culture,
+and by the rapid growth of this, other forms are held in check.</p>
+
+<p>The attempt has been made to use these culture starters in raw sweet
+cream, but it can scarcely be expected that the most beneficial results
+will be attained in this way. This method has been justified on the
+basis of the following experiments. Where cream is pasteurized and no
+starter is added, the spore-bearing forms frequently produce undesirable
+flavors. These can almost always be controlled if a culture starter is
+added, the obnoxious form being repressed by the presence of the added
+starter. This condition is interpreted as indicating that the addition
+of a starter to cream which already contains developing bacteria will<span class='pagenum'><a name="Page_144" id="Page_144">[Pg 144]</a></span>
+prevent those originally present in the cream from growing.<a name="FNanchor_163_163" id="FNanchor_163_163"></a><a href="#Footnote_163_163" class="fnanchor">[163]</a> This
+repressive action of one species on another is a well-known
+bacteriological fact, but it must be remembered that such an explanation
+is only applicable in those cases where the culture organism is better
+able to develop than those forms that already exist in the cream.</p>
+
+<p>If the culture organism is added to raw milk or cream which already
+contains a flora that is well suited to develop in this medium, it is
+quite doubtful whether it would gain the supremacy in the ripening
+cream. The above method of adding a culture to raw cream renders
+cream-ripening details less burdensome, but at the same time Danish
+experience, which is entitled to most credence on this question, is
+opposed to this method.</p>
+
+<p><b>Reputed advantages of culture starters.</b> <i>1. Flavor and aroma.</i> Naturally
+the flavor produced by pure-culture ferments depends upon the character
+of the organism used. Those which are most extensively used are able to
+produce a perfectly clean but mild flavor, and a delicate but not
+pronounced aroma. The "high, quick" flavor and aroma that is so much
+desired in the American market is not readily obtained by the use of
+cultures. It is quite problematical whether the use of any single
+species will give any more marked aroma than normally occurs in natural
+ripening.</p>
+
+<p><i>2. Uniformity of product.</i> Culture starters produce a more uniform
+product because the type of fermentation is under more complete control,
+and herein is the greatest advantage to be derived from their use. Even
+the best butter-maker at times will fail to secure uniform results if
+his starter is not perfectly satisfactory.</p>
+<p><span class='pagenum'><a name="Page_145" id="Page_145">[Pg 145]</a></span></p>
+<p><i>3. Keeping quality of product.</i> Butter made from pasteurized cream to
+which a pure-culture starter has been added will keep much better than
+the ordinary product, because the diversity of the bacterial flora is
+less and the milk is therefore not so likely to contain those organisms
+that produce an "off" condition.</p>
+
+<p><i>4. Elimination of taints.</i> Many defective conditions in butter are
+attributable to the growth of undesirable bacteria in the cream that
+result in the formation of "off" flavors and taints. If cream is
+pasteurized, thereby destroying these organisms, then ripened with pure
+ferments, it is generally possible to eliminate the abnormal
+conditions.<a name="FNanchor_164_164" id="FNanchor_164_164"></a><a href="#Footnote_164_164" class="fnanchor">[164]</a> Taints may also be present in cream due to direct
+absorption from the cow or through exposure to foul odors.<a name="FNanchor_165_165" id="FNanchor_165_165"></a><a href="#Footnote_165_165" class="fnanchor">[165]</a> Troubles
+of this sort may thus be carried over to the butter. This is
+particularly true in regions where leeks and wild onions abound, as in
+some of the Atlantic States. The heating of the cream tends to expel
+these volatile taints, so that a fairly good article of butter can be
+made from what would otherwise be a relatively worthless product.</p>
+
+<p><b>Characteristics desired in culture starters.</b> Certain conditions as the
+following are desirable in starters made from pure cultures:</p>
+
+<p>1. Vigorous growth in milk at ordinary ripening temperatures.</p>
+
+<p>2. Ability to form acid so as to facilitate churning and increase the
+yield of butter.</p>
+
+<p>3. Able to produce a clean flavor and desirable aroma.</p>
+
+<p>4. Impart a good keeping quality to butter.</p>
+
+<p><span class='pagenum'><a name="Page_146" id="Page_146">[Pg 146]</a></span></p><p>5. Not easily modified in its flavor-producing qualities by artificial
+cultivation.</p>
+
+<p>These different conditions are difficult to attain, for the reason that
+some of them seem to be in part incompatible. Weigmann<a name="FNanchor_166_166" id="FNanchor_166_166"></a><a href="#Footnote_166_166" class="fnanchor">[166]</a> found that a
+good aroma was generally an evanescent property, and therefore opposed
+to good keeping quality. Conn has shown that the functions of
+acid-formation, flavor and aroma production are not necessarily related,
+and therefore the chances of finding a single organism that possesses
+all the desirable attributes are not very good.</p>
+
+<p>In all probability no one germ possesses all of these desirable
+qualities, but natural ripening is the resultant of the action of
+several forms.<a name="FNanchor_167_167" id="FNanchor_167_167"></a><a href="#Footnote_167_167" class="fnanchor">[167]</a> This idea has led to the attempt at mixing selected
+organisms that have been chosen on account of certain favorable
+characteristics which they might possess. The difficulty of maintaining
+such a composite culture in its correct proportions when it is
+propagated in the creamery is seemingly well nigh insuperable, as one
+organism is very apt to develop more or less rapidly than the other.</p>
+
+<p>A very satisfactory way in which these cultures are marketed is to mix
+the bacterial growth with some sterile, inert, dry substance. This is
+the method used in most of the Danish cultures. In this country, some of
+the more prominent cultures employed are marketed in a liquid form.</p>
+
+<p><b>Culture vs. home-made starters.</b> One great advantage which has accrued
+from the use of culture or commercial starters has been that in
+emphasizing the need of closer control of the ripening process, greater
+attention has been<span class='pagenum'><a name="Page_147" id="Page_147">[Pg 147]</a></span> paid to the carrying out of the details. In the
+hands of the better operators, the differences in flavor of butter made
+with a culture or a natural starter are not marked,<a name="FNanchor_168_168" id="FNanchor_168_168"></a><a href="#Footnote_168_168" class="fnanchor">[168]</a> but in the
+hands of those who fail to make a good product under ordinary
+conditions, an improvement is often secured where a commercial culture
+is used.</p>
+
+<p><b>Pasteurization as applied to butter-making.</b> This process, as applied to
+butter making, is often confounded with the treatment of milk and cream
+for direct consumption. It is unfortunate that the same term is used in
+connection with the two methods, for they have but little in common
+except in the use of heat to destroy the germ life of the milk. In
+pasteurizing cream for butter-making, it is not necessary to observe the
+stringent precautions that are to be noted in the preservation of milk;
+for the addition of a rapidly developing starter controls at once the
+fermentative changes that subsequently occur. Then again, the physical
+requirement as to the production of a cooked taste is not so stringent
+in butter-making. While a cooked taste is imparted to milk or even cream
+at about 158&deg; F., it is possible to make butter that shows no permanent
+cooked taste from cream that has been raised as high as 185&deg; or even
+195&deg; F. This is due to the fact that the fat does not readily take up
+those substances that give to scalded milk its peculiar flavor.</p>
+
+<p>Unless care is taken in the manipulation of the heated cream, the grain
+or body of the butter may be injured. This tendency can be overcome if
+the ripened cream is<span class='pagenum'><a name="Page_148" id="Page_148">[Pg 148]</a></span> chilled to 48&deg; F. for about two hours before
+churning. It is also essential that the heated cream should be quickly
+and thoroughly chilled after being pasteurized.</p>
+
+<p>The Danes, who were the first to employ pasteurization in butter-making,
+used, in the beginning, a temperature ranging from 158&deg; to 167&deg; F., but
+owing to the prevalence of such diseases as tuberculosis and
+foot-and-mouth disease, it became necessary to treat all of the skim
+milk that was returned from the creameries. For this purpose the skim
+milk is heated to a temperature of 176&deg; F., it having been more recently
+determined that this degree of heat is sufficient to destroy the seeds
+of disease. With the use of this higher temperature the capacity of the
+pasteurizing apparatus is considerably reduced, but the higher
+temperature is rendered necessary by the prevailing conditions as to
+disease.</p>
+
+<p>When the system was first introduced in Denmark, two methods of
+procedure were followed: the whole milk was heated to a sufficiently
+high temperature to thoroughly pasteurize it before it was separated, or
+it was separated first, and the cream pasteurized afterwards. In the
+latter case, it is necessary to heat the skim milk after separation to
+destroy the disease organisms, but this can be quickly done by the use
+of steam directly. Much more care must be used in heating the cream in
+order to prevent injury to the grain of the butter. In spite of the
+extra trouble of heating the cream and skim milk separately, this method
+has practically supplanted the single heating. With the continual spread
+of tuberculosis in America the heating of skim milk separately is
+beginning to be introduced.<a name="FNanchor_169_169" id="FNanchor_169_169"></a><a href="#Footnote_169_169" class="fnanchor">[169]</a></p>
+<p><span class='pagenum'><a name="Page_149" id="Page_149">[Pg 149]</a></span></p>
+<p><b>Use of starters in pasteurized and unpasteurized cream.</b> In order to
+secure the beneficial results presumably attributable to the use of a
+starter, natural as well as a pure culture, it should be employed in
+cream in which the bacteria have first been killed out by
+pasteurization. This is certainly the most logical and scientific method
+and is the way in which the process has been developed in Denmark.</p>
+
+<p>Here in this country, the use of pure cultures has been quite rapidly
+extended, but the system of heating the cream has been used in only a
+slight measure. The increased labor and expense incurred in pasteurizing
+the cream has naturally militated somewhat against the wide-spread use
+of the process, but doubtless the main factor has been the inability to
+secure as high a flavor where the cream was heated as in the unheated
+product. As the demands of the market change from a high, quick flavor
+to one that is somewhat milder but of better keeping quality, doubtless
+pasteurization of the cream will become more and more popular. That such
+a change is gradually occurring is already evident, although as yet only
+a small proportion of butter made in this country is now made in this
+way. Where the cream is unheated, a considerable number of species will
+be found, and even the addition of a pure culture, if that culture is of
+the lactic acid-producing species, will to some extent control the type
+of fermentation that occurs. Such would not be the case with a culture
+composed of the casein-digesting type of bacteria. Only those forms
+could thus be used which are especially well suited to development in
+raw cream. For this reason the pure culture ferments that are generally
+employed in creamery practice are organisms of the lactic acid type,
+able to grow rapidly in cream and produce a pure cream flavor in the
+butter.<span class='pagenum'><a name="Page_150" id="Page_150">[Pg 150]</a></span></p>
+
+<p><b>Purity of commercial starters.</b> Naturally the butter maker is forced to
+rely on the laboratory for his commercial starter, and the question will
+often arise as to the purity and vigor of the various ferments employed.
+As there is no way for the factory operator to ascertain the actual
+condition of the starter, except by using the same, the greatest care
+should be taken by the manufacturer to insure the absolute purity of the
+seed used.</p>
+
+<p>A bacteriological examination of the various cultures which have been
+placed on the market not infrequently reveals an impure condition. In
+several cases the writer has found a not inconsiderable number of
+liquefying bacteria mixed with the selected organism. Molds not
+infrequently are found in cultures put up in the dry form. Doubtless the
+effect of these accidental contaminations is considerably less in the
+case of a starter composed of a distinctively lactic acid-producing
+organism than with a form which is less capable of thriving vigorously
+in milk, and it should be said that these impurities can frequently be
+eliminated by continued propagation.</p>
+
+<p>The virility and vigor of the starter is also a fluctuating factor,
+dependent in part at least, upon the conditions under which the organism
+is grown. In some cases the germ is cultivated in solutions in which
+acid cannot be formed in abundance. Where the conditions permit of the
+formation of acid, as would be the case if sugar was present with a
+lactic acid-producing species, the vitality of the culture is often
+impaired by the action of the gradually accumulating acid. Some
+manufacturers attempt to minimize this deleterious condition by adding
+carbonate of lime which unites with the acid that is formed.<span class='pagenum'><a name="Page_151" id="Page_151">[Pg 151]</a></span></p>
+
+<p><b>Propagation of starters for cream-ripening.</b> The preparation and
+propagation of a starter for cream-ripening is a process involving
+considerable bacteriological knowledge, whether the starter is of
+domestic origin or prepared from a pure-culture ferment. In any event,
+it is necessary that the starter should be handled in a way so as to
+prevent the introduction of foreign bacteria as far as possible. It
+should be remembered at all times that the starter is a live thing and
+must be handled throughout its entire history in a way so as to retain
+its vitality and vigor unimpaired. The following points should be taken
+into consideration in growing the starter and transferring it from day
+to day:</p>
+
+<p>1. If a commercial starter is used, see that it is fresh and that the
+seal has not been broken. If the culture is too old, the larger part of
+the organisms may have died out before it is transferred, in which case
+the effect of its addition to the sterilized milk would be of little
+value.</p>
+
+<p>When the commercial ferment is received, it should be stored in the
+refrigerator pending its use so as to retard as much as possible the
+changes that naturally go on in the culture liquid. Be careful that the
+bottle is not exposed to the influence of direct sunlight for in a
+transparent medium the organisms may be readily killed by the
+disinfecting action of the sun's rays.</p>
+
+<p>2. If a home-made starter is employed, use the greatest possible care in
+selecting the milk that is to be used as a basis for the starter.</p>
+
+<p>3. For the propagation and perpetuation of the starter from day to day,
+it is necessary that the same should be grown in milk that is as
+germ-free as it is possible to secure it. For this purpose sterilize
+some fresh skim-milk in a<span class='pagenum'><a name="Page_152" id="Page_152">[Pg 152]</a></span> covered can that has previously been well
+steamed. This can be done easily by setting cans containing skim-milk in
+a vat filled with water and heating the same to 180&deg; F. or above for
+one-half hour or more. Steam should not be introduced directly. This
+process destroys all but a few of the most resistant spore-bearing
+organisms. This will give a cooked flavor to the milk, but will not
+affect the cream to which the starter is added. Dairy supply houses are
+now introducing the use of starter cans that are specially made for this
+purpose.</p>
+
+<p>4. After the heated milk is cooled down to about 70&deg; or 80&deg; F., it can
+be inoculated with the desired culture. Sometimes it is desirable to
+"build up" the starter by propagating it first in a smaller volume of
+milk, and then after this has developed, adding it to a larger amount.</p>
+
+<p>This method is of particular value where a large amount of starter is
+needed for the cream-ripening.</p>
+
+<p>5. After the milk has been inoculated, it should be kept at a
+temperature that is suitable for the rapid development of the contained
+bacteria, 65&deg;-75&deg; F., which temperature should be kept as uniform as
+possible.</p>
+
+<p>This can best be done by setting the covered can in a vat filled with
+warm water. The starter cans are often arranged so that temperature can
+be controlled by circulating water.</p>
+
+<p>6. The starter should not be too thoroughly curdled when it is needed
+for use, but should be well soured and only partially curdled for it is
+difficult to break up thoroughly the curd particles if the starter is
+completely curdled. If<span class='pagenum'><a name="Page_153" id="Page_153">[Pg 153]</a></span> these curd masses are added to ripening cream,
+white specks may appear in the butter.</p>
+
+<p>7. The vigor of the starter is in all probability stronger when the milk
+is on the point of curdling than it is after the curd has been formed
+some time. The continued formation of lactic acid kills many of the
+bacteria and thus weakens the fermentative action. It is therefore
+highly important that the acidity of the starter should be closely
+watched.</p>
+
+<p>8. Do not refrigerate the starter when it has reached the proper stage
+of development, as this retards the bacterial growth in the same manner
+as cold weather checks the growth of grain. It is preferable to dilute
+the starter, if it cannot be used when ready, with sufficient freshly
+sterilized sweet milk to hold the acidity at the proper point and thus
+keep the bacteria in the starter in a condition which will favor
+vigorous growth.</p>
+
+<p>9. The starter should be propagated from day to day by adding a small
+quantity to a new lot of freshly prepared milk. For this purpose two
+propagating cans should be provided so that one starter may be in use
+while the other is being prepared.</p>
+
+<p><b>How long should a starter be propagated?</b> No hard-and-fast rule can be
+given for this, for it depends largely upon how carefully the starter is
+handled during its propagation. If the starter is grown in sterilized
+milk kept in steamed vessels and is handled with sterile dippers, it is
+possible to maintain it in a state of relative purity for a considerable
+period of time; if, however, no especial care is given, it will soon
+become infected by the air, and the<span class='pagenum'><a name="Page_154" id="Page_154">[Pg 154]</a></span> retention of its purity will depend
+more upon the ability of the contained organism to choke out foreign
+growths than upon any other factor. Experience seems to indicate that
+pure-culture starters "run out" sooner than domestic starters. While it
+is possible, by bacteriological methods, to determine with accuracy the
+actual condition of a starter as to its germ content, still such methods
+are inapplicable in creamery practice. Here the maker must rely largely
+upon the general appearance of the starter as determined by taste and
+smell. The supply houses that deal in cultures of this class generally
+expect to supply a new culture at least every month.</p>
+
+<p><b>Bacteria in butter.</b> As ripened cream is necessarily rich in bacteria, it
+follows that butter will also contain germ life in varying amounts, but
+as butter-fat is not well adapted for bacterial food, the number of
+germs in butter is usually less than in ripened cream.</p>
+
+<p>Sweet-cream butter is naturally poorer in germ life than that made from
+ripened cream. Grotenfelt reports in sweet-cream butter, the so-called
+"Paris butter," only a few bacteria while in acid cream butter the germ
+content runs from scores to hundreds of thousands.</p>
+
+<p><b>Effect of bacteria in wash water.</b> An important factor in contamination
+may be the wash water that is used. Much carelessness often prevails
+regarding the location and drainage of the creamery well, and if same
+becomes polluted with organic matter, bacterial growth goes on apace.
+Melick<a name="FNanchor_170_170" id="FNanchor_170_170"></a><a href="#Footnote_170_170" class="fnanchor">[170]</a> has made some interesting studies on using pasteurized and
+sterilized well waters for washing. He found a direct relation to exist
+between the bacterial content of the wash water and the keeping quality
+of the butter. Some creameries have tried filtered water but under
+ordinary conditions a filter, unless it is tended to with great
+regularity, becomes a source of infection rather than otherwise.</p>
+<p><span class='pagenum'><a name="Page_155" id="Page_155">[Pg 155]</a></span></p>
+<p><b>Changes in germ content.</b> The bacteria that are incorporated with the
+butter as it first "comes" undergo a slight increase for the first few
+days. The duration of this period of increase is dependent largely upon
+the condition of the butter. If the buttermilk is well worked out of the
+butter, the increase is slight and lasts for a few days only, while the
+presence of so nutritious a medium as buttermilk affords conditions much
+more favorable for the continued growth of the organisms.</p>
+
+<p>While there may be many varieties in butter when it is fresh, they are
+very soon reduced in kind as well as number. The lactic acid group of
+organisms disappear quite rapidly; the spore-bearing species remaining
+for a somewhat longer time. Butter examined after it is several months
+old is often found to be almost free from germs.</p>
+
+<p>In the manufacture of butter there is much that is dependent upon the
+mechanical processes of churning, washing, salting and working the
+product. These processes do not involve any bacteriological principles
+other than those that are incident to cleanliness. The cream, if ripened
+properly, will contain such enormous numbers of favorable forms that the
+access of the few organisms that are derived from the churn, the air, or
+the water in washing will have little effect, unless the conditions are
+abnormal.</p>
+
+
+<h4>BACTERIAL DEFECTS IN BUTTER.</h4>
+
+<p><b>Rancid change in butter.</b> Fresh butter has a peculiar aroma that is very
+desirable and one that enhances the market price, if it can be retained;
+but this delicate flavor is more or less evanescent, soon disappearing,
+even in the best makes. While a good butter loses with age some of the
+peculiar aroma that it possesses when first made, yet a<span class='pagenum'><a name="Page_156" id="Page_156">[Pg 156]</a></span> gilt-edged
+product should retain its good keeping qualities for some length of
+time. All butters, however, sooner or later undergo a change that
+renders them worthless for table use. This change is usually a rancidity
+that is observed in all stale products of this class. The cause of this
+rancid condition in butter was at first attributed to the formation of
+butyric acid, but it is now recognized that other changes also enter
+in.<a name="FNanchor_171_171" id="FNanchor_171_171"></a><a href="#Footnote_171_171" class="fnanchor">[171]</a> Light and especially air also exert a marked effect on the
+flavor of butter. Where butter is kept in small packages it is much more
+prone to develop off flavors than when packed in large tubs. From the
+carefully executed experiments of Jensen it appears that some of the
+molds as well as certain species of bacteria are able to incite these
+changes. These organisms are common in the air and water and it
+therefore readily follows that inoculation occurs.</p>
+
+<p>Practically, rancidity is held in check by storing butter at low
+temperatures where germ growth is quite suspended.</p>
+
+<p><b>Lack of flavor.</b> Often this may be due to improper handling of the cream
+in not allowing it to ripen far enough, but sometimes it is impossible
+to produce a high flavor. The lack of flavor in this case is due to the
+absence of the proper flavor-producing organisms. This condition can
+usually be overcome by the addition of a proper starter.</p>
+
+<p><b>Putrid butter.</b> This specific butter trouble has been observed in
+Denmark, where it has been studied by Jensen.<a name="FNanchor_172_172" id="FNanchor_172_172"></a><a href="#Footnote_172_172" class="fnanchor">[172]</a> Butter affected by it
+rapidly acquires a peculiar putrid odor that ruins it for table use.
+Sometimes, this flavor may be developed in the cream previous to
+churning.<span class='pagenum'><a name="Page_157" id="Page_157">[Pg 157]</a></span></p>
+
+<p>Jensen found the trouble to be due to several different putrefactive
+bacteria. One form which he called <i>Bacillus f[oe]tidus lactis</i>, a close
+ally of the common feces bacillus, produced this rotten odor and taste
+in milk in a very short time. Fortunately, this organism was easily
+killed by a comparatively low heat, so that pasteurization of the cream
+and use of a culture starter quickly eliminated the trouble, where it
+was tried.</p>
+
+<p><b>Turnip-flavored butter.</b> Butter sometimes acquires a peculiar flavor
+recalling the order of turnips, rutabagas, and other root crops. Often
+this trouble is due to feeding, there being in several of these crops,
+aromatic substances that pass directly into the milk, but in some
+instances the trouble arises from bacteria that are able to produce
+decomposition products,<a name="FNanchor_173_173" id="FNanchor_173_173"></a><a href="#Footnote_173_173" class="fnanchor">[173]</a> the odor and taste of which strongly
+recalls these vegetables.</p>
+
+<p><b>"Cowy" butter.</b> Frequently there is to be noted in milk a peculiar odor
+that resembles that of the cow stable. Usually this defect in milk has
+been ascribed to the absorption of impure gases by the milk as it cools,
+although the gases and odors naturally present in fresh milk have this
+peculiar property that is demonstrable by certain methods of aeration.
+Occasionally it is transmitted to butter, and recently Pammel<a name="FNanchor_174_174" id="FNanchor_174_174"></a><a href="#Footnote_174_174" class="fnanchor">[174]</a> has
+isolated from butter a bacillus that produced in milk the same peculiar
+odor so commonly present in stables.</p>
+
+<p><b>Lardy and tallowy butter.</b> The presence of this unpleasant taste in
+butter may be due to a variety of causes. In some instances, improper
+food seems to be the source of the trouble; then again, butter exposed
+to direct sunlight<span class='pagenum'><a name="Page_158" id="Page_158">[Pg 158]</a></span> bleaches in color and develops a lardy flavor.<a name="FNanchor_175_175" id="FNanchor_175_175"></a><a href="#Footnote_175_175" class="fnanchor">[175]</a>
+In addition to these, cases have been found in which the defect has been
+traced to the action of bacteria. Storch<a name="FNanchor_176_176" id="FNanchor_176_176"></a><a href="#Footnote_176_176" class="fnanchor">[176]</a> has described a
+lactic-acid form in a sample of tallowy butter that was able to produce
+this disagreeable odor.</p>
+
+<p><b>Oily butter.</b> Jensen has isolated one of the causes of the dreaded oily
+butter that is reported quite frequently in Denmark. The specific
+organism that he found belongs to the sour-milk bacteria. In twenty-four
+hours it curdles milk, the curd being solid like that of ordinary sour
+milk. There is produced, however, in addition to this, an unpleasant
+odor and taste resembling that of machine oil, a peculiarity that is
+transmitted directly to butter made from affected cream.</p>
+
+<p><b>Bitter butter.</b> Now and then butter develops a bitter taste that may be
+due to a variety of different bacterial forms. In most cases, the bitter
+flavor in the butter is derived primarily from the bacteria present in
+the cream or milk. Several of the fermentations of this character in
+milk are also to be found in butter. In addition to these defects
+produced by a biological cause, bitter flavors in butter are sometimes
+produced by the milk being impregnated with volatile, bitter substances
+derived from weeds.</p>
+
+<p><b>Moldy butter.</b> This defect is perhaps the most serious because most
+common. It is produced by the development of a number of different
+varieties of molds. The trouble appears most frequently in packed butter
+on the outside of the mass of butter in contact with the tub. Mold
+spores are so widely disseminated that if proper conditions are<span class='pagenum'><a name="Page_159" id="Page_159">[Pg 159]</a></span> given
+for their germination, they are almost sure to develop. In some cases
+the mold is due to the growth of the ordinary bread mold, <i>Penicillium
+glaucum</i>; in other cases a black mold develops, due often to
+<i>Cladosporium butyri</i>. Not infrequently trouble of this character is
+associated with the use of parchment wrappers. The difficulty can easily
+be held in check by soaking the parchment linings and the tubs in a
+strong brine, or paraffining the inside of the tub.</p>
+
+<p><b>Fishy butter.</b> Considerable trouble has been experienced in Australian
+butter exported to Europe in which a fishy flavor developed. It was
+noted that the production of this defect seemed to be dependent upon the
+storage temperature at which the butter was kept. When the butter was
+refrigerated at 15&deg; F. no further difficulty was experienced. It is
+claimed that the cause of this condition is due to the formation of
+trimethylamine (herring brine odor) due to the growth of the mold fungus
+<i>Oidium lactis</i>, developing in combination with the lactic-acid
+bacteria.</p>
+
+<p>A fishy taste is sometimes noted in canned butter. Rogers<a name="FNanchor_177_177" id="FNanchor_177_177"></a><a href="#Footnote_177_177" class="fnanchor">[177]</a> has
+determined that this flavor is caused by yeasts (<i>Torula</i>) which produce
+fat-splitting enzyms capable of producing this undesirable change.</p>
+
+<div class="footnotes"><h3>FOOTNOTES:</h3>
+
+<div class="footnote"><p><a name="Footnote_153_153" id="Footnote_153_153"></a><a href="#FNanchor_153_153"><span class="label">[153]</span></a> Conn and Esten, Cent. f. Bakt., II Abt., 1901, 7:746.</p></div>
+
+<div class="footnote"><p><a name="Footnote_154_154" id="Footnote_154_154"></a><a href="#FNanchor_154_154"><span class="label">[154]</span></a> Tiemann, Milch Zeit., 23:701.</p></div>
+
+<div class="footnote"><p><a name="Footnote_155_155" id="Footnote_155_155"></a><a href="#FNanchor_155_155"><span class="label">[155]</span></a> Milch Zeit., 1889, p. 7; 1894, p. 624; 1895, p. 383.</p></div>
+
+<div class="footnote"><p><a name="Footnote_156_156" id="Footnote_156_156"></a><a href="#FNanchor_156_156"><span class="label">[156]</span></a> Dean, Ont. Agr. Coll., 1897, p. 66.</p></div>
+
+<div class="footnote"><p><a name="Footnote_157_157" id="Footnote_157_157"></a><a href="#FNanchor_157_157"><span class="label">[157]</span></a> Storch, Nogle, Unders. over Floed. Syrning, 1890.</p></div>
+
+<div class="footnote"><p><a name="Footnote_158_158" id="Footnote_158_158"></a><a href="#FNanchor_158_158"><span class="label">[158]</span></a> Conn, 6 Storrs Expt. Stat., 1893, p. 66.</p></div>
+
+<div class="footnote"><p><a name="Footnote_159_159" id="Footnote_159_159"></a><a href="#FNanchor_159_159"><span class="label">[159]</span></a> Conn, 9 Storrs Expt. Stat., 1896, p. 17.</p></div>
+
+<div class="footnote"><p><a name="Footnote_160_160" id="Footnote_160_160"></a><a href="#FNanchor_160_160"><span class="label">[160]</span></a> Weigmann, Milch Zeit., 1891, p. 793</p></div>
+
+<div class="footnote"><p><a name="Footnote_161_161" id="Footnote_161_161"></a><a href="#FNanchor_161_161"><span class="label">[161]</span></a> McDonnell, &uuml;. Milchs&auml;ure Bakterien (Diss. Kiel, 1899), p.
+43.</p></div>
+
+<div class="footnote"><p><a name="Footnote_162_162" id="Footnote_162_162"></a><a href="#FNanchor_162_162"><span class="label">[162]</span></a> Storch, Milch Zeit., 1890, p. 304.</p></div>
+
+<div class="footnote"><p><a name="Footnote_163_163" id="Footnote_163_163"></a><a href="#FNanchor_163_163"><span class="label">[163]</span></a> Conn, 9 Storrs Expt. Stat., 1896, p. 25.</p></div>
+
+<div class="footnote"><p><a name="Footnote_164_164" id="Footnote_164_164"></a><a href="#FNanchor_164_164"><span class="label">[164]</span></a> Milch Zeit., 1891, p. 122; 1894, p. 284; 1895, p. 56;
+1896, p. 163.</p></div>
+
+<div class="footnote"><p><a name="Footnote_165_165" id="Footnote_165_165"></a><a href="#FNanchor_165_165"><span class="label">[165]</span></a> McKay, Bull. 32, Iowa Expt. Stat., p. 47</p></div>
+
+<div class="footnote"><p><a name="Footnote_166_166" id="Footnote_166_166"></a><a href="#FNanchor_166_166"><span class="label">[166]</span></a> Weigmann, Landw. Woch. f. Schl. Hol., No. 2, 1890.</p></div>
+
+<div class="footnote"><p><a name="Footnote_167_167" id="Footnote_167_167"></a><a href="#FNanchor_167_167"><span class="label">[167]</span></a> Weigmann, Cent. f. Bakt., II Abt., 3:497, 1897.</p></div>
+
+<div class="footnote"><p><a name="Footnote_168_168" id="Footnote_168_168"></a><a href="#FNanchor_168_168"><span class="label">[168]</span></a> At the National Creamery Buttermakers' Association for
+1901, 193 out of 240 exhibitors used starters. Of those that employed
+starters, nearly one-half used commercial cultures. There was
+practically no difference in the average score of the two classes of
+starters, but those using starters ranked nearly two points higher in
+flavor than those that did not.</p></div>
+
+<div class="footnote"><p><a name="Footnote_169_169" id="Footnote_169_169"></a><a href="#FNanchor_169_169"><span class="label">[169]</span></a> Russell, Bull. 143, Wis. Expt. Stat., Feb. 1907.</p></div>
+
+<div class="footnote"><p><a name="Footnote_170_170" id="Footnote_170_170"></a><a href="#FNanchor_170_170"><span class="label">[170]</span></a> Melick, Bull. 138, Kansas Expt. Stat., June 1906.</p></div>
+
+<div class="footnote"><p><a name="Footnote_171_171" id="Footnote_171_171"></a><a href="#FNanchor_171_171"><span class="label">[171]</span></a> Reinmann, Cent. f. Bakt., 1900, 6:131; Jensen, Landw.
+Jahr. d. Schweiz, 1901.</p></div>
+
+<div class="footnote"><p><a name="Footnote_172_172" id="Footnote_172_172"></a><a href="#FNanchor_172_172"><span class="label">[172]</span></a> Jensen, Cent. f. Bakt., 1891, 11:409.</p></div>
+
+<div class="footnote"><p><a name="Footnote_173_173" id="Footnote_173_173"></a><a href="#FNanchor_173_173"><span class="label">[173]</span></a> Jensen, Milch Zeit., 1892, 6, Nos. 5 and 6.</p></div>
+
+<div class="footnote"><p><a name="Footnote_174_174" id="Footnote_174_174"></a><a href="#FNanchor_174_174"><span class="label">[174]</span></a> Pammel, Bull. 21, Iowa Expt. Stat., p. 803.</p></div>
+
+<div class="footnote"><p><a name="Footnote_175_175" id="Footnote_175_175"></a><a href="#FNanchor_175_175"><span class="label">[175]</span></a> Fischer, Hyg. Rund., 5:573.</p></div>
+
+<div class="footnote"><p><a name="Footnote_176_176" id="Footnote_176_176"></a><a href="#FNanchor_176_176"><span class="label">[176]</span></a> Storch, 18 Rept. Danish Agric. Expt. Stat., 1890.</p></div>
+
+<div class="footnote"><p><a name="Footnote_177_177" id="Footnote_177_177"></a><a href="#FNanchor_177_177"><span class="label">[177]</span></a> Rogers Bull. 57, B. A. I. U. S. Dept Agric., 1904.</p></div>
+</div>
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_160" id="Page_160">[Pg 160]</a></span></p>
+<h2>CHAPTER VIII.</h2>
+
+<h3>BACTERIA IN CHEESE.</h3>
+
+
+<p>The art of cheese-making, like all other phases of dairying, has been
+developed mainly as a result of empirical methods. Within the last
+decade or so, the subject has received more attention from the
+scientific point of view and the underlying causes determined to some
+extent. Since the subject has been investigated from the bacteriological
+point of view, much light has been thrown on the cause of many changes
+that were heretofore inexplicable. Our knowledge, as yet, is quite
+meager, but enough has already been determined to indicate that the
+whole industry is largely based on the phenomena of ferment action, and
+that the application of bacteriological principles and ideas is sure to
+yield more than ordinary results, in explaining, in a rational way, the
+reasons underlying many of the processes to be observed in this
+industry.</p>
+
+<p>The problem of good milk is a vital one in any phase of dairy activity,
+but it is pre-eminently so in cheese-making, for the ability to make a
+first-class product depends to a large extent on the quality of the raw
+material. Cheese contains so large a proportion of nitrogenous
+constituents that it is admirably suited, as a food medium, to the
+development of bacteria; much better, in fact, than butter.</p>
+
+
+<h4>INFLUENCE OF BACTERIA IN NORMAL CHEESE PROCESSES.</h4>
+
+<p>In the manufacture of cheddar cheese bacteria exert a marked influence
+in the initial stages of the process. To produce the proper texture that
+characterizes cheddar cheese, it is necessary to develop a certain
+amount of acid<span class='pagenum'><a name="Page_161" id="Page_161">[Pg 161]</a></span> which acts upon the casein. This acidity is measured by
+the development of the lactic-acid bacteria that normally abound in the
+milk; or, as the cheese-maker expresses it, the milk is "ripened" to the
+proper point. The action of the rennet, which is added to precipitate
+the casein of the milk, is markedly affected by the amount of acid
+present, as well as the temperature. Hence it is desirable to have a
+standard amount of acidity as well as a standard temperature for
+coagulation, so as to unify conditions. It frequently happens that the
+milk is abnormal with reference to its bacterial content, on account of
+the absence of the proper lactic bacteria, or the presence of forms
+capable of producing fermentative changes of an undesirable character.
+In such cases the maker attempts to overcome the effect of the unwelcome
+bacteria by adding a "starter;" or he must vary his method of
+manufacture to some extent to meet these new conditions.</p>
+
+<p><b>Use of starters.</b> A starter may be employed to hasten the ripening of
+milk that is extremely sweet, so as to curtail the time necessary to get
+the cheese to press; or it may be used to overcome the effect of
+abnormal conditions.</p>
+
+<p>The starter that is employed is generally one of domestic origin, and is
+usually taken from skim milk that has been allowed to ferment and sour
+under carefully controlled conditions. Of course much depends upon the
+quality of the starter, and in a natural starter there is always the
+possibility that it may not be perfectly pure.</p>
+
+<p>Within recent years the attempt has been made to control the effect of
+the starter more thoroughly by using pure cultures of some desirable
+lactic-acid form.<a name="FNanchor_178_178" id="FNanchor_178_178"></a><a href="#Footnote_178_178" class="fnanchor">[178]</a> This has rendered<span class='pagenum'><a name="Page_162" id="Page_162">[Pg 162]</a></span> the making of cheese not only
+more uniform, but has aided in repressing abnormal fermentations
+particularly those that are characterized by the production of gas.</p>
+
+<p>Recently, pure cultures of Adametz's <i>B. nobilis</i>, a digesting organism
+that is claimed to be the cause of the breaking down of the casein and
+also of the peculiar aroma of Emmenthaler cheese, has been placed on the
+market under the name <i>Tyrogen</i>. It is claimed that the use of this
+starter, which is added directly to the milk and also rubbed on the
+surface of the cheese, results in the improvement of the curds, assists
+in the development of the proper holes, imparts a favorable aroma and
+hastens ripening.<a name="FNanchor_179_179" id="FNanchor_179_179"></a><a href="#Footnote_179_179" class="fnanchor">[179]</a></p>
+
+<p>Campbell<a name="FNanchor_180_180" id="FNanchor_180_180"></a><a href="#Footnote_180_180" class="fnanchor">[180]</a> states that the discoloration of cheese in England, which
+is due to the formation of white spots that are produced by the
+bleaching of the coloring matter in the cheese, may be overcome by the
+use of lactic-acid starters.</p>
+
+<p>The use of stringy or slimy whey has been advocated in Holland for some
+years as a means of overcoming the tendency toward gas formation in Edam
+cheese which is made from practically sweet milk. This fermentation, the
+essential feature of which is produced by a culture of <i>Streptococcus
+Hollandicus</i>,<a name="FNanchor_181_181" id="FNanchor_181_181"></a><a href="#Footnote_181_181" class="fnanchor">[181]</a> develops acid in a marked degree, thereby inhibiting
+the production of gas.</p>
+
+<p>The use of masses of moldy bread in directing the fermentation of
+Roquefort cheese is another illustration of the empirical development of
+starters, although in this instance it is added after the curds have
+been prepared for the press.</p>
+
+<p><b>Pasteurizing milk for cheese-making.</b> If it were possible to use properly
+pasteurized milk in cheese-making,<span class='pagenum'><a name="Page_163" id="Page_163">[Pg 163]</a></span> then practically all abnormal
+conditions could be controlled by the use of properly selected starters.
+Numerous attempts have been made to perfect this system with reference
+to cheddar cheese, but so far they have been attended with imperfect
+success. The reason for this is that in pasteurizing milk, the soluble
+lime salts are precipitated by the action of heat, and under these
+conditions rennet extract does not curdle the casein in a normal manner.
+This condition can be restored, in part at least, by the addition of
+soluble lime salts, such as calcium chlorid; but in our experience,
+desirable results were not obtained where heated milks to which this
+calcium solution had been added were made into cheddar cheese.
+Considerable experience has been gained in the use of heated milks in
+the manufacture of certain types of foreign cheese. Klein<a name="FNanchor_182_182" id="FNanchor_182_182"></a><a href="#Footnote_182_182" class="fnanchor">[182]</a> finds
+that Brick cheese can be successfully made even where the milk is heated
+as high as 185&deg; F. An increased weight is secured by the addition of the
+coagulated albumin and also increased moisture.</p>
+
+<p><b>Bacteria in rennet.</b> In the use of natural rennets, such as are
+frequently employed in the making of Swiss cheese, considerable numbers
+of bacteria are added to the milk. Although these rennets are preserved
+in salt, alcohol or boric acid, they are never free from bacteria.
+Adametz<a name="FNanchor_183_183" id="FNanchor_183_183"></a><a href="#Footnote_183_183" class="fnanchor">[183]</a> found ten different species and from 640,000 to 900,000
+bacteria per cc. in natural rennets. Freudenreich has shown that rennet
+extract solutions can be used in Swiss cheese-making quite as well as
+natural rennets; but to secure the best results, a small quantity of
+pure lactic ferment must be added to simulate the conditions that
+prevail when natural rennets are soaked in whey, which, it must be
+remembered, is a fluid rich in bacterial life.</p>
+
+<p><span class='pagenum'><a name="Page_164" id="Page_164">[Pg 164]</a></span></p><p>Where rennet extract or tablets are used, as is generally the case in
+cheddar making, the number of bacteria added is so infinitesimal as to
+be negligible.</p>
+
+<p><b>Development of acid.</b> In the manufacture of cheddar cheese, the
+development of acid exerts an important influence on the character of
+the product. This is brought about by holding the curds at temperatures
+favorable to the growth of the bacteria in the same. Under these
+conditions the lactic-acid organisms, which usually predominate, develop
+very rapidly, producing thereby considerable quantities of acid which
+change materially the texture of the curds. The lactic acid acts upon
+the casein in solutions containing salt, causing it to dissolve to some
+extent, thus forming the initial compounds of digestion.<a name="FNanchor_184_184" id="FNanchor_184_184"></a><a href="#Footnote_184_184" class="fnanchor">[184]</a> This
+solution of the casein is expressed physically by the "stringing" of the
+curds on a hot iron. This causes the curds to mat, producing a close,
+solid body, free from mechanical holes. Still further, the development
+of this acid is necessary for the digestive activity of the pepsin in
+the rennet extract.</p>
+
+<p>In some varieties of cheese, as the Swiss, acid is not developed and the
+character of the cheese is much different from that of cheddar. In all
+such varieties, a great deal more trouble is experienced from the
+production of "gassy" curds, because the development of the
+gas-producing bacteria is held in check by the rapid growth of the
+lactic acid-producing species.</p>
+
+<p><b>Bacteria in green cheese.</b> The conditions under which cheese is made
+permit of the development of bacteria throughout the entire process. The
+cooking or heating of curds to expel the excessive moisture is never so
+high as to be fatal to germ life; on the contrary, the acidity of the<span class='pagenum'><a name="Page_165" id="Page_165">[Pg 165]</a></span>
+curd and whey is continually increased by the development of bacteria in
+the same.</p>
+
+<p>The body of green cheese fresh from the press is, to a considerable
+extent, dependent upon the acid produced in the curds. If the curds are
+put to press in a relatively sweet condition the texture is open and
+porous. The curd particles do not mat closely together and "mechanical
+holes," rough and irregular in outline, occur. Very often, at relatively
+high temperatures, such cheese begin to "huff," soon after being taken
+from the press, a condition due to the development of gas, produced by
+gas-generating bacteria acting on the sugar in the curd. This gas finds
+its way readily into these ragged holes, greatly distending them, as in
+Fig. 30.</p>
+
+<div class="figcenter" style="width: 600px;">
+<img src="images/fig30.jpg" width="600" height="338" alt="Fig. 30." title="" />
+<span class="caption">Fig. 30.</span>
+</div>
+<div class="blockquot"><p> <i>L</i>, a sweet curd cheese direct from the press.
+"Mechanical" holes due to lack of acid development; <i>P</i>, same cheese
+four days later, mechanical holes distended by development of gas.</p></div>
+
+<p><b>Physical changes in ripening cheese.</b> When a green cheese is taken from
+the press, the curd is tough, firm, but elastic. It has no value as a
+food product for immediate use,<span class='pagenum'><a name="Page_166" id="Page_166">[Pg 166]</a></span> because it lacks a desirable flavor and
+is not readily digestible. It is nothing but precipitated casein and
+fat. In a short time, a deep-seated change occurs. Physically this
+change is demonstrated in the modification that the curd undergoes.
+Gradually it breaks down and becomes plastic, the elastic, tough curd
+being changed into a softened mass. This change in texture of the cheese
+is also accompanied by a marked change in flavor. The green cheese has
+no distinctively cheese flavor, but in course of time, with the gradual
+change of texture, the peculiar flavor incident to ripe cheese is
+developed.</p>
+
+<p>The characteristic texture and flavor are susceptible of considerable
+modification that is induced not only by variation in methods of
+manufacture, but by the conditions under which the cheese are cured. The
+amount of moisture incorporated with the curd materially affects the
+physical appearance of the cheese, and the rate of change in the same.
+The ripening temperature, likewise the moisture content of the
+surrounding air, also exerts a marked influence on the physical
+properties of the cheese. To some extent the action of these forces is
+purely physical, as in the gradual loss by drying, but in other respects
+they are associated with chemical transformations.</p>
+
+<p><b>Chemical changes in ripening cheese.</b> Coincident with the physical
+breaking down of the curd comes a change in the chemical nature of the
+casein. The hitherto insoluble casein is gradually transformed into
+soluble nitrogenous substances (<i>caseone</i> of Duclaux, or <i>caseogluten</i>
+of Weigmann). This chemical phenomenon is a breaking-down process that
+is analogous to the peptonization of proteids, although in addition to
+the peptones and albumoses characteristic of peptic digestion,
+amido-acids and ammonia are<span class='pagenum'><a name="Page_167" id="Page_167">[Pg 167]</a></span> to be found. The quantity of these lower
+products increases with the age of the cheese.</p>
+
+<p>The chemical reaction of cheese is normally acid to phenolphthalein,
+although there is generally no free acid, as shown by Congo red, the
+lactic acid being converted into salts as fast as formed. In very old
+cheese, undergoing putrefactive changes, especially on the outside, an
+alkaline reaction may be present, due to the formation of free ammonia.</p>
+
+<p>The changes that occur in a ripening cheese are for the most part
+confined to the proteids. According to most investigators the fat
+remains practically unchanged, although the researches of Weigmann and
+Backe<a name="FNanchor_185_185" id="FNanchor_185_185"></a><a href="#Footnote_185_185" class="fnanchor">[185]</a> show that fatty acids are formed from the fat. In the green
+cheese considerable milk-sugar is present, but, as a result of the
+fermentation that occurs, this is rapidly converted into acid products.</p>
+
+<p><b>Bacterial flora of cheese.</b> It might naturally be expected that the green
+cheese, fresh from the press, would contain practically the same kind of
+bacteria that are in the milk, but a study of cheese shows a peculiar
+change in the character of the flora. In the first place, fresh cottage
+cheese, made by the coagulation of the casein through the action of
+acid, has a more diversified flora than cheese made with rennet, for the
+reason, as given by Lafar,<a name="FNanchor_186_186" id="FNanchor_186_186"></a><a href="#Footnote_186_186" class="fnanchor">[186]</a> that the fermentative process is farther
+advanced.</p>
+
+<p>When different varieties of cheese are made from milk in the same
+locality, the germ content of even the ripened product has a marked
+similarity, as is illustrated by Adametz's work<a name="FNanchor_187_187" id="FNanchor_187_187"></a><a href="#Footnote_187_187" class="fnanchor">[187]</a> on Emmenthaler or
+Swiss hard cheese,<span class='pagenum'><a name="Page_168" id="Page_168">[Pg 168]</a></span> and Schweitzer Hausk&auml;se, a soft variety. Of the nine
+species of bacilli and cocci found in mature Emmenthaler, eight of them
+were also present in ripened Hausk&auml;se.</p>
+
+<p>Different investigators have studied the bacterial flora of various
+kinds of cheese, but as yet little comparative systematic work has been
+done. Freudenreich<a name="FNanchor_188_188" id="FNanchor_188_188"></a><a href="#Footnote_188_188" class="fnanchor">[188]</a> has determined the character and number of
+bacteria in Emmenthaler cheese, and Russell<a name="FNanchor_189_189" id="FNanchor_189_189"></a><a href="#Footnote_189_189" class="fnanchor">[189]</a> the same for cheddar
+cheese. The same general law has also been noted in Canadian<a name="FNanchor_190_190" id="FNanchor_190_190"></a><a href="#Footnote_190_190" class="fnanchor">[190]</a> and
+English<a name="FNanchor_191_191" id="FNanchor_191_191"></a><a href="#Footnote_191_191" class="fnanchor">[191]</a> cheese. At first a marked decrease in numbers is usually
+noted, lasting for a day or two. This is followed by an enormous
+increase, caused by the rapid growth of the lactic-acid type. The
+development may reach scores of millions and often over a hundred
+million organisms per gram. Synchronous with this increase, the
+peptonizing and gas-producing bacteria gradually disappear. This rapid
+development, which lasts only for a few weeks, is followed by a general
+decline.</p>
+
+<p>In the ripening of cheese a question arises as to whether the process
+goes on throughout the entire mass of cheese, or whether it is more
+active at or near the surface. In the case of many of the soft cheese,
+such as Brie and limburger, bacterial and mold development is
+exceedingly active on the exterior, and the enzyms secreted by these
+organisms diffuse toward the interior. That such a condition occurs in
+the hard type of cheese made with rennet is extremely improbable. Most
+observers agree that in this type of cheese the ripening progresses
+throughout the entire mass, although Adametz opposes this view and
+considers that in<span class='pagenum'><a name="Page_169" id="Page_169">[Pg 169]</a></span> Emmenthaler cheese the development of the specific
+aroma-producing organism occurs in the superficial layers. Jensen has
+shown, however, that the greatest amount of soluble nitrogenous products
+are to be found in the innermost part of the cheese, a condition that is
+not reconcilable with the view that the most active ripening is on the
+exterior.<a name="FNanchor_192_192" id="FNanchor_192_192"></a><a href="#Footnote_192_192" class="fnanchor">[192]</a></p>
+
+<p>The course of development of bacteria in cheddar cheese is materially
+influenced by the ripening temperature. In cheese ripened at relatively
+low temperatures (50&deg;-55&deg; F.),<a name="FNanchor_193_193" id="FNanchor_193_193"></a><a href="#Footnote_193_193" class="fnanchor">[193]</a> a high germ content is maintained
+for a much longer period of time than at higher temperatures. Under
+these conditions the lactic-acid type continues in the ascendancy as
+usual. In cheese cured at high temperatures (80&deg;-86&deg; F.) the number of
+organisms is greatly diminished, and they fail to persist in appreciable
+numbers for as long a time as in cheese cured at temperatures more
+frequently employed.</p>
+
+<p><b>Influence of temperature on curing.</b> Temperature exerts a most potent
+influence on the quality of the cheese, as determined not only by the
+rate of ripening but the nature of the process itself. Much of the poor
+quality of cheese is attributable to the effect of improper curing
+conditions. Probably in the initial stage of this industry cheese were
+allowed to ripen without any sort of control, with the inevitable result
+that during the summer months the temperature generally fluctuated so
+much as to impair seriously the quality. The effect of high temperatures
+(70&deg; F. and above) is to produce a rapid curing, and, therefore, a short
+lived cheese; also a sharp, strong flavor, and generally a<span class='pagenum'><a name="Page_170" id="Page_170">[Pg 170]</a></span> more or less
+open texture. Unless the cheese is made from the best quality of milk,
+it is very apt to undergo abnormal fermentations, more especially those
+of a gassy character.</p>
+
+<div class="figcenter" style="width: 600px;">
+<img src="images/fig31.jpg" width="600" height="286" alt="Fig. 31. Influence of curing temperature on texture of
+cheese. Upper row ripened eight months at 60&deg; F.; lower row at 40&deg; F." title="" />
+<span class="caption">Fig. 31. Influence of curing temperature on texture of
+cheese. Upper row ripened eight months at 60&deg; F.; lower row at 40&deg; F.</span>
+</div>
+
+<p>Where cheese is ripened at low temperatures, ranging from 50&deg; F. down to
+nearly the freezing temperatures, it is found that the quality is
+greatly improved.<a name="FNanchor_194_194" id="FNanchor_194_194"></a><a href="#Footnote_194_194" class="fnanchor">[194]</a> Such cheese are thoroughly broken down from a
+physical point of view even though they may not show such a high per
+cent of soluble nitrogenous products. They have an excellent texture,
+generally solid and firm, free from all tendency to openness; and,
+moreover, their flavor is clean and entirely devoid of the sharp,
+undesirable tang that so frequently appears in old cheese. The keeping
+quality of such cheese is much superior to the ordinary product. The
+introduction<span class='pagenum'><a name="Page_171" id="Page_171">[Pg 171]</a></span> of this new system of cheese-curing promises much from a
+practical point of view, and undoubtedly a more complete study of the
+subject from a scientific point of view will aid materially in
+unraveling some of the problems as to flavor production.</p>
+
+<p><b>Theories of cheese curing.</b> Within the last few years considerable study
+has been given the subject of cheese curing or ripening, in order to
+explain how this physical and chemical transformation is brought about.</p>
+
+<p>Much of the misconception that has arisen relative to the cause of
+cheese ripening comes from a confusion of terms. In the ordinary use of
+the word, ripening or curing of cheese is intended to signify the sum
+total of all the changes that result in converting the green product as
+it comes from the press into the edible substance that is known as cured
+cheese. As previously shown, the most marked chemical transformation
+that occurs is that which has to do with the peptonization or breaking
+down of the casein. It is true that under ordinary conditions this
+decomposition process is also accompanied with the formation of certain
+flavor-producing substances, more or less aromatic in character; but it
+by no means follows that these two processes are necessarily due to the
+same cause. The majority of investigators have failed to consider these
+two questions of casein decomposition and flavor as independent, or at
+least as not necessarily related. They are undoubtedly closely bound
+together, but it will be shown later that the problems are quite
+different and possibly susceptible of more thorough understanding when
+considered separately.</p>
+
+<p>In the earlier theories of cheese ripening it was thought to be purely a
+chemical change, but, with the growth of<span class='pagenum'><a name="Page_172" id="Page_172">[Pg 172]</a></span> bacteriological science,
+evidence was forthcoming that seemed to indicate that the activity of
+organisms entered into the problem. Schaffer<a name="FNanchor_195_195" id="FNanchor_195_195"></a><a href="#Footnote_195_195" class="fnanchor">[195]</a> showed that if milk
+was boiled and made into cheese, the casein failed to break down.
+Adametz<a name="FNanchor_196_196" id="FNanchor_196_196"></a><a href="#Footnote_196_196" class="fnanchor">[196]</a> added to green cheese various disinfectants, as creolin and
+thymol, and found that this practically stopped the curing process. From
+these experiments he drew the conclusion that bacteria must be the cause
+of the change, because these organisms were killed; but when it is
+considered that such treatment would also destroy the activity of enzyms
+as well as vital ferments, it is evident that these experiments were
+quite indecisive.</p>
+
+<p>A determination of the nature of the by-products found in maturing
+cheese indicates that the general character of the ripening change is a
+peptonization or digestion of the casein.</p>
+
+<p>Until recently the most widely accepted views relating to the cause of
+this change have been those which ascribed the transformation to the
+activity of micro-organisms, although concerning the nature of these
+organisms there has been no unanimity of opinion. The overwhelming
+development of bacteria in all cheeses naturally gave support to this
+view; and such experiments as detailed above strengthened the idea that
+the casein transformation could not occur where these ferment organisms
+were destroyed.</p>
+
+<p>The very nature of the changes produced in the casein signified that to
+take part in this process any organism must possess the property of
+dissolving the proteid molecule, casein, and forming therefrom
+by-products that are most generally found in other digestive or
+peptonizing changes of this class.</p>
+
+<p><span class='pagenum'><a name="Page_173" id="Page_173">[Pg 173]</a></span></p><p><b>Digestive bacterial theory.</b> The first theory propounded was that of
+Duclaux,<a name="FNanchor_197_197" id="FNanchor_197_197"></a><a href="#Footnote_197_197" class="fnanchor">[197]</a> who in 1887 advanced the idea that this change was due to
+that type of bacteria which is able to liquefy gelatin, peptonize milk,
+and cause a hydrolytic change in proteids. To this widely-spread group
+that he found in cheese, he gave the generic name <i>Tyrothrix</i> (cheese
+hairs). According to him, these organisms do not function directly as
+ripening agents, but they secrete an enzym or unorganized ferment to
+which he applies the name <i>casease</i>. This ferment acts upon the casein
+of milk, converting it into a soluble product known as <i>caseone</i>. These
+organisms are found in normal milk, and if they function as casein
+transformers, one would naturally expect them to be present, at least
+frequently, if not predominating in the ripening cheese; but such is not
+the case. In typical cheddar or Swiss cheese, they rapidly disappear (p.
+168), although in the moister, softer varieties, they persist for
+considerable periods of time. According to Freudenreich, even where
+these organisms are added in large numbers to the curd, they soon
+perish, an observation that is not regarded as correct by the later
+adherents to the digestive bacterial theory, as Adametz and Winkler.</p>
+
+<p>Duclaux's experiments were made with liquid media for isolation
+purposes, and his work, therefore, cannot be regarded as satisfactory as
+that carried out with more modern technical methods. Recently this
+theory has been revived by Adametz,<a name="FNanchor_198_198" id="FNanchor_198_198"></a><a href="#Footnote_198_198" class="fnanchor">[198]</a> who claims to have found in
+Emmenthaler cheese a digesting species, one of the Tyrothrix type, which
+is capable of peptonizing the casein and at the same time producing the
+characteristic flavor of this class of cheese.<span class='pagenum'><a name="Page_174" id="Page_174">[Pg 174]</a></span> This organism, called by
+him <i>Bacillus nobilis</i>, the Edelpilz of Emmenthaler cheese, has been
+subjected to comparative experiments, and in the cheese made with pure
+cultures of this germ better results are claimed to have been secured.
+Sufficient experiments have not as yet been reported by other
+investigators to warrant the acceptance of the claims made relative to
+the effect of this organism.</p>
+
+<p><b>Lactic-acid bacterial theory.</b> It has already been shown that the
+lactic-acid bacteria seems to find in the green cheese the optimum
+conditions of development; that they increase enormously in numbers for
+a short period, and then finally decline. This marked development,
+coincident with the breaking down of the casein, has led to the view
+which has been so ably expounded by Freudenreich<a name="FNanchor_199_199" id="FNanchor_199_199"></a><a href="#Footnote_199_199" class="fnanchor">[199]</a> that this type of
+bacterial action is concerned in the ripening of cheese. This group of
+bacteria is, under ordinary conditions, unable to liquefy gelatin, or
+digest milk, or, in fact, to exert, under ordinary conditions, any
+proteolytic or peptonizing properties. This has been the stumbling-block
+to the acceptance of this hypothesis, as an explanation of the breaking
+down of the casein. Freudenreich has recently carried on experiments
+which he believes solve the problem. By growing cultures of these
+organisms in milk, to which sterile, freshly precipitated chalk had been
+added, he was able to prolong the development of bacteria for a
+considerable period of time, and as a result finds that an appreciable
+part of the casein is digested; but this action is so slow compared with
+what normally occurs in a cheese, that exception may well be taken to
+this type of experiment alone. Weigmann<a name="FNanchor_200_200" id="FNanchor_200_200"></a><a href="#Footnote_200_200" class="fnanchor">[200]</a> inclines to the view that
+the<span class='pagenum'><a name="Page_175" id="Page_175">[Pg 175]</a></span> lactic-acid bacteria are not the true cause of the peptonizing
+process, but that their development prepares the soil, as it were, for
+those forms that are more directly concerned in the peptonizing process.
+This they do by developing an acid substratum that renders possible the
+more luxuriant growth of the aroma-producing species. According to
+Gorini,<a name="FNanchor_201_201" id="FNanchor_201_201"></a><a href="#Footnote_201_201" class="fnanchor">[201]</a> certain of the Tyrothrix forms function at high
+temperatures as lactic acid producing bacteria, while at lower
+temperatures they act as peptonizers. On this basis he seeks to
+reconcile the discrepancies that appear in the experiments of other
+investigators.</p>
+
+<p><b>Digestive milk enzym theory.</b> In 1897 Babcock and the writer<a name="FNanchor_202_202" id="FNanchor_202_202"></a><a href="#Footnote_202_202" class="fnanchor">[202]</a> showed
+that milk underwent digestive changes spontaneously when bacterial
+activity was suspended by the addition of such anaesthetics as ether,
+chloroform and benzol. The chemical nature of the by-products produced
+by this auto-digestion of milk resembles quite closely those found in
+ripened cheese, except that ammonia is not produced as is the case in
+old cheese. The cause of the decomposition of the casein, they found to
+be due to the action of a milk enzym which is inherent to the milk
+itself. This digestive ferment may be separated from fresh milk by
+concentrating centrifuge slime extracts by the usual physiological
+reagents. This ferment, called by them <i>galactase</i>, on account of its
+origin in milk, is a proteolytic enzym of the tryptic type. Its activity
+is destroyed by strong chemicals such as formaldehyde, corrosive
+sublimate, also when heated to 175&deg; F. or above. When such extracts are
+added to boiled milk, the digestive process is started anew, and the
+by-products produced are very similar to those noted in a normal cheese.</p>
+<p><span class='pagenum'><a name="Page_176" id="Page_176">[Pg 176]</a></span></p>
+<p>Jensen<a name="FNanchor_203_203" id="FNanchor_203_203"></a><a href="#Footnote_203_203" class="fnanchor">[203]</a> has also shown that the addition of pancreatic extracts to
+cheese accelerated the formation of soluble nitrogenous products.</p>
+
+<p>The action of galactase in milk and cheese has been confirmed by
+Freudenreich<a name="FNanchor_204_204" id="FNanchor_204_204"></a><a href="#Footnote_204_204" class="fnanchor">[204]</a> and Jensen,<a name="FNanchor_205_205" id="FNanchor_205_205"></a><a href="#Footnote_205_205" class="fnanchor">[205]</a> as well as by American investigators,
+and this enzym is now generally accepted as one of the factors concerned
+in the decomposition of the casein. Freudenreich believes it is able to
+change casein into albumose and peptones, but that the lactic-acid
+bacteria are chiefly responsible for the further decomposition of the
+nitrogen to amid form.</p>
+
+<p>Failure before to recognize the presence of galactase in milk is
+attributable to the fact that all attempts to secure sterile milk had
+been made by heating the same, in which case galactase was necessarily
+destroyed. A brief exposure at 176&deg; F. is sufficient to destroy its
+activity, and even an exposure at lower temperatures weakens its action
+considerably, especially if the reaction of the medium is acid. This
+undoubtedly explains the contradictory results obtained in the ripening
+of cheese from pasteurized milk, such cheese occasionally breaking down
+in an abnormal manner.</p>
+
+<p>The results mentioned on page 172, in which cheese failed to ripen when
+treated with disinfectants,&mdash;experiments which were supposed at that
+time to be the foundation of the bacterial theory of casein
+digestion&mdash;are now explicable on an entirely different basis. In these
+cases the casein was not peptonized, because these strong disinfectants
+destroyed the activity of the enzyms as well as the bacteria.</p>
+
+<p>Another important factor in the breaking down of the casein is the
+<i>pepsin</i> in the rennet extract. The digestive<span class='pagenum'><a name="Page_177" id="Page_177">[Pg 177]</a></span> influence of this agent
+was first demonstrated for cheddar cheese by Babcock, Russell and
+Vivian,<a name="FNanchor_206_206" id="FNanchor_206_206"></a><a href="#Footnote_206_206" class="fnanchor">[206]</a> and simultaneously, although independently, by Jensen<a name="FNanchor_207_207" id="FNanchor_207_207"></a><a href="#Footnote_207_207" class="fnanchor">[207]</a>
+in Emmenthaler cheese. In this digestive action, only albumoses and
+higher peptones are produced. The activity of pepsin does not become
+manifest until there is about 0.3 per cent. acid which is approximately
+the amount developed in the cheddar process. These two factors
+undoubtedly account for by far the larger proportion of the changes in
+the casein; and yet, the formation of ammonia in well ripened cheese is
+not accounted for by these factors. This by-product is the main end
+product of proteid digestion by the liquefying bacteria but their
+apparent infrequency in cheese makes it difficult to understand how they
+can function prominently in the change, unless the small quantity of
+digestive enzyms excreted by them in their growth in milk is capable of
+continuing its action until a cumulative effect is obtained. Although
+much light has been thrown on this question by the researches of the
+last few years, the matter is far from being satisfactorily settled at
+the present time and the subject needs much more critical work. If
+liquefying bacteria abound in the milk, doubtless they exert some
+action, but the r&ocirc;le of bacteria is doubtless much greater in the
+production of flavor than in the decomposition of the curd.</p>
+
+<p><b>Conditions determining quality.</b> In determining the quality of cheese,
+several factors are to be taken into consideration. First and foremost
+is the flavor, which determines more than anything else the value of the
+product. This should be mild and pleasant, although with age the
+intensity of the same generally increases but at no time should it have
+any bitter, sour, or otherwise undesirable taste or<span class='pagenum'><a name="Page_178" id="Page_178">[Pg 178]</a></span> aroma. Texture
+registers more accurately the physical nature of the ripening. The
+cheese should not be curdy and harsh, but should yield quite readily to
+pressure under the thumb, becoming on manipulation waxy and plastic
+instead of crumbly or mealy. Body refers to the openness or closeness of
+the curd particles, a close, compact mass being most desirable. The
+color of cheese should be even, not wavy, streaked or bleached.</p>
+
+<p>For a cheese to possess all of these characteristics in an optimum
+degree is to be perfect in every respect&mdash;a condition that is rarely
+reached.</p>
+
+<p>So many factors influence this condition that the problem of making a
+perfect cheese becomes exceedingly difficult. Not only must the quality
+of the milk&mdash;the raw material to be used in the manufacture&mdash;be
+perfectly satisfactory, but the factory management while the curds are
+in the vat demands great skill and careful attention; and finally, the
+long period of curing in which variation in temperature or moisture
+conditions may seriously affect the quality,&mdash;all of these stages, more
+or less critical, must be successfully gone through, before the product
+reaches its highest state of development.</p>
+
+<p>It is of course true that many phases of this complex series of
+processes have no direct relation to bacteria, yet it frequently happens
+that the result attained is influenced at some preceding stage by the
+action of bacteria in one way or another. Thus the influence of the
+acidity developed in the curds is felt throughout the whole life of the
+cheese, an over-development of lactic-acid bacteria producing a sour
+condition that leaves its impress not only on flavor but texture. An
+insufficient development of acid fails to soften the curd-particles so
+as to permit of<span class='pagenum'><a name="Page_179" id="Page_179">[Pg 179]</a></span> close matting, the consequence being that the body of
+the cheese remains loose and open, a condition favorable to the
+development of gas-generating organisms.</p>
+
+<p><b>Production of flavor.</b> The importance of flavor as determining the
+quality of cheese makes it imperative that the nature of the substances
+that confer on cheese its peculiar aromatic qualities and taste be
+thoroughly understood. It is to be regretted that the results obtained
+so far are not more satisfactory, for improvement in technique is hardly
+to be expected until the reason for the process is thoroughly
+understood.</p>
+
+<p>The view that is most generally accepted is that this most important
+phase of cheese curing is dependent upon bacterial activity, but the
+organisms that are concerned in this process have not as yet been
+satisfactorily determined. In a number of cases, different species of
+bacteria have been separated from milk and cheese that have the power of
+producing aromatic compounds that resemble, in some cases, the peculiar
+flavors and odors that characterize some of the foreign kinds of cheese;
+but an introduction of these into curd has not resulted in the
+production of the peculiar variety, even though the methods of
+manufacture and curing were closely followed. The similarity in germ
+content in different varieties of cheese made in the same locality has
+perhaps a bearing on this question of flavor as related to bacteria. Of
+the nine different species of bacteria found in Emmenthaler cheese by
+Adametz, eight of them were also present in ripened Hausk&auml;se. If
+specific flavors are solely the result of specific bacterial action, it
+might naturally be expected that the character of the flora would
+differ.</p>
+
+<p>Some suggestive experiments were made by Babcock and Russell on the
+question of flavor as related to bacterial<span class='pagenum'><a name="Page_180" id="Page_180">[Pg 180]</a></span> growth, by changing the
+nature of the environment in cheese by washing the curds on the racks
+with warm water. In this way the sugar and most of the ash were removed.
+Under such conditions the character of the bacterial flora was
+materially modified. While the liquefying type of bacteria was very
+sparse in normal cheddar, they developed luxuriantly in the washed
+cheese. The flavor at the same time was markedly affected. The control
+cheddar was of good quality, while that made from the washed curds was
+decidedly off, and in the course of ripening became vile. It may be
+these two results are simply coincidences, but other data<a name="FNanchor_208_208" id="FNanchor_208_208"></a><a href="#Footnote_208_208" class="fnanchor">[208]</a> bear out
+the view that the flavor was to some extent related to the nature of the
+bacteria developing in the cheese. This was strengthened materially by
+adding different sugars to washed curds, in which case it was found that
+the flavor was much improved, while the more normal lactic-acid type of
+bacteria again became predominant.</p>
+
+<p><b>Ripening of moldy cheese.</b> In a number of foreign cheeses, the peculiar
+flavor obtained is in part due to the action of various fungi which grow
+in the cheese, and there produce certain by-products that flavor the
+cheese. Among the most important of these are the Roquefort cheese of
+France, Stilton of England, and Gorgonzola of Italy.</p>
+
+<p>Roquefort cheese is made from goat's or cow's milk, and in order to
+introduce the desired mold, which is the ordinary bread-mold,
+<i>Penicillium glaucum</i>, carefully-prepared moldy bread-crumbs are added
+to the curd.</p>
+
+<p>At ordinary temperatures this organism develops too rapidly, so that the
+cheese to ripen properly must be kept at a low temperature. The town of
+Roquefort is situated<span class='pagenum'><a name="Page_181" id="Page_181">[Pg 181]</a></span> in a limestone country, in a region full of
+caves, and it is in these natural caves that most of the ripening is
+done. These caverns are always very moist and have a temperature ranging
+from 35&deg; to 44&deg; F., so that the growth of the fungus is retarded
+considerably. The spread of the mold throughout the ripening mass is
+also assisted in a mechanical way. The partially-matured cheese are run
+through a machine that pricks them full of small holes. These slender
+canals allow the mold organism to penetrate the whole mass more
+thoroughly, the moldy straw matting upon which the ripening cheese are
+placed helping to furnish an abundant seeding of the desired germ.</p>
+
+<p>When new factories are constructed it is of advantage to introduce this
+necessary germ in quantities, and the practice is sometimes followed of
+rubbing the walls and cellars of the new location with material taken
+from the old established factory. In this custom, developed in purely an
+empirical manner, is to be seen a striking illustration of a
+bacteriological process crudely carried out.</p>
+
+<p>In the Stilton cheese, one of the highly prized moldy cheeses of
+England, the desired mold fungus is introduced into the green cheese by
+exchanging plugs taken with a cheese trier from a ripe Stilton.</p>
+
+<p><b>Ripening of soft cheese.</b> The type of ripening which takes place in the
+soft cheeses is materially different from that which occurs in the hard
+type. The peptonizing action does not go on uniformly throughout the
+cheese, but is hastened by the development of molds and bacteria on the
+outside that exert a solvent action on the casein. For this reason, soft
+cheeses are usually made up in small sizes, so that this action may be
+hastened. The organisms that take part in this process are those that
+are<span class='pagenum'><a name="Page_182" id="Page_182">[Pg 182]</a></span> able to form enzyms (similar in their action to trypsin, galactase,
+etc.), and these soluble ferments gradually diffuse from the outside
+through the cheese.</p>
+
+<p>Most of these peptonizing bacteria are hindered in their growth by the
+presence of lactic acid, so that in many cases the appearance of the
+digesting organisms on the surface is delayed until the acidity of the
+mass is reduced to the proper point by the development of other
+organisms, principally molds, which prefer an acid substratum for their
+growth.</p>
+
+<p>In Brie cheese a blue coating of mold develops on the surface. In the
+course of a few weeks, a white felting appears which later changes to
+red. This slimy coat below the mold layer is made up of diverse species
+of bacteria and fungi that are able to grow after the acid is reduced by
+the blue mold. The organisms in the red slimy coat act upon the casein,
+producing an alkaline reaction that is unfavorable to the growth of the
+blue mold. Two sets of organisms are, therefore essential in the
+ripening process, one preparing the soil for the ferment that later
+produces the requisite ripening changes. As ordinarily carried on, the
+process is an empirical one, and if the red coat does not develop as
+expected, the maker resorts to all kinds of devices to bring out the
+desired ferment. The appearance of the right form is dependent, however,
+upon the proper reaction of the cheese, and if this is not suitable, the
+wished-for growth will not appear.</p>
+
+
+<h4>INFLUENCE OF BACTERIA IN ABNORMAL CHEESE PROCESSES.</h4>
+
+<p>The reason why cheese is more subject to abnormal fermentation than
+butter is because its high nitrogen content favors the continued
+development of bacteria for some time<span class='pagenum'><a name="Page_183" id="Page_183">[Pg 183]</a></span> after it is made. It must be
+borne in mind, in considering the more important of these changes, that
+not all defective conditions in cheese are attributable to the influence
+of living organisms. Troubles frequently arise from errors in
+manufacturing details, as too prolonged cooking of curds, too high
+heating, or the development of insufficient or too much acid. Then
+again, the production of undesirable flavors or impairment in texture
+may arise from imperfect curing conditions.</p>
+
+<p>Our knowledge regarding the exact nature of these indefinite faults is
+as yet too inadequate to enable many of these undesirable conditions to
+be traced to their proper source; but in many cases the taints observed
+in a factory are due to the abnormal development of certain bacteria,
+capable of evolving unpleasant or even putrid odors. Most of them are
+seeded in the milk before it comes to the factory and are due to
+careless manipulation of the milk while it is still on the farm. Others
+gain access to the milk in the factory, owing to unclean conditions of
+one sort or another. Sometimes the cheese-maker is able to overcome
+these taints by vigorous treatment, but often they pass on into the
+cheese, only to detract from the market value of the product. Most
+frequently these "off" flavors appear in cheese that are cured at too
+high temperatures, say above 65&deg; F.</p>
+
+<p><b>"Gassy" fermentations in cheese.</b> One of the worst and at the same time
+most common troubles in cheese-making is where the cheese undergoes a
+fermentation marked by the evolution of gas. The presence of gas is
+recognized by the appearance either of spherical or lens-shaped holes of
+various sizes in the green cheese; often they appear in the curd before
+it is put to press. Usually in this condition<span class='pagenum'><a name="Page_184" id="Page_184">[Pg 184]</a></span> the curds look as if they
+had been punctured with a pin, and are known as "pin holey" curds. Where
+the gas holes are larger, they are known as "Swiss holes" from their
+resemblance to the normal holes in the Swiss product. If the development
+of gas is abundant, these holes are restricted in size. Often the
+formation of gas may be so intense as to cause the curds to float on the
+surface of the whey before they are removed. Such curds are known as
+"floaters" or "bloaters."</p>
+
+<p>If "gassy" curds are put to press, the abnormal fermentation may
+continue. The further production of gas causes the green cheese to
+"huff" or swell, until it may be considerably distorted as in Fig. 33.
+In such cases the texture of the cheese is greatly injured, and the
+flavor is generally impaired.</p>
+
+<div class="figcenter" style="width: 450px;">
+<img src="images/fig33.jpg" width="450" height="368" alt="Fig. 33. Cheese made from gassy milk." title="" />
+<span class="caption">Fig. 33. Cheese made from gassy milk.</span>
+</div>
+
+<p>Such abnormal changes may occur at any season of the year, but the
+trouble is most common in summer, especially in the latter part.</p>
+
+<p>This defect is less likely to occur in cheese that is well<span class='pagenum'><a name="Page_185" id="Page_185">[Pg 185]</a></span> cheddared
+than in sweet curd cheese. When acidity is produced, these gassy
+fermentations are checked, and in good cheddar the body is so close and
+firm as not readily to permit of gaseous changes.</p>
+
+<p>In Swiss cheese, which is essentially a sweet curd cheese, these
+fermentations are very troublesome. Where large holes are formed in
+abundance (bl&auml;hen), the trouble reaches its maximum. If the gas holes
+are very numerous and therefore small it is called a "nissler."
+Sometimes the normal "eyes" are even wanting when it is said to be
+"blind" or a "gl&auml;sler."</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/fig34.jpg" width="500" height="249" alt="Fig. 34. Block Swiss cheese showing &quot;gassy&quot;
+fermentation." title="" />
+<span class="caption">Fig. 34. Block Swiss cheese showing &quot;gassy&quot;
+fermentation.</span>
+</div>
+
+<p>One method of procedure which is likely to cause trouble in Swiss
+factories is often produced by the use of sour, fermented whey in which
+to soak the natural rennets. Freudenreich and Steinegger<a name="FNanchor_209_209" id="FNanchor_209_209"></a><a href="#Footnote_209_209" class="fnanchor">[209]</a> have shown
+that a much more uniform quality of cheese can be made with rennet
+extract if it is prepared with a starter made from a pure lactic
+ferment.</p>
+
+<p>The cause of the difficulty has long been charged to various<span class='pagenum'><a name="Page_186" id="Page_186">[Pg 186]</a></span> sources,
+such as a lack of aeration, improper feeding, retention of animal gases,
+etc., but in all these cases it was nothing more than a surmise. Very
+often the milk does not betray any visible symptom of fermentation when
+received, and the trouble is not to be recognized until the process of
+cheese-making is well advanced.</p>
+
+<p>Studies from a biological standpoint have, however, thrown much light on
+this troublesome problem; and it is now known that the formation of gas,
+either in the curd or after it has been put to press, is due entirely to
+the breaking down of certain elements, such as the sugar of milk, due to
+the influence of various living germs. This trouble is, then, a type
+fermentation, and is, therefore, much more widely distributed than it
+would be if it was caused by a single specific organism. These
+gas-producing organisms are to be found, sparingly at least, in almost
+all milks, but are normally held in check by the ordinary lactic
+species. Among them are a large number of the bacteria, although yeasts
+and allied germs are often present and are likewise able to set up
+fermentative changes of this sort. In these cases the milk-sugar is
+decomposed in such a way as to give off CO<sub>2</sub> and H, and in some cases,
+alcohol. Russell and Hastings<a name="FNanchor_210_210" id="FNanchor_210_210"></a><a href="#Footnote_210_210" class="fnanchor">[210]</a> found a lactose-splitting yeast in a
+severe outbreak of gassy cheese in a Swiss factory. In this case the gas
+did not develop until the cheese were a few weeks old. In severe cases
+the cheese actually cracked to pieces.</p>
+
+<p>According to Guillebeau, a close relation exists between those germs
+that are able to produce an infectious inflammation (mastitis) in the
+udder of the cow and some forms capable of gas evolution.</p>
+
+<p>If pure cultures of these gas-producing bacteria are added<span class='pagenum'><a name="Page_187" id="Page_187">[Pg 187]</a></span> to perfectly
+sweet milk, it is possible to artificially produce the conditions in
+cheese that so frequently appear in practice.</p>
+
+<p><b>Treatment of "pin-holey" curds.</b> When this type of fermentation appears
+during the manufacture of the cheese, the maker can control it in part
+within certain limits. These methods of treatment are, as a rule, purely
+mechanical, as when the curds are piled and turned, and subsequently
+ground in a curd mill. After the gas has been forced out, the curds are
+then put to press and the whole mats into a compact mass.</p>
+
+<p>Another method of treatment based upon bacteriological principles is the
+addition of a starter to induce the formation of acid. Where acid is
+developed as a result of the growth of the lactic-acid bacteria, the
+gas-producing species do not readily thrive. Another reason why acid
+aids in repressing the development of gas is that the curd particles are
+partially softened or digested by the action of the acid. This causes
+them to mat together more closely, and there is not left in the cheese
+the irregular mechanical openings in which the developing gas may find
+lodgment.</p>
+
+<p>Another method that is also useful with these curds is to employ salt.
+This represses gaseous fermentations, and the use of more salt than
+usual in making the cheese will very often restrain the production of
+gas. Tendency to form gas in Edam cheese is controlled by the addition
+of a starter prepared from slimy whey (lange wei) which is caused by the
+development of an acid-forming organism.</p>
+
+<p>Some have recommended the custom of washing the curds to remove the whey
+and the gas-producing bacteria contained therein. Care must be taken not
+to carry this<span class='pagenum'><a name="Page_188" id="Page_188">[Pg 188]</a></span> too far, for the removal of the sugar permits
+taint-producing organisms to thrive.<a name="FNanchor_211_211" id="FNanchor_211_211"></a><a href="#Footnote_211_211" class="fnanchor">[211]</a></p>
+
+<p>The temperature at which the cheese is cured also materially affects the
+development of gas. At high curing temperatures, gas-producing organisms
+develop rapidly; therefore more trouble is experienced in summer than at
+other seasons.</p>
+
+<p>If milks which are prone to undergo "gassy" development are excluded
+from the general supply, it would be possible to eliminate the source of
+the entire trouble. To aid in the early recognition of such milks that
+are not apparently affected when brought to the factory, fermentation or
+curd tests (p. 76) are of great value. The use of this test in the hands
+of the factory operator often enables him to detect the exact source of
+the trouble, which may frequently be confined to the milk delivered by a
+single patron.</p>
+
+<p><b>"Fruity" or "sweet" flavor.</b> Not infrequently the product of a factory
+may acquire during the process of ripening what is known as a "sweet" or
+"fruity" flavor. This flavor resembles the odor of fermented fruit or
+the bouquet of certain kinds of wine. It has been noted in widely
+different sections of the country and its presence bears no relation to
+the other qualities of the cheese. The cause of this trouble has
+recently been traced<a name="FNanchor_212_212" id="FNanchor_212_212"></a><a href="#Footnote_212_212" class="fnanchor">[212]</a> to the presence of various kinds of yeasts.
+Ordinarily yeasts are rarely present in good cheese, but in cheese
+affected with this trouble they abound. The addition of starters made
+from yeast cultures resulted in the production of the undesirable
+condition.</p>
+
+<p><span class='pagenum'><a name="Page_189" id="Page_189">[Pg 189]</a></span></p><p><b>Mottled cheese.</b> The color of cheese is sometimes cut to that extent that
+the cheese presents a wavy or mottled appearance. This condition is apt
+to appear if the ripening temperature is somewhat high, or larger
+quantities of rennet used than usual. The cause of the defect is
+obscure, but it has been demonstrated that the same is communicable if a
+starter is made by grating some of this mottled cheese into milk. The
+bacteriology of the trouble has not yet been worked out, but the defect
+is undoubtedly due to an organism that is able to grow in the ripening
+cheese. It has been claimed that the use of a pure lactic ferment as a
+starter enables one to overcome this defect.</p>
+
+<p><b>Bitter cheese.</b> Bitter flavors are sometimes developed in cheese
+especially where the ripening process is carried on at a low temperature
+in the presence of an excess of moisture for a considerable length of
+time.</p>
+
+<p>Guillebeau<a name="FNanchor_213_213" id="FNanchor_213_213"></a><a href="#Footnote_213_213" class="fnanchor">[213]</a> isolated several forms from Emmenthaler cheese which he
+connected with udder inflammation that were able to produce a bitter
+substance in cheese.</p>
+
+<p>Von Freudenreich<a name="FNanchor_214_214" id="FNanchor_214_214"></a><a href="#Footnote_214_214" class="fnanchor">[214]</a> has described a new form <i>Micrococcus casei amari</i>
+(micrococcus of bitter cheese) that was found in a sample of bitter
+cheese. This germ is closely related to Conn's micrococcus of bitter
+milk. It develops lactic acid rapidly, coagulating the milk and
+producing an intensely bitter taste in the course of one to three days.
+When milk infected with this organism is made into cheese, there is
+formed in a few days a decomposition product that imparts a marked
+bitter flavor to the cheese.</p>
+
+<p>Harrison<a name="FNanchor_215_215" id="FNanchor_215_215"></a><a href="#Footnote_215_215" class="fnanchor">[215]</a> has recently found a yeast that grows in the milk and also
+in the cheese which produces an undesirable bitter change.</p>
+
+<p><span class='pagenum'><a name="Page_190" id="Page_190">[Pg 190]</a></span></p><p>It is peculiar that some of the organisms that are able to produce
+bitter products in milk do not retain this property when the milk is
+worked up into cheese.</p>
+
+<p><b>Putrid or rotten cheese.</b> Sometimes cheese undergoes a putrefactive
+decomposition in which the texture is profoundly modified and various
+foul smelling gases are evolved. These often begin on the exterior as
+small circumscribed spots that slowly extend into the cheese, changing
+the casein into a soft slimy mass. Then, again, the interior of the
+cheese undergoes this slimy decomposition. The soft varieties are more
+prone toward this fermentation than the hard, although the firm cheeses
+are by no means exempt from the trouble. The "Verlaufen" or "running" of
+limburger cheese is a fermentation allied to this. It is where the
+inside of the cheese breaks down into a soft semi-fluid mass. In severe
+cases, the rind may even be ruptured, in which case the whole interior
+of the cheese flows out as a thick slimy mass, having sometimes a putrid
+odor. The conditions favoring this putrid decomposition are usually
+associated with an excess of moisture, and an abnormally low ripening
+temperature.</p>
+
+<p><b>Rusty spot.</b> This name is applied to the development of small
+yellowish-red or orange spots that are formed sometimes throughout the
+whole mass of cheddar cheese. A close inspection shows the colored
+points to be located along the edges of the curd particles. According to
+Harding,<a name="FNanchor_216_216" id="FNanchor_216_216"></a><a href="#Footnote_216_216" class="fnanchor">[216]</a> this trouble is most common in spring and fall. The cause
+of the difficulty has been traced by Connell<a name="FNanchor_217_217" id="FNanchor_217_217"></a><a href="#Footnote_217_217" class="fnanchor">[217]</a> to the development of
+a chromogenic bacterium, <i>Bacillus rudensis</i>. The organism can be most
+readily isolated on a potato<span class='pagenum'><a name="Page_191" id="Page_191">[Pg 191]</a></span> surface rather than with the usual
+isolating media, agar or gelatin.</p>
+
+<p><b>Other pigment changes.</b> Occasionally, with the hard type of cheese, but
+more frequently with the softer foreign varieties, various abnormal
+conditions arise that are marked by the production of different pigments
+in or on the cheese. More frequently these are merely superficial and
+affect only the outer layers of the cheese. Generally they are
+attributable to the development of certain chromogenic organisms
+(bacteria, molds and yeasts), although occasionally due to other causes,
+as in the case of a blue discoloration sometimes noted in foreign cheese
+made in copper kettles.<a name="FNanchor_218_218" id="FNanchor_218_218"></a><a href="#Footnote_218_218" class="fnanchor">[218]</a></p>
+
+<p>De Vries<a name="FNanchor_219_219" id="FNanchor_219_219"></a><a href="#Footnote_219_219" class="fnanchor">[219]</a> has described a blue condition that is found in Edam
+cheese. It appears first as a small blue spot on the inside, increasing
+rapidly in size until the whole mass is affected. This defect he was
+able to show was produced by a pigment-forming organism, <i>B.
+cyaneo-fuscus</i>. By the use of slimy whey (lange wei) this abnormal
+change was controlled.</p>
+
+<p><b>Moldy cheese.</b> With many varieties of cheese, especially some of the
+foreign types, the presence of mold on the exterior is not regarded as
+detrimental; in fact a limited development is much desired. In hard
+rennet cheese as cheddar or Swiss, the market demands a product free
+from mold, although it should be said that this condition is imposed by
+the desire to secure a good-looking cheese rather than any injury in
+flavor that the mold causes. Mold spores are so widely distributed that,
+if proper temperature and moisture conditions prevail, these spores will
+always develop. At temperatures in the neighborhood of 40&deg; F.<span class='pagenum'><a name="Page_192" id="Page_192">[Pg 192]</a></span> and
+below, mold growth is exceedingly slow, and often fructification does
+not occur, the only evidence of the mold being the white, felt-like
+covering that is made up of the vegetating filaments. The use of
+paraffin has been suggested as a means of overcoming this growth, the
+cheese being dipped at an early stage into melted paraffin. Recent
+experiments have shown that "off" flavors sometimes develop where cheese
+are paraffined directly from the press. If paraffin is too hard, it has
+a tendency to crack and separate from the rind, thus allowing molds to
+develop beneath the paraffin coat, where the conditions are ideal as to
+moisture, for evaporation is excluded and the air consequently
+saturated. The use of formalin (2% solution) has been suggested as a
+wash for the outside of the cheese. This substance or sulfur is also
+applied in a gaseous form. Double bandaging is also resorted to as a
+means of making the cheese more presentable through the removal of the
+outer bandage.</p>
+
+<p>The nature of these molds has not been thoroughly studied as yet. The
+ordinary blue-green bread mold, <i>Penicillium glaucum</i>, is most
+frequently found, but there are numerous other forms that appear,
+especially at low temperatures.</p>
+
+<p><b>Poisonous cheese.</b> Cases of acute poisoning arising from the ingestion of
+cheese are reported from time to time. Vaughan has succeeded in showing
+that this condition is due to the formation of a highly poisonous
+alkaloid which he has isolated, and which he calls <i>tyrotoxicon</i>.<a name="FNanchor_220_220" id="FNanchor_220_220"></a><a href="#Footnote_220_220" class="fnanchor">[220]</a>
+This poisonous ptomaine has also been demonstrated in milk and other
+milk products, and is undoubtedly due to the development of various
+putrefactive bacteria that find their way into the milk. It seems quite
+probable that the development<span class='pagenum'><a name="Page_193" id="Page_193">[Pg 193]</a></span> of these toxic organisms can also go on
+in the cheese after it is taken from the press.</p>
+
+<p><b>Prevention or cheese defects.</b> The defective conditions previously
+referred to can rarely be overcome in cheese so as to improve the
+affected product, for they only become manifest in most cases during the
+later stages of the curing process. The only remedy against future loss
+is to recognize the conditions that are apt to prevail during the
+occurrence of an outbreak and see that the cheese are handled in such a
+way as to prevent a recurrence of the difficulty.</p>
+
+<p>Many abnormal and undesirable results are incident to the manufacture of
+the product, such as "sour" or "mealy" cheese, conditions due to the
+development of too much acid in the milk or too high a "cook." These are
+under the direct control of the maker and for them he alone is
+responsible. The development of taints due to the growth of unwelcome
+bacteria that have gained access to the milk while it is yet on the farm
+are generally beyond the control of the cheese maker, unless they are so
+pronounced as to appear during the handling of the curds. If this does
+occur he is sometimes able, through the intervention of a starter or by
+varying some detail in making, to handle the milk in such a way as to
+minimize the trouble, but rarely is he able to eliminate it entirely.</p>
+
+<p>One of the most strenuous duties which the maker must perform at all
+times is to point out to his patrons the absolute necessity of their
+handling the milk in such a way as to prevent the introduction of
+organisms of a baleful type.</p>
+
+<div class="footnotes"><h3>FOOTNOTES:</h3>
+
+<div class="footnote"><p><a name="Footnote_178_178" id="Footnote_178_178"></a><a href="#FNanchor_178_178"><span class="label">[178]</span></a> Russell, 13 Rept. Wis. Expt. Stat., 1896, p. 112;
+Campbell, Trans. High. &amp; Agr. Soc. Scotland, 5 ser., 1898, 10:181.</p></div>
+
+<div class="footnote"><p><a name="Footnote_179_179" id="Footnote_179_179"></a><a href="#FNanchor_179_179"><span class="label">[179]</span></a> Winkler, Milch Zeit. (Hildesheim), Nov. 24, 1900.</p></div>
+
+<div class="footnote"><p><a name="Footnote_180_180" id="Footnote_180_180"></a><a href="#FNanchor_180_180"><span class="label">[180]</span></a> Campbell, No. Brit., Agric., May 12, 1897.</p></div>
+
+<div class="footnote"><p><a name="Footnote_181_181" id="Footnote_181_181"></a><a href="#FNanchor_181_181"><span class="label">[181]</span></a> Weigmann, Milch Zeit., No. 50, 1889.</p></div>
+
+<div class="footnote"><p><a name="Footnote_182_182" id="Footnote_182_182"></a><a href="#FNanchor_182_182"><span class="label">[182]</span></a> Klein, Milch Zeit. (Hildesheim), No. 17, 1900.</p></div>
+
+<div class="footnote"><p><a name="Footnote_183_183" id="Footnote_183_183"></a><a href="#FNanchor_183_183"><span class="label">[183]</span></a> Adametz, Landw. Jahr., 18:256.</p></div>
+
+<div class="footnote"><p><a name="Footnote_184_184" id="Footnote_184_184"></a><a href="#FNanchor_184_184"><span class="label">[184]</span></a> Van Slyke and Hart, Bull. 214, N. Y. Expt. Stat., July
+1902.</p></div>
+
+<div class="footnote"><p><a name="Footnote_185_185" id="Footnote_185_185"></a><a href="#FNanchor_185_185"><span class="label">[185]</span></a> Milch Zeit., 1898, No. 49.</p></div>
+
+<div class="footnote"><p><a name="Footnote_186_186" id="Footnote_186_186"></a><a href="#FNanchor_186_186"><span class="label">[186]</span></a> Lafar, Technical Mycology, p. 216.</p></div>
+
+<div class="footnote"><p><a name="Footnote_187_187" id="Footnote_187_187"></a><a href="#FNanchor_187_187"><span class="label">[187]</span></a> Adametz, Landw. Jahr., 18:228.</p></div>
+
+<div class="footnote"><p><a name="Footnote_188_188" id="Footnote_188_188"></a><a href="#FNanchor_188_188"><span class="label">[188]</span></a> Freudenreich, Landw. Jahr. d. Schweiz, 4:17; 5:16.</p></div>
+
+<div class="footnote"><p><a name="Footnote_189_189" id="Footnote_189_189"></a><a href="#FNanchor_189_189"><span class="label">[189]</span></a> Russell, 13 Rept. Wis. Expt. Stat., 1896, p. 95.</p></div>
+
+<div class="footnote"><p><a name="Footnote_190_190" id="Footnote_190_190"></a><a href="#FNanchor_190_190"><span class="label">[190]</span></a> Harrison and Connell, Rev. gen. du Lait, Nos. 4, 5, 6, 7
+and 8, 1903-04.</p></div>
+
+<div class="footnote"><p><a name="Footnote_191_191" id="Footnote_191_191"></a><a href="#FNanchor_191_191"><span class="label">[191]</span></a> Lloyd, Bath and West of Eng. Soc. Rept., 1892, 2:180.</p></div>
+
+<div class="footnote"><p><a name="Footnote_192_192" id="Footnote_192_192"></a><a href="#FNanchor_192_192"><span class="label">[192]</span></a> Freudenreich, Landw. Jahr. d. Schweiz, 1900; Adametz,
+Oest. Molk. Zeit., 1899, No. 7.</p></div>
+
+<div class="footnote"><p><a name="Footnote_193_193" id="Footnote_193_193"></a><a href="#FNanchor_193_193"><span class="label">[193]</span></a> Russell, 14 Wis. Expt. Stat., 1897, p. 203. Harrison and
+Connell, Rev. gen. du Lait Nos. 4, etc., 1903-04.</p></div>
+
+<div class="footnote"><p><a name="Footnote_194_194" id="Footnote_194_194"></a><a href="#FNanchor_194_194"><span class="label">[194]</span></a> Babcock and Russell, 18 Rept. Wis. Expt. Stat., 1901.
+Dean, Harrison and Harcourt, Bull. 121, Ont. Agr'l. Coll., June 1902.</p></div>
+
+<div class="footnote"><p><a name="Footnote_195_195" id="Footnote_195_195"></a><a href="#FNanchor_195_195"><span class="label">[195]</span></a> Schaffer, Milch Zeit., 1889, p. 146.</p></div>
+
+<div class="footnote"><p><a name="Footnote_196_196" id="Footnote_196_196"></a><a href="#FNanchor_196_196"><span class="label">[196]</span></a> Adametz, Landw. Jahr., 18:261.</p></div>
+
+<div class="footnote"><p><a name="Footnote_197_197" id="Footnote_197_197"></a><a href="#FNanchor_197_197"><span class="label">[197]</span></a> Duclaux, Le Lait, p. 213.</p></div>
+
+<div class="footnote"><p><a name="Footnote_198_198" id="Footnote_198_198"></a><a href="#FNanchor_198_198"><span class="label">[198]</span></a> Adametz, Oest. Molk. Zeit., 1900, Nos. 16-18.</p></div>
+
+<div class="footnote"><p><a name="Footnote_199_199" id="Footnote_199_199"></a><a href="#FNanchor_199_199"><span class="label">[199]</span></a> Freudenreich, Landw. Jahr. d. Schweiz, 1897, p. 85.</p></div>
+
+<div class="footnote"><p><a name="Footnote_200_200" id="Footnote_200_200"></a><a href="#FNanchor_200_200"><span class="label">[200]</span></a> Weigmann, Cent. f. Bakt., II Abt., 1898, 4:593; also
+1899, 5:630.</p></div>
+
+<div class="footnote"><p><a name="Footnote_201_201" id="Footnote_201_201"></a><a href="#FNanchor_201_201"><span class="label">[201]</span></a> Gorini, Abs. in Expt. Stat. Rec., 11:388.</p></div>
+
+<div class="footnote"><p><a name="Footnote_202_202" id="Footnote_202_202"></a><a href="#FNanchor_202_202"><span class="label">[202]</span></a> Babcock and Russell, 14 Rept. Wis. Expt. Stat., 1897, p.
+161.</p></div>
+
+<div class="footnote"><p><a name="Footnote_203_203" id="Footnote_203_203"></a><a href="#FNanchor_203_203"><span class="label">[203]</span></a> Jensen, Cent. f. Bakt., II Abt., 3:752.</p></div>
+
+<div class="footnote"><p><a name="Footnote_204_204" id="Footnote_204_204"></a><a href="#FNanchor_204_204"><span class="label">[204]</span></a> Freudenreich, Cent. f. Bakt., II Abt., 1900, 6:332.</p></div>
+
+<div class="footnote"><p><a name="Footnote_205_205" id="Footnote_205_205"></a><a href="#FNanchor_205_205"><span class="label">[205]</span></a> Jensen, Ibid., 1900, 6:734.</p></div>
+
+<div class="footnote"><p><a name="Footnote_206_206" id="Footnote_206_206"></a><a href="#FNanchor_206_206"><span class="label">[206]</span></a> 17 Rept. Wis. Expt. Stat., 1900, p. 102.</p></div>
+
+<div class="footnote"><p><a name="Footnote_207_207" id="Footnote_207_207"></a><a href="#FNanchor_207_207"><span class="label">[207]</span></a> Jensen, Landw. Jahr. d. Schweiz, 1900.</p></div>
+
+<div class="footnote"><p><a name="Footnote_208_208" id="Footnote_208_208"></a><a href="#FNanchor_208_208"><span class="label">[208]</span></a> Babcock and Russell, 18 Rept. Wis. Expt. Stat., 1901.</p></div>
+
+<div class="footnote"><p><a name="Footnote_209_209" id="Footnote_209_209"></a><a href="#FNanchor_209_209"><span class="label">[209]</span></a> Cent. f. Bakt. 1899, p. 14.</p></div>
+
+<div class="footnote"><p><a name="Footnote_210_210" id="Footnote_210_210"></a><a href="#FNanchor_210_210"><span class="label">[210]</span></a> Bull. 128, Wis. Expt. Stat., Sept. 1905.</p></div>
+
+<div class="footnote"><p><a name="Footnote_211_211" id="Footnote_211_211"></a><a href="#FNanchor_211_211"><span class="label">[211]</span></a> Babcock and Russell, 18 Rept. Wis. Expt. Stat., 1901.</p></div>
+
+<div class="footnote"><p><a name="Footnote_212_212" id="Footnote_212_212"></a><a href="#FNanchor_212_212"><span class="label">[212]</span></a> Harding, Rogers and Smith, Bull. 183, N. Y. (Geneva)
+Expt. Stat., Dec., 1900.</p></div>
+
+<div class="footnote"><p><a name="Footnote_213_213" id="Footnote_213_213"></a><a href="#FNanchor_213_213"><span class="label">[213]</span></a> Guillebeau, Landw. Jahr., 1890, p. 27.</p></div>
+
+<div class="footnote"><p><a name="Footnote_214_214" id="Footnote_214_214"></a><a href="#FNanchor_214_214"><span class="label">[214]</span></a> Freudenreich, F&uuml;ehl. Landw. Ztg., 43:361.</p></div>
+
+<div class="footnote"><p><a name="Footnote_215_215" id="Footnote_215_215"></a><a href="#FNanchor_215_215"><span class="label">[215]</span></a> Harrison, Bull. 123 Ont. Agr'l. Coll., May, 1902.</p></div>
+
+<div class="footnote"><p><a name="Footnote_216_216" id="Footnote_216_216"></a><a href="#FNanchor_216_216"><span class="label">[216]</span></a> Bull. 183, N. Y. (Geneva) Expt. Stat., Dec. 1900.</p></div>
+
+<div class="footnote"><p><a name="Footnote_217_217" id="Footnote_217_217"></a><a href="#FNanchor_217_217"><span class="label">[217]</span></a> Connell, Bull. Canadian Dept. of Agr., 1897.</p></div>
+
+<div class="footnote"><p><a name="Footnote_218_218" id="Footnote_218_218"></a><a href="#FNanchor_218_218"><span class="label">[218]</span></a> Schm&ouml;ger, Milch Zeit., 1883, p. 483.</p></div>
+
+<div class="footnote"><p><a name="Footnote_219_219" id="Footnote_219_219"></a><a href="#FNanchor_219_219"><span class="label">[219]</span></a> De Vries, Milch Zeit., 1888, pp. 861, 885.</p></div>
+
+<div class="footnote"><p><a name="Footnote_220_220" id="Footnote_220_220"></a><a href="#FNanchor_220_220"><span class="label">[220]</span></a> Zeit. f. physiol. Chemie, 10:146.</p></div>
+</div>
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_195" id="Page_195">[Pg 195]</a></span></p>
+<h2>INDEX.</h2>
+
+
+<p>
+Acid, effect of, on churning, <a href='#Page_137'>137</a>;<br />
+<span style="margin-left: 1em;">in butter-making, <a href='#Page_138'>138</a>.</span><br />
+<br />
+Acid test, <a href='#Page_52'>52</a>.<br />
+<br />
+Aeration of milk, <a href='#Page_59'>59</a>.<br />
+<br />
+Aerobic bacteria, <a href='#Page_7'>7</a>.<br />
+<br />
+Alcoholic fermentation in milk, <a href='#Page_72'>72</a>.<br />
+<br />
+Anaerobic bacteria, <a href='#Page_7'>7</a>.<br />
+<br />
+Animal, influence of, on milk infection, <a href='#Page_34'>34</a>.<br />
+<br />
+Animal odor, <a href='#Page_56'>56</a>.<br />
+<br />
+Anthrax, <a href='#Page_94'>94</a>.<br />
+<br />
+Antiseptics, <a href='#Page_9'>9</a>, <a href='#Page_88'>88</a>.<br />
+<br />
+Aroma, of butter, <a href='#Page_140'>140</a>.<br />
+<br />
+<br />
+Bacillus: definition of, <a href='#Page_2'>2</a>.<br />
+<span style="margin-left: 1em;"><i>acidi lactici</i>, <a href='#Page_64'>64</a>;</span><br />
+<span style="margin-left: 1em;"><i>cyaneo-fuscus</i>, <a href='#Page_188'>188</a>;</span><br />
+<span style="margin-left: 1em;"><i>cyanogenus</i>, <a href='#Page_74'>74</a>;</span><br />
+<span style="margin-left: 1em;"><i>foetidus lactis</i>, <a href='#Page_157'>157</a>;</span><br />
+<span style="margin-left: 1em;"><i>lactis aerogenes</i>, <a href='#Page_65'>65</a>;</span><br />
+<span style="margin-left: 1em;"><i>lactis erythrogenes</i>, <a href='#Page_74'>74</a>;</span><br />
+<span style="margin-left: 1em;"><i>lactis saponacei</i>, <a href='#Page_67'>67</a>;</span><br />
+<span style="margin-left: 1em;"><i>lactis viscosus</i>, <a href='#Page_71'>71</a>;</span><br />
+<span style="margin-left: 1em;"><i>nobilis</i>, <a href='#Page_162'>162</a>, <a href='#Page_174'>174</a>;</span><br />
+<span style="margin-left: 1em;"><i>prodigiosus</i>, <a href='#Page_74'>74</a>;</span><br />
+<span style="margin-left: 1em;"><i>rudensis</i>, <a href='#Page_188'>188</a>;</span><br />
+<span style="margin-left: 1em;"><i>synxanthus</i>, <a href='#Page_75'>75</a>;</span><br />
+<span style="margin-left: 1em;"><i>tuberculosis</i>, <a href='#Page_84'>84</a>.</span><br />
+<br />
+Bacteria:<br />
+<span style="margin-left: 1em;">on hairs, <a href='#Page_35'>35</a>;</span><br />
+<span style="margin-left: 1em;">kinds in milk, <a href='#Page_63'>63</a>;</span><br />
+<span style="margin-left: 1em;">in barn air, <a href='#Page_42'>42</a>;</span><br />
+<span style="margin-left: 1em;">in milk pails, <a href='#Page_27'>27</a>;</span><br />
+<span style="margin-left: 1em;">in butter, <a href='#Page_154'>154</a>;</span><br />
+<span style="margin-left: 1em;">classification of, <a href='#Page_4'>4</a>;</span><br />
+<span style="margin-left: 1em;">in cheese, <a href='#Page_160'>160</a>;</span><br />
+<span style="margin-left: 1em;">culture of, <a href='#Page_17'>17</a>;</span><br />
+<span style="margin-left: 1em;">in cream, <a href='#Page_128'>128</a>;</span><br />
+<span style="margin-left: 1em;">discovery of, <a href='#Page_1'>1</a>;</span><br />
+<span style="margin-left: 1em;">external conditions affecting, <a href='#Page_8'>8</a>;</span><br />
+<span style="margin-left: 1em;">form of, <a href='#Page_2'>2</a>;</span><br />
+<span style="margin-left: 1em;">in butter, <a href='#Page_142'>142</a>;</span><br />
+<span style="margin-left: 1em;">in butter-making, <a href='#Page_127'>127</a>;</span><br />
+<span style="margin-left: 1em;">in centrifuge slime, <a href='#Page_39'>39</a>;</span><br />
+<span style="margin-left: 1em;">In fore milk, <a href='#Page_28'>28</a>;</span><br />
+<span style="margin-left: 1em;">in rennet, <a href='#Page_163'>163</a>;</span><br />
+<span style="margin-left: 1em;">In separator slime, <a href='#Page_39'>39</a>;</span><br />
+<span style="margin-left: 1em;">manure, <a href='#Page_37'>37</a>;</span><br />
+<span style="margin-left: 1em;">number of, in milk, <a href='#Page_50'>50</a>.</span><br />
+<span style="margin-left: 1em;">Distribution of:</span><br />
+<span style="margin-left: 2em;">milk of American cities, <a href='#Page_50'>50</a>;</span><br />
+<span style="margin-left: 2em;">European cities, <a href='#Page_50'>50</a>;</span><br />
+<span style="margin-left: 2em;">in relation to cheese, <a href='#Page_168'>168</a>.</span><br />
+<span style="margin-left: 1em;">Of disease:</span><br />
+<span style="margin-left: 2em;">anthrax, <a href='#Page_94'>94</a>;</span><br />
+<span style="margin-left: 2em;">cholera, <a href='#Page_98'>98</a>;</span><br />
+<span style="margin-left: 2em;">diphtheria, <a href='#Page_99'>99</a>;</span><br />
+<span style="margin-left: 2em;">lockjaw, <a href='#Page_94'>94</a>;</span><br />
+<span style="margin-left: 2em;">toxic, <a href='#Page_100'>100</a>;</span><br />
+<span style="margin-left: 2em;">tuberculosis, <a href='#Page_84'>84</a>;</span><br />
+<span style="margin-left: 2em;">typhoid fever, <a href='#Page_98'>98</a>.</span><br />
+<span style="margin-left: 1em;">Methods of study of:</span><br />
+<span style="margin-left: 2em;">culture, <a href='#Page_15'>15</a>;</span><br />
+<span style="margin-left: 2em;">culture media, <a href='#Page_13'>13</a>;</span><br />
+<span style="margin-left: 2em;">isolation, <a href='#Page_14'>14</a>.</span><br />
+<br />
+Bitter butter, <a href='#Page_158'>158</a>;<br />
+<span style="margin-left: 1em;">cheese, <a href='#Page_189'>189</a>;</span><br />
+<span style="margin-left: 1em;">milk, <a href='#Page_72'>72</a>.</span><br />
+<br />
+Bloody milk, <a href='#Page_74'>74</a>.<br />
+<br />
+Blue cheese, <a href='#Page_191'>191</a>;<br />
+<span style="margin-left: 1em;">milk, <a href='#Page_74'>74</a>.</span><br />
+<br />
+Bovine tuberculosis, <a href='#Page_84'>84</a>.<br />
+<br />
+Brie cheese, <a href='#Page_182'>182</a>.<br />
+<br />
+Butter:<br />
+<span style="margin-left: 1em;">bacteria in, <a href='#Page_154'>154</a>;</span><br />
+<span style="margin-left: 1em;">bitter, <a href='#Page_158'>158</a>;</span><br />
+<span style="margin-left: 1em;">"cowy,"<a href='#Page_157'>157</a>;</span><br />
+<span style="margin-left: 1em;">fishy, <a href='#Page_159'>159</a>;</span><br />
+<span style="margin-left: 1em;">lardy, <a href='#Page_157'>157</a>;</span><br />
+<span style="margin-left: 1em;">moldy, <a href='#Page_158'>158</a>;</span><br />
+<span style="margin-left: 1em;">mottled, <a href='#Page_156'>156</a>;</span><br />
+<span style="margin-left: 1em;">oily, <a href='#Page_158'>158</a>;</span><br />
+<span style="margin-left: 1em;">putrid, <a href='#Page_156'>156</a>;</span><br />
+<span style="margin-left: 1em;">rancid, <a href='#Page_155'>155</a>;</span><br />
+<span style="margin-left: 1em;">tallowy, <a href='#Page_157'>157</a>;</span><br />
+<span style="margin-left: 1em;">turnip flavor in, <a href='#Page_157'>157</a>.</span><br />
+<span style="margin-left: 1em;">Making:</span><br />
+<span style="margin-left: 2em;">aroma, <a href='#Page_140'>140</a>;</span><br />
+<span style="margin-left: 2em;">flavor in, <a href='#Page_140'>140</a>;</span><br />
+<span style="margin-left: 2em;">pure culture, <a href='#Page_143'>143</a>;</span><br />
+<span style="margin-left: 2em;">in ripening of cream, <a href='#Page_136'>136</a>.</span><br />
+<br />
+Butyric acid fermentation, <a href='#Page_69'>69</a>.<br />
+<br />
+By-products of factory, methods of preserving, <a href='#Page_25'>25</a>.<br />
+<br />
+<br />
+Casease, <a href='#Page_68'>68</a>.<br />
+<br />
+Caseone, <a href='#Page_68'>68</a>.<br />
+<br />
+Centrifugal force, cleaning milk by, <a href='#Page_38'>38</a>.<br />
+<br />
+Cheese:<br />
+<span style="margin-left: 1em;">bacterial flora of, <a href='#Page_168'>168</a>;</span><br />
+<span style="margin-left: 1em;">bitter, <a href='#Page_189'>189</a>;</span><br />
+<span style="margin-left: 1em;">blue, <a href='#Page_187'>187</a>;</span><br />
+<span style="margin-left: 1em;">Brie, <a href='#Page_182'>182</a>;</span><br />
+<span style="margin-left: 1em;">Edam, <a href='#Page_72'>72</a>, <a href='#Page_162'>162</a>;</span><br />
+<span style="margin-left: 1em;">Emmenthaler, <a href='#Page_185'>185</a>;</span><br />
+<span style="margin-left: 1em;">flavor of, <a href='#Page_179'>179</a>;</span><br />
+<span style="margin-left: 1em;">gassy fermentations in, <a href='#Page_183'>183</a>;</span><br />
+<span style="margin-left: 1em;">Gorgonzola, <a href='#Page_180'>180</a>;</span><br />
+<span style="margin-left: 1em;">molds on, <a href='#Page_191'>191</a>;</span><br />
+<span style="margin-left: 1em;">mottled, <a href='#Page_189'>189</a>;</span><br />
+<span style="margin-left: 1em;">"nissler,"<a href='#Page_185'>185</a>;</span><br />
+<span style="margin-left: 1em;">poisonous, <a href='#Page_192'>192</a>;</span><br />
+<span style="margin-left: 1em;">putrid, <a href='#Page_190'>190</a>;</span><br />
+<span style="margin-left: 1em;">ripening of moldy, <a href='#Page_180'>180</a>;</span><br />
+<span style="margin-left: 1em;">ripening of soft, <a href='#Page_181'>181</a>;</span><br />
+<span style="margin-left: 1em;">Roquefort, <a href='#Page_180'>180</a>;</span><br />
+<span style="margin-left: 1em;">rusty spot in, <a href='#Page_188'>188</a>;</span><br />
+<span style="margin-left: 1em;">Stilton, <a href='#Page_180'>180</a>;</span><br />
+<span style="margin-left: 1em;">Swiss, <a href='#Page_185'>185</a>.</span><br />
+<span style="margin-left: 1em;">Making and curing:</span><br />
+<span style="margin-left: 2em;">chemical changes in curing, <a href='#Page_166'>166</a>;</span><br />
+<span style="margin-left: 2em;">influence of temperature on curing, <a href='#Page_169'>169</a>;</span><br />
+<span class='pagenum'><a name="Page_196" id="Page_196">[Pg 196]</a></span><span style="margin-left: 2em;">influence of rennet, <a href='#Page_177'>177</a>;</span><br />
+<span style="margin-left: 2em;">physical changes in curing, <a href='#Page_165'>165</a>;</span><br />
+<span style="margin-left: 2em;">prevention of defects, <a href='#Page_193'>193</a>;</span><br />
+<span style="margin-left: 2em;">starters in, <a href='#Page_161'>161</a>;</span><br />
+<span style="margin-left: 2em;">temperature in relation to bacterial influence, <a href='#Page_169'>169</a>.</span><br />
+<span style="margin-left: 1em;">Theories of curing:</span><br />
+<span style="margin-left: 2em;">digestive, <a href='#Page_173'>173</a>;</span><br />
+<span style="margin-left: 2em;">galactase, <a href='#Page_175'>175</a>, <a href='#Page_177'>177</a>;</span><br />
+<span style="margin-left: 2em;">lactic acid, <a href='#Page_174'>174</a>.</span><br />
+<br />
+Chemical changes in cheese-ripening, <a href='#Page_166'>166</a>.<br />
+<br />
+Chemical disinfectants in milk:<br />
+<span style="margin-left: 1em;">bleaching powder, <a href='#Page_81'>81</a>;</span><br />
+<span style="margin-left: 1em;">corrosive sublimate, <a href='#Page_81'>81</a>;</span><br />
+<span style="margin-left: 1em;">formalin, <a href='#Page_80'>80</a>;</span><br />
+<span style="margin-left: 1em;">sulfur, <a href='#Page_80'>80</a>;</span><br />
+<span style="margin-left: 1em;">whitewash, <a href='#Page_81'>81</a>;</span><br />
+<span style="margin-left: 1em;">vitriol, <a href='#Page_81'>81</a>.</span><br />
+<br />
+Chemical preservatives, <a href='#Page_80'>80</a>.<br />
+<br />
+Children, milk for, <a href='#Page_45'>45</a>.<br />
+<br />
+Cholera in milk, <a href='#Page_98'>98</a>.<br />
+<br />
+Classification by separator, <a href='#Page_38'>38</a>.<br />
+<br />
+Coccus, definition of, <a href='#Page_2'>2</a>.<br />
+<br />
+Cold, influence on bacteria, <a href='#Page_8'>8</a>, <a href='#Page_48'>48</a>.<br />
+<br />
+Contamination of milk through disease germs, <a href='#Page_95'>95</a>, <a href='#Page_191'>191</a>.<br />
+<br />
+Covered milk pails, <a href='#Page_41'>41</a>.<br />
+<br />
+Cream, bacterial changes in, <a href='#Page_135'>135</a>;<br />
+<span style="margin-left: 1em;">mechanical causes for bacteria in, <a href='#Page_135'>135</a>;</span><br />
+<span style="margin-left: 1em;">pasteurized, <a href='#Page_113'>113</a>;</span><br />
+<span style="margin-left: 1em;">restoration of consistency of pasteurized, <a href='#Page_132'>132</a>.</span><br />
+<span style="margin-left: 1em;">Ripening of, <a href='#Page_136'>136</a>;</span><br />
+<span style="margin-left: 2em;">advantage of pure cultures in, <a href='#Page_144'>144</a>;</span><br />
+<span style="margin-left: 2em;">by natural starters, <a href='#Page_142'>142</a>;</span><br />
+<span style="margin-left: 2em;">characteristics of pure cultures in, <a href='#Page_145'>145</a>;</span><br />
+<span style="margin-left: 2em;">objections to pure cultures in, <a href='#Page_146'>146</a>;</span><br />
+<span style="margin-left: 2em;">principles of pure cultures in, <a href='#Page_143'>143</a>;</span><br />
+<span style="margin-left: 2em;">propagation of pure cultures, <a href='#Page_151'>151</a>;</span><br />
+<span style="margin-left: 2em;">purity of commercial starters, <a href='#Page_150'>150</a>;</span><br />
+<span style="margin-left: 2em;">home-made starters in, <a href='#Page_146'>146</a>.</span><br />
+<br />
+Creaming methods, <a href='#Page_134'>134</a>.<br />
+<br />
+Curd test, <a href='#Page_76'>76</a>.<br />
+<br />
+<br />
+Dairy utensils a source of contamination, <a href='#Page_21'>21</a>.<br />
+<br />
+Diarrhoeal diseases, <a href='#Page_100'>100</a>.<br />
+<br />
+Digesting bacteria, <a href='#Page_67'>67</a>.<br />
+<br />
+Digestibility of heated milk, <a href='#Page_111'>111</a>.<br />
+<br />
+Diphtheria, <a href='#Page_99'>99</a>.<br />
+<br />
+Dirt in milk, <a href='#Page_34'>34</a>.<br />
+<br />
+Dirt, exclusion of, <a href='#Page_36'>36</a>.<br />
+<br />
+Disease germs in milk, <a href='#Page_95'>95</a>;<br />
+<span style="margin-left: 1em;">effect of heat on, <a href='#Page_91'>91</a>;</span><br />
+<span style="margin-left: 1em;">origin of, <a href='#Page_83'>83</a>.</span><br />
+<br />
+Disinfectants, <a href='#Page_9'>9</a>:<br />
+<span style="margin-left: 1em;">carbolic acid, <a href='#Page_81'>81</a>;</span><br />
+<span style="margin-left: 1em;">chloride of lime, <a href='#Page_81'>81</a>;</span><br />
+<span style="margin-left: 1em;">corrosive sublimate, <a href='#Page_81'>81</a>;</span><br />
+<span style="margin-left: 1em;">formalin, <a href='#Page_80'>80</a>;</span><br />
+<span style="margin-left: 1em;">sulfur, <a href='#Page_80'>80</a>;</span><br />
+<span style="margin-left: 1em;">vitriol salts, <a href='#Page_81'>81</a>;</span><br />
+<span style="margin-left: 1em;">whitewash, <a href='#Page_79'>79</a>.</span><br />
+<br />
+Disinfectants in milk:<br />
+<span style="margin-left: 1em;">alkaline salts, <a href='#Page_106'>106</a>;</span><br />
+<span style="margin-left: 1em;">boracic acid, <a href='#Page_106'>106</a>;</span><br />
+<span style="margin-left: 1em;">formalin, <a href='#Page_106'>106</a>;</span><br />
+<span style="margin-left: 1em;">preservaline, <a href='#Page_107'>107</a>;</span><br />
+<span style="margin-left: 1em;">salicylic acid, <a href='#Page_106'>106</a>.</span><br />
+<br />
+Domestic pasteurizing apparatus, <a href='#Page_119'>119</a>.<br />
+<br />
+Drugs, taints in milk due to, <a href='#Page_56'>56</a>.<br />
+<br />
+Drying, effect of, <a href='#Page_8'>8</a>.<br />
+<br />
+<br />
+Edam cheese, <a href='#Page_72'>72</a>, <a href='#Page_162'>162</a>.<br />
+<br />
+Emmenthaler cheese, <a href='#Page_185'>185</a>.<br />
+<br />
+Endospores, <a href='#Page_3'>3</a>.<br />
+<br />
+Enzyms, <a href='#Page_10'>10</a>.<br />
+<br />
+<br />
+Factory by-products, <a href='#Page_22'>22</a>;<br />
+<span style="margin-left: 1em;">treatment of, <a href='#Page_25'>25</a>.</span><br />
+<br />
+Farrington alkaline tablet, <a href='#Page_52'>52</a>.<br />
+<br />
+Fecal bacteria, effect of, on butter, <a href='#Page_35'>35</a>.<br />
+<br />
+Fermentation:<br />
+<span style="margin-left: 1em;">In cheese: gassy, <a href='#Page_183'>183</a>.</span><br />
+<span style="margin-left: 1em;">In milk:</span><br />
+<span style="margin-left: 2em;">alcoholic, <a href='#Page_72'>72</a>;</span><br />
+<span style="margin-left: 2em;">bitter, <a href='#Page_72'>72</a>;</span><br />
+<span style="margin-left: 2em;">blue, <a href='#Page_74'>74</a>;</span><br />
+<span style="margin-left: 2em;">butyric, <a href='#Page_69'>69</a>;</span><br />
+<span style="margin-left: 2em;">digesting, <a href='#Page_67'>67</a>;</span><br />
+<span style="margin-left: 2em;">gassy, <a href='#Page_66'>66</a>;</span><br />
+<span style="margin-left: 2em;">kephir, <a href='#Page_72'>72</a>;</span><br />
+<span style="margin-left: 2em;">koumiss, <a href='#Page_72'>72</a>;</span><br />
+<span style="margin-left: 2em;">lactic acid, <a href='#Page_63'>63</a>;</span><br />
+<span style="margin-left: 2em;">lange-wei, <a href='#Page_72'>72</a>;</span><br />
+<span style="margin-left: 2em;">red, <a href='#Page_74'>74</a>;</span><br />
+<span style="margin-left: 2em;">ropy, <a href='#Page_69'>69</a>;</span><br />
+<span style="margin-left: 2em;">slimy, <a href='#Page_69'>69</a>;</span><br />
+<span style="margin-left: 2em;">soapy, <a href='#Page_73'>73</a>;</span><br />
+<span style="margin-left: 2em;">souring, <a href='#Page_63'>63</a>;</span><br />
+<span style="margin-left: 2em;">sweet curdling, <a href='#Page_67'>67</a>;</span><br />
+<span style="margin-left: 2em;">treatment of, <a href='#Page_75'>75</a>.</span><br />
+<span style="margin-left: 1em;">Tests, <a href='#Page_76'>76</a>;</span><br />
+<span style="margin-left: 2em;">Gerber's, <a href='#Page_76'>76</a>;</span><br />
+<span style="margin-left: 2em;">Walther's, <a href='#Page_76'>76</a>;</span><br />
+<span style="margin-left: 2em;">Wisconsin curd, <a href='#Page_76'>76</a>.</span><br />
+<br />
+Filtration of milk, <a href='#Page_38'>38</a>.<br />
+<br />
+Fishy butter, <a href='#Page_159'>159</a>.<br />
+<br />
+Flavor:<br />
+<span style="margin-left: 1em;">of butter, <a href='#Page_140'>140</a>;</span><br />
+<span style="margin-left: 1em;">of cheese, <a href='#Page_179'>179</a>.</span><br />
+<br />
+Foot and mouth disease, <a href='#Page_93'>93</a>.<br />
+<br />
+Fore milk, <a href='#Page_28'>28</a>.<br />
+<br />
+Formaldehyde, <a href='#Page_80'>80</a>.<br />
+<br />
+Formalin, <a href='#Page_80'>80</a>.<br />
+<br />
+<span class='pagenum'><a name="Page_197" id="Page_197">[Pg 197]</a></span>Fruity flavor in cheese, <a href='#Page_188'>188</a>.<br />
+<br />
+<br />
+Galactase in cheese, <a href='#Page_175'>175</a>.<br />
+<br />
+Gassy fermentations:<br />
+<span style="margin-left: 1em;">in cheese, <a href='#Page_183'>183</a>;</span><br />
+<span style="margin-left: 1em;">in milk, <a href='#Page_67'>67</a>;</span><br />
+<span style="margin-left: 1em;">in Swiss cheese, <a href='#Page_167'>167</a>.</span><br />
+<br />
+Gl&auml;sler, <a href='#Page_185'>185</a>.<br />
+<br />
+Gorgonzola cheese, <a href='#Page_180'>180</a>.<br />
+<br />
+Growth of bacteria, essential conditions for, <a href='#Page_4'>4</a>;<br />
+<span style="margin-left: 1em;">in milk, <a href='#Page_46'>46</a>.</span><br />
+<br />
+<br />
+Hair, bacteria on, <a href='#Page_35'>35</a>.<br />
+<br />
+Heat, influence on bacterial growth, <a href='#Page_8'>8</a>.<br />
+<br />
+Heated milk:<br />
+<span style="margin-left: 1em;">characteristics of, <a href='#Page_109'>109</a>;</span><br />
+<span style="margin-left: 1em;">action toward rennet, <a href='#Page_112'>112</a>;</span><br />
+<span style="margin-left: 1em;">body, <a href='#Page_110'>110</a>;</span><br />
+<span style="margin-left: 1em;">digestibility, <a href='#Page_111'>111</a>;</span><br />
+<span style="margin-left: 1em;">fermentative changes, <a href='#Page_111'>111</a>;</span><br />
+<span style="margin-left: 1em;">flavor, <a href='#Page_110'>110</a>;</span><br />
+<span style="margin-left: 1em;">hydrogen peroxid test in, <a href='#Page_23'>23</a>;</span><br />
+<span style="margin-left: 1em;">Storch's test, <a href='#Page_23'>23</a>.</span><br />
+<br />
+Hygienic milk, bacteria in, <a href='#Page_45'>45</a>.<br />
+<br />
+<br />
+Infection of milk:<br />
+<span style="margin-left: 1em;">animal, <a href='#Page_34'>34</a>;</span><br />
+<span style="margin-left: 1em;">dairy utensils, <a href='#Page_21'>21</a>;</span><br />
+<span style="margin-left: 1em;">fore milk, <a href='#Page_28'>28</a>;</span><br />
+<span style="margin-left: 1em;">milker, <a href='#Page_36'>36</a>.</span><br />
+<br />
+Isolation of bacteria, methods of, <a href='#Page_14'>14</a>.<br />
+<br />
+<br />
+Kephir, <a href='#Page_72'>72</a>.<br />
+<br />
+Koumiss, <a href='#Page_72'>72</a>.<br />
+<br />
+<br />
+Lactic acid:<br />
+<span style="margin-left: 1em;">fermentation in milk, <a href='#Page_63'>63</a>;</span><br />
+<span style="margin-left: 1em;">theory in cheese-curing, <a href='#Page_174'>174</a>.</span><br />
+<br />
+Lange-wei, <a href='#Page_72'>72</a>.<br />
+<br />
+Lardy butter, <a href='#Page_157'>157</a>.<br />
+<br />
+Light, action on bacteria, <a href='#Page_9'>9</a>.<br />
+<br />
+<br />
+Manure, bacteria in, <a href='#Page_33'>33</a>.<br />
+<br />
+Methods:<br />
+<span style="margin-left: 1em;">of isolation, <a href='#Page_14'>14</a>;</span><br />
+<span style="margin-left: 1em;">culture, <a href='#Page_15'>15</a>.</span><br />
+<br />
+<i>Micrococcus casei amari</i>, <a href='#Page_189'>189</a>.<br />
+<br />
+Microscope, use of, <a href='#Page_17'>17</a>.<br />
+<br />
+Milk:<br />
+<span style="margin-left: 1em;">a bacterial food medium, <a href='#Page_19'>19</a>;</span><br />
+<span style="margin-left: 1em;">bacteria in, <a href='#Page_48'>48</a>.</span><br />
+<span style="margin-left: 1em;">Disease organisms in:</span><br />
+<span style="margin-left: 2em;">anthrax, <a href='#Page_94'>94</a>;</span><br />
+<span style="margin-left: 2em;">cholera, <a href='#Page_98'>98</a>;</span><br />
+<span style="margin-left: 2em;">diphtheria, <a href='#Page_99'>99</a>;</span><br />
+<span style="margin-left: 2em;">foot and mouth disease, <a href='#Page_93'>93</a>;</span><br />
+<span style="margin-left: 2em;">poisonous, <a href='#Page_101'>101</a>;</span><br />
+<span style="margin-left: 2em;">ptomaines, <a href='#Page_101'>101</a>;</span><br />
+<span style="margin-left: 2em;">scarlet fever, <a href='#Page_99'>99</a>;</span><br />
+<span style="margin-left: 2em;">tuberculosis, <a href='#Page_84'>84</a>;</span><br />
+<span style="margin-left: 2em;">typhoid fever, <a href='#Page_98'>98</a>.</span><br />
+<span style="margin-left: 1em;">Contamination, <a href='#Page_20'>20</a>:</span><br />
+<span style="margin-left: 2em;">from air, <a href='#Page_42'>42</a>;</span><br />
+<span style="margin-left: 2em;">from animal odors, <a href='#Page_55'>55</a>;</span><br />
+<span style="margin-left: 2em;">dirt, <a href='#Page_34'>34</a>;</span><br />
+<span style="margin-left: 2em;">distinction between bacterial and non-bacterial, <a href='#Page_57'>57</a>;</span><br />
+<span style="margin-left: 2em;">fore milk, <a href='#Page_28'>28</a>;</span><br />
+<span style="margin-left: 2em;">infection in factory, <a href='#Page_59'>59</a>;</span><br />
+<span style="margin-left: 2em;">milker, <a href='#Page_36'>36</a>;</span><br />
+<span style="margin-left: 2em;">relative importance of various kinds, <a href='#Page_43'>43</a>;</span><br />
+<span style="margin-left: 2em;">utensils, <a href='#Page_21'>21</a>.</span><br />
+<br />
+Milk fermentations:<br />
+<span style="margin-left: 1em;">alcoholic, <a href='#Page_72'>72</a>;</span><br />
+<span style="margin-left: 1em;">bitter, <a href='#Page_72'>72</a>;</span><br />
+<span style="margin-left: 1em;">bloody, <a href='#Page_74'>74</a>;</span><br />
+<span style="margin-left: 1em;">blue, <a href='#Page_74'>74</a>;</span><br />
+<span style="margin-left: 1em;">butyric acid, <a href='#Page_69'>69</a>;</span><br />
+<span style="margin-left: 1em;">gassy, <a href='#Page_66'>66</a>, <a href='#Page_167'>167</a>;</span><br />
+<span style="margin-left: 1em;">kephir, <a href='#Page_72'>72</a>;</span><br />
+<span style="margin-left: 1em;">koumiss, <a href='#Page_72'>72</a>;</span><br />
+<span style="margin-left: 1em;">lactic acid, <a href='#Page_63'>63</a>;</span><br />
+<span style="margin-left: 1em;">red, <a href='#Page_72'>72</a>;</span><br />
+<span style="margin-left: 1em;">ropy, <a href='#Page_69'>69</a>;</span><br />
+<span style="margin-left: 1em;">slimy, <a href='#Page_69'>69</a>;</span><br />
+<span style="margin-left: 1em;">soapy, <a href='#Page_74'>74</a>;</span><br />
+<span style="margin-left: 1em;">souring, <a href='#Page_63'>63</a>;</span><br />
+<span style="margin-left: 1em;">sweet curdling, <a href='#Page_67'>67</a>;</span><br />
+<span style="margin-left: 1em;">tests for, <a href='#Page_76'>76</a>;</span><br />
+<span style="margin-left: 1em;">treatment of, <a href='#Page_75'>75</a>;</span><br />
+<span style="margin-left: 1em;">yellow, <a href='#Page_75'>75</a>.</span><br />
+<br />
+Milk, heated:<br />
+<span style="margin-left: 1em;">action towards rennet, <a href='#Page_112'>112</a>;</span><br />
+<span style="margin-left: 1em;">digestibility, <a href='#Page_111'>111</a>;</span><br />
+<span style="margin-left: 1em;">flavor of, <a href='#Page_110'>110</a>;</span><br />
+<span style="margin-left: 1em;">fermentative changes in, <a href='#Page_111'>111</a>;</span><br />
+<span style="margin-left: 1em;">hydrogen peroxid test, <a href='#Page_110'>110</a>.</span><br />
+<br />
+Milking machines, influence of, on germ content, <a href='#Page_37'>37</a>.<br />
+<br />
+Milk preservation:<br />
+<span style="margin-left: 1em;">chemical agents in, <a href='#Page_106'>106</a>;</span><br />
+<span style="margin-left: 1em;">condensation, <a href='#Page_107'>107</a>;</span><br />
+<span style="margin-left: 1em;">freezing, <a href='#Page_108'>108</a>;</span><br />
+<span style="margin-left: 1em;">heat, <a href='#Page_108'>108</a>;</span><br />
+<span style="margin-left: 1em;">pasteurization, <a href='#Page_113'>113</a>;</span><br />
+<span style="margin-left: 1em;">sterilization, <a href='#Page_112'>112</a>.</span><br />
+<br />
+Milk-sugar as bacterial food, <a href='#Page_19'>19</a>.<br />
+<br />
+Mold, in butter, <a href='#Page_158'>158</a>;<br />
+<span style="margin-left: 1em;">in cheese, <a href='#Page_191'>191</a>.</span><br />
+<br />
+Mottled cheese, <a href='#Page_189'>189</a>.<br />
+<br />
+<br />
+"Nissler" cheese, <a href='#Page_185'>185</a>.<br />
+<br />
+<br />
+Odors, direct absorption of, in milk, <a href='#Page_55'>55</a>.<br />
+<br />
+<i>Oidium lactis</i>, <a href='#Page_159'>159</a>.<br />
+<br />
+Oily butter, <a href='#Page_158'>158</a>.<br />
+<br />
+<br />
+Pasteurization of milk;<br />
+<span style="margin-left: 1em;">acid test in, <a href='#Page_128'>128</a>;</span><br />
+<span style="margin-left: 1em;">bacteriological study of, <a href='#Page_124'>124</a>, <a href='#Page_126'>126</a>, <a href='#Page_149'>149</a>;</span><br />
+<span style="margin-left: 1em;">for butter, <a href='#Page_147'>147</a>;</span><br />
+<span style="margin-left: 1em;">for cheese, <a href='#Page_162'>162</a>;</span><br />
+<span style="margin-left: 1em;">for direct use, <a href='#Page_113'>113</a>;</span><br />
+<span style="margin-left: 1em;">of skim milk, <a href='#Page_25'>25</a>;</span><br />
+<span style="margin-left: 1em;">details of, <a href='#Page_128'>128</a>;</span><br />
+<span style="margin-left: 1em;">temperature and time limit in, <a href='#Page_118'>118</a>.</span><br />
+<br />
+Pasteurizing apparatus:<br />
+<span style="margin-left: 1em;">continuous flow, <a href='#Page_122'>122</a>;</span><br />
+<span style="margin-left: 1em;">coolers, <a href='#Page_131'>131</a>;</span><br />
+<span style="margin-left: 1em;">Danish, <a href='#Page_123'>123</a>;</span><br />
+<span style="margin-left: 1em;">domestic, <a href='#Page_119'>119</a>;</span><br />
+<span style="margin-left: 1em;">Farrington, <a href='#Page_122'>122</a>;</span><br />
+<span style="margin-left: 1em;">intermittant flow, <a href='#Page_121'>121</a>;</span><br />
+<span style="margin-left: 1em;">Miller, <a href='#Page_122'>122</a>;</span><br />
+<span style="margin-left: 1em;">Potts, <a href='#Page_121'>121</a>;</span><br />
+<span style="margin-left: 1em;">regenerator, <a href='#Page_122'>122</a>;</span><br />
+<span style="margin-left: 1em;">Reid, <a href='#Page_126'>126</a>;</span><br />
+<span style="margin-left: 1em;">Russell, <a href='#Page_121'>121</a>;</span><br />
+<span class='pagenum'><a name="Page_198" id="Page_198">[Pg 198]</a></span><span style="margin-left: 1em;">testing rate of flow, <a href='#Page_124'>124</a>.</span><br />
+<br />
+<i>Penicillium glaucum</i>, <a href='#Page_159'>159</a>, <a href='#Page_180'>180</a>, <a href='#Page_190'>190</a>.<br />
+<br />
+Pepsin, <a href='#Page_10'>10</a>.<br />
+<br />
+Physical changes in cheese-ripening, <a href='#Page_165'>165</a>.<br />
+<br />
+Poisonous bacteria:<br />
+<span style="margin-left: 1em;">in cheese, <a href='#Page_192'>192</a>;</span><br />
+<span style="margin-left: 1em;">in milk, <a href='#Page_100'>100</a>, <a href='#Page_101'>101</a>.</span><br />
+<br />
+Preservaline, <a href='#Page_167'>167</a>.<br />
+<br />
+Preservation of milk:<br />
+<span style="margin-left: 1em;">by exclusion, <a href='#Page_103'>103</a>;</span><br />
+<span style="margin-left: 1em;">chemical agents, <a href='#Page_106'>106</a>;</span><br />
+<span style="margin-left: 1em;">condensing, <a href='#Page_107'>107</a>;</span><br />
+<span style="margin-left: 1em;">filtration, <a href='#Page_38'>38</a>;</span><br />
+<span style="margin-left: 1em;">freezing, <a href='#Page_108'>108</a>;</span><br />
+<span style="margin-left: 1em;">pasteurization, <a href='#Page_112'>112</a>;</span><br />
+<span style="margin-left: 1em;">physical agents, <a href='#Page_107'>107</a>;</span><br />
+<span style="margin-left: 1em;">sterilization, <a href='#Page_112'>112</a>.</span><br />
+<br />
+Ptomaine poisoning, <a href='#Page_101'>101</a>.<br />
+<br />
+Pure cultures, <a href='#Page_15'>15</a>.<br />
+<br />
+Pure culture starters:<br />
+<span style="margin-left: 1em;">advantages of, <a href='#Page_144'>144</a>;</span><br />
+<span style="margin-left: 1em;">characteristics of, <a href='#Page_145'>145</a>;</span><br />
+<span style="margin-left: 1em;">home-made cultures compared with, <a href='#Page_146'>146</a>;</span><br />
+<span style="margin-left: 1em;">propagation of, <a href='#Page_151'>151</a>.</span><br />
+<br />
+Putrid cheese, <a href='#Page_190'>190</a>;<br />
+<span style="margin-left: 1em;">butter, <a href='#Page_156'>156</a>.</span><br />
+<br />
+<br />
+Rancidity in butter, <a href='#Page_155'>155</a>.<br />
+<br />
+Red milk, <a href='#Page_74'>74</a>.<br />
+<br />
+Rennet:<br />
+<span style="margin-left: 1em;">action in heated milk, <a href='#Page_112'>112</a>;</span><br />
+<span style="margin-left: 1em;">bacteria in, <a href='#Page_163'>163</a>;</span><br />
+<span style="margin-left: 1em;">influence of, on cheese-ripening, <a href='#Page_177'>177</a>.</span><br />
+<br />
+Restoration of consistency in pasteurized cream, <a href='#Page_132'>132</a>.<br />
+<br />
+Ripening of cheese:<br />
+<span style="margin-left: 1em;">moldy cheese, <a href='#Page_180'>180</a>;</span><br />
+<span style="margin-left: 1em;">soft cheese, <a href='#Page_181'>181</a>.</span><br />
+<span style="margin-left: 1em;">Of cream, <a href='#Page_136'>136</a>;</span><br />
+<span style="margin-left: 2em;">artificial starters, <a href='#Page_143'>143</a>;</span><br />
+<span style="margin-left: 2em;">natural starters, <a href='#Page_142'>142</a>;</span><br />
+<span style="margin-left: 2em;">principles of pure culture starters in, <a href='#Page_143'>143</a>.</span><br />
+<br />
+Ropy milk, <a href='#Page_69'>69</a>.<br />
+<br />
+Roquefort cheese, <a href='#Page_180'>180</a>.<br />
+<br />
+Rusty spot in cheese, <a href='#Page_190'>190</a>.<br />
+<br />
+Rusty cans: effect of, on acidity, <a href='#Page_53'>53</a>.<br />
+<br />
+<br />
+Sanitary milk, <a href='#Page_45'>45</a>, <a href='#Page_104'>104</a>.<br />
+<br />
+Sanitary pails, <a href='#Page_41'>41</a>.<br />
+<br />
+Scarlet fever in milk, <a href='#Page_99'>99</a>.<br />
+<br />
+Separator slime:<br />
+<span style="margin-left: 1em;">bacteria in, <a href='#Page_39'>39</a>;</span><br />
+<span style="margin-left: 1em;">tubercle bacillus in, <a href='#Page_93'>93</a>.</span><br />
+<br />
+Scalded layer, resistance of bacteria in, <a href='#Page_91'>91</a>.<br />
+<br />
+Skim-milk, a distributor of disease, <a href='#Page_96'>96</a>.<br />
+<br />
+Slimy milk, <a href='#Page_69'>69</a>.<br />
+<br />
+Soapy milk, <a href='#Page_74'>74</a>.<br />
+<br />
+Soft cheese, ripening of, <a href='#Page_186'>186</a>.<br />
+<br />
+Sources of contamination in milk:<br />
+<span style="margin-left: 1em;">barn air, <a href='#Page_42'>42</a>;</span><br />
+<span style="margin-left: 1em;">dairy utensils, <a href='#Page_21'>21</a>;</span><br />
+<span style="margin-left: 1em;">dirt from animals, <a href='#Page_34'>34</a>;</span><br />
+<span style="margin-left: 1em;">factory cans, <a href='#Page_25'>25</a>;</span><br />
+<span style="margin-left: 1em;">fore-milk, <a href='#Page_28'>28</a>;</span><br />
+<span style="margin-left: 1em;">milker, <a href='#Page_36'>36</a>.</span><br />
+<br />
+Souring of milk, <a href='#Page_63'>63</a>.<br />
+<br />
+Spirillum, definition of, <a href='#Page_2'>2</a>.<br />
+<br />
+Spores, <a href='#Page_3'>3</a>.<br />
+<br />
+Starters:<br />
+<span style="margin-left: 1em;">in cheese-making, <a href='#Page_161'>161</a>;</span><br />
+<span style="margin-left: 1em;">in butter-making, <a href='#Page_142'>142</a>;</span><br />
+<span style="margin-left: 1em;">propagation of, <a href='#Page_151'>151</a>;</span><br />
+<span style="margin-left: 1em;">pure cultures in cream-ripening, <a href='#Page_143'>143</a>.</span><br />
+<br />
+Sterilization of milk, <a href='#Page_112'>112</a>.<br />
+<br />
+<i>Streptococcus Hollandicus</i>, <a href='#Page_72'>72</a>, <a href='#Page_162'>162</a>.<br />
+<br />
+Stilton cheese, <a href='#Page_181'>181</a>.<br />
+<br />
+Storch's test, <a href='#Page_23'>23</a>.<br />
+<br />
+Sulfur as a disinfectant, <a href='#Page_81'>81</a>.<br />
+<br />
+Sweet curdling milk, <a href='#Page_68'>68</a>.<br />
+<br />
+Sweet flavor in cheese, <a href='#Page_188'>188</a>.<br />
+<br />
+Swiss cheese, <a href='#Page_177'>177</a>;<br />
+<span style="margin-left: 1em;">gassy fermentations in, <a href='#Page_24'>24</a>, <a href='#Page_185'>185</a>.</span><br />
+<br />
+<br />
+Taints, absorption of, <a href='#Page_55'>55</a>.<br />
+<br />
+Taints, bacterial vs. physical, <a href='#Page_58'>58</a>.<br />
+<br />
+Taints in milk, absorption of, <a href='#Page_55'>55</a>.<br />
+<br />
+Taints, use of starters in overcoming, <a href='#Page_79'>79</a>.<br />
+<br />
+Taints in butter:<br />
+<span style="margin-left: 1em;">putrid, <a href='#Page_156'>156</a>;</span><br />
+<span style="margin-left: 1em;">rancidity, <a href='#Page_155'>155</a>;</span><br />
+<span style="margin-left: 1em;">turnip flavor, <a href='#Page_157'>157</a>.</span><br />
+<br />
+Tallowy butter, <a href='#Page_157'>157</a>.<br />
+<br />
+Temperature:<br />
+<span style="margin-left: 1em;">effect on bacterial development, <a href='#Page_6'>6</a>, <a href='#Page_48'>48</a>;</span><br />
+<span style="margin-left: 1em;">effect of low, <a href='#Page_108'>108</a>;</span><br />
+<span style="margin-left: 1em;">effect of high, <a href='#Page_108'>108</a>;</span><br />
+<span style="margin-left: 1em;">and time limit in milk pasteurization, <a href='#Page_113'>113</a>.</span><br />
+<br />
+Tests for milk:<br />
+<span style="margin-left: 1em;">fermentation, <a href='#Page_76'>76</a>;</span><br />
+<span style="margin-left: 1em;">Storch's, <a href='#Page_23'>23</a>;</span><br />
+<span style="margin-left: 1em;">acid, <a href='#Page_52'>52</a>.</span><br />
+<br />
+Theories in cheese-curing:<br />
+<span style="margin-left: 1em;">digestive, <a href='#Page_171'>171</a>;</span><br />
+<span style="margin-left: 1em;">galactase, <a href='#Page_175'>175</a>, <a href='#Page_177'>177</a>;</span><br />
+<span style="margin-left: 1em;">lactic acid, <a href='#Page_174'>174</a>.</span><br />
+<br />
+<span class='pagenum'><a name="Page_199" id="Page_199">[Pg 199]</a></span>Trypsin, <a href='#Page_10'>10</a>.<br />
+<br />
+Tubercle bacillus:<br />
+<span style="margin-left: 1em;">in milk, <a href='#Page_88'>88</a>;</span><br />
+<span style="margin-left: 1em;">in separator slime, <a href='#Page_93'>93</a>;</span><br />
+<span style="margin-left: 1em;">thermal death limits, <a href='#Page_117'>117</a>.</span><br />
+<br />
+Tuberculin test, <a href='#Page_86'>86</a>.<br />
+<br />
+Tuberculosis, bovine, <a href='#Page_84'>84</a>.<br />
+<br />
+Turnip flavor in butter, <a href='#Page_157'>157</a>.<br />
+<br />
+Typhoid fever, <a href='#Page_98'>98</a>.<br />
+<br />
+Tyrogen, <a href='#Page_162'>162</a>.<br />
+<br />
+Tyrotoxicon, <a href='#Page_101'>101</a>, <a href='#Page_190'>190</a>.<br />
+<br />
+<br />
+Udder:<br />
+<span style="margin-left: 1em;">artificial introduction of bacteria into, <a href='#Page_32'>32</a>;</span><br />
+<span style="margin-left: 1em;">milk germ-free in, <a href='#Page_19'>19</a>;</span><br />
+<span style="margin-left: 1em;">infection of, <a href='#Page_28'>28</a>;</span><br />
+<span style="margin-left: 1em;">washing, <a href='#Page_89'>89</a>;</span><br />
+<span style="margin-left: 1em;">tuberculosis in, <a href='#Page_87'>87</a>.</span><br />
+<br />
+<br />
+Viscogen, <a href='#Page_132'>132</a>.<br />
+<br />
+<br />
+Water: as a source of infection, <a href='#Page_61'>61</a>.<br />
+<br />
+Whey, pollution of vats, <a href='#Page_23'>23</a>;<br />
+<span style="margin-left: 1em;">method of preserving, <a href='#Page_25'>25</a>;</span><br />
+<span style="margin-left: 1em;">treatment of, in vats, <a href='#Page_25'>25</a>.</span><br />
+<br />
+Whitewash, <a href='#Page_81'>81</a>.<br />
+<br />
+Wisconsin curd test, <a href='#Page_76'>76</a>.<br />
+<br />
+<br />
+Yeasts:<br />
+<span style="margin-left: 1em;">alcoholic ferments in milk, <a href='#Page_73'>73</a>;</span><br />
+<span style="margin-left: 1em;">fruity flavor in cheese, <a href='#Page_186'>186</a>;</span><br />
+<span style="margin-left: 1em;">gassy due to yeasts, <a href='#Page_186'>186</a>;</span><br />
+<span style="margin-left: 1em;">in bitter cheese, <a href='#Page_189'>189</a>;</span><br />
+<span style="margin-left: 1em;">in canned butter, <a href='#Page_159'>159</a>;</span><br />
+<span style="margin-left: 1em;">kephir, <a href='#Page_72'>72</a>.</span><br />
+</p>
+
+
+
+
+
+
+
+
+
+<pre>
+
+
+
+
+
+End of the Project Gutenberg EBook of Outlines of Dairy Bacteriology, 8th
+edition, by H. L. Russell
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@@ -0,0 +1,6976 @@
+The Project Gutenberg EBook of Outlines of Dairy Bacteriology, 8th edition, by
+H. L. Russell
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
+
+Title: Outlines of Dairy Bacteriology, 8th edition
+       A Concise Manual for the Use of Students in Dairying
+
+Author: H. L. Russell
+
+Release Date: January 11, 2009 [EBook #27778]
+
+Language: English
+
+Character set encoding: ASCII
+
+*** START OF THIS PROJECT GUTENBERG EBOOK DAIRY BACTERIOLOGY, 8TH EDITION ***
+
+
+
+
+Produced by Mark C. Orton, Linda McKeown, Josephine Paolucci
+and the Online Distributed Proofreading Team at
+https://www.pgdp.net.
+
+
+
+
+
+
+
+OUTLINES
+
+OF
+
+DAIRY BACTERIOLOGY
+
+A CONCISE MANUAL FOR THE USE OF STUDENTS IN DAIRYING
+
+BY
+
+H. L. RUSSELL
+
+DEAN OF THE COLLEGE OF AGRICULTURE, UNIVERSITY OF WISCONSIN
+
+EIGHTH EDITION
+THOROUGHLY REVISED
+
+MADISON, WISCONSIN
+H. L. RUSSELL
+1907
+
+
+Copyrighted 1905
+BY
+H. L. RUSSELL
+
+
+STATE JOURNAL PRINTING COMPANY,
+Printers And Stereotypers,
+Madison, Wis.
+
+Transcriber's note:
+
+For Text: A word surrounded by a cedilla such as ~this~ signifies that
+the word is bolded in the text. A word surrounded by underscores like
+_this_ signifies the word is italics in the text. For numbers and
+equations, underscores before bracketed numbers in equations denote a
+subscript.
+
+Minor typos have been corrected.
+
+
+
+
+PREFACE.
+
+
+Knowledge in dairying, like all other technical industries, has grown
+mainly out of experience. Many facts have been learned by observation,
+but the _why_ of each is frequently shrouded in mystery.
+
+Modern dairying is attempting to build its more accurate knowledge upon
+a broader and surer foundation, and in doing this is seeking to
+ascertain the cause of well-established processes. In this, bacteriology
+is playing an important role. Indeed, it may be safely predicted that
+future progress in dairying will, to a large extent, depend upon
+bacteriological research. As Fleischmann, the eminent German dairy
+scientist, says: "The gradual abolition of uncertainty surrounding dairy
+manufacture is the present important duty which lies before us, and its
+solution can only be effected by bacteriology."
+
+It is therefore natural that the subject of Dairy Bacteriology has come
+to occupy an important place in the curriculum of almost every Dairy
+School. An exposition of its principles is now recognized as an integral
+part of dairy science, for modern dairy practice is rapidly adopting the
+methods that have been developed as the result of bacteriological study.
+The rapid development of the subject has necessitated a frequent
+revision of this work, and it is gratifying to the writer that the
+attempt which has been made to keep these Outlines abreast of
+bacteriological advance has been appreciated by students of dairying.
+
+While the text is prepared more especially for the practical dairy
+operator who wishes to understand the principles and reasons underlying
+his art, numerous references to original investigations have been added
+to aid the dairy investigator who wishes to work up the subject more
+thoroughly.
+
+My acknowledgments are due to the following for the loan of
+illustrations: Wisconsin Agricultural Experiment Station; Creamery
+Package Mfg. Co., Chicago, Ill.; and A. H. Reid, Philadelphia, Pa.
+
+        H. L. Russell.
+           University of Wisconsin.
+
+
+
+
+CONTENTS.
+
+
+CHAPTER I. Structure of the bacteria and conditions governing
+their development and distribution                                   1
+
+CHAPTER II. Methods of studying bacteria                            13
+
+CHAPTER III. Contamination of milk                                  19
+
+CHAPTER IV. Fermentations in milk and their treatment               62
+
+CHAPTER V. Relation of disease-bacteria to milk                     82
+
+  Diseases transmissible from animal to man
+  through diseased milk                                             84
+
+  Diseases transmissible to man through infection
+  of milk after withdrawal                                          94
+
+CHAPTER VI. Preservation of milk for commercial purposes           102
+
+CHAPTER VII. Bacteria and butter making                            134
+
+  Bacterial defects in butter                                      156
+
+CHAPTER VIII. Bacteria in cheese                                   160
+
+  Influence Of bacteria in normal cheese processes                 160
+
+  Influence of bacteria in abnormal cheese processes               182
+
+
+
+
+CHAPTER I.
+
+STRUCTURE OF THE BACTERIA AND CONDITIONS GOVERNING THEIR DEVELOPMENT AND
+DISTRIBUTION.
+
+
+Before one can gain any intelligent conception of the manner in which
+bacteria affect dairying, it is first necessary to know something of the
+life history of these organisms in general, how they live, move and
+react toward their environment.
+
+~Nature of Bacteria.~ Toadstools, smuts, rusts and mildews are known to
+even the casual observer, because they are of evident size. Their
+plant-like nature can be more readily understood from their general
+structure and habits of life. The bacteria, however, are so small, that
+under ordinary conditions, they only become evident to our unaided
+senses by the by-products of their activity.
+
+When Leeuwenhoek (pronounced Lave-en-hake) in 1675 first discovered
+these tiny, rapidly-moving organisms he thought they were animals.
+Indeed, under a microscope, many of them bear a close resemblance to
+those minute worms found in vinegar that are known as "vinegar-eels."
+The idea that they belonged to the animal kingdom continued to hold
+ground until after the middle of the nineteenth century; but with the
+improvement in microscopes, a more thorough study of these tiny
+structures was made possible, and their vegetable nature demonstrated.
+The bacteria as a class are separated from the fungi mainly by their
+method of growth; from the lower algae by the absence of chlorophyll,
+the green coloring matter of vegetable organisms.
+
+~Structure of bacteria.~ So far as structure is concerned the bacteria
+stand on the lowest plane of vegetable life. The single individual is
+composed of but a single cell, the structure of which does not differ
+essentially from that of many of the higher types of plant life. It is
+composed of a protoplasmic body which is surrounded by a thin membrane
+that separates it from neighboring cells that are alike in form and
+size.
+
+~Form and size.~ When a plant is composed of a single cell but little
+difference in form is to be expected. While there are intermediate
+stages that grade insensibly into each other, the bacteria may be
+grouped into three main types, so far as form is concerned. These are
+spherical, elongated, and spiral, and to these different types are given
+the names, respectively, _coccus_, _bacillus_ and _spirillum_ (plural,
+_cocci_, _bacilli_, _spirilla_) (fig. 1). A ball, a short rod, and a
+corkscrew serve as convenient models to illustrate these different
+forms.
+
+[Illustration: FIG. 1. Different forms of bacteria. _a_, _b_, _c_,
+represent different types as to form: _a_, coccus, _b_, bacillus, _c_,
+spirillum; _d_, diplococcus or twin coccus; _e_, staphylococcus or
+cluster coccus; _f_ and _g_, different forms of bacilli, _g_ shows
+internal endospores within cell; _h_ and _i_, bacilli with motile organs
+(cilia).]
+
+In size, the bacteria are the smallest organisms that are known to
+exist. Relatively there is considerable difference in size between the
+different species, yet in absolute amount this is so slight as to
+require the highest powers of the microscope to detect it. As an average
+diameter, one thirty-thousandth of an inch may be taken. It is difficult
+to comprehend such minute measurements, but if a hundred individual
+germs could be placed side by side, their total thickness would not
+equal that of a single sheet of paper upon which this page is printed.
+
+~Manner of Growth.~ As the cell increases in size as a result of growth,
+it elongates in one direction, and finally a new cell wall is formed,
+dividing the so-called mother-cell into two, equal-sized daughter-cells.
+This process of cell division, known as _fission_, is continued until
+growth ceases and is especially characteristic of bacteria.
+
+~Cell Arrangement.~ If fission goes on in the same plane continually, it
+results in the formation of a cell-row. A coccus forming such a chain of
+cells is called _strepto-coccus_ (chain-coccus). If only two cells
+cohere, it is called a _diplo-coccus_ (twin-coccus). If the second cell
+division plane is formed at right angles to the first, a _cell surface_
+or _tetrad_ is formed. If growth takes place in three dimensions of
+space, a _cell mass_ or _sarcina_ is produced. Frequently, these cell
+aggregates cohere so tenaciously that this arrangement is of value in
+distinguishing different species.
+
+~Spores.~ Some bacteria possess the property of forming _spores_ within
+the mother cell (called _endospores_, fig. 1g) that are analogous in
+function to the seeds of higher plants and spores of fungi. By means of
+these structures which are endowed with greater powers of resistance
+than the vegetating cell, the organism is able to protect itself from
+the effect of an unfavorable environment. Many of the bacilli form
+endospores but the cocci do not. It is these spore forms that make it
+so difficult to thoroughly sterilize milk.
+
+~Movement.~ Many bacteria are unable to move from place to place. They
+have, however, a vibrating movement known as the _Brownian_ motion that
+is purely physical. Many other kinds are endowed with powers of
+locomotion. Motion is produced by means of fine thread-like processes of
+protoplasm known as _cilia_ (sing. _cilium_) that are developed on the
+outer surface of the cell. By means of the rapid vibration of these
+organs, the cell is propelled through the medium. Nearly all cocci are
+immotile, while the bacilli may or may not be. These cilia are so
+delicate that it requires special treatment to demonstrate their
+presence.
+
+~Classification.~ In classifying or arranging the different members of any
+group of living objects, certain similarities and dissimilarities must
+be considered. These are usually those that pertain to the structure and
+form, as such are regarded as most constant. With the bacteria these
+differences are so slight that they alone do not suffice to distinguish
+distinctly one species from another. As far as these characters can be
+used, they are taken, but in addition, many characteristics of a
+physiological nature are added. The way that the organism grows in
+different kinds of cultures, the by-products produced in different
+media, and effect on the animal body when injected into the same are
+also used as data in distinguishing one species from another.
+
+~Conditions favoring bacterial growth.~ The bacteria, in common with all
+other living organisms are affected by external conditions, either
+favorably or unfavorably. Certain conditions must prevail before
+development can occur. Thus, the organism must be supplied with an
+adequate and suitable food supply and with moisture. The temperature
+must also range between certain limits, and finally, the oxygen
+requirements of the organism must be considered.
+
+~Food supply.~ Most bacteria are capable of living on dead, inert, organic
+matter, such as meats, milk and vegetable material, in which case, they
+are known as _saprophytes_. In contradistinction to this class is a
+smaller group known as _parasites_, which derive their nourishment from
+the living tissues of animals or plants. The first group comprise by far
+the larger number of known organisms which are concerned for the most
+part in the decomposition of organic matter. The parasitic group
+includes those which are the cause of various communicable diseases.
+Between these two groups there is no sharp line of division, and in some
+cases, certain species possess the faculty of growing either as
+parasites or saprophytes, in which case they are known as _facultative_
+parasites or saprophytes.
+
+The great majority of bacteria of interest in dairying belong to the
+saprophytic class; only those species capable of infecting milk through
+the development of disease in the animal are parasites in the strict
+sense of the term. Most disease-producing species, as diphtheria or
+typhoid fever, while parasitic in man lead a saprophytic method of life
+so far as their relation to milk is concerned.
+
+Bacteria require for their growth, nitrogen, hydrogen, carbon, oxygen,
+together with a limited amount of mineral matter. The nitrogen and
+carbon are most available in the form of organic compounds, such as
+albuminous material. Carbon in the form of carbohydrates, as sugar or
+starch, is most readily attacked by bacteria.
+
+Inasmuch as the bacteria are plant-cells, they must imbibe their food
+from material in solution. They are capable of living on solid
+substances, but in such cases, the food elements must be rendered
+soluble, before they can be appropriated. If nutritive liquids are too
+highly concentrated, as in the case of syrups and condensed milk,
+bacteria cannot grow therein, although all the necessary ingredients may
+be present. Generally, bacteria prefer a neutral or slightly alkaline
+medium, rather than one of acid reaction; but there are numerous
+exceptions to this general rule, especially among the bacteria found in
+milk.
+
+~Temperature.~ Growth of bacteria can only occur within certain
+temperature limits, the extremes of which are designated as the
+_minimum_ and _maximum_. Below and above these respective limits, life
+may be retained in the cell for a time, but actual cell-multiplication
+is stopped. Somewhere between these two cardinal temperature points, and
+generally nearer the maximum limit is the most favorable temperature for
+growth, known as the _optimum_. The temperature zone of most dairy
+bacteria in which growth occurs ranges from 40 deg.-45 deg. F. to somewhat
+above blood-heat, 105 deg.-110 deg. F., the optimum being from 80 deg.-95
+deg. F. Many parasitic species, because of their adaptation to the bodies
+of warm-blooded animals, generally have a narrower range, and a higher
+optimum, usually approximating the blood heat (98 deg.-99 deg. F). The
+broader growth limits of bacteria in comparison with other kinds of life
+explain why these organisms are so widely distributed in nature.
+
+~Air supply.~ Most bacteria require as do the green plants and animal
+life, the free oxygen of the air for their respiration. These are called
+_aerobic_. Some species, however, and some yeasts as well possess the
+peculiar property of taking the oxygen which they need from organic
+compounds such as sugar, etc., and are therefore able to live and grow
+under conditions where the atmospheric air is excluded. These are known
+as _anaerobic_. While some species grow strictly under one condition or
+the other, and hence are _obligate_ aerobes or anaerobes, others possess
+the ability of growing under either condition and are known as
+_facultative_ or optional forms. The great majority of milk bacteria are
+either obligate or facultative aerobes.
+
+~Rate of growth.~ The rate of bacterial development is naturally very much
+affected by external conditions, food supply and temperature exerting
+the most influence. In the neighborhood of the freezing point but little
+growth occurs. The rate increases with a rise in temperature until at
+the _optimum_ point, which is generally near the blood heat or slightly
+below (90 deg.-98 deg. F.), a single cell will form two cells in 20 to 30
+minutes. If temperature rises much above blood heat rate of growth is
+lessened and finally ceases. Under ideal conditions, rapidity of growth
+is astounding, but this initially rapid rate of development cannot be
+maintained indefinitely, for growth is soon limited by the accumulation
+of by-products of cell activity. Thus, milk sours rapidly at ordinary
+temperatures until the accumulation of acid checks its development.
+
+~Detrimental effect of external conditions.~ Environmental influences of a
+detrimental character are constantly at work on bacteria, tending to
+repress their development or destroy them. These act much more readily
+on the vegetating cell than on the more resistant spore. A thorough
+knowledge of the effect of these antagonistic forces is essential, for
+it is often by their means that undesirable bacteria may be killed out.
+
+~Effect of cold.~ While it is true that chilling largely prevents
+fermentative action, and actual freezing stops all growth processes,
+still it does not follow that exposure to low temperatures will
+effectually destroy the vitality of bacteria, even in the vegetative
+condition. Numerous non-spore-bearing species remain alive in ice for a
+prolonged period, and recent experiments with liquid air show that even
+a temperature of -310 deg. F. for hours does not effectually kill all
+exposed cells.
+
+~Effect of heat.~ High temperatures, on the other hand, will destroy any
+form of life, whether in the vegetative or latent stage. The temperature
+at which the vitality of the cell is lost is known as the _thermal death
+point_. This limit is not only dependent upon the nature of the
+organism, but varies with the time of exposure and the condition in
+which the heat is applied. In a moist atmosphere the penetrating power
+of heat is great; consequently cell-death occurs at a lower temperature
+than in a dry atmosphere. An increase in time of exposure lowers the
+temperature point at which death occurs.
+
+For vegetating forms the thermal death point of most bacteria ranges
+from 130 deg.-140 deg. F. where the exposure is made for ten minutes which
+is the standard arbitrarily selected. In the spore stage resistance is
+greatly increased, some forms being able to withstand steam at 210
+deg.-212 deg. F. from one to three hours. If dry heat is employed, 260
+deg.-300 deg. F. for an hour is necessary to kill spores. Where steam is
+confined under pressure, a temperature of 230 deg.-240 deg. F. for 15-20
+minutes suffices to kill all spores.
+
+~Drying.~ Spore-bearing bacteria like anthrax withstand drying with
+impunity; even tuberculous material, although not possessing spores
+retains its infectious properties for many months. Most of the dairy
+bacteria do not produce spores, and yet in a dry condition, they retain
+their vitality unimpaired for considerable periods, if they are not
+subjected to other detrimental influences.
+
+~Light.~ Bright sunlight exerts on many species a powerful disinfecting
+action, a few hours being sufficient to destroy all cells that are
+reached by the sun's rays. Even diffused light has a similar effect,
+although naturally less marked. The active rays in this disinfecting
+action are those of the chemical or violet end of the spectrum, and not
+the heat or red rays.
+
+~Influence of chemical substances.~ A great many chemical substances exert
+a more or less powerful toxic action of various kinds of life. Many of
+these are of great service in destroying or holding bacterial growth in
+check. Those that are toxic and result in the death of the cell are
+known as _disinfectants_; those that merely inhibit, or retard growth
+are known as _antiseptics_. All disinfectants must of necessity be
+antiseptic in their action, but not all antiseptics are disinfectants
+even when used in strong doses. Disinfectants have no place in dairy
+work, except to destroy disease bacteria, or preserve milk for
+analytical purposes. Corrosive sublimate or potassium bichromate are
+most frequently used for these purposes. The so-called chemical
+preservatives used to "keep" milk depend for their effect on the
+inhibition of bacterial growth. With a substance so violently toxic as
+formaldehyde (known as formalin, freezene) antiseptic doses are likely
+to be exceeded. In this country most states prohibit the use of these
+substances in milk. Their only function in the dairy should be to check
+fermentative or putrefactive processes outside of milk and so keep the
+air free from taints.
+
+~Products of growth.~ All bacteria in their development form certain more
+or less characteristic by-products. With most dairy bacteria, these
+products are formed from the decomposition of the medium in which the
+bacteria may happen to live. Such changes are known, collectively, as
+fermentations, and are characterised by the production of a large amount
+of by-products, as a result of the development of a relatively small
+amount of cell-life. The souring of milk, the formation of butyric acid,
+the making of vinegar from cider, are all examples of fermentative
+changes.
+
+With many bacteria, especially those that affect proteid matter,
+foul-smelling gases are formed. These are known as putrefactive changes.
+All organic matter, under the action of various organisms, sooner or
+later undergoes decay, and in different stages of these processes,
+acids, alkalies, gases and numerous other products are formed. Many of
+these changes in organic matter occur only when such material is brought
+in direct contact with the living bacterial cell.
+
+In other instances, soluble, non-vital ferments known as _enzyms_ are
+produced by the living cell, which are able to act on organic matter, in
+a medium free from live cells, or under conditions where the activity of
+the cell is wholly suspended. These enzyms are not confined to bacteria
+but are found throughout the animal and plant world, especially in those
+processes that are concerned in digestion. Among the better known of
+these non-vital ferments are rennet, the milk-curdling enzym; diastase
+or ptyalin of the saliva, the starch-converting enzym; pepsin and
+trypsin, the digestive ferments of the animal body.
+
+Enzyms of these types are frequently found among the bacteria and yeasts
+and it is by virtue of this characteristic that these organisms are
+able to break down such enormous quantities of organic matter. Most of
+these enzyms react toward heat, cold and chemical poisons in a manner
+quite similar to the living cells. In one respect they are readily
+differentiated, and that is, that practically all of them are capable of
+producing their characteristic chemical transformations under
+anaesthetic conditions, as in a saturated ether or chloroform
+atmosphere.
+
+~Distribution of bacteria.~ As bacteria possess greater powers of
+resistance than most other forms of life, they are to be found more
+widely distributed than any other type. At the surface of the earth,
+where conditions permit of their growth, they are found everywhere,
+except in the healthy tissues of animals and plants. In the superficial
+soil layers, they exist in myriads, as here they have abundance of
+nourishment. At the depth of several feet however, they diminish rapidly
+in numbers, and in the deeper soil layers, from six to ten feet or more,
+they are not present, because of the unsuitable growth conditions.
+
+The bacteria are found in the air because of their development in the
+soil below. They are unable to grow even in a moist atmosphere, but are
+so readily dislodged by wind currents that over land areas the lower
+strata of the air always contain them. They are more numerous in summer
+than in winter; city air contains larger numbers than country air.
+Wherever dried fecal matter is present, as in barns, the air contains
+many forms.
+
+Water contains generally enough organic matter in solution, so that
+certain types of bacterial life find favorable growth conditions. Water
+in contact with the soil surface takes up many impurities, and is of
+necessity rich in microbes. As the rain water percolates into the soil,
+it loses its germ content, so that the normal ground water, like the
+deeper soil layers, contains practically no bacterial life. Springs
+therefore are relatively deficient in germ life, except as they become
+infected with soil organisms, as the water issues from the soil. Water
+may serve to disseminate certain infectious diseases as typhoid fever
+and cholera among human beings, and a number of animal maladies.
+
+While the inner tissues of healthy animals are free from bacteria, the
+natural passages as the respiratory and digestive tracts, being in more
+direct contact with the exterior, become more readily infected. This is
+particularly true with reference to the intestinal tract, for in the
+undigested residue, bacterial activity is at a maximum. The result is
+that fecal matter contains enormous numbers of organisms so that the
+possibility of pollution of any food medium such as milk with such
+material is sure to introduce elements that seriously affect the quality
+of the product.
+
+
+
+
+CHAPTER II.
+
+METHODS OF STUDYING BACTERIA.
+
+
+~Necessity of bacterial masses for study.~ The bacteria are so extremely
+small that it is impossible to study individual germs separately without
+the aid of first-class microscopes. For this reason, but little advance
+was made in the knowledge of these lower forms of plant life, until the
+introduction of culture methods, whereby a single organism could be
+cultivated and the progeny of this cell increased to such an extent in a
+short course of time, that they would be visible to the unaided eye.
+
+This is done by growing the bacteria in masses on various kinds of food
+media that are prepared for the purpose, but inasmuch as bacteria are so
+universally distributed, it becomes an impossibility to cultivate any
+special form, unless the medium in which they are grown is first freed
+from all pre-existing forms of germ life. To accomplish this, it is
+necessary to subject the nutrient medium to some method of
+sterilization, such as heat or filtration, whereby all life is
+completely destroyed or eliminated. Such material after it has been
+rendered germ-free is kept in sterilized glass tubes and flasks, and is
+protected from infection by cotton stoppers.
+
+~Culture media.~ For culture media, many different substances are
+employed. In fact, bacteria will grow on almost any organic substance
+whether it is solid or fluid, provided the other essential conditions of
+growth are furnished. The food substances that are used for culture
+purposes are divided into two classes; solids and liquids.
+
+Solid media may be either permanently solid like potatoes, or they may
+retain their solid properties only at certain temperatures like gelatin
+or agar. The latter two are of utmost importance in bacteriological
+research, for their use, which was introduced by Koch, permits the
+separation of the different forms that may happen to be in any mixture.
+Gelatin is used advantageously because the majority of bacteria present
+wider differences due to growth upon this medium than upon any other. It
+remains solid at ordinary temperatures, becoming liquid at about 70 deg. 
+F. Agar, a gelatinous product derived from a Japanese sea-weed, has a much
+higher melting point, and can be successfully used, especially with
+those organisms whose optimum growth point is above the melting point of
+gelatin.
+
+Besides these solid media, different liquid substances are extensively
+used, such as beef broth, milk, and infusions of various vegetable and
+animal tissues. Skim-milk is of especial value in studying the milk
+bacteria and may be used in its natural condition, or a few drops of
+litmus solution may be added in order to detect any change in its
+chemical reaction due to the bacteria.
+
+[Illustration: FIG. 2. A gelatin plate culture showing appearance of
+different organisms in a sample of milk. Each mass represents a
+bacterial growth (colony) derived from a single cell. Different forms
+react differently toward the gelatin, some liquefying the same, others
+growing in a restricted mass. _a_, represents a colony of the ordinary
+bread mold; _b_, a liquefying bacterium; _c_, and _d_, solid forms.]
+
+~Methods of isolation.~ Suppose for instance one wishes to isolate the
+different varieties of bacteria found in milk. The method of procedure
+is as follows: Sterile gelatin in glass tubes is melted and cooled down
+so as to be barely warm. To this gelatin which is germ-free a drop of
+milk is added. The gelatin is then gently shaken so as to thoroughly
+distribute the milk particles, and poured out into a sterile flat glass
+dish and quickly covered. This is allowed to stand on a cool surface
+until the gelatin hardens. After the culture plate has been left for
+twenty-four to thirty-six hours at the proper temperature, tiny spots
+will begin to appear on the surface, or in the depth of the culture
+medium. These patches are called _colonies_ and are composed of an
+almost infinite number of individual germs, the result of the continued
+growth of a single organism that was in the drop of milk which was
+firmly held in place when the gelatin solidified. The number of these
+colonies represents approximately the number of germs that were present
+in the milk drop. If the plate is not too thickly sown with these germs,
+the colonies will continue to grow and increase in size, and as they do,
+minute differences will begin to appear. These differences may be in the
+color, the contour and the texture of the colony, or the manner in
+which it acts toward gelatin. In order to make sure that the seeding in
+not too copious so as to interfere with continued study, an
+_attenuation_ is usually made. This consists in taking a drop of the
+infected gelatin in the first tube, and transferring it to another tube
+of sterile media. Usually this operation is repeated again so that these
+culture plates are made with different amounts of seed with the
+expectation that in at least one plate the seeding will not be so thick
+as to prevent further study. For transferring the culture a loop made of
+platinum wire is used. By passing this through a gas flame, it can be
+sufficiently sterilized.
+
+[Illustration: FIG. 3. Profile view of gelatin plate culture; _b_, a
+liquefying form that dissolves the gelatin; _c_ and _d_, surface
+colonies that do not liquefy the gelatin.]
+
+To further study the peculiarities of different germs, the separate
+colonies are transferred to other sterile tubes of culture material and
+thus _pure cultures_ of the various germs are secured. These cultures
+then serve as a basis for continued study and must be planted and grown
+upon all the different kinds of media that are obtainable. In this way
+the slight variations in the growth of different forms are detected and
+the peculiar characteristics are determined, so that the student is able
+to recognize this form when he meets it again.
+
+These culture methods are of essential importance in bacteriology, as it
+is the only way in which it is possible to secure a quantity of germs of
+the same kind.
+
+~The microscope in bacterial investigation.~ In order to verify the purity
+of the cultures, the microscope is in constant demand throughout all the
+different stages of the isolating process. For this purpose, it is
+essential that the instrument used shall be one of strong magnifying
+powers (600-800 diameters), combined with sharp definition.
+
+[Illustration: FIG. 4. Pure cultures of different kinds of bacteria in
+gelatin tubes. _a_, growth slight in this medium; _b_, growth copious at
+and near surface. Fine parallel filaments growing out into medium
+liquefying at surface; _c_, a rapid liquefying form; _d_, a
+gas-producing form that grows equally well in lower part of tube as at
+surface (facultative anaerobe); _e_, an obligate anaerobe, that develops
+only in absence of air.]
+
+The microscopical examination of any germ is quite as essential as the
+determination of culture characteristics; in fact, the two must go hand
+in hand. The examination reveals not only the form and size of the
+individual germs, but the manner in which they are united with each
+other, as well as any peculiarities of movement that they may possess.
+
+In carrying out the microscopical part of the work, not only is the
+organism examined in a living condition, but preparations are made by
+using solutions of anilin dyes as staining agents. These are of great
+service in bringing out almost imperceptible differences. The art of
+staining has been carried to the highest degree of perfection in
+bacteriology, especially in the detection of germs that are found in
+diseased tissues in the animal or human body.
+
+In studying the peculiarities of any special organism, not only is it
+necessary that these cultural and microscopical characters should be
+closely observed, but special experiments must be carried out along
+different lines, in order to determine any special properties that the
+germ may possess. Thus, the ability of any form to act as a fermentative
+organism can be tested by fermentation experiments; the property of
+causing disease, studied by the inoculation of pure cultures into
+animals. A great many different methods have been devised for the
+purpose of studying special characteristics of different bacteria, but a
+full description of these would necessarily be so lengthy that in a work
+of this character they must be omitted. For details of this nature
+consult standard reference books on bacteriological technique.
+
+
+
+
+CHAPTER III.
+
+CONTAMINATION OF MILK.
+
+
+No more important lesson is to be learned than that which relates to the
+ways in which milk is contaminated with germ life of various kinds; for
+if these sources of infection are thoroughly recognized they can in
+large measure be prevented, and so the troubles which they engender
+overcome. Various organisms find in milk a congenial field for
+development. Yeasts and some fungi are capable of growth, but more
+particularly the bacteria.
+
+~Milk a suitable bacterial food.~ The readiness with which milk undergoes
+fermentative changes indicates that it is well adapted to nourish
+bacterial life. Not only does it contain all the necessary nutritive
+substances but they are diluted in proper proportions so as to render
+them available for bacterial as well as mammalian life.
+
+Of the nitrogenous compounds, the albumen is in readily assimilable
+form. The casein, being insoluble, is not directly available, until it
+is acted upon by proteid-dissolving enzyms like trypsin which may be
+secreted by bacteria. The fat is relatively resistant to change,
+although a few forms are capable of decomposing it. Milk sugar, however,
+is an admirable food for many species, acids and sometimes gases being
+generally produced.
+
+~Condition when secreted.~ When examined under normal conditions milk
+always reveals bacterial life, yet in the secreting cells of the udder
+of a healthy cow germ life is not found. Only when the gland is diseased
+are bacteria found in any abundance. In the passage of the milk from
+the secreting cells to the outside it receives its first infection, so
+that when drawn from the animal it generally contains a considerable
+number of organisms.
+
+[Illustration: FIG. 5. Microscopic appearance of milk showing relative
+size of fat globules and bacteria.]
+
+~Contamination of milk.~ From this time until it is consumed in one form
+or another, it is continually subjected to contamination. The major part
+of this infection occurs while the milk is on the farm and the degree of
+care which is exercised while the product is in the hands of the milk
+producer is the determining factor in the course of bacterial changes
+involved. This of course does not exclude the possibility of
+contamination in the factory, but usually milk is so thoroughly seeded
+by the time it reaches the factory that the infection which occurs here
+plays a relatively minor role to that which happens earlier. The great
+majority of the organisms in milk are in no wise dangerous to health,
+but many species are capable of producing various fermentative changes
+that injure the quality of the product for butter or cheese. To be able
+to control abnormal changes of an undesirable character one must know
+the sources of infection which permit of the introduction of these
+unwelcome intruders.
+
+~Sources of infection.~ The bacterial life that finds its way into milk
+while it is yet on the farm may be traced to several sources, which may
+be grouped under the following heads: unclean dairy utensils, fore milk,
+coat of animal, and general atmospheric surroundings. The relative
+importance of these various factors fluctuates in each individual
+instance.
+
+~Dairy utensils.~ Of first importance are the vessels that are used during
+milking, and also all storage cans and other dairy utensils that come in
+contact with the milk after it is drawn. By unclean utensils, actually
+_visible_ dirt need not always be considered, although such material is
+often present in cracks and angles of pails and cans. Unless cleansed
+with especial care, these are apt to be filled with foul and decomposing
+material that suffices to seed thoroughly the milk. Tin utensils are
+best. Where made with joints, they should be well flushed with solder so
+as to be easily cleaned (see Fig. 6). In much of the cheaper tin ware on
+the market, the soldering of joints and seams is very imperfect,
+affording a place of refuge for bacteria and dirt.
+
+Cans are often used when they are in a condition wholly unsuitable for
+sanitary handling of milk. When the tin coating becomes broken and the
+can is rusty, the quality of the milk is often profoundly affected.
+Olson[1] of the Wisconsin Station has shown that the action of rennet is
+greatly impaired where milk comes in contact with a rusty iron surface.
+
+[Illustration: FIG. 6.]
+
+With the introduction of the form or hand separator a new milk utensil
+has been added to those previously in use and one which is very
+frequently not well cleaned. Where water is run through the machine to
+rinse out the milk particles, gross bacterial contamination occurs and
+the use of the machine much increases the germ content of the milk.
+Every time the separator is used it should be taken apart and thoroughly
+cleaned and dried before reassembling.[2]
+
+~Use of milk-cans for transporting factory by-products.~ The general
+custom of using the milk-cans to carry back to the farm the factory
+by-products (skim-milk or whey) has much in it that is to be deprecated.
+These by-products are generally rich in bacterial life, more especially
+where the closest scrutiny is not given to the daily cleaning of the
+vats and tanks. Too frequently the cans are not cleaned immediately upon
+arrival at the farm, so that the conditions are favorable for rapid
+fermentation. Many of the taints that bother factories are directly
+traceable to such a cause. A few dirty patrons will thus seriously
+infect the whole supply. The responsibility for this defect should,
+however, not be laid entirely upon the shoulders of the producer. The
+factory operator should see that the refuse material does not accumulate
+in the waste vats from day to day and is not transformed into a more or
+less putrid mass. A dirty whey tank is not an especially good object
+lesson to the patron to keep his cans clean.
+
+It is possible that abnormal fermentations or even contagious diseases
+may thus be disseminated.
+
+Suppose there appears in a dairy an infectious milk trouble, such as
+bitter milk. This milk is taken to the factory and passes unnoticed into
+the general milk-supply. The skim-milk from the separator is of course
+infected with the germ, and if conditions favor its growth, the whole
+lot soon becomes tainted. If this waste product is returned to the
+different patrons in the same cans that are used for the fresh milk, the
+probabilities are strongly in favor of some of the cans being
+contaminated and thus infecting the milk supply of the patrons. If the
+organism is endowed with spores so that it can withstand unfavorable
+conditions, this taint may be spread from patron to patron simply
+through the infection of the vessels that are used in the transportation
+of the by-products. Connell has reported just such a case in a Canadian
+cheese factory where an outbreak of slimy milk was traced to infected
+whey vats. Typhoid fever among people, foot and mouth disease and
+tuberculosis among stock are not infrequently spread in this way. In
+Denmark, portions of Germany and some states in America, compulsory
+heating of factory by-products is practiced to eliminate this danger.[3]
+
+~Pollution of cans from whey tanks.~ The danger is greater in cheese
+factories than in creameries, for whey usually represents a more
+advanced stage of fermentation than skim-milk. The higher temperature at
+which it is drawn facilitates more rapid bacterial growth, and the
+conditions under which it is stored in many factories contribute to the
+ease with which fermentative changes can go on in it. Often this
+by-product is stored in wooden cisterns or tanks, situated below ground,
+where it becomes impossible to clean them out thoroughly. A custom that
+is almost universally followed in the Swiss cheese factories, here in
+this country, as in Switzerland, is fully as reprehensible as any dairy
+custom could well be. In Fig. 7 the arrangement in vogue for the
+disposal of the whey is shown. The hot whey is run out through the
+trough from the factory into the large trough that is placed over the
+row of barrels, as seen in the foreground. Each patron thus has allotted
+to him in his individual barrel his portion of the whey, which he is
+supposed to remove day by day. No attempt is made to clean out these
+receptacles, and the inevitable result is that they become filled with a
+foul, malodorous liquid, especially in summer. When such material is
+taken home in the same set of cans that is used to bring the fresh milk
+(twice a day as is the usual custom in Swiss factories), it is no wonder
+that this industry is seriously handicapped by "gassy" fermentations
+that injure materially the quality of the product. Not only is the above
+danger a very considerable one, but the quality of the factory
+by-product for feeding purposes, whether it is skim-milk or whey, is
+impaired through the development of fermentative changes.
+
+[Illustration: FIG. 7. Swiss cheese factory (Wisconsin), showing
+careless way in which whey is handled. Each patron's share is placed in
+a barrel, from which it is removed by him. No attempt is made to cleanse
+these receptacles.]
+
+~Improved methods of disposal of by-products.~ The difficulties which
+attend the distribution of these factory by-products have led to
+different methods of solution. One is to use another separate set of
+receptacles to carry back these products to the farm. This method has
+been tried, and while it is deemed impracticable by many to handle two
+sets of vessels, yet some of the most progressive factories report
+excellent results where this method is in use.
+
+Large barrels could be used for this purpose to economize in wagon
+space.
+
+Another method that has met with wider acceptance, especially in
+creameries, is the custom of pasteurizing or scalding the skim-milk
+immediately after it is separated, so that it is returned to the farmer
+in a hot condition. In factories where the whole milk is pasteurized,
+further treatment of the by-product is not necessary. In most factories
+steam, generally exhaust, is used directly in the milk, and experience
+has shown that such milk, without any cooling, will keep sweet for a
+considerable number of hours longer than the untreated product. It is
+noteworthy that the most advanced and progressive factories are the ones
+that appreciate the value of this work, and although it involves some
+time and expense, experience has shown the utility of the process in
+that a better grade of milk is furnished by the patrons of factories
+which follow this practice.[4] The exclusion of all danger of animal or
+human disease is also possible in this way.
+
+~Cleaning dairy utensils.~ The thorough cleaning of all dairy apparatus
+that in any way comes in contact with the milk is one of the most
+fundamental and important problems in dairying. All such apparatus
+should be so constructed as to permit of easy cleaning. Tinware,
+preferably of the pressed variety, gives the best surface for this
+purpose and is best suited for the handling of milk.
+
+Milk vessels should never be allowed to become dry when dirty, for dried
+particles of milk residue are extremely difficult to remove. In cleaning
+dairy utensils they should first be rinsed in lukewarm instead of hot
+water, so as to remove organic matter without coagulating the milk. Then
+wash thoroughly in hot water, using a good washing powder. The best
+washing powders possess considerable disinfecting action.[5] Strong
+alkalies should not be used. After washing rinse thoroughly in clean hot
+water. If steam is available, as it always is in creameries, cans and
+pails should be turned over jet for a few moments. While a momentary
+exposure will not suffice to completely sterilize such a vessel, yet
+many bacteria are killed in even a short exposure, and the cans dry more
+thoroughly and quickly when heated by steam.
+
+Not only should the greatest care be paid to the condition of the cans
+and milk-pails, but all dippers, strainers, and other utensils that come
+in contact with the milk must be kept equally clean. Cloth strainers,
+unless attended to, are objectionable, for the fine mesh of the cloth
+retains so much moisture that they become a veritable hot-bed of
+bacterial life, unless they are daily boiled or steamed.
+
+The inability to thoroughly render vessels bacteria-free with the
+conveniences which are generally to be found on the farm has led in some
+cases to the custom of washing and sterilizing the milk cans at the
+factory.
+
+~Germ content of milk utensils.~ Naturally the number of bacteria found in
+different milk utensils after they have received their regular cleaning
+will be subject to great fluctuations; but, nevertheless, such
+determinations are of value as giving a scientific foundation for
+practical methods of improvement. The following studies may serve to
+indicate the relative importance of the utensils as a factor in milk
+contamination.
+
+Two cans were taken, one of which had been cleaned in the ordinary way,
+while the other was sterilized by steaming. Before milking, the udder
+was thoroughly cleaned and special precautions taken to avoid raising of
+dust; the fore milk was rejected. Milk drawn into these two cans showed
+the following germ content:
+
+                      No. bacteria       Hours before
+                        per cc.           souring.
+
+    Steamed pail          165              28-1/2
+    Ordinary pail        4265              23
+
+Harrison[6] has shown how great a variation is in the bacterial content
+in milk cans. The utensils were rinsed with 100 cc. of sterile water and
+numerical determinations of this rinsing water made. In poorly cleaned
+cans, the average germ content was 442,000; in cans washed in tepid
+water and then scalded--the best farm practice--54,000, and in cans
+carefully washed and then steamed for five minutes, 880.
+
+Another method used by the writer is to wash the utensil with 100 cc.
+sterile wash water, using a sterile swab to remove dirt. Then repeat the
+process twice or more with fresh rinsing waters, making plate cultures
+from each. The following data were obtained from three such
+determinations:
+
+No. bacteria in different washings.     Total No.
+   I.           II.         III.        bacteria.
+7,800,000    1,450,000     49,000       9,299,000
+  283,000       43,400     35,000         361,400
+1,685,000      105,000     61,400       1,851,400
+
+~Infection of milk in udder cavity.~ A frequently neglected but
+considerable factor of infection is that which is attributable to the
+bacteria which are present in the udder and which are removed in large
+numbers during the milking process. An examination of the fore milk, i.
+e., the first few streams from each teat, and that which is subsequently
+withdrawn, generally reveals a very much larger number of organisms in
+the fore milk.[7] Not infrequently will this part of the milk when drawn
+under as careful conditions as possible, contain several score thousand
+organisms per cc. If successive bacterial determinations are made at
+different periods of the milking, as shown in the following experiment,
+a marked diminution is to be noted after the first portion of the milk
+is removed:
+
+    _Bacterial content at different periods of milking._
+
+                Fore     200th    2000th    4300th    6500th   Strippings.
+                milk.     cc.      cc.       cc.       cc.
+Expt. 1        6,500     1,700     475      220        75         5
+Expt. 2        8,100     1,650     400      240        50        10
+
+By some observers it has been claimed that it is possible to secure
+absolutely sterile milk in the strippings but this is rarely so. It is
+quite probable that such reported results are due to the fact that too
+small quantities of milk were used in the examinations and so erroneous
+conclusions were drawn. This marked diminution in numbers indicates that
+the larger proportion of the organisms found in the fore milk are
+present in the lower portion of the udder and milk ducts. When
+consideration is given to the structure of the udder, it is readily
+apparent that infection will be greater here than above.
+
+[Illustration: FIG. 8. Sectional view of udder showing teat with milk
+duct connecting exterior with the milk cistern. Milk sinuses are mostly
+shown in cross section interspersed and below the secreting tissue
+(Moore and Ward).]
+
+The udder is composed of secreting tissue (_gland cells_) held in place
+by fibrous connective tissue. Ramifying throughout this glandular
+structure are numerous channels (_milk sinuses_) that serve to convey
+the milk from the cells where it is produced into the _milk cistern_, a
+common receptacle just above the teats. This cavity is connected with
+the exterior through the milk duct in the teat, which is more or less
+tightly closed by the circular sphincter muscles, thus preventing the
+milk from flowing out. The mucous membranes of the milk duct and cistern
+are naturally moist. The habits of the animal render it impossible to
+prevent infection of the external opening at the end of the teat and
+there is no mechanical reason why bacteria cannot readily find their way
+along the moist lining membrane for some distance. If organisms are
+adapted to this kind of an environment, ideal conditions exist for their
+multiplication, as moisture, warmth and suitable food supply are
+present. The question arises how far up into this organ is penetration
+possible? Within late years numerous observations have been made on the
+presence of organisms in the upper portion of the udder in contact with
+the secreting tissue.[8]
+
+These investigations prove that bacteria are distributed throughout the
+whole of the udder, although numerically they are much less abundant in
+the region of the milk-secreting tissue than in the lower portion.
+Ward's conclusions are "that milk when secreted by the glands of a
+healthy udder is sterile. It may however, immediately become
+contaminated by the bacteria which are normally present in the smaller
+milk ducts of the udder."
+
+~Nature of bacteria in fore milk.~ Generally speaking the number of
+different species found in the fore milk is not large, and of those
+which do appear many occur at only occasional intervals. Moore[9] in the
+examination of 9 udders found 20 different forms, and of these only 3
+species predominated, all of which proved to be micrococci. Streptococci
+have also been quite frequently reported. Freudenreich[10] found the
+most common types to be cocci, belonging to both the liquefying and
+non-liquefying class.
+
+Peptonizing[11] and spore-bearing[12] species have also been reported as
+well as gas-producing[13] forms allied to the colon bacillus. Such
+findings are, however, due in all probability to accidental invasion.
+Most investigators report the absence of the distinctively lactic-acid
+group of organisms.[14]
+
+~Origin of bacteria in udder.~ There is no question but that many of the
+types of bacteria found in the udder gain access from the outside. Those
+belonging to the spore-bearing, digesting and intestinal types have such
+a favorable opportunity for introduction from outside and are so
+unlikely to have come directly from the body of the animal, that the
+external source of infection is much more probable. Whether this
+explanation answers the origin of the cocci that are so generally found
+in the upper portion of the udder is questionable. The statement is
+ordinarily made that the inner tissues of healthy organs are
+bacteria-free, but the studies of Ford[15] seem to indicate that 70 per
+cent. of such organs, removed under aseptic conditions from guinea pigs,
+rabbits, dogs and cats contained living organisms. Others have reported
+similar results in which cocci have been found[16] very similar to those
+occurring in the udder. These findings increase the probability that the
+origin of this type is from the blood. The persistence of certain
+species in the udder for months as noted by Ward indicates possibility
+of growth of some forms at least. Stocking[17] has shown where cows are
+not milked clean that the germ content of succeeding milkings is greatly
+increased.
+
+~Artificial introduction of bacteria into udder.~ If bacteria are capable
+of actually developing in the udder proper, it ought to be possible to
+easily demonstrate this by the artificial introduction of cultures. In a
+number of cases[18] such experiments have been made with various
+saprophytic forms, such as _B. prodigiosus_, lactic acid bacilli and
+others. In no case has it appeared evident that actual growth has
+occurred, although the introduced organism has been demonstrated in
+diminishing numbers for 5-6 days. Even the common lactic acid germ and a
+yellow liquefying coccus isolated from the fore milk failed to persist
+for more than a few days when thus artificially introduced. This failure
+to colonize is indeed curious and needs explanation. Is it due to
+unsuitable environmental conditions or attributable to the germicidal
+influence of the milk?
+
+Various body fluids are known to possess the property of destroying
+bacteria and it is claimed by Fokker[19] that this same property was
+found in freshly drawn milk. This peculiarity has also been investigated
+by Freudenreich,[20] and Hunziker[21] who find a similar property.
+
+No material increase in germ content takes place in milk for several
+hours when chilled to 40 deg.-70 deg. F.; on the other hand an actual,
+but usually not a marked decrease is observed for about 6 hours. This
+phenomenon varies with the milk of different cows. Nothing is known as
+to the cause of this apparent germicidal action. The question is yet by
+no means satisfactorily settled, although the facts on which the
+hypothesis is based are not in controversy. If such a peculiarity
+belongs to milk, it is not at all improbable that it may serve to keep
+down the germ content in the udder. Freudenreich[22] found that udders
+which were not examined for some time after death showed abundant
+growth, which fact he attributed to the loss of this germicidal
+property.
+
+The infection of the whole milk can be materially reduced by rejecting
+the fore milk, but it is questionable whether such rejection is worth
+while, except in the case of "sanitary" dairies where milk is produced
+with as low a germ content as possible. The intrinsic loss in butter fat
+in the fore milk is inconsiderable as the first few streams contain only
+about one-fifth the normal fat content.
+
+~Infection of milk after withdrawal from animal.~ The germ content of the
+milk, when it is being drawn from the animal is immediately increased
+upon contact with the atmosphere. These organisms are derived from the
+surrounding air and the utensils in which the milk is received and
+stored. The number of organisms which find their way into the milk
+depends largely upon the character of the surroundings. Bacteria are so
+intimately associated with dirt, dust and filth of all kinds that
+wherever the latter are found, the former are sure to be present in
+abundance.
+
+The most important factors in the infection of the milk after withdrawal
+are the pollution which is directly traceable to the animal herself and
+the condition of the milk utensils. Fortunately both of these sources of
+contamination are capable of being greatly minimized by more careful
+methods of handling.
+
+~Infection directly from the cow.~ It is a popular belief that the
+organisms found in milk are derived from the feed and water which the
+cow consumes, the same passing directly from the intestinal tract to the
+milk by the way of the blood circulation. Such a view has no foundation
+in fact as bacteria absorbed into the circulation are practically all
+destroyed in the tissues by the action of the body fluids and cells.[23]
+While organisms cannot pass readily from the intestine to the udder, yet
+this must not be interpreted as indicating that no attention should be
+given to the bacterial character of the material consumed. The water
+supply given should be pure and wholesome and no decomposed or spoiled
+food should be used.
+
+The infection traceable directly to the cow is modified materially by
+the conditions under which the animal is kept and the character of the
+feed consumed. The nature of the fecal matter is in part dependent upon
+the character of the food. The more nitrogenous rations with which
+animals are now fed leads to the production of softer fecal discharges
+which is more likely to soil the coat of the animal unless care is
+taken. The same is true with animals kept on pasture in comparison with
+those fed dry fodder.
+
+Stall-fed animals, however, are more likely to have their flanks fouled,
+unless special attention is paid to the removal of the manure. All dairy
+stalls should be provided with a manure drop which should be cleaned as
+frequently as circumstances will permit.
+
+[Illustration: FIG. 9. Showing the bacterial contamination arising from
+hair. These hairs were allowed to fall on a sterile gelatin surface. The
+adherent bacteria developed readily in this medium, and the number of
+bacteria thus introduced into the milk from these hairs can be estimated
+by the number of developing colonies.]
+
+The animal herself contributes materially to the quota of germ life
+finding its way into the milk through the dislodgment of dust and filth
+particles adhering to its hairy coat. The nature of this coat is such as
+to favor the retention of these particles. Unless care is taken the
+flanks and udder become polluted with fecal matter, which upon drying is
+displaced with every movement of the animal. Every hair or dirt particle
+so dislodged and finding its way into the milk-pail adds its quota of
+organisms to the liquid. This can be readily demonstrated by placing
+cow's hairs collected with care on the moist surface of gelatin culture
+plates. Almost invariably, bacteria will be found in considerable
+numbers adhering to such hairs as is indicated in Fig. 9. Dirt particles
+are even richer in germ life. Not only is there the dislodgment of
+hairs, epithelial scales and masses of dirt and filth, but during the
+milking process, as at all other times, every motion of the animal is
+accompanied by a shower of _invisible_ particles more or less teeming
+with bacterial life.
+
+The amount of actual impurities found in milk is often considerable and
+when it is remembered that about one-half of fresh manure dissolves in
+milk,[24] and thus does not appear as sediment, the presence of this
+undissolved residue bespeaks filthy conditions as to milking. From
+actual tests made, it is computed that the city of Berlin, Germany
+consumes about 300 pounds of such dirt and filth daily. Renk has laid
+down the following rule with reference to this insoluble dirt: If a
+sample of milk shows any evidence of impurity settling on a transparent
+bottom within two hours, it should be regarded as too dirty for use.
+
+While the number of organisms here introduced is at all times large, the
+character of the species is of even greater import. Derived primarily
+from dirt and fecal matter, it is no wonder that such forms are able to
+produce very undesirable fermentative changes.
+
+~Influence of milker.~ The condition of the milker is not to be ignored in
+determining all possible factors of infection, for when clothed in the
+dust-laden garments that have been worn all day, a favorable opportunity
+for direct contamination is possible. The filthy practice of wetting the
+hands with milk just before milking is to be condemned. The milker's
+hands should be washed immediately before milking in clean water and
+dried. A pinch of vaseline on hands is sometimes used to obtain a firmer
+grasp and prevents the ready dislodgment of scales.[25] It must also be
+borne in mind that the milker may spread disease through the milk. In
+typhoid fever and diphtheria, the germs often remain in the system for
+weeks and thus make infection possible. Stocking[26] has shown that the
+individual milker exerts a potent influence on the total germ content of
+milk, even where the procedure is quite the same. In sanitary dairies
+milkers are usually clad in white duck suits.
+
+~Milking by machinery.~ Several mechanical devices have been invented for
+milking, some of which have been tested bacteriologically as to their
+efficiency. Harrison[27] has examined the "Thistle" machine but found a
+much higher germ content than with hand-drawn milk. The recent
+introduction of the Burrel-Lawrence-Kennedy machine has led to numerous
+tests in which very satisfactory results have been obtained. If the
+rubber parts of the milker are thoroughly cleaned and kept in lime water
+solution, they remain nearly sterile. When milk is properly handled, the
+germ content may be greatly reduced.
+
+~Reduction in dirt and adherent bacteria.~ No factor of contamination is
+so susceptible of improvement as that which relates to the reduction in
+filth and dirt which gains access during and immediately subsequent to
+the milking. The care which is taken to keep the coat of the animal
+clean by grooming lessens very much the grosser portion of such
+contamination, but with a dry, hairy coat, fine scales and dust
+particles must of necessity be dislodged.[28] Ordinarily the patron
+thinks all evidence of such dirt is removed if the milk is strained, but
+this process only lessens the difficulty; it does not overcome it.
+Various methods are in use, the effectiveness of which is subject to
+considerable variation. Some of these look to the elimination of the
+bacteria after they are once introduced into the milk; others to the
+prevention of infection in the first place.
+
+_1. Straining the milk._ Most of the visible, solid particles of filth,
+such as hairs, dirt particles, etc., can be removed by simple straining,
+the time-honored process of purification. As ordinarily carried out,
+this process often contributes to instead of diminishing the germ life
+in milk. The strainer cloths unless washed and thoroughly sterilized by
+boiling harbor multitudes of organisms from day to day and may thus
+actually add to the organisms present. Various methods have been
+suggested for this simple process, but the most practical and efficient
+strainer is that made of fine wire gauze to which is added 3-4 layers of
+cheese cloth, the whole to set over the storage milk can.
+
+_2. Filtration._ In Europe especially, the system of cleaning milk by
+filtration through sand, gravel and other substances has been quite
+extensively used. These filters are built in sections and the milk
+passes from below upward. The filtering substance is washed in hot water
+immediately after use and then steamed and finally baked. While it is
+possible to remove the solid impurities in this way, the germ content
+cannot be greatly reduced.[29] Cellulose filters have also been
+suggested[30] as an improvement over the sand filters. Methods of
+filtration of this character have not been used under commercial
+conditions here in this country.
+
+_3. Clarification in separator._ Within recent years the custom has
+grown of clarifying milk or removing the visible dirt by passing the
+milk through a centrifugal separator the cream and skim milk being
+remixed after separation. This process naturally removes the solid
+impurities as dirt, hairs, epithelial scales and cells, also some of the
+casein, making what is known as centrifuge slime. This conglomerate mass
+is incomparably rich in germ life and the natural inference would be
+that the bacterial content of the milk would be greatly reduced by this
+procedure. Eckles and Barnes[31] noted a reduction of 37 to 56 per cent.
+of the bacteria but others have failed to observe such reductions.[32]
+This condition is explained by the more thorough breaking up of the
+bacterial masses in the process, thus apparently not reducing them in
+numbers.
+
+It is somewhat surprising that in spite of the elimination of much
+organic matter and bacteria, such clarified milk sours as rapidly as the
+untreated product.[33]
+
+The mechanical shock of separation ruptures the clusters of fat globules
+and so delays creaming and also lessens the consistency of cream derived
+from such milk. This practical disadvantage together with the increased
+expense of the operation and the failure to materially enhance the
+keeping quality of the product outweigh the advantage which might come
+from removal of solid impurities which can be largely accomplished on
+the farm by efficient straining.
+
+_4. Washing the udder._ If a surface is moist, bacteria adherent to it
+cannot be dislodged by ordinary movements. Thus the air over
+snow-covered mountains or oceans is relatively devoid of germ life. The
+method of moistening the udder is applied with success to the hairy coat
+of the animal thus subserving the double purpose of cleaning the animal
+and preventing in large measure the continual dislodgment of dust
+particles. After these parts have been well carded to remove loose hairs
+and dirt particles, the skin should be thoroughly moistened with clean
+water and then wiped. It has been urged that this procedure lessens the
+yield of milk but Eckles[34] concludes from experiments that when the
+animal becomes accustomed to this treatment, no noticeable change in
+amount of milk or butter-fat is produced.
+
+The effectiveness of this method in reducing the actual amount of dirt
+and filth introduced into the milk as well as the great diminution in
+germ life is shown by the instructive experiments of Fraser[35] who
+found that the actual amount of dirt dislodged from udders of apparently
+clean animals during the milking process was three and one-half times as
+much as when the cow's udders were washed. From udders visibly polluted
+one ounce of such filth was removed in 275 pounds of milk, while from
+cows whose udders had been washed, the same amount of dirt was
+distributed through 24,030 pounds.
+
+Fraser observed as a result of 420 examinations that the average germ
+content of 4-inch culture dishes under clean but unwashed udders was
+578, while under washed animals it was reduced to 192. From numerous
+tests made in the writer's laboratory, it is evident that the germ
+content of the milk in the pail is increased from 20,000-40,000 bacteria
+_per minute_ during the milking period. By far the larger part of this
+pollution can be easily prevented by cleaning and dampening the udder.
+
+_5. Diminishing exposed surface of pail._ The entrance of organisms into
+the milk can be greatly reduced by lessening the area of the milk pail
+directly exposed to the dust shower. A number of so-called sanitary or
+hygienic milk pails have been devised for this purpose. In one case the
+pail is smaller at the top than bottom, but in most of them the common
+form is kept and the exposed area is lessened by means of a cover, the
+milk being received through a narrower opening. In some cases, strainers
+are also interposed so as to remove more effectually the coarse
+particles. It is necessary to have these covers and strainers
+constructed in such a way so they can be easily removed and cleaned.
+
+[Illustration: FIG. 10. Sanitary milk pails designed to diminish the
+introduction of hairs, scales, dirt, etc., into milk.]
+
+Stocking tested one of these pails (A, Fig. 10) and found that 63 per
+cent of the dirt and 29 per cent. of the bacteria were prevented from
+passing into the milk. Eckles examined one in which the germ content was
+found to be 3200 per cc. as against 43200 per cc. in a common open
+pail. This milk did not sour until it was 64 hours old in the first case
+while in the latter it curdled in 43 hours.
+
+~Air in barn.~ The atmosphere of the barn where the milking is done may
+frequently contribute considerable infection. Germ life is incapable of
+development in the air, but in a dried condition, organisms may retain
+their vitality for long periods. Anything which contributes to the
+production of dust in the stable and aids in the stirring up of the same
+increases the number of organisms to be found in the air (Fig. 11).
+Thus, the feeding of dry fodder and the bedding of animals with straw
+adds greatly to the germ life floating in the air. Dust may vary much in
+its germ content depending upon its origin. Fraser found the dust from
+corn meal to contain only about one-sixth to one-eighth as much germ
+life as that from hay or bran.[36] In time most of these dust particles
+settle to the floor, but where the herd is kept in the barn, the
+constant movement of the animals keeps these particles more or less in
+motion. Much can be done by forethought to lessen the germ content of
+stables. Feeding dry feed should not be done until after milking.[37] In
+some of the better sanitary dairies, it is customary to have a special
+milking room that is arranged with special reference to the elimination
+of all dust. In this way this source of infection may be quite obviated
+as the air of a clean, still room is relatively free from bacteria,
+especially if the floor is moistened. It has often been noted that the
+milk of stall-fed animals does not keep as well as that milked out of
+doors, a condition in part attributable to the lessened contamination.
+
+[Illustration: FIG. 11. Effect of contaminated air. The number of spots
+indicate the colonies that have developed from the bacteria which fell
+in 30 seconds on the surface of the gelatin plate (3 inches in
+diameter). This exposure was made at time the cows were fed.]
+
+~Relative importance of different sources of infection.~ It is impossible
+to measure accurately the influence of the different sources of
+infection as these are continually subject to modification in each and
+every case. As a general rule, however, where milk is drawn and handled
+without any special care, the utensils and the animal contribute the
+larger proportion of dirt and bacteria that find their way into milk.
+Where the manner of milking and handling is designed to exclude the
+largest number of organisms possible, the bacteria appearing in the fore
+milk make up the major portion remaining. By putting into practice the
+various suggestions that have been made with reference to diminishing
+the bacterial content of milk, it is possible to greatly reduce the
+number of organisms found therein, and at the same time materially
+improve the keeping quality of the milk. Backhaus[38] estimates that
+the germ life in milk can be easily reduced to one-two thousandth of its
+original number by using care in milking. He reports a series of
+experiments covering two years in which milk was secured that averaged
+less than 10,000 bacteria per cc., while that secured under ordinary
+conditions averaged over 500,000.
+
+[Illustration: FIG. 12. Bacterial content of milk handled in ordinary
+way. Each spot represents a colony growing on gelatin plate. Compare
+with Fig. 13, where same quantity of milk is used in making culture.
+Over 15,000 bacteria per cc. in this milk.]
+
+Fig. 13 gives an illustration as to what care in milking will do in the
+way of eliminating bacteria. Fig. 12 shows a gelatin plate seeded with
+the same quantity of milk that was used in making the culture indicated
+by Fig. 13. The first plate was inoculated with milk drawn under good
+conditions, the germ content of which was found to be 15,500 bacteria
+per cc., while the sample secured under as nearly aseptic conditions as
+possible (Fig. 13) contained only 330 organisms in the same volume.
+
+[Illustration: FIG. 13. Bacterial content of milk drawn with care.
+Diminished germ content is shown by smaller number of colonies (330
+bacteria per cc.). Compare this culture with that shown in Fig. 12.]
+
+~"Sanitary" or "certified" milk.~ Within recent years there has been more
+or less generally introduced into many cities, the custom of supplying
+high grade milk that has been handled in a way so as to diminish its
+germ content as much as possible. Milk of this character is frequently
+known as "sanitary," "hygienic" or "certified," the last term being used
+in connection with a certification from veterinary authorities or boards
+of health as to the freedom of animals from contagious disease.
+Frequently a numerical bacterial standard is exacted as a pre-requisite
+to the recommendation of the board of examining physicians. Thus, the
+Pediatric Society of Philadelphia requires all children's milk that
+receives its recommendation to have not more than 10,000 bacteria per
+cc. Such a standard has its value in the scrupulous cleanliness that
+must prevail in order to secure these results. This in itself is
+practically a guarantee of the absence of those bacteria liable to
+produce trouble in children. The number of organisms found in such milks
+is surprisingly low when compared with ordinary milk. Naturally, there
+is considerable fluctuation from day to day, and occasionally the germ
+content is increased to a high figure without any apparent reason. The
+average results though, show a greatly reduced number of organisms. De
+Schweinitz[39] found in a Washington dairy in 113 examinations extending
+throughout a year, an average of 6,485 bacteria per cc. The daily
+analyses made of the Walker-Gordon supply sold in Philadelphia for an
+entire year, showed that the milk almost always contained less than
+5,000 bacteria per cc. and on 120 days out of the year the germ content
+was 1,000 organisms per cc. or less.
+
+From a practical point of view, the improvement in quality of sanitary
+milk, in comparison with the ordinary product is seen in the enhanced
+keeping quality. During the Paris Exposition in 1900, milk and cream
+from several such dairies in the United States were shipped to Paris,
+arriving in good condition after 15 to 18 days transit. When milk has
+been handled in such a way, it is evident that it is much better suited
+to serve as a food supply than where it has undergone the fermentative
+changes incident to the development of myriads of organisms.
+
+~Application of foregoing precautions to all milk producers.~ Milk is so
+susceptible to bacterial changes that it is necessary to protect it from
+invasion, if its original purity is to be maintained, and yet, from a
+practical point of view, the use to which it is destined has much to do
+with the care necessary to take in handling. The effect of the bacterial
+contamination of milk depends largely upon the way in which the product
+is used. To the milk-man engaged in the distribution of milk for direct
+consumption, all bacterial life is more or less of a detriment, while to
+the butter-maker and cheese-maker some forms are a direct necessity. It
+is unnecessary and impracticable to require the same degree of care in
+handling milk destined to be worked up into factory products as is done,
+for instance, in sanitary milk supplies, but this fact should not be
+interpreted to mean that the care of milk for factories is a matter of
+small consequence. In fact no more important dairy problem exists, and
+the purer and better the quality of the raw material the better the
+product will be. Particularly is this true with reference to
+cheese-making.
+
+Dairymen have learned many lessons in the severe school of experience,
+but it is earnestly to be hoped that future conditions will not be
+summed up in the words of the eminent German dairy scientist, Prof.
+Fleischmann, when he says that "all the results of scientific
+investigation which have found such great practical application in the
+treatment of disease, in disinfection, and in the preservation of
+various products, are almost entirely ignored in milking."
+
+~Growth of bacteria in milk.~ Milk is so well suited as a medium for the
+development of germ life that it might be expected that all
+microorganisms would develop rapidly therein, and yet, as a matter of
+fact, growth does not begin at once, even though the milk may be richly
+seeded. At ordinary temperatures, such as 70 deg. F., no appreciable
+increase is to be noted for a period of 6-9 hours; at lower temperatures
+(54 deg. F.) this period is prolonged to 30-40 hours or even longer. After
+this period has elapsed, active growth begins and continues more or less
+rapidly until after curdling.
+
+The cause of this suspended development is attributed to the germicidal
+properties inherent to the milk.[40]
+
+Milk is of course seeded with a considerable variety of organisms at
+first. The liquefying and inert species are the most abundant, the
+distinctively lactic acid class occurring sparsely, if at all. As milk
+increases in age, germ growth begins to occur. More or less development
+of all types go on, but soon the lactic species gain the ascendency,
+owing to their being better suited to this environment; they soon
+outstrip all other species, with the result that normal curdling
+generally supervenes. The growth of this type is largely conditioned by
+the presence of the milk sugar. If the sugar is removed from milk by
+dialysis, the liquid undergoes putrefactive changes due to the fact that
+the putrefactive bacteria are able to grow if no acid is produced.
+
+~Relation of temperature to growth.~ When growth does once begin in milk,
+the temperature at which it is stored exerts the most profound effect on
+the rate of development. When milk is not artificially cooled, it
+retains its heat for some hours, and consequently the conditions become
+very favorable for the rapid multiplication of the contained organisms,
+as is shown in following results obtained by Freudenreich[41]:
+
+    _No. of bacteria per cc. in milk kept at different temperatures._
+
+                                 77 deg. F.       95 deg. F.
+     5 hrs. after milking       10,000        30,000
+     8  "     "    "            25,000    12,000,000
+    12  "     "    "            46,000    35,280,000
+    26  "     "    "         5,700,000    50,000,000
+
+[Illustration: FIG. 14. Effect of cooling milk on the growth of
+bacteria.]
+
+Conn[42] is inclined to regard temperature of more significance in
+determining the keeping quality than the original infection of the milk
+itself. Milk which curdled in 18 hours at 98 deg. F., did not curdle in 48
+hours at 70 deg., and often did not change in two weeks, if the
+temperature was kept at 50 deg. F.
+
+Where kept for a considerable period at this low temperature, the milk
+becomes filled with bacteria of the undesirable putrefactive type, the
+lactic group being unable to form acid in any appreciable amounts.
+Running well water can be used for cooling, if it is possible to secure
+it at a temperature of 48 deg.-50 deg. F. The use of ice, of course, gives
+better results, and in summer is greatly to be desired. The influence of
+these lowered temperatures makes it possible to ship milk long
+distances[43] by rail for city supplies, if the temperature is kept low
+during transit.
+
+~Mixing night and morning milk.~ Not infrequently it happens when old milk
+is mixed with new, that the course of the fermentative changes is more
+rapid than would have been the case if the two milks had been kept
+apart. Thus, adding the cooled night milk to the warm morning milk
+sometimes produces more rapid changes in both. The explanation for this
+often imperfectly understood phenomenon is that germ growth may have
+gone on in the cooled milk, and when this material is added to the
+warmer, but bacteria-poor, fresh milk, the temperature of the whole mass
+is raised to a point suitable for the more rapid growth of all bacteria
+than would have occurred if the older milk had been kept chilled.
+
+~Number of bacteria in milk.~ The number of organisms found in milk
+depends upon (1) the original amount of contamination, (2) the age of
+the milk, and (3) the temperature at which it has been held. These
+factors all fluctuate greatly in different cases; consequently, the germ
+life is subject to exceedingly wide variations. Here in America, milk
+reaches the consumer with less bacteria than in Europe, although it may
+often be older. This is due largely to the more wide-spread use of ice
+for chilling the milk _en route_ to market. Examinations have been made
+of various supplies with the following results: Sedgwick and Batchelder
+found in 57 tests of Boston milk from 30,000-4,220,000 per cc. Jordan
+and Heineman found 30% of samples of Chicago milk to range from 100,000
+to 1,000,000 while nearly one half were from 1-20,000,000 per cc. The
+germ content of city milks increase rapidly in the summer months.
+Park[44] found 250,000 organisms per cc. in winter, about 1,000,000 in
+cool weather and 5,000,000 per cc. in hot summer weather. Knox and
+Bassett in Baltimore report 1,500,000 in spring and nearly 4,500,000 in
+summer. Eckles[45] studied milk under factory conditions. He finds from
+1,000,000 to 5,000,000 per cc. in winter, and in summer from 15-30
+millions.
+
+~Bacterial standards for city supplies.~ It would be very desirable to
+have a hygienic standard for city milk supplies, as there is a butter
+fat and milk-solid test, but the wide spread variation in germ content
+and the impracticability of utilizing ordinary bacterial determinations
+(on account of time required) makes the selection of such a standard
+difficult. Some hold, as Park, that such a standard is feasible. The New
+York City Milk commission has set a standard of 30,000 bacteria per cc.
+for their certified milk and 100,000 per cc. for inspected milk.
+Rochester, N. Y. has attempted the enforcement of such a standard
+(limit, 100,000 per cc.) with good results it is claimed while Boston
+has placed the legal limit at 500,000 per cc. Quantitative standards
+would seem more applicable to "certified" or sanitary supplies than to
+general city supplies, where the wide range in conditions lead to such
+enormous variations that the bacterial standard seems too refined a
+method for practical routine inspection.
+
+~Other tests.~ Any test to be of much service must be capable of being
+quickly applied. The writer believes for city milk inspectors that the
+acid test would serve a very useful purpose. This test measures the
+acidity of the milk. There is, of course, no close and direct
+relationship between the development of acidity and the growth of
+bacteria, yet in a general way one follows the other at normal
+temperatures. Where the temperature is kept rather low, bacterial growth
+might go on without much acid development, but in the great majority of
+cases a high degree of acidity means either old milk, in which there has
+been a long period of incubation, or high temperature, where rapid
+bacterial growth has been possible. Either of these conditions
+encourages germ growth and thus impairs the quality of the milk.
+
+The rapid determination of acidity may be made in an approximate manner
+so as to serve as a test at the weigh-can or intake. The test is best
+made by the use of the well known alkaline tablet which is composed of a
+solid alkali, and the indicator, phenolphthalein. The tablets are
+dissolved in water, one to each ounce used. A number of white cups are
+filled with the proper quantity of the solution necessary to neutralize
+say, 0.2 per cent. lactic acid. Then, as the milk is delivered, the
+proper quantity is taken from each can to which is added the tablet
+solution. A retention of the pink color shows that there is not
+sufficient acid in the milk to neutralize the alkali used; a
+disappearance of color indicates an excess of acid. The standard
+selected is of course arbitrarily chosen. In our experience, 0.2 per
+cent. acidity (figured as lactic), has proven a satisfactory point. With
+carefully handled milk the acidity ought to be reduced to about 0.15 per
+cent. The acidity of the milk may be abnormally reduced if milk is kept
+in rusty cans, owing to action of acid on the metal.
+
+[Illustration: FIG. 15. Apparatus used in making rapid acid test. A
+definite quantity of the alkali solution and indicator is placed in the
+white tea cup. To this is added the quantity of milk by means of the
+cartridge measure which would just be neutralized if the acidity was 0.2
+per cent. A retention of the pink color shows a low acid milk; its
+disappearance, a high acid milk.]
+
+~Kinds of bacteria in milk.~ The number of bacteria in milk is not of so
+much consequence as the kinds present. With reference to the number of
+different species, the more dirt and foreign matter the milk contains,
+the larger the number of varieties found in the same. While milk may
+contain forms that are injurious to man, still the great majority of
+them have no apparent effect on human health. In their effect on milk,
+the case is much different. Depending upon their action in milk, they
+may be grouped into three classes:
+
+1. Inert group, those producing no visible change in the milk.
+
+2. Sour milk forms, those breaking up the milk sugar with or without the
+formation of gas.
+
+3. Digesting or peptonizing group, those capable of rendering the casein
+of milk soluble and more or less completely digested.
+
+A surprisingly large number of bacteria that are found in milk belong to
+the first class. Undoubtedly they affect the chemical characteristics of
+the milk somewhat, but not to the extent that it becomes physically
+perceptible. Eckles[46] reports in a creamery supply from 20 to 55 per
+cent. of entire flora as included in this class.
+
+By far the most important group is that embraced under the second head.
+It includes not only the true lactic acid types in which no gas is
+formed, but those species capable of producing gases and various kinds
+of acids. These organisms are the distinctively milk bacteria, although
+they do not predominate when the milk is first drawn. Their adaptation
+to this medium is normally shown, however, by this extremely rapid
+growth, in which they soon gain the ascendency over all other species
+present. It is to this lactic acid class that the favorable
+flavor-producing organisms belong which are concerned in butter-making.
+They are also indispensable in cheese-making.
+
+The third class represents those capable of producing a liquefied or
+digested condition on gelatin or in milk. They are usually the
+spore-bearing species which gain access from filth and dirt. Their high
+powers of resistance due to spores makes it difficult to eradicate this
+type, although they are materially held in subjection by the lactic
+bacteria. The number of different kinds that have been found in milk is
+quite considerable, something over 200 species having been described
+more or less thoroughly. In all probability, however, many of these
+forms will be found to be identical when they are subjected to a more
+critical study.
+
+~Direct absorption of taints.~ A tainted condition in milk may result from
+the development of bacteria, acting upon various constituents of the
+milk, and transforming these in such a way as to produce by-products
+that impair the flavor or appearance of the liquid; or it may be
+produced by the milk being brought in contact with any odoriferous or
+aromatic substance, under conditions that permit of the direct
+absorption of such odors.
+
+This latter class of taints is entirely independent of bacterial action,
+and is largely attributable to the physical property which milk
+possesses of being able to absorb volatile odors, the fat in particular,
+having a great affinity for many of these substances. This direct
+absorption may occur before the milk is withdrawn from the animal, or
+afterwards if exposed to strong odors.
+
+It is not uncommon for the milk of animals advanced in lactation to have
+a more or less strongly marked odor and taste; sometimes this is apt to
+be bitter, at other times salty to the taste. It is a defect that is
+peculiar to individual animals and is liable to recur at approximately
+the same period in lactation.
+
+The peculiar "cowy" or "animal odor" of fresh milk is an inherent
+peculiarity that is due to the direct absorption of volatile elements
+from the animal herself. This condition is very much exaggerated when
+the animal consumes strong-flavored substances as garlic, leeks, turnips
+and cabbage. The volatile substances that give to these vegetables their
+characteristic odor are quickly diffused through the system, and if such
+foods are consumed some few hours before milking, the odor in the milk
+will be most pronounced. The intensity of such taints is diminished
+greatly and often wholly disappears, if the milking is not done for some
+hours (8-12) after such foods are consumed.
+
+This same principle applies in lesser degree to many green fodders that
+are more suitable as feed for animals, as silage, green rye, rape, etc.
+Not infrequently, such fodders as these produce so strong a taint in
+milk as to render it useless for human use. Troubles from such sources
+could be entirely obviated by feeding limited quantities of such
+material immediately after milking. Under such conditions the taint
+produced is usually eliminated before the next milking. The milk of
+swill-fed cows is said to possess a peculiar taste, and the urine of
+animals fed on this food is said to be abnormally acid. Brewers' grains
+and distillery slops when fed in excess also induce a similar condition
+in the milk.
+
+Milk may also acquire other than volatile substances directly from the
+animal, as in cases where drugs, as belladonna, castor oil, sulfur,
+turpentine, jalap, croton oil, and many others have been used as
+medicine. Such mineral poisons as arsenic have been known to appear
+eight hours after ingestion, and persist for a period of three weeks
+before being eliminated.
+
+~Absorption of odors after milking.~ If milk is brought in contact with
+strong odors after being drawn from the animal, it will absorb them
+readily, as in the barn, where frequently it is exposed to the odor of
+manure and other fermenting organic matter.
+
+It has long been a popular belief that milk evolves odors and cannot
+absorb them so long as it is warmer than the surrounding air, but from
+experimental evidence, the writer[47] has definitely shown that the
+direct absorption of odors takes place much more rapidly when the milk
+is warm than when cold, although under either condition, it absorbs
+volatile substances with considerable avidity. In this test fresh milk
+was exposed to an atmosphere impregnated with odors of various essential
+oils and other odor-bearing substances. Under these conditions, the
+cooler milk was tainted very much less than the milk at body temperature
+even where the period of exposure was brief. It is therefore evident
+that an exposure in the cow barn where the volatile emanations from the
+animals themselves and their excreta taint the air will often result in
+the absorption of these odors by the milk to such an extent as to
+seriously affect the flavor.
+
+The custom of straining the milk in the barn has long been deprecated as
+inconsistent with proper dairy practice, and in the light of the above
+experiments, an additional reason is evident why this should not be
+done.
+
+Even after milk is thoroughly cooled, it may absorb odors as seen where
+the same is stored in a refrigerator with certain fruits, meats, fish,
+etc.
+
+~Distinguishing bacterial from non-bacterial taints.~ In perfectly fresh
+milk, it is relatively easy to distinguish between taints caused by the
+growth of bacteria and those attributable to direct absorption.
+
+If the taint is evident at time of milking, it is in all probability due
+to character of feed consumed, or possibly to medicines. If, however,
+the intensity of the taint grows more pronounced as the milk becomes
+older, then it is probably due to living organisms, which require a
+certain period of incubation before their fermentative properties are
+most evident.
+
+Moreover, if the difficulty is of bacterial origin, it can be frequently
+transferred to another lot of milk (heated or sterilized is preferable)
+by inoculating same with some of the original milk. Not all abnormal
+fermentations are able though to compete with the lactic acid bacteria,
+and hence outbreaks of this sort soon die out by the re-establishment of
+more normal conditions.
+
+~Treatment of directly absorbed taints.~ Much can be done to overcome
+taints of this nature by exercising greater care in regard to the feed
+of animals, and especially as to the time of feeding and milking. But
+with milk already tainted, it is often possible to materially improve
+its condition. Thorough aeration has been frequently recommended, but
+most satisfactory results have been obtained where a combined process of
+aeration and pasteurization was resorted to. Where the milk is used in
+making butter, the difficulty has been successfully met by washing the
+cream with twice its volume of hot water in which a little saltpeter has
+been dissolved (one teaspoonful per gallon), and then separating it
+again.[48]
+
+The treatment of abnormal conditions due to bacteria has been given
+already under the respective sources of infection, and is also still
+further amplified in following chapter.
+
+~Aeration.~ It is a common belief that aeration is of great aid in
+improving the quality of milk, yet, when closely studied, no material
+improvement can be determined, either where the milk is made into butter
+or sold as milk. Dean in Canada and Storch in Denmark have both
+experimented on the influence of aeration in butter making, but with
+negative results. Marshall and Doane failed to observe any material
+improvement in keeping quality, but it is true that odors are eliminated
+from the milk during aeration. The infection of the milk during aeration
+often more than counterbalances the reputed advantage. Especially is
+this so if the aeration is carried out in an atmosphere that is not
+perfectly clean and pure.
+
+In practice aeration differs greatly. In some cases, air is forced into
+the milk; in others, the milk is allowed to distribute itself in a thin
+sheet over a broad surface and fall some distance so that it is brought
+intimately in contact with the air. This latter process is certainly
+much more effective if carried out under conditions which preclude
+infection. It must be remembered that aeration is frequently combined
+with cooling, in which case the reputed advantages may not be entirely
+attributable to the process of aeration.
+
+~Infection of milk in the factory.~ The problem of proper handling of milk
+is not entirely solved when the milk is delivered to the factory or
+creamery, although it might be said that the danger of infection is much
+greater while the milk is on the farm.
+
+In the factory, infection can be minimized because effective measures of
+cleanliness can be more easily applied. Steam is available in most
+cases, so that all vats, cans, churns and pails can be thoroughly
+scalded. Special emphasis should be given to the matter of cleaning
+pumps and pipes. The difficulty of keeping these utensils clean often
+leads to neglect and subsequent infection. In Swiss cheese factories,
+the custom of using home-made rennet solutions is responsible for
+considerable factory infection. Natural rennets are soaked in whey which
+is kept warm in order to extract the rennet ferment. This solution when
+used for curdling the milk often adds undesirable yeasts and other
+gas-generating organisms, which are later the cause of abnormal ferment
+action in the cheese (See page 186).
+
+The influence of the air on the germ content of the milk is, as a rule,
+overestimated. If the air is quiet, and free from dust, the amount of
+germ life in the same is not relatively large. In a creamery or factory,
+infection from this source ought to be much reduced, for the reason that
+the floors and wall are, as a rule, quite damp, and hence germ life
+cannot easily be dislodged. The majority of organisms found under such
+conditions come from the person of the operators and attendants. Any
+infection can easily be prevented by having the ripening cream-vats
+covered with a canvas cloth. The clothing of the operator should be
+different from the ordinary wearing-apparel. If made of white duck, the
+presence of dirt is more quickly recognized, and greater care will
+therefore be taken than if ordinary clothes are worn.
+
+The surroundings of the factory have much to do with the danger of germ
+infection. Many factories are poorly constructed and the drainage is
+poor, so that filth and slime collect about and especially under the
+factory. The emanations from these give the peculiar "factory odor" that
+indicates fermenting matter. Not only are these odors absorbed
+directly, but germ life from the same is apt to find its way into the
+milk. Connell[49] has recently reported a serious defect in cheese that
+was traced to germ infection from defective factory drains.
+
+The water supply of a factory is also a question of prime importance.
+When taken from a shallow well, especially if surface drainage from the
+factory is possible, the water may be contaminated to such an extent as
+to introduce undesirable bacteria in such numbers that the normal course
+of fermentation may be changed. The quality of the water, aside from
+flavor, can be best determined by making a curd test (p. 76) which is
+done by adding some of the water to boiled milk and incubating the same.
+If "gassy" fermentations occur, it signifies an abnormal condition. In
+deep wells, pumped as thoroughly as is generally the case with factory
+wells, the germ content should be very low, ranging from a few score to
+a few hundred bacteria per cc. at most.
+
+Harrison[50] has recently traced an off-flavor in cheese in a Canadian
+factory to an infection arising from the water-supply. He found the same
+germ in both water and cheese and by inoculating a culture into
+pasteurized milk succeeded in producing the undesirable flavor. The
+danger from ice is much less, for the reason that good dairy practice
+does not sanction using ice directly in contact with milk or cream.
+Then, too, ice is largely purified in the process of freezing, although
+if secured from a polluted source, reliance should not be placed in the
+method of purification; for even freezing does not destroy all
+vegetating bacteria.
+
+FOOTNOTES:
+
+[1] Olson. 24 Rept. Wis. Expt. Stat., 1907.
+
+[2] Erf and Melick Bull. 131, Kan. Expt. Stat., Apr. 1905.
+
+[3] Storch (40 Rept. Danish Expt. Stat., Copenhagen, 1898) has devised a
+test whereby it can be determined whether this treatment has been
+carried out or not: Milk contains a soluble enzym known as galactase
+which has the property of decomposing hydrogen peroxid. If milk is
+heated to 176 deg. F. (80 deg. C.) or above, this enzym is destroyed so
+that the above reaction no longer takes place. If potassium iodid and
+starch are added to unheated milk and the same treated with hydrogen
+peroxid, the decomposition of the latter agent releases oxygen which
+acts on the potassium salt, which in turn gives off free iodine that
+turns the starch blue.
+
+[4] McKay, N. Y. Prod. Rev., Mch. 22, 1899.
+
+[5] Doane, Bull. 79, Md. Expt. Stat., Jan. 1902.
+
+[6] Harrison, 22 Rept. Ont. Agr'l Coll., 1896, p. 113.
+
+[7] Moore and Ward, Bull. 158, Cornell Expt. Stat., Jan. 1899; Ward,
+Bull. 178, Cornell Expt. Stat., Jan. 1900.
+
+[8] Harrison, 22 Rept. Ont. Agr. Coll., 1896, p. 108; Moore, 12 Rept.
+Bur. Animal Ind., U. S. Dept. Ag., 1895-6, p. 261.
+
+[9] Moore, Bacteria in Milk, N. Y. Dept. Ag., 1902.
+
+[10] Freudenreich, Cent. f. Bakt., II Abt., 10: 418, 1903.
+
+[11] Harrison, 22 Rept. Ont. Agr. Coll., 1896, p. 108.
+
+[12] Marshall, Bull. 147, Mich. Expt. Stat., p. 42.
+
+[13] Moore and Ward, Bull. 158, Cornell Expt. Stat., Jan. 1899.
+
+[14] Burr, R. H. Cent. f. Bakt., II Abt., 8: 236, 1902. Freudenreich, l.
+c. p. 418. Ward, Bull. 178, Cornell Expt. Stat., p. 277. Bolley (Cent.
+f. Bakt., II Abt., 1: 795, 1895), in 30 experiments found 12 out of 16
+species to belong to lactic class. Harrison (Trans. Can. Inst., 7: 474,
+1902-3) records the lactic type as most commonly present.
+
+[15] Ford, Journ. of Hyg., 1901, 1: 277.
+
+[16] Freudenreich, l. c. p. 421.
+
+[17] Stocking, Bull. 42, Storrs Expt. Stat., June, 1906.
+
+[18] Dinwiddie, Bull, 45 Ark. Expt. Stat., p. 57. Ward, Journ. Appld.
+Mic. 1: 205, 1898. Appel, Milch Zeit., No. 17, 1900. Harrison and
+Cumming, Journ. Appld. Mic. 5: 2087. Russell and Hastings, 21 Rept. Wis.
+Expt. Stat., 158, 1904.
+
+[19] Fokker, Zeit. f. Hyg., 9: 41, 1890.
+
+[20] Freudenreich, Ann. de Microg., 3: 118, 1891.
+
+[21] Hunziker, Bull. 197, Cornell Expt. Stat., Dec. 1901.
+
+[22] Freudenreich, Cent. f. Bakt., II Abt., 10: 417, 1903.
+
+[23] This general statement is in the main correct, although Ford
+(Journ. of Hyg., 1: 277, 1901) claims to have found organisms sparingly
+present in healthy tissues.
+
+[24] Backhaus, Milch Zeit., 26: 357, 1897.
+
+[25] Freudenreich, Die Bakteriologie, p. 30.
+
+[26] Stocking, Bull. 42, Storrs Expt. Stat., June 1906.
+
+[27] Harrison, Cent. f. Bakt., II Abt., 5: 183, 1899.
+
+[28] Drysdale, Trans. High. and Agr. Soc. Scotland. 5 Series, 10: 166,
+1898.
+
+[29] Schuppan, (Cent. f. Bakt., 13: 155, 1893) claims to have found a
+reduction of 48 per cent. in the Copenhagen filters while in the more
+extended work of Dunbar and Kister (Milch Zeit., pp. 753, 787, 1899) the
+bacterial content was higher in the filtered milk in 17 cases out of 22.
+
+[30] Backhaus and Cronheim, Journ. f. Landw., 45: 222, 1897.
+
+[31] Eckles and Barnes, Bull. 159 Iowa Expt. Stat., Aug. 1901.
+
+[32] Dunbar and Kister, Milch Zeit., p. 753, 1899. Harrison and Streit,
+Trans. Can. Inst., 7: 488, 1902-3.
+
+[33] Doane, Bull. 88 Md. Expt. Stat., May 1903.
+
+[34] Eckles, Hoard's Dairyman, July 8, 1898.
+
+[35] Fraser, Bull. 91, Ill. Expt. Stat.
+
+[36] Fraser, Bull. 91, Ill. Expt. Stat., Dec. 1903.
+
+[37] Stocking, Bull. 42, Storrs Expt. Stat., June, 1906.
+
+[38] Backhaus. Ber. Landw. Inst. Univ. Koenigsberg 2: 12, 1897.
+
+[39] De Schweinitz, Nat. Med. Rev., April, 1899.
+
+[40] Conn, Proc. Soc. Amer. Bacteriologists, 1902.
+
+[41] Freudenreich, Ann. de Microg., 2:115, 1890.
+
+[42] Conn, Bull. 26, Storrs Expt. Stat.
+
+[43] New York City is supplied with milk that is shipped 350 miles.
+
+[44] Park, N. Y. Univ. Bull., 1: 85, 1901.
+
+[45] Eckles, Bull. 59, Iowa Expt. Stat., Aug. 1901.
+
+[46] Eckles, Bull. 59, Iowa Expt. Stat., Aug. 1901.
+
+[47] Russell, 15 Rept. Wis. Expt. Stat. 1898, p. 104.
+
+[48] Alvord, Circ. No. 9, U. S. Dept. Agric. (Div. of Bot.).
+
+[49] Connell, Rept. of Commissioner of Agr., Canada, 1897, part XVI, p.
+15.
+
+[50] Harrison, Hoard's Dairyman, March 4, 1898.
+
+
+
+
+CHAPTER IV.
+
+FERMENTATIONS IN MILK AND THEIR TREATMENT.
+
+
+Under the conditions in which milk is drawn, it is practically
+impossible to secure the same without bacterial contamination. The
+result of the introduction of these organisms often changes its
+character materially as most bacteria cause the production of more or
+less pronounced fermentative processes. Under normal conditions, milk
+sours, i. e., develops lactic acid, but at times this more common
+fermentation may be replaced by other changes which are marked by the
+production of some other more or less undesirable flavor, odor or change
+in appearance.
+
+In referring to these changes, it is usually customary to designate them
+after the most prominent by-product formed, but it must be kept in mind
+that generally some other decomposition products are usually produced.
+Whether the organisms producing this or that series of changes prevail
+or not depends upon the initial seeding, and the conditions under which
+the milk is kept. Ordinarily, the lactic acid organisms grow so
+luxuriantly in the milk that they overpower all competitors and so
+determine the nature of the fermentation; but occasionally the milk
+becomes infected with other types of bacteria in relatively large
+numbers and the conditions may be especially suitable to the development
+of these forms, thereby modifying the course of the normal changes that
+occur.
+
+The kinds of bacteria that find it possible to develop in milk may be
+included under two heads:
+
+1. Those which cause no appreciable change in the milk, either in taste,
+odor or appearance. While these are frequently designated as the inert
+bacteria, it must not be supposed that they have absolutely no effect on
+milk. It is probably true in most cases that slight changes of a
+chemical nature are produced, but the nature of the changes do not
+permit of ready recognition.
+
+2. This class embraces all those organisms which, as a result of their
+growth, are capable of producing evident changes. These transformations
+may be such as to affect the taste, as in the sour milk or in the bitter
+fermentations, or the odor, as in some of the fetid changes, or the
+appearance of the milk, as in the slimy and color changes later
+described.
+
+~Souring of milk.~ Ordinarily if milk is allowed to stand for several days
+at ordinary temperatures it turns sour. This is due to the formation of
+lactic acid, which is produced by the decomposition of the milk-sugar.
+While this change is well nigh universal, it does not occur without a
+pre-existing cause, and that is the presence of certain living bacterial
+forms. These organisms develop in milk with great rapidity, and the
+decomposition changes that are noted in souring are due to the
+by-products of their development.
+
+The milk-sugar undergoes fermentation, the chief product being lactic
+acid, although various other by-products, as other organic acids
+(acetic, formic and succinic), different alcohols and gaseous products,
+as CO_{2}, H, N and methane (CH_{4}) are produced in small amounts.
+
+In this fermentation, the acidity begins to be evident to the taste when
+it reaches about 0.3 per cent., calculated as lactic acid. As the
+formation of acid goes on, the casein is precipitated and incipient
+curdling or lobbering of the milk occurs. This begins to be apparent
+when the acidity is about 0.4 per cent., but the curd becomes more solid
+with increasing acidity. The rapidity of curdling is also dependent upon
+the temperature of the milk. Thus milk which at ordinary temperatures
+might remain fluid often curdles when heated. The growth of the bacteria
+is continued until about 0.8 to 1.0 per cent. acid is formed, although
+the maximum amount fluctuates considerably with different lactic acid
+species. Further formation then ceases even though all of the milk-sugar
+is not used up, because of the inability of the lactic bacteria to
+continue their growth in such acid solutions.
+
+As this acidity is really in the milk serum, cream never develops so
+much acid as milk, because a larger proportion of its volume is made up
+of butter-fat globules. This fact must be considered in the ripening of
+cream in butter-making where the per cent. of fat is subject to wide
+fluctuations.
+
+The formation of lactic acid is a characteristic that is possessed by a
+large number of bacteria, micrococci as well as bacilli being numerously
+represented. Still the preponderance of evidence is in favor of the view
+that a few types are responsible for most of these changes. The most
+common type found in spontaneously soured milk changes the milk-sugar
+into lactic acid without the production of any gas. This type has been
+described by various workers on European as well as American milks, and
+is designated by Conn as the _Bact. lactis acidi_ type.[51] It is
+subject to considerable variation under different conditions.
+
+Curiously enough if milk which has been drawn with special care is
+examined immediately after milking, the lactic organisms are not usually
+found. They are incapable of development in the udder itself, as shown
+by injections into the milk cistern. They abound, however, on hay, in
+dust, in the barn air, on the hairy coat of the animal, and from these
+sources easily gain access to the milk. In this medium they find an
+exceptionally favorable environment and soon begin a very rapid growth,
+so that by the time milk is consumed, either in the form of milk or milk
+products, they make up numerically the larger portion of the bacteria
+present.
+
+Another widely disseminated, although numerically less prevalent, type
+is _B. lactis aerogenes_. This type forms gas in milk so that the soured
+milk is torn by the presence of gas bubbles. It also grows more
+luxuriantly in contact with the air.
+
+Other types occur more or less sporadically, some of which are capable
+of liquefying the casein of milk while at the same time they also
+develop lactic acid. Conn and Aikman refer to the fact that over one
+hundred species capable of producing variable quantities of lactic acid
+are already known. It is fair to presume, however, that a careful
+comparative study of these would show that simply racial differences
+exist in many cases, and therefore, that they are not distinct species.
+
+As a group these bacteria are characterized by their inability to
+liquefy gelatin or develop spores. On account of this latter
+characteristic they are easily destroyed when milk is pasteurized. They
+live under aerobic or anaerobic conditions, many of them being able to
+grow in either environment, although, according to McDonnell,[52] they
+are more virulent when air is not excluded.
+
+While growth of these lactic forms may go on in milk throughout a
+relatively wide range in temperature, appreciable quantities of acid are
+not produced except very slowly at temperatures below 50 deg. F.[53]
+
+From the standpoint of frequency the most common abnormal changes that
+occur in milk are those in which gases of varying character are
+developed in connection with acids, from the milk sugar. Other volatile
+products imparting bad flavors usually accompany gas production. These
+fermentations are of most serious import in the cheese industry, as they
+are especially prone to develop in the manufacture of milk into certain
+types of cheese. Not often is their development so rapid that they
+appear in the milk while it is yet in the hands of the milk producer,
+but almost invariably the introduction of the causal organisms takes
+place while the milk is on the farm. Numerous varieties of bacteria
+possess this property of producing gas (H and CO_{2} are most common
+although N and methane (CH_{4}) are sometimes produced). The more common
+forms are those represented by _B. lactis aerogenes_ and the common
+fecal type, _B. coli commune_. The ordinary habitat of this type is dirt
+and intestinal filth. Hence careless methods of milk handling invite
+this type of abnormal change in milk.
+
+It is a wide-spread belief that thunder storms cause milk to sour
+prematurely, but this idea has no scientific foundation. Experiments[54]
+with the electric spark, ozone and loud detonations show no effect on
+acid development, but the atmospheric conditions usually incident to a
+thunder storm are such as permit of a more rapid growth of organisms.
+There is no reason to believe but that the phenomenon of souring is
+wholly related to the development of bacteria. Sterile milks are never
+affected by the action of electric storms.
+
+~"Gassy" milks.~ Where these gas bacteria abound, the amount of lactic
+acid is generally reduced, due to the splitting up of some of the sugar
+into gaseous products. This type of germ life does not seem to be able
+to develop well in the presence of the typical lactic acid non
+gas-forming bacteria.
+
+[Illustration: FIG. 16. Cheese made from "gassy" milk.]
+
+~"Sweet curdling" and digesting fermentations.~ Not infrequently milk,
+instead of undergoing spontaneous souring, curdles in a weakly acid or
+neutral condition, in which state it is said to have undergone "sweet
+curdling." The coagulation of the milk is caused by the action of enzyms
+of a rennet type that are formed by the growth of various species of
+bacteria. Later the whey separates more or less perfectly from the curd,
+producing a "wheyed off" condition. Generally the coagulum in these
+cases is soft and somewhat slimy. The curd usually diminishes in bulk,
+due to the gradual digestion or peptonization of the casein by
+proteid-dissolving enzyms (tryptic type) that are also produced by the
+bacteria causing the change.
+
+A large number of bacteria possess the property of affecting milk in
+this way. So far as known they are able to liquefy gelatin (also a
+peptonizing process) and form spores. The Tyrothrix type of bacteria (so
+named by Duclaux on account of the supposed relation to cheese ripening)
+belongs to this class. The hay and potato forms are also digesters.
+Organisms of this type are generally associated with filth and manure,
+and find their way into the milk from the accumulations on the coat of
+the animal.
+
+Conn[55] has separated the rennet enzym from bacterial cultures in a
+relatively pure condition, while Fermi[56] has isolated the digestive
+ferment from several species.
+
+Duclaux[57] has given to this digesting enzym the name _casease_ or
+cheese ferment. These isolated ferments when added to fresh milk possess
+the power of causing the characteristic curdling and subsequent
+digestion quite independent of cell development. The quantity of ferment
+produced by different species differs materially in some cases. In these
+digestive fermentations, the chemical transformations are profound, the
+complex proteid molecule being broken down into albumoses, peptones,
+amido-acids (tyrosin and leucin) and ammonia as well as fatty acids.
+
+Not infrequently these fermentations gain the ascendency over the normal
+souring change, but under ordinary conditions they are held in abeyance,
+although this type of bacteria is always present to some extent in milk.
+When the lactic acid bacteria are destroyed, as in boiled, sterilized
+or pasteurized milk, these rennet-producing, digesting species develop.
+
+~Butyric acid fermentations.~ The formation of butyric acid in milk which
+may be recognized by the "rancid butter" odor is not infrequently seen
+in old, sour milk, and for a long time was thought to be a continuation
+of the lactic fermentation, but it is now believed that these organisms
+find more favorable conditions for growth, not so much on account of the
+lactic acid formed as in the absence of dissolved oxygen in the milk
+which is consumed by the sour-milk organisms.
+
+Most of the butyric class of bacteria are spore-bearing, and hence they
+are frequently present in boiled or sterilized milk. The by-products
+formed in this series of changes are quite numerous. In most cases,
+butyric acid is prominent, but in addition to this, other organic acids,
+as lactic, succinic, and acetic, are produced, likewise different
+alcohols. Concerning the chemical origin of butyric acid there is yet
+some doubt. Duclaux[58] affirms that the fat, sugar and casein are all
+decomposed by various forms. In some cases, the reaction of the milk is
+alkaline, with other species it may be neutral or acid. This type of
+fermentation has not received the study it deserves.
+
+In milk these organisms are not of great importance, as this
+fermentation does not readily gain the ascendency over the lactic
+bacteria.
+
+~Ropy or slimy milk.~ The viscosity of milk is often markedly increased
+over that which it normally possesses. The intensity of this abnormal
+condition may vary much; in some cases the milk becoming viscous or
+slimy; in others stringing out into long threads, several feet in
+length, as in Fig. 17. Two sets of conditions are responsible for these
+ropy or slimy milks. The most common is where the milk is clotted or
+stringy when drawn, as in some forms of garget. This is generally due to
+the presence of viscid pus, and is often accompanied by a bloody
+discharge, such a condition representing an inflamed state of the udder.
+Ropiness of this character is not usually communicable from one lot of
+milk to another.
+
+[Illustration: FIG. 17. Ropy milk.]
+
+The communicable form of ropy milk only appears after the milk has been
+drawn from the udder for a day or so, and is caused by the development
+of various species of bacteria which find their way into the milk after
+it is drawn. These defects are liable to occur at any season of the
+year. Their presence in a dairy is a source of much trouble, as the
+unsightly appearance of the milk precludes its use as food, although
+there is no evidence that these ropy fermentations are dangerous to
+health.
+
+There are undoubtedly a number of different species of bacteria that are
+capable of producing these viscid changes,[59] but it is quite probable
+that they are not of equal importance in infecting milk under natural
+conditions.
+
+In the majority of cases studied in this country,[60] the causal
+organism seems to be _B. lactis viscosus_, a form first found by Adametz
+in surface waters.[61] This organism possesses the property of
+developing at low temperatures (45 deg.-50 deg. F.), and consequently it
+is often able in winter to supplant the lactic-acid forms. Ward has found
+this germ repeatedly in water tanks where milk cans are cooled; and
+under these conditions it is easy to see how infection of the milk might
+occur. Marshall[62] reports an outbreak which he traced to an external
+infection of the udder; in another case, the slime-forming organism was
+abundant in the barn dust. A defect of this character is often
+perpetuated in a dairy for some time, and may therefore become
+exceedingly troublesome. In one instance in the writer's experience, a
+milk dealer lost over $150 a month for several months from ropy cream.
+Failure to properly sterilize cans, and particularly strainer cloths, is
+frequently responsible for a continuance of trouble of this sort.
+
+The slimy substance formed in milk comes from various constituents of
+the milk, and the chemical character of the slime produced also varies
+with different germs. In some cases the slimy material is merely the
+swollen outer cell membrane of the bacteria themselves as in the case of
+_B. lactis viscosus_; in others it is due to the decomposition of the
+proteids, but often the chief decomposition product appears to come from
+a viscous fermentation of the milk-sugar.
+
+An interesting case of a fermentation of this class being utilized in
+dairying is seen in the use of "lange wei" (long or stringy whey) which
+is employed as a starter in Holland to control the gassy fermentations
+in Edam cheese. This slimy change is due to the growth of
+_Streptococcus Hollandicus_.[63]
+
+~Alcoholic fermentations.~ Although glucose or cane-sugar solutions are
+extremely prone to undergo alcoholic fermentation, milk sugar does not
+readily undergo this change. Where such changes are produced it is due
+to yeasts. Several outbreaks attributable to such a cause have been
+reported.[64] Russell and Hastings[65] have found these milk-sugar
+splitting yeasts particularly abundant in regions where Swiss cheese is
+made, a condition made possible by the use of whey-soaked rennets in
+making such cheese.
+
+Kephir and Koumiss are liquors much used in the Orient which are made
+from milk that has undergone alcoholic fermentation. Koumiss was
+originally made from mare's milk but is now often made from cows' milk
+by adding cane sugar and yeast. In addition to the CO_{2} developed,
+alcohol, lactic acid, and casein-dissolving ferments are formed. Kephir
+is made by adding to milk Kephir grains, which are a mass of yeast and
+bacterial cells. The yeasts produce alcohol and CO_{2} while the
+bacteria change the casein of milk, rendering it more digestible. These
+beverages are frequently recommended to persons who seem to be unable to
+digest raw milk readily. The exact nature of the changes produced are
+not yet well understood.[66]
+
+~Bitter milk.~ The presence of bitter substances in milk may be ascribed
+to a variety of causes. A number of plants, such as lupines, ragweed and
+chicory, possess the property of affecting milk when the same are
+consumed by animals. At certain stages in lactation, a bitter salty
+taste is occasionally to be noted that is peculiar to individual
+animals.
+
+A considerable number of cases of bitter milk have, however, been traced
+to bacterial origin. For a number of years the bitter fermentation of
+milk was thought to be associated with the butyric fermentation, but
+Weigmann[67] showed that the two conditions were not dependent upon each
+other. He found that the organism which produced the bitter taste acted
+upon the casein.
+
+Conn[68] observed a coccus form in bitter cream that was able to impart
+a bitter flavor to milk. Sometimes a bitter condition does not develop
+in the milk, but may appear later in the milk products, as in the case
+of a micrococcus which Freudenreich[69] found in cheese.
+
+Harrison[70] has traced a common bitter condition in Canadian milk to a
+milk-sugar splitting yeast, _Torula amara_ which not only grows rapidly
+in milk but produces an undesirable bitterness in cheddar cheese.
+
+Cream ripened at low temperatures not infrequently develops a bitter
+flavor, showing that the optimum temperature for this type of
+fermentation is below the typical lactic acid change.
+
+Milk that has been heated often develops a bitter condition. The
+explanation of this is that the bacteria producing the bitter substances
+usually possess endospores, and that while the boiling or sterilizing of
+milk easily kills the lactic acid germs, these forms on account of their
+greater resisting powers are not destroyed by the heat.
+
+~Soapy milk:~ A soapy flavor in milk was traced by Weigmann and Zirn[71]
+to a specific bacillus, _B. lactis saponacei_, that they found gained
+access to the milk in one case from the bedding and in another instance
+from hay. A similar outbreak has been reported in this country,[72] due
+to a germ acting on the casein and albumen.
+
+~Red milk.~ The most common trouble of this nature in milk is due to
+presence of blood, which is most frequently caused by some wound in the
+udder. The ingestion of certain plants as sedges and scouring rushes is
+also said to cause a bloody condition; madders impart a reddish tinge
+due to coloring matter absorbed. Defects of this class can be readily
+distinguished from those due to germ growth because they are apparent at
+time of milking. Where blood is actually present, the corpuscles settle
+out in a short time if left undisturbed.
+
+There are a number of chromogenic or color-producing bacteria that are
+able to grow in milk, but their action is so slow that generally they
+are not of much consequence. Moreover their development is usually
+confined to the surface of the milk as it stands in a vessel. The most
+important is the well-known _B. prodigiosus_. Another form found at
+times in milk possessing low acidity[73] is _B. lactis erythrogenes_.
+This species only develops the red color in the dark. In the light, it
+forms a yellow pigment. Various other organisms have been reported at
+different times.[74]
+
+~Blue milk.~ Blue milk has been known for many years, its communicable
+nature being established as long ago as 1838. It appears on the surface
+of milk first as isolated particles of bluish or grey color, which
+later become confluent, the blue color increasing in intensity as the
+acidity increases. The causal organism, _B. cyanogenes_, is very
+resistant toward drying,[75] thus accounting for its persistence. In
+Mecklenberg an outbreak of this sort once continued for several years.
+It has frequently been observed in Europe in the past, but is not now so
+often reported. Occasional outbreaks have been reported in this country.
+
+~Other kinds of colored milk.~ Two or three chromogenic forms producing
+still other colors have occasionally been found in milk. Adametz[76]
+discovered in a sample of cooked milk a peculiar form (_Bacillus
+synxanthus_) that produced a citron-yellow appearance which precipitated
+and finally rendered soluble the casein. Adametz, Conn, and List have
+described other species that confer tints of yellow on milk. Some of
+these are bright lemon, others orange, and some amber in color.
+
+Still other color-producing bacteria, such as those that produce violet
+or green changes in the milk, have been observed. In fact, almost any of
+the chromogenic bacteria are able to produce their color changes in milk
+as it is such an excellent food medium. Under ordinary conditions, these
+do not gain access to milk in sufficient numbers so that they modify the
+appearance of it except in occasional instances.
+
+~Treatment of abnormal fermentations.~ If the taint is recognized as of
+bacterial origin (see p. 57) and is found in the mixed milk of the herd,
+it is necessary to ascertain, first, whether it is a general trouble, or
+restricted to one or more animals. This can sometimes be done by
+separating the milk of the different cows and noting whether any
+abnormal condition develops in the respective samples.
+
+~Fermentation tests.~ The most satisfactory way to detect the presence of
+the taints more often present is to make a fermentation test of one kind
+or another. These tests are most frequently used at the factory, to
+enable the maker to detect the presence of milk that is likely to prove
+unfit for use, especially in cheese making. They are based upon the
+principle that if milk is held at a moderately high temperature, the
+bacteria will develop rapidly. A number of different methods have been
+devised for this purpose. In Walther's lacto-fermentator samples of milk
+are simply allowed to stand in bottles or glass jars until they sour.
+They are examined at intervals of several hours. If the curdled milk is
+homogeneous and has a pure acid smell, the milk is regarded as all
+right. If it floats in a turbid serum, is full of gas or ragged holes,
+it is abnormal. As generally carried out, no attempt is made to have
+these vessels sterile. Gerber's test is a similar test that has been
+extensively employed in Switzerland. Sometimes a few drops of rennet are
+added to the milk so as to curdle the same, and thus permit of the more
+ready detection of the gas that is evolved.
+
+~Wisconsin curd test.~ The method of testing milk described below was
+devised at the Wisconsin Experiment Station in 1895 by Babcock, Russell
+and Decker.[77] It was used first in connection with experimental work
+on the influence of gas-generating bacteria in cheese making, but its
+applicability to the detection of all taints in milk produced by
+bacteria makes it a valuable test for abnormal fermentations in general.
+
+In the curd test a small pat of curd is made in a glass jar from each
+sample of milk. These tests may be made in any receptacle that has been
+cleaned in boiling water, and to keep the temperature more nearly
+uniform these jars should be immersed in warm water, as in a wash tub or
+some other receptacle. When the milk is about 95 deg. F., about ten drops
+of rennet extract are added to each sample and mixed thoroughly with the
+milk. The jars should then remain undisturbed until the milk is
+completely curdled; then the curd is cut into small pieces with a case
+knife and stirred to expel the whey. The whey should be poured off at
+frequent intervals until the curd mats. If the sample be kept at blood
+heat (98 deg. F.) for six to eight hours, it will be ready to examine.
+
+[Illustration: FIG. 18. Improved bottles for making curd test. _A_, test
+bottle complete; _B_, bottle showing construction of cover; _S_, sieve
+to hold back the curd when bottle is inverted; _C_, outer cover with _(D
+H)_ drain holes to permit of removal of whey.]
+
+More convenient types of this test than the improvised apparatus just
+alluded to have been devised by different dairy manufacturers.
+Generally, they consist of a special bottle having a full-sized top,
+thus permitting the easy removal of the curd. The one shown in Fig. 18
+is provided with a sieve of such construction that the bottles will
+drain thoroughly if inclined in an inverted position.
+
+~Interpretation of results of test.~ The curd from a good milk has a firm,
+solid texture, and should contain at most only a few small pin holes. It
+may have some large, irregular, "mechanical" holes where the curd
+particles have failed to cement, as is seen in Fig. 19. If gas-producing
+bacteria are very prevalent in the milk, the conditions under which the
+test is made cause such a rapid growth of the same that the evidence of
+the abnormal fermentation may be readily seen in the spongy texture of
+the curd (Fig. 20). If the undesirable organisms are not very abundant
+and the conditions not especially suited to their growth, the "pin
+holes" will be less frequent.
+
+[Illustration: FIG. 19. Curd from a good milk. The large irregular holes
+are mechanical.]
+
+Sometimes the curds show no evidence of gas, but their abnormal
+condition can be recognized by the "mushy" texture and the presence of
+"off" flavors that are rendered more apparent by keeping them in closed
+bottles. This condition is abnormal and is apt to produce quite as
+serious results as if gas was formed.
+
+~Overcoming taints by use of starters.~ Another method of combatting
+abnormal fermentations that is often fruitful, is that which rests upon
+the inability of one kind of bacteria to grow in the same medium in
+competition with certain other species.
+
+Some of the undesirable taints in factories can be controlled in large
+part by the introduction of starters made from certain organisms that
+are able to obtain the ascendency over the taint-producing germ. Such a
+method is commonly followed when a lactic ferment, either a commercial
+pure culture, or a home-made starter, is added to milk to overcome the
+effect of gas-generating bacteria.
+
+[Illustration: FIG. 20. Curd from a badly tainted milk. Large ragged
+holes are mechanical; numerous small holes due to gas. This curd was a
+"floater."]
+
+A similar illustration is seen in the case of the "lange wei" (slimy
+whey), that is used in the manufacture of Edam cheese to control the
+character of the fermentation of the milk.
+
+This same method is sometimes applied in dealing with certain abnormal
+fermentations that are apt to occur on the farm. It is particularly
+useful with those tainted milks known as "sweet curdling." The ferment
+organisms concerned in this change are unable to develop in the
+presence of lactic acid bacteria, so the addition of a clean sour milk
+as a starter restores the normal conditions by giving the ordinary milk
+bacteria the ascendency.
+
+~Chemical disinfection.~ In exceptional instances it may be necessary to
+employ chemical disinfectants to restore the normal conditions. Of
+course with such diseases as tuberculosis, very stringent measures are
+required, as they are such a direct menace to human life, but with these
+abnormal or taint-producing fermentations, care and cleanliness, well
+directed, will usually overcome the trouble.
+
+If it becomes necessary to employ chemical substances as disinfecting
+agents, their use should always be preceded by a thorough cleansing with
+hot water so that the germicide may come in direct contact with the
+surface to be disinfected.
+
+It must be borne in mind that many chemicals act as deodorants, _i.e._,
+destroy the offensive odor, without destroying the cause of the trouble.
+
+_Sulfur_ is often recommended as a disinfecting agent, but its use
+should be carefully controlled, otherwise the vapors have but little
+germicidal power. The common practice of burning a small quantity in a
+room or any closed space for a few moments has little or no effect upon
+germ life. The effect of sulfur vapor (SO_{2}) alone upon germ life is
+relatively slight, but if this gas is produced in the presence of
+moisture, sulfurous acid (H_{2}SO_{3}) is formed, which is much more
+efficient. To use this agent effectively, it must be burned in large
+quantities in a moist atmosphere (three lbs. to every 1,000 cubic feet
+of space), for at least twelve hours. After this operation, the space
+should be thoroughly aired.
+
+_Formalin_, a watery solution of a gas known as formaldehyde, is a new
+disinfectant that recent experience has demonstrated to be very useful.
+It may be used as a gas where rooms are to be disinfected, or applied as
+a liquid where desired. It is much more powerful in its action than
+sulfur, and it has a great advantage over mercury and other strong
+disinfectants, as it is not so poisonous to man as it is to the lower
+forms of life.
+
+_Bleaching powder or chloride of lime_ is often recommended where a
+chemical can be advantageously used. This substance is a good
+disinfectant as well as a deodorant, and if applied as a wash, in the
+proportion of four to six ounces of the powder to one gallon of water,
+it will destroy most forms of life. In many cases this agent is
+inapplicable on account of its odor.
+
+_Corrosive sublimate_ (HgCl_{2}) for most purposes is a good
+disinfectant, but it is such an intense poison that its use is dangerous
+in places that are at all accessible to stock.
+
+For the disinfection of walls in stables and barns, common thin _white
+wash_ Ca(OH)_{2} is admirably adapted if made from freshly-burned quick
+lime. It possesses strong germicidal powers, increases the amount of
+light in the barn, is a good absorbent of odors, and is exceedingly
+cheap.
+
+Carbolic acid, creosote, and such products, while excellent
+disinfectants, cannot well be used on account of their odor, especially
+in factories.
+
+For gutters, drains, and waste pipes in factories, _vitriol salts_
+(sulfates of copper, iron and zinc) are sometimes used. These are
+deodorants as well as disinfectants, and are not so objectionable to use
+on account of their odor.
+
+These suggestions as to the use of chemicals, however, only apply to
+extreme cases and should not be brought into requisition until a
+thorough application of hot water, soap, a little soda, and the
+scrubbing brush have failed to do their work.
+
+FOOTNOTES:
+
+[51] Guenther and Thierfelder, Arch. f. Hyg., 25:164, 1895; Leichmann,
+Cent. f. Bakt., 2:281, 1896; Esten, 9 Rept. Storrs Expt. Stat., p. 44,
+1896; Dinwiddie, Bull. 45, Ark. Expt. Stat., May, 1897; Kozai, Zeit. f.
+Hyg., 38:386, 1901; Weigmann, Hyg. Milk Congress, Hamburg, 1903, p. 375.
+
+[52] McDonnell, Inaug. Diss., Kiel. 1899, p. 39.
+
+[53] Kayser, Cent. f. Bakt. II. Abt. 1:436.
+
+[54] Treadwell, Science, 1894, 17:178.
+
+[55] Conn, 5 Rept. Storrs Expt. Stat., 1892, p. 396.
+
+[56] Fermi, Arch. f. Hyg., 1892, 14:1.
+
+[57] Duclaux, Le Lait, p. 121.
+
+[58] Duclaux, Principes de Laiterie, p. 67.
+
+[59] Guillebeau (Milch Zeit., 1892, p. 808) has studied over a dozen
+different forms that possess this property.
+
+[60] Ward, Bull. 165, Cornell Expt. Stat., Mch., 1899; also Bull. 195,
+Ibid., Nov., 1901.
+
+[61] Adametz, Landw. Jahr., 1891, p. 185.
+
+[62] Marshall, Mich. Expt. Stat., Bull. 140.
+
+[63] Milch Zeit., 1899, p. 982.
+
+[64] Duclaux, Principes de Laiterie, p. 60. Heinze and Cohn, Zeit. f.
+Hyg., 46: 286, 1904.
+
+[65] Bull. 128, Wis. Expt. Stat., Sept. 1905.
+
+[66] Freudenreich, Landw. Jahr. d. Schweiz, 1896, 10; 1.
+
+[67] Weigmann, Milch Zeit., 1890, p. 881.
+
+[68] Conn, 3 Rept. Storrs Expt. Stat., 1890, p. 158.
+
+[69] Freudenreich, Fuehl. Landw. Ztg. 43: 361.
+
+[70] Harrison, Bull. 120 Ont. Agr'l. Coll., May, 1902.
+
+[71] Milch Zeit. 22:569.
+
+[72] Marshall, Bull. 146, Mich. Expt. Stat., p. 16.
+
+[73] Grotenfelt, Milch Zeit., 1889, p. 263.
+
+[74] Menge, Cent. f. Bakt., 6:596; Keferstein, Cent. f. Bakt., 21:177.
+
+[75] Heim, Arb. a. d. Kais. Gesundheitsamte, 5:578.
+
+[76] Adametz, Milch Zeit., 1890, p. 225.
+
+[77] 12 Rept. Wis. Expt. Stat., 1895, p. 148; also Bull. 67, Ibid.,
+June, 1898.
+
+
+
+
+CHAPTER V.
+
+RELATION OF DISEASE-BACTERIA TO MILK.
+
+
+Practical experience with epidemic disease has abundantly demonstrated
+the fact that milk not infrequently serves as a vehicle for the
+dissemination of contagion. Attention has been prominently called to
+this relation by Ernest Hart,[78] who in 1880 compiled statistical
+evidence showing the numerous outbreaks of various contagious diseases
+that had been associated with milk infection up to that time. Since
+then, further compilations have been made by Freeman,[79] and also by
+Busey and Kober,[80] who have collected the data with reference to
+outbreaks from 1880 to 1899.
+
+These statistics indicate the relative importance of milk as a factor in
+the dissemination of disease.
+
+The danger from this source is much intensified for the reason that
+milk, generally speaking, is consumed in a raw state; and also because a
+considerable number of disease-producing bacteria are able, not merely
+to exist, but actually thrive and grow in milk, even though the normal
+milk bacteria are also present. Moreover the recognition of the presence
+of such pathogenic forms is complicated by the fact that often they do
+not alter the appearance of the milk sufficiently so that their
+presence can be detected by a physical examination. These facts which
+have been experimentally determined, coupled with the numerous clinical
+cases on record, make a strong case against milk serving as an agent in
+the dissemination of disease.
+
+~Origin of pathogenic bacteria in milk.~ Disease-producing bacteria may be
+grouped with reference to their relation toward milk into two classes,
+depending upon the manner in which infection occurs:
+
+Class I. Disease-producing bacteria capable of being transmitted
+directly from a diseased animal to man through the medium of infected
+milk.
+
+Class II. Bacteria pathogenic for man but not for cattle which are
+capable of thriving in milk after it is drawn from the animal.
+
+In the first group the disease produced by the specific organism must be
+common to both cattle and man. The organism must live a parasitic life
+in the animal, developing in the udder, and so infect the milk supply.
+It may, of course, happen that diseases toward which domestic animals
+alone are susceptible may be spread from one animal to another in this
+way without affecting human beings.
+
+In the second group, the bacterial species lives a saprophytic
+existence, growing in milk, if it happens to find its way therein. In
+such cases milk indirectly serves as an agent in the dissemination of
+disease, by giving conditions favorable to the growth of the disease
+germ.
+
+By far the most important of diseases that may be transmitted directly
+from animal to man through a diseased milk supply is tuberculosis, but
+in addition to this, foot and mouth disease (aphthous fever in
+children), anthrax and acute enteric troubles have also been traced to a
+similar source of infection.
+
+The most important specific diseases that have been disseminated through
+subsequent pollution of the milk are typhoid fever, diphtheria, scarlet
+fever and cholera, but, of course, the possibility exists that any
+disease germ capable of living and thriving in milk may be spread in
+this way. In addition to these diseases that are caused by the
+introduction of specific organisms (the causal organism of scarlet fever
+has not yet been definitely determined), there are a large number of
+more or less illy-defined troubles of an intestinal character that occur
+especially in infants and young children that are undoubtedly
+attributable to the activity of microorganisms that gain access to milk
+during and subsequent to the milking, and which produce changes in milk
+before or after its ingestion that result in the formation of toxic
+products.
+
+
+DISEASES TRANSMISSIBLE FROM ANIMAL TO MAN THROUGH DISEASED MILK.
+
+~Tuberculosis.~ In view of the wide-spread distribution of this disease in
+both the human and the bovine race, the relation of the same to milk
+supplies is a question of great importance. It is now generally admitted
+that the different types of tubercular disease found in different kinds
+of animals and man are attributable to the development of the same
+organism, _Bacillus tuberculosis_, although there are varieties of this
+organism found in different species of animals that are sufficiently
+distinct to permit of recognition.
+
+The question of prime importance is, whether the bovine type is
+transmissible to the human or not. Artificial inoculation of cattle with
+tuberculous human sputum as well as pure cultures of this variety show
+that the human type is able to make but slight headway in cattle. This
+would indicate that the danger of cattle acquiring the infection from
+man would in all probability be very slight, but these experiments offer
+no answer as to the possibility of transmission from the bovine to the
+human. Manifestly it is impossible to solve this problem by direct
+experiment upon man except by artificial inoculation, but comparative
+experiments upon animals throw some light on the question.
+
+Theo. Smith[81] and others[82] have made parallel experiments with
+animals such as guinea pigs, rabbits and pigeons, inoculated with both
+bovine and human cultures of this organism. The results obtained in the
+case of all animals tested show that the virulence of the two types was
+much different, but that the bovine cultures were much more severe.
+While of course this does not prove that transmission from bovine to
+human is possible, still the importance of the fact must not be
+overlooked.
+
+In a number of cases record of accidental infection from cattle to man
+has been noted.[83] These have occurred with persons engaged in making
+post-mortem examinations on tuberculous animals, and the tubercular
+nature of the wound was proven in some cases by excision and
+inoculation.
+
+In addition to data of this sort that is practically experimental in
+character, there are also strong clinical reasons for considering that
+infection of human beings may occur through the medium of milk.
+Naturally such infection should produce intestinal tuberculosis, and it
+is noteworthy that this phase of the disease is quite common in
+children especially between the ages of two and five.[84] It is
+difficult to determine, though, whether primary infection occurred
+through the intestine, for, usually, other organs also become involved.
+In a considerable number of cases in which tubercular infection by the
+most common channel, inhalation, seems to be excluded, the evidence is
+strong that the disease was contracted through the medium of the milk,
+but it is always very difficult to exclude the possibility of pulmonary
+infection.
+
+Tuberculosis as a bovine disease has increased rapidly during recent
+decades throughout many portions of the world. This has been most marked
+in dairy regions. Its extremely insidious nature does not permit of an
+early recognition by physical means, and it was not until the
+introduction of the tuberculin test[85] in 1892, as a diagnostic aid
+that accurate knowledge of its distribution was possible. The quite
+general introduction of this test in many regions has revealed an
+alarmingly large percentage of animals as affected. In Denmark in 1894
+over forty per cent were diagnosed as tubercular. In some parts of
+Germany almost as bad a condition has been revealed. Slaughter-house
+statistics also show that the disease has increased rapidly since 1890.
+In this country the disease on the average is much less than in Europe
+and is also very irregularly distributed. In herds where it gained a
+foothold some years ago, often the majority of animals are frequently
+infected; many herds, in fact the great majority, are wholly free from
+all taint. The disease has undoubtedly been most frequently introduced
+through the purchase of apparently healthy but incipiently affected
+animals. Consequently in the older dairy regions where stock has been
+improved the most by breeding, more of the disease exists than among the
+western and southern cattle.
+
+[Illustration FIG. 21: Front view of a tuberculous udder, showing extent
+of swelling in single quarter.]
+
+~Infectiousness of milk of reacting animals.~ Where the disease appears in
+the udder the milk almost invariably contains the tubercle organism.
+Under such conditions the appearance of the milk is not materially
+altered at first, but as the disease progresses the percentage of fat
+generally diminishes, and at times in the more advanced stages where the
+physical condition of the udder is changed (Fig. 21), the milk may
+become "watery"; but the percentage of animals showing such udder
+lesions is not large, usually not more than a few per cent. (4 per cent.
+according to Ostertag.)
+
+On the other hand, in the earlier phases of the disease, where its
+presence has been recognized solely by the aid of the tuberculin test,
+before there are any recognizable physical symptoms in any part of the
+animal, the milk is generally unaffected. Between these extremes,
+however, is found a large proportion of cases, concerning which so
+definite data are not available. The results of investigators on this
+point are conflicting and further information is much desired. Some have
+asserted so long as the udder itself shows no lesions that no tubercle
+bacilli would be present,[86] but the findings of a considerable number
+of investigators[87] indicate that even when the udder is apparently not
+diseased the milk may contain the specific organism as revealed by
+inoculation experiments upon animals. In some cases, however, it has
+been demonstrated by post-mortem examination that discoverable udder
+lesions existed that were not recognizable before autopsy was made. In
+the experimental evidence collected, a varying percentage of reacting
+animals were found that gave positive results; and this number was
+generally sufficient to indicate that the danger of using milk from
+reacting animals was considerable, even though apparently no disease
+could be found in the udder.
+
+The infectiousness of milk can also be proven by the frequent
+contraction of the disease in other animals, such as calves and pigs
+which may be fed on the skim milk. The very rapid increase of the
+disease among the swine of Germany and Denmark,[88] and the frequently
+reported cases of intestinal infection of young stock also attest the
+presence of the organism in milk.
+
+The tubercle bacillus is so markedly parasitic in its habits, that,
+under ordinary conditions, it is incapable of growing at normal air
+temperatures. There is, therefore, no danger of the germ developing in
+milk after it is drawn from the animal, unless the same is kept at
+practically blood heat.
+
+Even though the milk of some reacting animals may not contain the
+dangerous organism at the time of making the test, it is quite
+impossible to foretell how long it will remain free. As the disease
+becomes more generalized, or if tuberculous lesions should develop in
+the udder, the milk may pass from a healthy to an infectious state.
+
+This fact makes it advisable to exclude from milk supplies intended for
+human use, all milk of animals that respond to the tuberculin test; or
+at least to treat it in a manner so as to render it safe. Whether it is
+necessary to do this or not if the milk is made into butter or cheese is
+a somewhat different question. Exclusion or treatment is rendered more
+imperative in milk supplies, because the danger is greater with children
+with whom milk is often a prominent constituent of their diet, and also
+for the reason that the child is more susceptible to intestinal
+infection than the adult.
+
+The danger of infection is much lessened in butter or cheese, because
+the processes of manufacture tend to diminish the number of organisms
+originally present in the milk, and inasmuch as no growth can ordinarily
+take place in these products the danger is minimized. Moreover, the fact
+that these foods are consumed by the individual in smaller amounts than
+is generally the case where milk is used, and also to a greater extent
+by adults, lessens still further the danger of infection.
+
+Notwithstanding this, numerous observers[89] especially in Germany have
+succeeded in finding the tubercle bacillus in market butter, but this
+fact is not so surprising when it is remembered that a very large
+fraction of their cattle show the presence of the disease as indicated
+by the tuberculin test, a condition that does not obtain in any large
+section in this country.
+
+The observations on the presence of the tubercle bacillus in butter have
+been questioned somewhat of late[2] by the determination of the fact
+that butter may contain an organism that possesses the property of being
+stained in the same way as the tubercle organism. Differentiation
+between the two forms is rendered more difficult by the fact that this
+tubercle-like organism is also capable of producing in animals lesions
+that stimulate those of tuberculosis, although a careful examination
+reveals definite differences. Petri[90] has recently determined that
+both the true tubercle and the acid-resisting butter organism may be
+readily found in market butter.
+
+In the various milk products it has been experimentally determined that
+the true tubercle bacillus is able to retain its vitality in butter for
+a number of months and in cheese for nearly a year.
+
+~Treatment of milk from tuberculosis cows.~ While it has been shown that
+it is practically impossible to foretell whether the milk of any
+reacting animal actually contains tubercle bacilli or not, still the
+interests of public health demand that no milk from such stock be used
+for human food until it has been rendered safe by some satisfactory
+treatment.
+
+_1. Heating._ By far the best treatment that can be given such milk is
+to heat it. The temperature at which this should be done depends upon
+the thermal death point of the tubercle bacillus, a question concerning
+which there has been considerable difference of opinion until very
+recently. According to the work of some of the earlier investigators,
+the tubercle bacillus in its vegetative stage is endowed with powers of
+resistance greater than those possessed by any other pathogenic
+organism. This work has not been substantiated by the most recent
+investigations on this subject. In determining the thermal death point
+of this organism, as of any other, not only must the temperature be
+considered, but the period of exposure as well, and where that exposure
+is made in milk, another factor must be considered, viz., the presence
+of conditions permitting of the formation of a "scalded layer," for as
+Smith[91] first pointed out, the resistance of the tubercle organism
+toward heat is greatly increased under these conditions. If tuberculous
+milk is heated in a closed receptacle where this scalded membrane cannot
+be produced, the tubercle bacillus is killed at 140 deg. F. in 15 to 20
+minutes. These results which were first determined by Smith, under
+laboratory conditions, and confirmed by Russell and Hastings,[92] where
+tuberculous milk was heated in commercial pasteurizers, have also been
+verified by Hesse.[93] A great practical advantage which accrues from
+the treatment of milk at 140 deg. F. is that the natural creaming is
+practically unaffected. Of course, where a higher temperature is
+employed, the period of exposure may be materially lessened. If milk is
+momentarily heated to 176 deg. F., it is certainly sufficient to destroy
+the tubercle bacillus. This is the plan practiced in Denmark where all
+skim milk and whey must be heated to this temperature before it can be
+taken back to the farm, a plan which is designed to prevent the
+dissemination of tuberculosis and foot and mouth disease by means of the
+mixed creamery by-products. This course renders it possible to utilize
+with perfect safety, for milk supplies, the milk of herds reacting to the
+tuberculin test, and as butter of the best quality can be made from
+cream or milk heated to even high temperatures,[94] it thus becomes
+possible to prevent with slight expense what would otherwise entail a
+large loss.
+
+_2. Dilution._ Another method that has been suggested for the treatment
+of this suspected milk is dilution with a relatively large volume of
+perfectly healthy milk. It is a well known fact that to produce
+infection, it requires the simultaneous introduction of a number of
+organisms, and in the case of tuberculosis, especially that produced by
+ingestion, this number is thought to be considerable. Gebhardt[95] found
+that the milk of tuberculous cows, which was virulent when injected by
+itself into animals, was innocuous when diluted with 40 to 100 times its
+volume of healthy milk. This fact is hardly to be relied upon in
+practice, unless the proportion of reacting to healthy cows is
+positively known.
+
+It has also been claimed in the centrifugal separation of cream from
+milk[96] that by far the larger number of tubercle bacilli were thrown
+out with the separator slime. Moore[97] has shown that the tubercle
+bacilli in an artificially infected milk might be reduced in this way,
+so as to be no longer microscopically demonstrable, yet the purification
+was not complete enough to prevent the infection of animals inoculated
+with the milk.
+
+Another way to exclude all possibility of tubercular infection in milk
+supplies is to reject all milk from reacting animals. This method is
+often followed where pasteurization or sterilization is not desired. In
+dairies where the keeping quality is dependent upon the exclusion of
+bacteria by stringent conditions as to milking and handling ("sanitary"
+or "hygienic" milk), the tuberculin test is frequently used as a basis
+to insure healthy milk.
+
+~Foot and mouth disease.~ The wide-spread extension of this disease
+throughout Europe in recent years has given abundant opportunity to show
+that while it is distinctively an animal malady, it is also
+transmissible to man, although the disease is rarely fatal. The causal
+organism has not been determined with certainty, but it has been shown
+that the milk of affected animals possesses infectious properties[98]
+although appearing unchanged in earlier phases of the disease.
+
+Hertwig showed the direct transmissibility of the disease to man by
+experiments made on himself and others. By ingesting milk from an
+affected animal, he was able to produce the symptoms of the disease, the
+mucous membrane of the mouth being covered with the small vesicles that
+characterize the malady. It has also been shown that the virus of the
+disease may be conveyed in butter.[99] This disease is practically
+unknown in this country, although widely spread in Europe.
+
+There are a number of other bovine diseases such as anthrax,[100]
+lockjaw,[101] and hydrophobia[102] in which it has been shown that the
+virus of the disease is at times to be found in the milk supply, but
+often the milk becomes visibly affected, so that the danger of using the
+same is greatly minimized.
+
+There are also a number of inflammatory udder troubles known as garget
+or mammitis. In most of these, the physical appearance of the milk is so
+changed, and often pus is present to such a degree as to give a very
+disagreeable appearance to the milk. Pus-forming bacteria (staphylococci
+and streptococci) are to be found associated with such troubles. A
+number of cases of gastric and intestinal catarrh have been reported as
+caused by such milks.[103]
+
+
+DISEASES TRANSMISSIBLE TO MAN THROUGH INFECTION OF MILK AFTER
+WITHDRAWAL.
+
+Milk is so well adapted to the development of bacteria in general, that
+it is not surprising to find it a suitable medium for the growth of many
+pathogenic species even at ordinary temperatures. Not infrequently,
+disease-producing bacteria are able to grow in raw milk in competition
+with the normal milk bacteria, so that even a slight contamination may
+suffice to produce infection.
+
+The diseases that are most frequently disseminated in this way are
+typhoid fever, diphtheria, scarlet fever and cholera, together with the
+various illy-defined intestinal troubles of a toxic character that occur
+in children, especially under the name of cholera infantum, summer
+complaint, etc.
+
+Diseases of this class are not derived directly from animals because
+cattle are not susceptible to the same.
+
+~Modes of infection.~ In a variety of ways, however, the milk may be
+subject to contaminating influences after it is drawn from the animal,
+and so give opportunity for the development of disease-producing
+bacteria. The more important methods of infection are as follows:
+
+_1. Infection directly from a pre-existing case of disease on premises._
+Quite frequently a person in the early stage of a diseased condition may
+continue at his usual vocation as helper in the barn or dairy, and so
+give opportunity for direct infection to occur. In the so-called cases
+of "walking typhoid," this danger is emphasized. It is noteworthy in
+typhoid fever that the bacilli frequently persist in the urine and in
+diphtheria they often remain in the throat until after convalescence. In
+some cases infection has been traced to storage of the milk in rooms in
+the house where it became polluted directly by the emanations of the
+patient.[104] Among the dwellings of the lower classes where a single
+room has to be used in common this source of infection has been most
+frequently observed.
+
+_2. Infection through the medium of another person._ Not infrequently
+another individual may serve in the capacity of nurse or attendant to a
+sick person, and also assist in the handling of the milk, either in
+milking the animals or caring for the milk after it has been drawn.
+Busey and Kober report twenty-one outbreaks of typhoid fever in which
+dairy employees also acted in the capacity of nurses.
+
+_3. Pollution of milk utensils._ The most frequent method of infection
+of cans, pails, etc., is in cleaning them with water that may be
+polluted with disease organisms. Often wells may be contaminated with
+diseased matter of intestinal origin, as in typhoid fever, and the use
+of water at normal temperatures, or even in a lukewarm condition, give
+conditions permitting of infection. Intentional adulteration of milk
+with water inadvertently taken from polluted sources has caused quite a
+number of typhoid outbreaks.[105] Sedgwick and Chapin[106] found in the
+Springfield, Mass., epidemic of typhoid that the milk cans were placed
+in a well to cool the milk, and it was subsequently shown that the well
+was polluted with typhoid fecal matter.
+
+_4. Pollution of udder_ of animal _by wading in infected water_, or by
+washing same with contaminated water. This method of infection would
+only be likely to occur in case of typhoid. An outbreak at the
+University of Virginia in 1893[107] was ascribed to the latter cause.
+
+_5. Pollution of creamery by-products, skim-milk, etc._ Where the milk
+supply of one patron becomes infected with pathogenic bacteria, it is
+possible that disease may be disseminated through the medium of the
+creamery, the infective agent remaining in the skim milk after
+separation and so polluting the mixed supply. This condition is more
+likely to prevail with typhoid because of the greater tolerance of this
+organism for acids such as would be found in raw milk. The outbreaks at
+Brandon,[108] England, in 1893, Castle Island,[109] Ireland, and
+Marlboro,[110] Mass., in 1894, were traced to such an origin.
+
+While most outbreaks of disease associated with a polluted milk supply
+originate in the use of the milk itself, yet infected milk may serve to
+cause disease even when used in other ways. Several outbreaks of typhoid
+fever have been traced to the use of ice cream where there were strong
+reasons for believing that the milk used in the manufacture of the
+product was polluted.[111] Hankin[112] details a case of an Indian
+confection made largely from milk that caused a typhoid outbreak in a
+British regiment.
+
+Although the evidence that milk may not infrequently serve as an agent
+in spreading disease is conclusive enough to satisfactorily prove the
+proposition, yet it should be borne in mind that the organism of any
+specific disease in question has rarely ever been found. The reasons for
+this are quite the same as those that govern the situation in the case
+of polluted waters, except that the difficulties of the problem are much
+greater in the case of milk than with water. The inability to readily
+separate the typhoid germ, for instance, from the colon bacillus, an
+organism frequently found in milk, presents technical difficulties not
+easily overcome. The most potent reason of failure to find disease
+bacteria is the fact that infection in any case must occur sometime
+previous to the appearance of the outbreak. Not only is there the usual
+period of incubation, but it rarely happens that an outbreak is
+investigated until a number of cases have occurred. In this interim the
+original cause of infection may have ceased to be operative.
+
+~Typhoid fever.~ With reference to the diseases likely to to be
+disseminated through the medium of milk, infected after being drawn from
+the animal, typhoid fever is the most important. The reason for this is
+due (1) to the wide spread distribution of the disease; (2) to the fact
+that the typhoid bacillus is one that is capable of withstanding
+considerable amounts of acid, and consequently finds even in raw milk
+containing the normal lactic acid bacteria conditions favorable for its
+growth.[113] Ability to grow under these conditions can be shown not
+only experimentally, but there is abundant clinical evidence that even a
+slight infection often causes extensive outbreaks, as in the Stamford,
+Conn., outbreak in 1895 where 386 cases developed in a few weeks, 97 per
+cent. of which occurred on the route of one milk-man. In this case the
+milk cans were thoroughly and properly cleaned, but were rinsed out with
+_cold_ water from a shallow well that was found to be polluted.
+
+The most common mode of pollution of milk with typhoid organisms is
+where the milk utensils are infected in one way or another.[114] Second
+in importance is the carrying of infection by persons serving in the
+dual capacity of nurse and dairy attendant.
+
+~Cholera.~ This germ does not find milk so favorable a nutrient medium as
+the typhoid organism, because it is much more sensitive toward the
+action of acids. Kitasato[115] found, however, that it could live in
+raw milk from one to four days, depending upon the amount of acid
+present. In boiled or sterilized milk it grows more freely, as the
+acid-producing forms are thereby eliminated. In butter it dies out in a
+few days (4 to 5).
+
+On account of the above relation not a large number of cholera outbreaks
+have been traced to milk, but Simpson[116] records a very striking case
+in India where a number of sailors, upon reaching port, secured a
+quantity of milk. Of the crew which consumed this, every one was taken
+ill, and four out of ten died, while those who did not partake escaped
+without any disease. It was later shown that the milk was adulterated
+with water taken from an open pool in a cholera infected district.
+
+~Diphtheria.~ Milk occasionally, though not often, serves as a medium for
+the dissemination of diphtheria. Swithinbank and Newman[117] cites four
+cases in which the causal organism has been isolated from milk. It has
+been observed that growth occurs more rapidly in raw than in sterilized
+milk.[118]
+
+Infection in this disease is more frequently attributable to direct
+infection from patient on account of the long persistence of this germ
+in the throat, or indirectly through the medium of an attendant.
+
+~Scarlet fever.~ Although it is more difficult to study the relation of
+this disease to contaminated milk supplies, because the causal germ of
+scarlet fever is not yet known, yet the origin of a considerable number
+of epidemics has been traced to polluted milk supplies. Milk doubtless
+is infected most frequently from persons in the earlier stages of the
+disease when the infectivity of the disease is greater.
+
+~Diarrhoeal diseases.~ Milk not infrequently acquires the property of
+producing diseases of the digestive tract by reason of the development
+of various bacteria that form more or less poisonous by-products. These
+troubles occur most frequently during the summer months, especially with
+infants and children, as in cholera infantum and summer complaint. The
+higher mortality of bottle-fed infants[119] in comparison with those
+that are nursed directly is explicable on the theory that cows' milk is
+the carrier of the infection, because in many cases it is not consumed
+until there has been ample time for the development of organisms in it.
+Where milk is pasteurized or boiled it is found that the mortality among
+children is greatly reduced. As a cause of sickness and death these
+diseases exceed in importance all other specific diseases previously
+referred to. These troubles have generally been explained as produced by
+bacteria of the putrefactive class which find their way into the milk
+through the introduction of filth and dirt at time of milking.[120]
+Fluegge[121] has demonstrated that certain peptonizing species possess
+toxic properties for animals. Recent experimental inquiry[122] has
+demonstrated that the dysentery bacillus (Shiga) probably bears a causal
+relation to some of these summer complaints.
+
+~Ptomaine poisoning.~ Many cases of poisoning from food products are also
+reported with adults. These are due to the formation of various toxic
+products, generally ptomaines, that are produced as a result of
+infection of foods by different bacteria. One of these substances,
+_tyrotoxicon_, was isolated by Vaughan[123] from cheese and various
+other products of milk, and found to possess the property of producing
+symptoms of poisoning similar to those that are noted in such cases. He
+attributes the production of this toxic effect to the decomposition of
+the elements in the milk induced by putrefactive forms of bacteria that
+develop where milk is improperly kept.[124] Often outbreaks of this
+character[125] assume the proportions of an epidemic, where a large
+number of persons use the tainted food.
+
+FOOTNOTES:
+
+[78] Hart, Trans. Int. Med. Cong., London, 1881, 4:491-544.
+
+[79] Freeman, Med. Rec., March 28, 1896.
+
+[80] Busey and Kober, Rept. Health Off. of Dist. of Col., Washington, D.
+C., 1895, p. 299. These authors present in this report an elaborate
+article on morbific and infectious milk, giving a very complete
+bibliography of 180 numbers. They append to Hart's list (which is
+published in full) additional outbreaks which have occurred since,
+together with full data as to extent of epidemic, circumstances
+governing the outbreak, as well as name of original reporter and
+reference.
+
+[81] Smith, Theo., Journ. of Expt. Med., 1898, 3:451.
+
+[82] Dinwiddie, Bull. 57, Ark. Expt. Stat., June, 1899; Ravenel, Univ.
+of Penn. Med. Bull., Sept. 1901.
+
+[83] Ravenel, Journ. of Comp. Med. & Vet. Arch., Dec. 1897; Hartzell,
+Journ. Amer. Med. Ass'n, April 16, 1898.
+
+[84] Stille, Brit. Med. Journ., Aug. 19, 1899.
+
+[85] This test is made by injecting into the animal a small quantity of
+tuberculin, which is a sterilized glycerin extract of cultures of the
+tubercle bacillus. In a tuberculous animal, even in the very earliest
+phases of the disease, tuberculin causes a temporary fever that lasts
+for a few hours. By taking the temperature a number of times before and
+after injection it is possible to readily recognize any febrile
+condition. A positive diagnosis is made where the temperature after
+inoculation is at least 2.0 deg. F. above the average normal, and where
+the reaction fever is continued for a period of some hours.
+
+[86] Martin, Brit. Med. Journ. 1895, 1:937; Nocard, Les Tuberculoses
+animales, 1895.
+
+[87] C. O. Jensen, Milch Kunde und Milch hygiene, p. 69.
+
+[88] Ostertag, Milch Zeit., 22:672.
+
+[89] Obermueller, Hyg. Rund., 1897, p. 712; Petri, Arb. a. d. Kais. Ges.
+Amte, 1898, 14: 1; Hormann und Morgenroth, Hyg. Rund., 1898, p. 217.
+
+[90] Rabinowitsch, Zeit. f. Hyg., 1897, 26: 90.
+
+[91] Th. Smith. Journ. of Expt. Med., 1899, 4:217.
+
+[92] Russell and Hastings, 18 Rept. Wis. Expt. Stat., 1901.
+
+[93] Hesse, Zeit. f. Hyg., 1900, 34:346.
+
+[94] Practically all of the finest butter made in Denmark is made from
+cream that has been pasteurized at temperatures varying from 160 deg.-185
+deg. F.
+
+[95] Gebhardt, Virch. Arch., 1890, 119:12.
+
+[96] Scheurlen, Arb. a. d. k. Ges. Amte, 1891, 7:269; Bang, Milch Zeit.,
+1893, p. 672.
+
+[97] Moore, Year Book of U. S. Dept. Agr., 1895, p. 432.
+
+[98] Weigel and Noack, Jahres. d. Ges. Med., 1890, p. 642; Weissenberg,
+Allg. med. Cent. Zeit., 1890, p. 1; Baum, Arch. f. Thierheilkunde, 1892,
+18:16.
+
+[99] Schneider, Muench, med. Wochenschr., 1893, No. 27; Froehner, Zeit f.
+Fleisch u. Milchhygiene, 1891, p. 55.
+
+[100] Feser, Deutsche Zeit. f. Thiermed., 1880, 6:166.
+
+[101] Nocard, Bull. Gen., 1885, p. 54.
+
+[102] Deutsche Viertelsjahr. f. offentl. Gesundheitspflege, 1890,
+20:444.
+
+[103] Zeit. f. Fleisch und Milch hygiene, 11:114.
+
+[104] E. Roth, Deutsche Vierteljahresschr. f. offentl. Gesundheitspfl.,
+1890, 22:238
+
+[105] S. W. North, London Practitioner, 1889, 43:393.
+
+[106] Sedgwick and Chapin, Boston Med. & Surg. Journ., 1893, 129:485.
+
+[107] Dabney, Phila. Med. News, 1893, 63:630.
+
+[108] Welphy, London Lancet, 1894, 2:1085.
+
+[109] Brit. Med. Journ., 1894, 1:815.
+
+[110] Mass. Bd. Health Rept., 1894, p. 765.
+
+[111] Turner, London Practitioner, 1892, 49:141; Munro, Brit. Med.
+Journ., 1894, 2:829.
+
+[112] Hankin, Brit. Med. Journ., 1894, 2:613.
+
+[113] Heim (Arb. a. d. Kais. Gesundheitsamte, 1889, 5:303) finds it
+capable of living from 20-30 days in milk.
+
+[114] Schueder (Zeit. f. Hyg., 1902, 38:34) examined the statistics of
+638 typhoid epidemics. He found 71 per cent. due to infected drinking
+water, 17 per cent. to infected milk, and 3.5 per cent. caused by other
+forms of food.
+
+[115] Kitasato. Arb. a. d. Kais. Gesundheitsamte, 1:470.
+
+[116] Simpson, London Practitioner, 1887, 39:144.
+
+[117] Swithinbank and Newman, Bacteriology of Milk, p. 341.
+
+[118] Schottelius and Ellerhorst. Milch Zeit., 1897, pp. 40 and 73.
+
+[119] Baginsky, Hyg. Rund., 1895, p. 176.
+
+[120] Gaffky, Deutsch. med. Wochen., 18:14.
+
+[121] Fluegge. Zeit., f. Hyg., 17:272, 1894.
+
+[122] Duval and Bassett, Studies from the Rockefeller Inst. for Med.
+Research, 2:7, 1904.
+
+[123] Zeit. f. physiol. Chemie, 10:146; 9 Intern. Hyg. Cong. (London),
+1891, p. 118.
+
+[124] Vaughan and Perkins, Arch. f. Hyg., 27:308.
+
+[125] Newton and Wallace (Phila. Med. News, 1887, 50:570) report three
+outbreaks at Long Branch, N. J., two of which occurred in summer hotels.
+
+
+
+
+CHAPTER VI.
+
+BACTERIA AND MILK SUPPLIES WITH ESPECIAL REFERENCE TO METHODS OF
+PRESERVATION.
+
+
+To the milk dealer or distributor, bacteria are more or less of a
+detriment. None of the organisms that find their way into milk, nor the
+by-products formed by their growth, improve the quality of milk
+supplies. It is therefore especially desirable from the milk-dealer's
+point of view that these changes should be held in abeyance as much as
+possible. Then too, the possibility that milk may serve as a medium for
+the dissemination of disease-breeding bacteria makes it advisable to
+protect this food supply from all possible infection from suspicious
+sources.
+
+In considering, therefore, the relation of bacteria to general milk
+supplies, the _economic_ and the _hygienic_ standpoints must be taken
+into consideration. Ordinarily much more emphasis is laid upon the first
+requirement. If the supply presents no abnormal feature as to taste,
+odor and appearance, unfortunately but little attention is paid to the
+possibility of infection by disease germs. The methods of control which
+are applicable to general milk supplies are based on the following
+foundations: (1) the exclusion of all bacterial life, as far as
+practicable, at the time the milk is drawn, and the subsequent storage
+of the same at temperatures unfavorable for the growth of the organisms
+that do gain access; (2) the removal of the bacteria, wholly or in part,
+after they have once gained access.
+
+Until within comparatively recent years, practically no attention was
+given to the character of milk supplies, except possibly as to the
+percentage of butter fat, and sometimes the milk solids which it
+contained. So long as the product could be placed in the hands of the
+consumer in such shape as not to be rejected by him as unfit for food,
+no further attention was likely to be given to its character. At
+present, however, much more emphasis is being given to the quality of
+milk, especially as to its germ content; and the milk dealer is
+beginning to recognize the necessity of a greater degree of control.
+This control must not merely concern the handling of the product after
+it reaches him, but should go back to the milk producer on the farm.
+Here especially, it is necessary to inculcate those methods of
+cleanliness which will prevent in large measure the wholesale infection
+that ordinarily occurs.
+
+The two watch words which are of the utmost importance to the milk
+dealer are _cleanliness_ and _cold_. If the milk is properly drawn from
+the animal in a clean manner and is immediately and thoroughly chilled,
+the dealer has little to fear as to his product. Whenever serious
+difficulties do arise, attributable to bacterial changes, it is because
+negligence has been permitted in one or both directions. The influence
+of cleanliness in diminishing the bacterial life in milk and that of low
+temperatures in repressing the growth of those forms which inevitably
+gain access has been fully dealt with in preceding chapters. It is of
+course not practicable to take all of these precautions to which
+reference has been made in the securing of large supplies of market milk
+for city use, but great improvement over existing conditions could be
+secured if the public would demand a better supervision of this
+important food article. Boards of health in our larger cities are
+awakening to the importance of this question and are becoming
+increasingly active in the matter of better regulations and the
+enforcement of the same.
+
+New York City Board of Health has taken an advanced position in
+requiring that all milk sold in the city shall be chilled down to 45 deg.
+F. immediately after milking and shall be transported to the city in
+refrigerator cars.
+
+Reference has already been made to the application of the acid test
+(page 52) in the inspection of city milk supplies, and it is the opinion
+of the writer that the curd test (see page 76) could also be used with
+advantage in determining the sanitary character of milk. This test
+reveals the presence of bacteria usually associated with dirt and
+permits of the recognition of milks that have been carelessly handled.
+From personal knowledge of examinations made of the milk supplies in a
+number of Wisconsin cities it appears that this test could be utilized
+with evident advantage.
+
+~"Sanitary" or "certified" milk supplies.~ In a number of the larger
+cities, the attempt has been made to improve the quality of the milk
+supplies by the installation of dairies in which is produced an
+especially high grade of milk. Frequently the inspection of the dairy as
+well as the examination of the milk at stated intervals is under the
+control of milk commissions or medical societies and as it is customary
+to distribute the certificate of the examining board with the product,
+such milks are frequently known as "certified." In such dairies the
+tuberculin test is used at regular intervals, and the herd inspected
+frequently by competent veterinarians. The methods of control
+inaugurated as to clean milking and subsequent handling are such as to
+insure the diminution of the bacteria to the lowest possible point. The
+bacterial limit set by the Pediatric Society of Philadelphia is 10,000
+organisms per cc. Often it is possible to improve very materially on
+this standard and not infrequently is the supply produced where it
+contains only a few thousand organisms per cc. Where such a degree of
+care is exercised, naturally a considerably higher price must be paid
+for the product,[126] and it should be remembered that the development
+of such a system is only possible in relatively large centers where the
+dealer can cater to a selected high-class trade. Moreover, it should
+also be borne in mind that such a method of control is only feasible in
+dairies that are under individual control. The impossibility of
+exercising adequate control with reference to the milking process and
+the care which should be given the milk immediately thereafter, when the
+same is produced on different farms under various auspices is evident.
+
+
+PRESERVATION OF MILK SUPPLIES.
+
+While much can be done to improve the quality of milk supplies by
+excluding a large proportion of the bacteria which normally gain access
+to the milk, and preventing the rapid growth of those that do find their
+way therein, yet for general municipal purposes, any practical method of
+preservation[127] that is applicable on a commercial scale must rest
+largely upon the destruction of bacteria that are present in the milk.
+
+The two possible methods by which bacteria can be destroyed after they
+have once gained access is (1) by the use of chemical preservatives; (2)
+by the aid of physical methods.
+
+~Chemical preservatives.~ Numerous attempts have been made to find some
+chemical substance that could be added to milk which would preserve it
+without interfering with its nutritive properties, but as a general rule
+a substance that is toxic enough to destroy or inhibit the growth of
+bacterial life exerts a prejudicial effect on the tissues of the body.
+The use of chemicals, such as carbolic acid, mercury salts and mineral
+acids, that are able to entirely destroy all life, is of course
+excluded, except when milk is preserved for analytical purposes; but a
+number of milder substances are more or less extensively employed,
+although the statutes of practically all states forbid their use.
+
+The substances so used may be grouped in two classes:
+
+1. Those that unite chemically with certain by-products of bacterial
+growth to form inert substances. Thus bicarbonate of soda neutralizes
+the acid in souring milk, although it does not destroy the lactic acid
+bacteria.
+
+2. Those that act directly upon the bacteria in milk, restraining or
+inhibiting their development. The substances most frequently utilized
+are salicylic acid, formaldehyde and boracic acid. These are nearly
+always sold to the milk handler, under some proprietary name, at prices
+greatly in excess of what the crude chemicals could be bought for in the
+open market. Formaldehyde has been widely advertised of late, but its
+use is fraught with the greatest danger, for it practically renders
+insoluble all albuminous matter and its toxic effect is greatly
+increased in larger doses.
+
+These substances are generally used by milk handlers who know nothing of
+their poisonous action, and although it may be possible for adults to
+withstand their use in dilute form, without serious results, yet their
+addition to general milk supplies that may be used by children is
+little short of criminal. The sale of these preparations for use in milk
+finds its only outlet with those dairymen who are anxious to escape the
+exactions that must be met by all who attempt to handle milk in the best
+possible manner. Farrington has suggested a simple means for the
+detection of preservalin (boracic acid).[128] When this substance is
+added to fresh milk, it increases the acidity of milk without affecting
+its taste. As normal milk tastes sour when it contains about 0.3 per
+cent lactic acid, a milk that tests as much or more than this without
+tasting sour has been probably treated with this antiseptic agent.
+
+~Physical methods of preservation.~ Methods based upon the application of
+physical forces are less likely to injure the nutritive value of milk,
+and are consequently more effective, if of any value whatever. A number
+of methods have been tried more or less thoroughly in an experimental
+way that have not yet been reduced to a practical basis, as electricity,
+use of a vacuum, and increased pressure.[129] Condensation has long been
+used with great success, but in this process the nature of the milk is
+materially changed. The keeping quality in condensed milk often depends
+upon the action of another principle, viz., the inhibition of bacterial
+growth by reason of the concentration of the medium. This condition is
+reached either by adding sugar and so increasing the soluble solids, or
+by driving off the water by evaporation, preferably in a vacuum pan.
+Temperature changes are, however, of the most value in preserving milk,
+for by a variation in temperature all bacterial growth can be brought to
+a standstill, and under proper conditions thoroughly destroyed.
+
+~Use of low temperatures.~ The effect of chilling or rapid cooling on the
+keeping quality of milk is well known. When the temperature of milk is
+lowered to the neighborhood of 45 deg. F., the development of bacterial
+life is so slow as to materially increase the period that milk remains
+sweet. Within recent years, attempts have been made to preserve milk so
+that it could be shipped long distances by freezing the product, which
+in the form of milk-ice could be held for an indefinite period without
+change.[130] A modification of this process known as Casse's system has
+been in use more or less extensively in Copenhagen and in several places
+in Germany. This consists of adding a small block of milk-ice (frozen
+milk) to large cans of milk (one part to about fifty of milk) which may
+or may not be pasteurized.[131] This reduces the temperature so that the
+milk remains sweet considerably longer. Such a process might permit of
+the shipment of milk for long distances with safety but as a matter of
+fact, the system has not met with especial favor.
+
+[Illustration: FIG. 22. Microscopic appearance of normal milk showing
+the fat-globules aggregated in clusters.]
+
+~Use of high temperatures.~ Heat has long been used as a preserving agent.
+Milk has been scalded or cooked to keep it from time immemorial. Heat
+may be used at different temperatures, and when so applied exerts a
+varying effect, depending upon temperature employed. All methods of
+preservation by heat rest, however, upon the application of the heat
+under the following conditions:
+
+1. A temperature above the maximum growing-point (105 deg.-115 deg. F.)
+and below the thermal death-point (130 deg.-140 deg. F.) will prevent
+further growth, and consequently fermentative action.
+
+2. A temperature above the thermal death-point destroys bacteria, and
+thereby stops all changes. This temperature varies, however, with the
+condition of the bacteria, and for spores is much higher than for
+vegetative forms.
+
+Attempts have been made to employ the first principle in shipping milk
+by rail, viz., prolonged heating above growing temperature, but when
+milk is so heated, its physical appearance is changed.[132] The methods
+of heating most satisfactorily used are known as sterilization and
+pasteurization, in which a degree of temperature is used approximating
+the boiling and scalding points respectively.
+
+[Illustration: FIG. 23. Microscopic appearance of milk heated above 140
+deg. F., showing the homogeneous distribution of fat-globules. The
+physical change noted in comparison with Fig. 22 causes the diminished
+consistency of pasteurized cream.]
+
+~Effect of heat on milk.~ When milk is subjected to the action of heat, a
+number of changes in its physical and chemical properties are to be
+noted.
+
+_1. Diminished "body."_ When milk, but more especially cream, is heated
+to 140 deg. F. or above, it becomes thinner in consistency or "body," a
+condition which is due to a change in the grouping of the fat globules.
+In normal milk, the butter fat for the most part is massed in
+microscopic clots as (Fig. 22). When exposed to 140 deg. F. or above for
+ten minutes these fat-globule clots break down, and the globules become
+homogeneously distributed (Fig. 23). A _momentary_ exposure to heat as
+high as 158 deg.-160 deg. may be made without serious effect on the cream
+lime; but above this the cream rises so poorly and slowly that it gives
+the impression of thinner milk.
+
+_2. Cooked Taste._ If milk is heated for some minutes to 160 deg. F., it
+acquires a cooked taste that becomes more pronounced as the temperature
+is further raised. Milk so heated develops on its surface a pellicle or
+"skin." The cause of this change in taste is not well known. Usually it
+has been explained as being produced by changes in the nitrogenous
+elements in the milk, particularly in the albumen. Thoerner[133] has
+pointed out the coincidence that exists between the appearance of a
+cooked taste and the loss of certain gases that are expelled by heating.
+He finds that the milk heated in closed vessels from which the gas
+cannot escape has a much less pronounced cooked flavor than if heated in
+an open vessel. The so-called "skin" on the surface of heated milk is
+not formed when the milk is heated in a tightly-closed receptacle. By
+some[134] it is asserted that this layer is composed of albumen, but
+there is evidence to show that it is modified casein due to the rapid
+evaporation of the milk serum at the surface of the milk.
+
+_3. Digestibility._ Considerable difference of opinion has existed in
+the minds of medical men as to the relative digestibility of raw and
+heated milks. A considerable amount of experimental work has been done
+by making artificial digestion experiments with enzyms, also digestion
+experiments with animals, and in a few cases with children. The results
+obtained by different investigators are quite contradictory, although
+the preponderance of evidence seems to be in favor of the view that
+heating does impair the digestibility of milk, especially if the
+temperature attains the sterilizing point.[135] It has been observed
+that there is a noteworthy increase in amount of rickets,[136] scurvy
+and marasmus in children where highly-heated milks are employed. These
+objections do not obtain with reference to milk heated to moderate
+temperatures, as in pasteurization, although even this lower temperature
+lessens slightly its digestibility. The successful use of pasteurized
+milks in children's hospitals is evidence of its usefulness.
+
+_4. Fermentative changes._ The normal souring change in milk is due to
+the predominance of the lactic acid bacteria, but as these organisms as
+a class do not possess spores, they are readily killed when heated above
+the thermal death-point of the developing cell. The destruction of the
+lactic forms leaves the spore-bearing types possessors of the field, and
+consequently the fermentative changes in heated milk are not those that
+usually occur, but are characterized by the curdling of the milk from
+the action of rennet enzyms.
+
+_5. Action of rennet._ Heating milk causes the soluble lime salts to be
+precipitated, and as the curdling of milk by rennet (in cheese-making)
+is dependent upon the presence of these salts, their absence in heated
+milks greatly retards the action of rennet. This renders it difficult to
+utilize heated milks in cheese-making unless the soluble lime salts are
+restored, which can be done by adding solutions of calcium chlorid.
+
+~Sterilization.~ As ordinarily used in dairying, sterilization means the
+application of heat at temperatures approximating, if not exceeding,
+212 deg. F. It does not necessarily imply that milk so treated is sterile,
+i. e., germ-free; for, on account of the resistance of spores, it is
+practically impossible to destroy entirely _all_ these hardy forms. If
+milk is heated at temperatures above the boiling point, as is done where
+steam pressure is utilized, it can be rendered practically germ-free.
+Such methods are employed where it is designed to keep milk sweet for a
+long period of time. The treatment of milk by sterilization has not met
+with any general favor in this country, although it has been more widely
+introduced abroad. In most cases the process is carried out after the
+milk is bottled; and considerable ingenuity has been exercised in the
+construction of devices which will permit of the closure of the bottles
+after the sterilizing process has been completed. Milks heated to so
+high a temperature have a more or less pronounced boiled or cooked
+taste, a condition that does not meet with general favor in this
+country. The apparatus suitable for this purpose must, of necessity, be
+so constructed as to withstand steam pressure, and consequently is
+considerably more expensive than that required for the simpler
+pasteurizing process.
+
+~Pasteurization.~ In this method the degree of heat used ranges from 140
+deg. to 185 deg. F. and the application is made for only a limited length
+of time. The process was first extensively used by Pasteur (from whom it
+derives its name) in combating various maladies of beer and wine. Its
+importance as a means of increasing the keeping quality of milk was not
+generally recognized until a few years ago; but the method is now
+growing rapidly in favor as a means of preserving milk for commercial
+purposes. The method does not destroy all germ-life in milk; it affects
+only those organisms that are in a growing, vegetative condition; but if
+the milk is quickly cooled, it enhances the keeping quality very
+materially. It is unfortunate that this same term is used in connection
+with the heating of cream as a preparatory step to the use of pure
+cultures in cream-ripening in butter-making. The objects to be
+accomplished vary materially and the details of the two processes are
+also quite different.
+
+While pasteurizing can be performed on a small scale by the individual,
+the process can also be adapted to the commercial treatment of large
+quantities of milk. The apparatus necessary for this purpose is not
+nearly so expensive as that used in sterilizing, a factor of importance
+when other advantages are considered. In this country pasteurization has
+made considerable headway, not only in supplying a milk that is designed
+to serve as children's food, but even for general purposes.
+
+~Requirements essential in pasteurization.~ While considerable latitude
+with reference to pasteurizing limits is permitted, yet there are
+certain conditions which should be observed, and these, in a sense, fix
+the limits that should be employed. These may be designated as (1) the
+_physical_, and (2) the _biological_ requirements.
+
+~Physical requirements.~ _1. Avoidance of scalded or cooked taste._ The
+English and American people are so averse to a scalded or cooked flavor
+in milk that it is practically impossible for a highly heated product to
+be sold in competition with ordinary raw milk. In pasteurization then,
+care must be taken not to exceed the temperature at which a permanently
+cooked flavor is developed. As previously observed, this point varies
+with the period of exposure. A momentary exposure to a temperature of
+about 170 deg. F. may be made without any material alteration, but if
+the heat is maintained for a few minutes (ten minutes or over), a
+temperature of 158 deg. to 160 deg. F. is about the maximum that can be
+employed with safety.
+
+_2. Normal creaming of the milk._ It is especially desirable that a
+sharp and definite cream line be evident on the milk soon after
+pasteurization. If this fails to appear, the natural inference of the
+consumer is that the milk is skimmed. If the milk be heated to a
+temperature sufficiently high to cause the fat-globule clusters to
+disintegrate (see Figs. 22 and 23), the globules do not rise to the
+surface as readily as before and the cream line remains indistinct.
+Where the exposure is made for a considerable period of time (10 minutes
+or more), the maximum temperature which can be used without producing
+this change is about 140 deg. F.; if the exposure is made for a very
+brief time, a minute or less, the milk may be heated to 158 deg.-160 F.
+deg. without injuring the creaming property.
+
+_3. No diminution in cream "body."_ Coincident with this change which
+takes place in the creaming of the milk is the change in body or
+consistency which is noted where cream is pasteurized at too high a
+temperature. For the same reason as given under (2) cream heated above
+these temperatures is reduced in apparent thickness and appears to
+contain less butter-fat. Of course the pasteurizing process does not
+change the fat content, but its "body" is apparently so affected. Thus a
+25 per cent. cream may seem to be no thicker or heavier than an 18 per
+cent. raw cream. This real reduction in consistency naturally affects
+the readiness with which the cream can be whipped.
+
+~Biological requirements.~ _1. Enhanced keeping quality._ In commercial
+practice the essential biological requirement is expressed in the
+enhanced keeping quality of the pasteurized milk. This expresses in a
+practical way the reduction in germ life accomplished by the
+pasteurizing process. The improvement in keeping quality depends upon
+the temperature and time of exposure, but fully as much also on the way
+in which the pasteurized product is handled after heating. The lowest
+temperature which can be used with success to kill the active,
+vegetative bacteria is about 140 deg. F., at which point it requires
+about ten minutes exposure. If this period is curtailed the temperature
+must be raised accordingly. An exposure to a temperature of 175 deg. F.
+for a minute has approximately the same effect as the lower degree of
+heat for the longer time.
+
+The following bacteriological studies as to the effect which a variation
+in temperature exerts on bacterial life in milk are of importance as
+indicating the foundation for the selection of the proper limits. In the
+following table the exposures were made for a uniform period (20
+minutes):
+
+_The bacterial content of milk heated at different temperatures._
+
+                          Number of bacteria per cc. in milk.
+                            45 deg. C.     50 deg. C.   55 deg. C.   60 deg. C.   65 deg. C.  70 deg. C.
+               Unheated    113 deg. F.    122 deg. F.  131 deg. F.  140 deg. F.  149 deg. F. 158 deg. F.
+Series I.     2,895,000    ----         1,260,000      798,000       32,000        5,770       3,900
+Series II.      750,000    665,000        262,400      201,000          950          700         705
+Series III.   1,350,000  1,100,000        260,000      215,000          575          610         650
+Series IV.    1,750,000    ----            87,360       ----          4,000        3,500       3,600
+
+It appears from these results that the most marked decrease in
+temperature occurs at 140 deg. F. (60 deg. C.). It should also be observed
+that an increase in heat above this temperature did not materially
+diminish the number of organisms present, indicating that those forms
+remaining were in a spore or resistant condition. It was noted, however,
+that the developing colonies grew more slowly in the plates made from the
+highly heated milk, showing that their vitality was injured to a greater
+extent even though not killed.
+
+_2. Destruction of disease bacteria._ While milk should be pasteurized
+so as to destroy all active, multiplying bacteria, it is particularly
+important to destroy any organisms of a disease nature that might find
+their way into the same. Fortunately most of the bacteria capable of
+thriving in milk before or after it is drawn from the animal are not
+able to form spores and hence succumb to proper pasteurization. Such is
+the case with the diphtheria, cholera and typhoid organisms.
+
+The organism that is invested with most interest in this connection is
+the tubercle bacillus. On account of its more or less frequent
+occurrence in milk and its reputed high powers of resistance, it may
+well be taken as a standard in pasteurizing.
+
+~Thermal death limits of tubercle bacillus.~ Concerning the exact
+temperature at which this germ is destroyed there is considerable
+difference of opinion. Part of this arises from the inherent difficulty
+in determining exactly when the organism is killed (due to its failure
+to grow readily on artificial media), and part from the lack of uniform
+conditions of exposure. The standards that previously have been most
+generally accepted are those of De Man,[137] who found that thirty
+minutes exposure at 149 deg. F., fifteen minutes at 155 deg. F., or ten
+minutes at 167 deg. F., sufficed to destroy this germ.
+
+More recently it has been demonstrated,[138] and these results
+confirmed,[139] that if tuberculous milk is heated in closed receptacles
+where the surface pellicle does not form, the vitality of this disease
+germ is destroyed at 140 deg. F. in 10-15 minutes, while an exposure at
+160 deg. F. requires only about one minute.[140] If the conditions of
+heating are such that the surface of the milk is exposed to the air, the
+resistance of bacteria is greatly increased. When heated in open vessels
+Smith found that the tubercle organism was not killed in some cases where
+the exposure was made for at least an hour. Russell and Hastings[141]
+have shown an instance where the thermal death-point of a micrococcus
+isolated from pasteurized milk was increased 12.5 deg. F., by heating it
+under conditions that permitted of the formation of the scalded layer.
+It is therefore apparent that apparatus used for pasteurization should
+be constructed so as to avoid this defect.
+
+~Methods of treatment.~ Two different systems of pasteurization have grown
+up in the treatment of milk. One of these has been developed from the
+hygienic or sanitary aspect of the problem and is used more particularly
+in the treatment of cream and relatively small milk supplies. The other
+system has been developed primarily from the commercial point of view
+where a large amount of milk must be treated in the minimum time. In the
+first method the milk is heated for a longer period of time, about
+fifteen minutes at a relatively low temperature from 140 deg.-155 deg.
+F.; in the other, the milk is exposed to the source of heat only while
+it is passing rapidly through the apparatus. Naturally, the exposure
+under such conditions must be made at a considerably higher temperature,
+usually in the neighborhood of 160 deg. F.
+
+The types of apparatus used in these respective processes naturally
+varies. Where the heating is prolonged, the apparatus employed is built
+on the principle of a _tank_ or _reservoir_ in which a given volume of
+milk may be held at any given temperature for any given period of time.
+
+When the heat is applied for a much shorter period of time, the milk is
+passed in a continuous stream through the machine. Naturally the
+capacity of a continuous-flow apparatus is much greater than a machine
+that operates on the intermittent principle; hence, for large supplies,
+as in city distribution, this system has a great advantage. The question
+as to relative efficiency is however one which should be given most
+careful consideration.
+
+~Pasteurizing apparatus.~ The problems to be solved in the pasteurization
+of milk and cream designed for direct consumption are so materially
+different from where the process is used in butter-making that the type
+of machinery for each purpose is quite different. The equipment
+necessary for the first purpose may be divided into two general classes:
+
+1. Apparatus of limited capacity designed for family use.
+
+2. Apparatus of sufficient capacity to pasteurize on a commercial scale.
+
+~Domestic pasteurizers.~ In pasteurizing milk for individual use, it is
+not desirable to treat at one time more than will be consumed in one
+day; hence an apparatus holding a few bottles will suffice. In this case
+the treatment can best be performed in the bottle itself, thereby
+lessening the danger of infection. Several different types of
+pasteurizers are on the market; but special apparatus is by no means
+necessary for the purpose. The process can be efficiently performed by
+any one with the addition of an ordinary dairy thermometer to the common
+utensils found in the kitchen. Fig. 24 indicates a simple contrivance
+that can be readily arranged for this purpose.
+
+The following suggestions indicate the different steps of the process:
+
+1. Use only fresh milk.
+
+2. Place milk in clean bottles or fruit cans, filling to a uniform
+level, closing bottles tightly with a cork or cover. If pint and quart
+cans are used at the same time, an inverted bowl will equalize the
+level. Set these in a flat-bottomed tin pail and fill with warm water to
+same level as milk. An inverted pie tin punched with holes will serve as
+a stand on which to place the bottles during the heating process.
+
+3. Heat water in pail until the temperature of same reaches 155 deg. to
+160 deg. F.; then remove from source of direct heat, cover with a cloth
+or tin cover, and allow the whole to stand for half an hour. In the
+preparation of milk for children, it is not advisable to use the
+low-temperature treatment (140 deg. F.) that is recommended for
+commercial city delivery.
+
+[Illustration: FIG. 24. A home-made pasteurizer.]
+
+4. Remove bottles of milk and cool them as rapidly as possible without
+danger to bottles and store in a refrigerator.
+
+~Commercial pasteurizers.~ The two methods of pasteurization practiced
+commercially for the preservation of milk and cream have been developed
+because of the two types of machinery now in use. Apparatus constructed
+on the reservoir or tank principle permits of the retention of the milk
+for any desired period of time. Therefore, a lower temperature can be
+employed in the treatment. In those machines where the milk flows
+through the heater in a more or less continuous stream, the period of
+exposure is necessarily curtailed, thereby necessitating a higher
+temperature.
+
+~Reservoir pasteurizers.~ The simplest type of apparatus suitable for
+pasteurizing on this principle is where the milk is placed in shotgun
+cans and immersed in water heated by steam. Ordinary tanks surrounded
+with water spaces can also be used successfully. The Boyd cream ripening
+vat has also been tried. In this the milk is heated by a swinging coil
+immersed in the vat through which hot water circulates.
+
+In 1894 the writer[142] constructed a tank pasteurizer which consisted
+of a long, narrow vat surrounded by a steam-heated water chamber. Both
+the milk and the water chambers were provided with mechanical agitators
+having a to-and-fro movement.
+
+[Illustration: FIG. 25. Pott's pasteurizer.]
+
+Another machine which has been quite generally introduced is the Potts'
+rotating pasteurizer. This apparatus has a central milk chamber that is
+surrounded with an outer shell containing hot water. The whole machine
+revolves on a horizontal axis, and the cream or milk is thus thoroughly
+agitated during the heating process.
+
+~Continuous-flow pasteurizers.~ The demand for greater capacity than can
+be secured in the reservoir machines has led to the perfection of
+several kinds of apparatus where the milk is heated momentarily as it
+flows through the apparatus. Most of these were primarily introduced for
+the treatment of cream for butter-making purposes, but they are
+frequently employed for the treatment of milk on a large scale in city
+milk trade. Many of them are of European origin although of late years
+several have been devised in this country.
+
+The general principle of construction is much the same in most of them.
+The milk is spread out in a thin sheet, and is treated by passing it
+over a surface, heated either with steam directly or preferably with hot
+water.
+
+Where steam is used directly, it is impossible to prevent the "scalding
+on" of the milk proteids to the heated surface.
+
+In some of these machines (Thiel, Kuehne, Lawrence, De Laval, and
+Hochmuth), a ribbed surface is employed over which the milk flows, while
+the opposite surface is heated with hot water or steam. Monrad, Lefeldt
+and Lentsch employ a centrifugal apparatus in which a thin layer of milk
+is heated in a revolving drum.
+
+In some types of apparatus, as in the Miller machine, an American
+pasteurizer, the milk is forced in a thin sheet between two heated
+surfaces, thereby facilitating the heating process. In the Farrington
+machine heated discs rotate in a reservoir through which the milk flows
+in a continuous stream.
+
+One of the most economical types of apparatus is the regenerator type (a
+German machine), in which the milk passes over the heating surface in a
+thin stream and then is carried back over the incoming cold milk so
+that the heated liquid is partially cooled by the inflowing fresh milk.
+In machines of this class it requires very much less steam to heat up
+the milk than in those in which the cold milk is heated wholly by the
+hot water.
+
+A number of machines have been constructed on the principle of a
+reservoir which is fed by a constantly flowing stream. In some kinds of
+apparatus of this type no attempt is made to prevent the mixing of the
+recently introduced milk with that which has been partially heated. The
+pattern for this reservoir type is Fjord's heater, in which the milk is
+stirred by a stirrer. This apparatus was originally designed as a heater
+for milk before separation, but it has since been materially modified so
+that it is better adapted to the purposes of pasteurization. Reid was
+the first to introduce this type of machine into America.
+
+~Objections to continuous flow pasteurizers.~ In all continuous flow
+pasteurizers certain defects are more or less evident. While they
+fulfill the important requirement of large capacity, an absolute
+essential where large volumes of milk are being handled, it does not of
+necessity follow that they conform to all the hygienic and physical
+requirements that should be kept in mind. The greatest difficulty is the
+shortened period of exposure. The period which the milk is actually
+heated is often not more than a minute or so. Another serious defect is
+the inability to heat _all_ of the milk for a uniform period of time. At
+best, the milk is exposed for an extremely short time, but even then
+portions pass through the machine much more quickly than do the
+remainder. Those portions in contact with the walls of the apparatus are
+retarded by friction and are materially delayed in their passage, while
+the particles in the center of the stream, however thin, flow through
+in the least possible time.
+
+The following simple method enables the factory operator to test the
+period of exposure in the machine: Start the machine full of water, and
+after the same has become heated to the proper temperature, change the
+inflow to full-cream milk, continuing at the same rate. Note the exact
+time of change and also when first evidence of milkiness begins to
+appear at outflow. If samples are taken from first appearance of milky
+condition and thereafter at different intervals for several minutes, it
+is possible, by determining the amount of butter-fat in the same, to
+calculate with exactness how long it takes for the milk to entirely
+replace the water.
+
+Tests made by the writer[143] on the Miller pasteurizer showed, when fed
+at the rate of 1,700 pounds per hour, the minimum period of exposure to
+be 15 seconds, and the maximum about 60-70 seconds, while about
+two-thirds of the milk passed the machine in 40-50 seconds. This
+manifest variation in the rate of flow of the milk through the machine
+is undoubtedly the reason why the results of this type of treatment are
+subject to so much variation. Naturally, even a fatal temperature to
+bacterial life can be reduced to a point where actual destruction of
+even vegetating cells does not occur.
+
+~Bacterial efficiency of reservoir pasteurizers.~ The bacterial content of
+pasteurized milk and cream will depend somewhat on the number of
+organisms originally present in the same. Naturally, if mixed milk
+brought to a creamery is pasteurized, the number of organisms remaining
+after treatment would be greater than if the raw material was fresh and
+produced on a single farm.
+
+An examination of milk and cream pasteurized on a commercial scale in
+the Russell vat at the Wisconsin Dairy school showed that over 99.8 per
+cent of the bacterial life in raw milk or cream was destroyed by the
+heat employed, i. e., 155 deg. F. for twenty minutes duration.[144] In
+nearly one-half of the samples of milk, the germ content in the
+pasteurized sample fell below 1,000 bacteria per cc., and the average of
+twenty-five samples contained 6,140 bacteria per cc. In cream the germ
+content was higher, averaging about 25,000 bacteria per cc. This milk
+was taken from the general creamery supply, which was high in organisms,
+containing on an average 3,675,000 bacteria per cc. De Schweinitz[145]
+has reported the germ content of a supply furnished in Washington which
+was treated at 158 deg. to 160 deg. F. for fifteen minutes. This supply
+came from a single source. Figures reported were from 48-hour-old agar
+plates. Undoubtedly these would have been higher if a longer period of
+incubation had been maintained. The average of 82 samples, taken for the
+period of one year, showed 325 bacteria per cc.
+
+[Illustration: FIG. 26. Effect of pasteurizing on germ content of milk.
+Black square represents bacteria of raw milk; small white square, those
+remaining after pasteurization.]
+
+~Bacterial efficiency of continuous-flow pasteurizers.~ A quantitative
+determination of the bacteria found in milk and cream when treated in
+machinery of this class almost always shows a degree of variation in
+results that is not to be noted in the discontinuous apparatus.
+
+[Illustration: FIG. 27. Reid's Continuous Pasteurizer.]
+
+Harding and Rogers[146] have tested the efficiency of one of the Danish
+type of continuous pasteurizers. These experiments were made at 158 deg.,
+176 deg. and 185 deg. F. They found the efficiency of the machine not
+wholly satisfactory at the lower temperatures. At 158 deg. F. the average
+of fourteen tests gave 15,300 bacteria per cc., with a maximum to minimum
+range from 62,790 to 120. Twenty-five examinations at 176 deg. F. showed
+an average of only 117, with a range from 300 to 20. The results at 185
+deg. F. showed practically the same results as noted at 176 deg. F.
+Considerable trouble was experienced with the "scalding on" of the milk
+to the walls of the machine when milk of high acidity was used.
+
+Jensen[147] details the results of 139 tests in 1899, made by the
+Copenhagen Health Commission. In 66 samples from one hundred thousand to
+one million organisms per cc. were found, and in 22 cases from one to
+five millions. Nineteen tests showed less than 10,000 per cc.
+
+In a series of tests conducted by the writer[148] on a Miller
+pasteurizer in commercial operation, an average of 21 tests showed
+12,350 bacteria remaining in the milk when the milk was pasteurized from
+156 deg.-164 deg. F. The raw milk in these tests ran from 115,000 to
+about one million organisms per cc.
+
+A recently devised machine of this type (Pasteur) has been tested by
+Lehmann, who found that it was necessary to heat the milk as high as
+176 deg. to 185 deg. F., in order to secure satisfactory results on the
+bacterial content of the cream.
+
+The writer tested Reid's pasteurizer at 155 deg. to 165 deg. F. with the
+following results: in some cases as many as 40 per cent. of the bacteria
+survived, which number in some cases exceeded 2,000,000 bacteria per
+cc.
+
+~Pasteurizing details.~ While the pasteurizing process is exceedingly
+simple, yet, in order to secure the best results, certain conditions
+must be rigidly observed in the treatment before and after the heating
+process.
+
+It is important to select the best possible milk for pasteurizing, for
+if the milk has not been milked under clean conditions, it is likely to
+be rich in the spore-bearing bacteria. Old milk, or milk that has not
+been kept at a low temperature, is much richer in germ-life than
+perfectly fresh or thoroughly chilled milk.
+
+The true standard for selecting milk for pasteurization should be to
+determine the actual number of bacterial _spores_ that are able to
+resist the heating process, but this method is impracticable under
+commercial conditions.
+
+The following method, while only approximate in its results, will be
+found helpful: Assuming that the age or treatment of the milk bears a
+certain relation to the presence of spores, and that the acid increases
+in a general way with an increase in age or temperature, the amount of
+acid present may be taken as an approximate index of the suitability of
+the milk for pasteurizing purposes. Biological tests were carried out in
+the author's laboratory[149] on milks having a high and low acid
+content, and it was shown that the milk with the least acid was, as a
+rule, the freest from spore-bearing bacteria.
+
+This acid determination can be made at the weigh-can by employing the
+Farrington alkaline tablet which is used in cream-ripening. Where milk
+is pasteurized under general creamery conditions, none should be used
+containing more than 0.2 per cent acidity. If only perfectly fresh milk
+is used, the amount of acid will generally be about 0.15 per cent with
+phenolphthalein as indicator.
+
+[Illustration: FIG. 28. Diagram showing temperature changes in
+pasteurizing, and the relation of same to bacterial growth.
+
+Shaded zone represents limits of bacterial growth, 50 deg.-109 deg. F.
+(10 deg.-43 deg. C.), the intensity of shading indicating rapidity of
+development. The solid black line shows temperature of milk during the
+process. The necessity for rapid cooling is evident as the milk falls
+in temperature to that of growing zone.]
+
+Emphasis has already been laid on the selection of a proper limit of
+pasteurizing (p. 114). It should be kept constantly in mind that the
+thermal death-point of any organism depends not alone on the temperature
+used, but on the period of exposure. With the lower limits given, 140
+deg. F., it is necessary to expose the milk for not less than fifteen
+minutes. If a higher heat is employed (and the cooked flavor
+disregarded) the period of exposure may be curtailed.
+
+~Chilling the milk.~ It is very essential in pasteurizing that the heated
+milk be immediately chilled in order to prevent the germination of the
+resistant spores, for if germination once occurs, growth can go on at
+relatively low temperatures.
+
+The following experiments by Marshall[150] are of interest as showing
+the influence of refrigeration on germination of spores:
+
+Cultures of organisms that had been isolated from pasteurized milk were
+inoculated into bouillon. One set was left to grow at room temperature,
+another was pasteurized and allowed to stand at same temperature, while
+another heated set was kept in a refrigerator. The unheated cultures at
+room temperature showed evidence of growth in thirty trials in an
+average of 26 hours; 29 heated cultures at room temperature all
+developed in an average of 50 hours, while the heated cultures kept in
+refrigerator showed no growth in 45 days with but four exceptions.
+
+Practically all of the rapid-process machines are provided with
+especially constructed cooling devices. In some of them, as in the
+Miller and Farrington, the cooling is effected by passing the milk
+through two separate coolers that are constructed in the same general
+way as the heater. With the first cooler, cold running water is
+employed, the temperature often being lowered in this way to 58 deg. or
+60 deg. F. Further lessening of the temperature is secured by an
+additional ice water or brine cooler which brings the temperature down
+to 40 deg.-50 deg. F.
+
+In the economical use of ice the ice itself should be applied as closely
+as possibly to the milk to be cooled, for the larger part of the
+chilling value of ice comes from the melting of the same. To convert a
+pound of ice at 32 deg. F. into a pound of water at the same temperature,
+if we disregard radiation, would require as much heat as would suffice to
+raise 142 pounds of water one degree F., or one pound of water 142 deg. F.
+The absorptive capacity of milk for heat (specific heat) is not quite
+the same as it is with water, being .847 for milk in comparison with 1.0
+for water.[151] Hot milk would therefore require somewhat less ice to
+cool it than would be required by any equal volume of water at the same
+temperature.
+
+~Bottling the product.~ If the milk has been properly pasteurized, it
+should, of course, be dispensed in sterilized bottles. Glass bottles
+with plain pulp caps are best, and these should be thoroughly sterilized
+in steam before using. The bottling can best be done in a commercial
+bottling machine. Care must be taken to thoroughly clean this apparatus
+after use each day. Rubber valves in these machines suffer deterioration
+rapidly.
+
+[Illustration: FIG. 29. Relative consistency of pasteurized cream before
+(A) and after (B) treatment with viscogen as shown by rate of flow down
+inclined glass plate.]
+
+~Restoration of "body" of pasteurized cream.~ The action of heat causes
+the tiny groupings of fat globules in normal milk (Fig. 22) to break up,
+and with this change, which occurs in the neighborhood of 140 deg. F.,
+where the milk is heated for about 15 minutes and at about 160-165 deg.
+F. where rapidly heated in a continuous stream, the consistency of the
+liquid is diminished, notwithstanding the fact that the fat-content
+remains unchanged. Babcock and the writer[152] devised the following
+"cure" for this apparent defect. If a strong solution of cane sugar is
+added to freshly slacked lime and the mixture allowed to stand, a clear
+fluid can be decanted off. The addition of this alkaline liquid, which
+is called "viscogen," to pasteurized cream in proportions of about one
+part of sugar-lime solution to 100 to 150 of cream, restores the
+consistency of the cream, as it causes the fat globules to cluster
+together in small groups.
+
+The relative viscosity of creams can easily be determined by the
+following method (Fig. 29):
+
+Take a perfectly clean piece of glass (plate or picture glass is
+preferable, as it is less liable to be wavy). Drop on one edge two or
+three drops of cream at intervals of an inch or so. Then incline piece
+of glass at such an angle as to cause the cream to flow down surface of
+glass. The cream, having the heavier body or viscosity, will move more
+slowly. If several samples of each cream are taken, then the aggregate
+lengths of the different cream paths may be taken, thereby eliminating
+slight differences due to condition of glass.
+
+FOOTNOTES:
+
+[126] From 10 to 16 cents per quart is usually paid for such milks.
+
+[127] Much improvement in quality could be made by more careful control
+of milk during shipment, especially as to refrigeration; also as to the
+care taken on the farms. The use of the ordinary milking machine (see
+page 37), would go far to reduce the germ content of milk.
+
+[128] Farrington, Journ. Amer. Chem. Soc., Sept., 1896.
+
+[129] Hite, Bull. 58, West Va. Expt. Stat., 1899.
+
+[130] Milch Zeit., 1895, No. 9.
+
+[131] Ibid., 1897, No. 33.
+
+[132] Bernstein, Milch Zeit., 1894, pp. 184, 200.
+
+[133] Thoerner, Chem. Zeit., 18:845.
+
+[134] Snyder, Chemistry of Dairying, p. 59.
+
+[135] Doane and Price (Bull. 77, Md. Expt. Stat., Aug. 1901) give quite
+a full resume of the work on this subject in connection with rather
+extensive experiments made by them on feeding animals with raw,
+pasteurized and sterilized milks.
+
+[136] Rickets is a disease in which the bones lack sufficient mineral
+matter to give them proper firmness. Marasmus is a condition in which
+the ingested food seems to fail to nourish the body and gradual wasting
+away occurs.
+
+[137] De Man, Arch. f. Hyg., 1893, 18:133.
+
+[138] Th. Smith, Journ. of Expt. Med., 1899, 4:217.
+
+[139] Russell and Hastings, 17 Rept. Wis. Expt. Stat., 1900, p. 147.
+
+[140] Russell and Hastings, 21 Rept. Ibid., 1904.
+
+[141] Russell and Hastings, 18 Rept. Ibid., 1901.
+
+[142] Russell, Bull. 44, Wis. Expt. Stat.
+
+[143] Russell, 22 Wis. Expt. Stat. Rept., 1905, p. 232.
+
+[144] Russell, 12 Wis. Expt. Stat. Rept., 1895, p. 160.
+
+[145] De Schweinitz, Nat. Med. Rev., 1899, No. 11.
+
+[146] Harding and Rogers. Bull. 182, N. Y. (Geneva) Expt. Stat., Dec.,
+1899.
+
+[147] Jensen, Milchkunde und Milch Hygiene, p. 132.
+
+[148] 22 Wis. Expt. Stat. Rept., 1905, p. 236.
+
+[149] Shockley, Thesis, Univ. of Wis., 1896.
+
+[150] Marshall, Mich. Expt. Stat., Bull. 147, p. 47.
+
+[151] Fleischmann, Landw. Versuchts Stat., 17:251.
+
+[152] Babcock and Russell, Bull. 54, Wis. Expt. Stat., Aug. 1896.
+
+
+
+
+CHAPTER VII.
+
+BACTERIA AND BUTTER-MAKING.
+
+
+In making butter from the butter fat in milk, it is necessary to
+concentrate the fat globules into cream, preliminary to the churning
+process. The cream may be raised by the gravity process or separated
+from the milk by centrifugal action. In either case the bacteria that
+are normally present in the milk differentiate themselves in varying
+numbers in the cream and the skim-milk. The cream always contains per
+cc. a great many more than the skim-milk, the reason for this being that
+the bacteria are caught and held in the masses of fat globules, which,
+on account of their lighter specific gravity, move toward the surface of
+the milk or toward the interior of the separator bowl. This filtering
+action of the fat globules is similar to what happens in muddy water
+upon standing. As the suspended particles fall to the bottom they carry
+with them a large number of the organisms that are in the liquid.
+
+~Various creaming methods.~ The creaming method has an important bearing
+on the kind as well as the number of the bacteria that are to be found
+in the cream. The difference in species is largely determined by the
+difference in ripening temperature, while the varying number is governed
+more by the age of the milk.
+
+_1. Primitive gravity methods._ In the old shallow-pan process, the
+temperature of the milk is relatively high, as the milk is allowed to
+cool naturally. This comparatively high temperature favors especially
+the development of those forms whose optimum growing-point is near the
+air temperature. By this method the cream layer is exposed to the air
+for a longer time than with any other, and consequently the
+contamination from this source is greater. Usually cream obtained by the
+shallow-pan process will contain a larger number of species and also
+have a higher acid content.
+
+_2. Modern gravity methods._ In the Cooley process, or any of the modern
+gravity methods where cold water or ice is used to lower the
+temperature, the conditions do not favor the growth of a large variety
+of species. The number of bacteria in the cream will depend largely upon
+the manner in which the milk is handled previous to setting. If care is
+used in milking, and the milk is kept so as to exclude outside
+contamination, the cream will be freer from bacteria than if
+carelessness prevails in handling the milk. Only those forms will
+develop in abundance that are able to grow at the low temperature at
+which the milk is set. Cream raised by this method is less frequently
+infected with undesirable forms than that which is creamed at a higher
+temperature.
+
+_3. Centrifugal method._ Separator cream should contain less germ-life
+than that which is secured in the old way. It should contain only those
+forms that have found their way into the milk during and subsequent to
+the milking, for the cream is ordinarily separated so soon that there is
+but little opportunity of infection, if care is taken in the handling.
+As a consequence, the number of species found therein is smaller.
+
+Where milk is separated, it is always prudent to cool the cream so as to
+check growth, as the milk is generally heated before separating in order
+to skim efficiently.
+
+Although cream is numerically much richer in bacteria than milk, yet
+the changes due to bacterial action are slower; hence milk sours more
+rapidly than cream. For this same reason, cream will sour sooner when it
+remains on the milk than it will if it is separated as soon as possible.
+This fact indicates the necessity of early creaming, so as to increase
+the keeping quality of the product, and is another argument in favor of
+the separator process.
+
+~Ripening of cream.~ If cream is allowed to remain at ordinary
+temperatures, it undergoes a series of fermentation changes that are
+exceedingly complex in character, the result of which is to produce in
+butter made from the same the characteristic flavor and aroma that are
+so well known in this article. We are so accustomed to the development
+of these flavors in butter that they are not generally recognized as
+being intimately associated with bacterial activity unless compared with
+butter made from perfectly fresh cream. Sweet-cream butter lacks the
+aromatic principle that is prominent in the ripened product, and while
+the flavor is delicate, it is relatively unpronounced.
+
+In the primitive method of butter-making, where the butter was made on
+the farm, the ripening of cream became a necessity in order that
+sufficient material might be accumulated to make a churning. The
+ripening change occurred spontaneously without the exercise of any
+especial control. With the development of the creamery system came the
+necessity of exercising a control of this process, and therefore the
+modern butter-maker must understand the principles which are involved in
+this series of complex changes that largely give to his product its
+commercial value.
+
+In these ripening changes three different factors are to be taken into
+consideration: the development of acid, flavor and aroma. Much confusion
+in the past has arisen from a failure to discriminate between these
+qualities. While all three are produced simultaneously in ordinary
+ripening, it does not necessarily follow that they are produced by the
+same cause. If the ripening changes are allowed to go too far,
+undesirable rather than beneficial decomposition products are produced.
+These greatly impair the value of butter, so that it becomes necessary
+to know just to what extent this process should be carried.
+
+In cream ripening there is a very marked bacterial growth, the extent of
+which is determined mainly by the temperature of the cream. Conn and
+Esten[153] find that the number of organisms may vary widely in
+unripened cream, but that the germ content of the ripened product is
+more uniform. When cream is ready for the churn, it often contains
+500,000,000 organisms per cc., and frequently even a higher number. This
+represents a germ content that has no parallel in any natural material.
+
+The larger proportion of bacteria in cream as it is found in the
+creamery belong to the acid-producing class, but in the process of
+ripening, these forms seem to thrive still better, so that when it is
+ready for churning the germ content of the cream is practically made up
+of this type.
+
+~Effect on churning.~ In fresh cream the fat globules which are suspended
+in the milk serum are surrounded by a film of albuminous material which
+prevents them from coalescing readily. During the ripening changes, this
+enveloping substance is modified, probably by partial solution, so that
+the globules cohere when agitated, as in churning. The result is that
+ripened cream churns more easily, and as it is possible to cause a
+larger number of the smaller fat-globules to cohere to the butter
+granules, the yield is slightly larger--a point of considerable
+economic importance where large quantities of butter are made.
+
+~Development of acid.~ The result of this enormous bacterial
+multiplication is that acid is produced in cream, lactic being the
+principal acid so formed.
+
+Other organic acids are undoubtedly formed as well as certain aromatic
+products. While the production of acid as a result of fermentative
+activity is usually accompanied with a development of flavor, the flavor
+is not directly produced by the formation of acid. If cream is treated
+in proper proportions with a commercial acid, as hydrochloric,[154] it
+assumes the same churning properties as found in normally ripened cream,
+but is devoid of the desired aromatic qualities. Lactic acid[155] has
+also been used in a similar way but with no better results.
+
+The amount of acidity that should be developed under natural conditions
+so as to secure the optimum quality as to flavor and aroma is the most
+important question in cream ripening. Concerning this there have been
+two somewhat divergent views as to what is best in practice, some
+holding that better results were obtained with cream ripened to a high
+degree of acidity than where a less amount was developed.[156] The
+present tendency seems to be to develop somewhat more than formerly, as
+it is thought that this secures more of the "high, quick" flavor wanted
+in the market. On the average, cream is ripened to about 0.5 to 0.65 per
+cent. acidity, a higher percentage than this giving a strong-flavored
+butter. In the determination of acidity, the most convenient method is
+to employ the Farrington alkaline tablet, which permits of an accurate
+and rapid estimation of the acidity in the ripening cream. The amount of
+acidity to be produced must of necessity be governed by the amount of
+butter-fat present, for the formation of acid is confined to the serum
+of the cream; consequently, a rich cream would show less acid by
+titration than a thinner cream, and still contain really as much acid as
+the other. The importance of this factor is evident in gathered-cream
+factories.
+
+The rate of ripening is dependent upon the conditions that affect the
+rate of growth of bacterial life, such as time and temperature, number
+of organisms in cream and also the per cent of butter fat in the cream.
+Some years ago it was customary to ripen cream at about 50 deg. to 60 deg.
+F., but more recently better results have been obtained, it is claimed,
+where the ripening temperature is increased and the period of ripening
+lessened. As high a temperature as 70 deg. to 75 deg. F. has been
+recommended. It should be said that this variation in practice may have
+a valid scientific foundation, for the temperature of the ripening cream
+is undoubtedly the most potent factor in determining what kind of bacteria
+will develop most luxuriantly. It is well known that those forms that
+are capable of producing bitter flavors are able to thrive better at a
+lower temperature than some of the desirable ripening species.
+
+The importance of this factor would be lessened where a pure culture was
+used in pasteurized cream, because here practically the selected
+organism alone controls the field.
+
+It is frequently asserted that better results are obtained by stirring
+the cream and so exposing it to the air as much as possible. Experiments
+made at the Ontario Agricultural College, however, show practically no
+difference in the quality of the butter made by these two methods. The
+great majority of the bacteria in the cream belong to the facultative
+class, and are able to grow under conditions where they are not in
+direct contact with the air.
+
+~Flavor and aroma.~ The basis for the peculiar flavor or taste which
+ripened cream-butter possesses is due, in large part, to the formation
+of certain decomposition products formed by various bacteria. Aroma is a
+quality often confounded with flavor, but this is produced by volatile
+products only, which appeal to the sense of smell rather than taste.
+Generally a good flavor is accompanied by a desirable aroma, but the
+origin of the two qualities is not necessarily dependent on the same
+organisms. The quality of flavor and aroma in butter is, of course, also
+affected by other conditions, as, for instance, the presence or absence
+of salt, as well as the inherent qualities of the milk, that are
+controlled, to some extent at least, by the character of the feed which
+is consumed by the animal. The exact source of these desirable but
+evanescent qualities in butter is not yet satisfactorily determined.
+According to Storch,[157] flavors are produced by the decomposition of
+the milk sugar and the absorption of the volatile flavors by the butter
+fat. Conn[158] holds that the nitrogenous elements in cream serve as
+food for bacteria, and in the decomposition of which the desired
+aromatic substance is produced. The change is unquestionably a complex
+one, and cannot be explained as a single fermentation.
+
+There is no longer much doubt but that both acid-forming and
+casein-digesting species can take part in the production of proper
+flavors as well as desirable aromas. The researches of Conn,[159] who
+has studied this question most exhaustively, indicate that both of these
+types of decomposition participate in the production of flavor and
+aroma. He has shown that both flavor and aroma production are
+independent of acid; that many good flavor-producing forms belong to
+that class which renders milk alkaline, or do not change the reaction at
+all. Some of these species liquefied gelatin and would therefore belong
+to the casein-dissolving class. Those species that produced bad flavors
+are also included in both fermentative types. Conn has found a number of
+organisms that are favorable flavor-producers; in fact they were much
+more numerous than desirable aroma-yielding species. None of the
+favorable aroma forms according to his investigations were lactic-acid
+species,--a view which is also shared by Weigmann.[160]
+
+McDonnell[161] has found that the production of aroma in certain cases
+varies at different temperatures, the most pronounced being evolved near
+the optimum growing temperature, which, as a general rule, is too high
+for cream ripening.
+
+The majority of bacteria in ripening cream do not seem to exert any
+marked influence in butter. A considerable number of species are
+positively beneficial, inasmuch as they produce a good flavor or aroma.
+A more limited number are concerned in the production of undesirable
+ripening changes. This condition being true, it may seem strange that
+butter is as good as it is, because so frequently the requisite care is
+not given to the development of proper ripening. In all probability the
+chief reason why this is so is that those bacteria that find milk and
+cream pre-eminently suited to their development, e. g. the lactic-acid
+class, are either neutral or beneficial in their effect on butter.
+
+~Use of starters.~ Experience has amply demonstrated that it is possible
+to control the nature of the fermentative changes that occur in ripening
+cream to such an extent as to materially improve the quality of the
+butter. This is frequently done by the addition of a "starter." While
+starters have been employed for many years for the purpose mentioned, it
+is only recently that their nature has been understood. A starter may be
+selected from widely divergent sources, but in all cases it is sure to
+contain a large number of bacteria, and the presumption is that they are
+of such a nature as to produce desirable fermentative changes in the
+cream.
+
+In the selection of these so-called natural starters, it follows that
+they must be chosen under such conditions as experience has shown to
+give favorable results. For this purpose, whole milk from a single
+animal is often used where the same is observed to sour with the
+production of no gas or other undesirable taint. A skim-milk starter
+from a mixed supply is recommended by many. Butter milk is frequently
+employed, but in the opinion of butter experts is not as suitable as the
+others mentioned.
+
+It not infrequently happens that the practical operator may be misled in
+selecting a starter that is not desirable, or by continuing its use
+after it has become contaminated.
+
+In 1890[162] a new system of cream ripening was introduced in Denmark by
+Storch that possesses the merit of being a truly scientific and at the
+same time practical method. This consisted in the use of pure cultures
+of specific organisms that were selected on account of their ability to
+produce a desirable ripening change in cream. The introduction of these
+so-called culture starters has become universal in Denmark, and in parts
+of Germany. Their use is also rapidly extending in this country,
+Australia and New Zealand.
+
+~Principles of pure-culture cream-ripening.~ In the proper use of pure
+cultures for ripening cream, it is necessary first to eliminate as far
+as possible the bacteria already present in cream before the culture
+starter is added. This result is accomplished by heating the cream to a
+temperature sufficiently high to destroy the vegetating organisms. The
+addition of a properly selected starter will then give the chosen
+organism such an impetus as will generally enable it to gain the
+ascendency over any other bacteria and so control the character of the
+ripening. The principle employed is quite like that practiced in raising
+grain. The farmer prepares his soil by plowing, in this way killing the
+weeds. Then he sows his selected grain, which is merely a pure culture,
+and by the rapid growth of this, other forms are held in check.
+
+The attempt has been made to use these culture starters in raw sweet
+cream, but it can scarcely be expected that the most beneficial results
+will be attained in this way. This method has been justified on the
+basis of the following experiments. Where cream is pasteurized and no
+starter is added, the spore-bearing forms frequently produce undesirable
+flavors. These can almost always be controlled if a culture starter is
+added, the obnoxious form being repressed by the presence of the added
+starter. This condition is interpreted as indicating that the addition
+of a starter to cream which already contains developing bacteria will
+prevent those originally present in the cream from growing.[163] This
+repressive action of one species on another is a well-known
+bacteriological fact, but it must be remembered that such an explanation
+is only applicable in those cases where the culture organism is better
+able to develop than those forms that already exist in the cream.
+
+If the culture organism is added to raw milk or cream which already
+contains a flora that is well suited to develop in this medium, it is
+quite doubtful whether it would gain the supremacy in the ripening
+cream. The above method of adding a culture to raw cream renders
+cream-ripening details less burdensome, but at the same time Danish
+experience, which is entitled to most credence on this question, is
+opposed to this method.
+
+~Reputed advantages of culture starters.~ _1. Flavor and aroma._ Naturally
+the flavor produced by pure-culture ferments depends upon the character
+of the organism used. Those which are most extensively used are able to
+produce a perfectly clean but mild flavor, and a delicate but not
+pronounced aroma. The "high, quick" flavor and aroma that is so much
+desired in the American market is not readily obtained by the use of
+cultures. It is quite problematical whether the use of any single
+species will give any more marked aroma than normally occurs in natural
+ripening.
+
+_2. Uniformity of product._ Culture starters produce a more uniform
+product because the type of fermentation is under more complete control,
+and herein is the greatest advantage to be derived from their use. Even
+the best butter-maker at times will fail to secure uniform results if
+his starter is not perfectly satisfactory.
+
+_3. Keeping quality of product._ Butter made from pasteurized cream to
+which a pure-culture starter has been added will keep much better than
+the ordinary product, because the diversity of the bacterial flora is
+less and the milk is therefore not so likely to contain those organisms
+that produce an "off" condition.
+
+_4. Elimination of taints._ Many defective conditions in butter are
+attributable to the growth of undesirable bacteria in the cream that
+result in the formation of "off" flavors and taints. If cream is
+pasteurized, thereby destroying these organisms, then ripened with pure
+ferments, it is generally possible to eliminate the abnormal
+conditions.[164] Taints may also be present in cream due to direct
+absorption from the cow or through exposure to foul odors.[165] Troubles
+of this sort may thus be carried over to the butter. This is
+particularly true in regions where leeks and wild onions abound, as in
+some of the Atlantic States. The heating of the cream tends to expel
+these volatile taints, so that a fairly good article of butter can be
+made from what would otherwise be a relatively worthless product.
+
+~Characteristics desired in culture starters.~ Certain conditions as the
+following are desirable in starters made from pure cultures:
+
+1. Vigorous growth in milk at ordinary ripening temperatures.
+
+2. Ability to form acid so as to facilitate churning and increase the
+yield of butter.
+
+3. Able to produce a clean flavor and desirable aroma.
+
+4. Impart a good keeping quality to butter.
+
+5. Not easily modified in its flavor-producing qualities by artificial
+cultivation.
+
+These different conditions are difficult to attain, for the reason that
+some of them seem to be in part incompatible. Weigmann[166] found that a
+good aroma was generally an evanescent property, and therefore opposed
+to good keeping quality. Conn has shown that the functions of
+acid-formation, flavor and aroma production are not necessarily related,
+and therefore the chances of finding a single organism that possesses
+all the desirable attributes are not very good.
+
+In all probability no one germ possesses all of these desirable
+qualities, but natural ripening is the resultant of the action of
+several forms.[167] This idea has led to the attempt at mixing selected
+organisms that have been chosen on account of certain favorable
+characteristics which they might possess. The difficulty of maintaining
+such a composite culture in its correct proportions when it is
+propagated in the creamery is seemingly well nigh insuperable, as one
+organism is very apt to develop more or less rapidly than the other.
+
+A very satisfactory way in which these cultures are marketed is to mix
+the bacterial growth with some sterile, inert, dry substance. This is
+the method used in most of the Danish cultures. In this country, some of
+the more prominent cultures employed are marketed in a liquid form.
+
+~Culture vs. home-made starters.~ One great advantage which has accrued
+from the use of culture or commercial starters has been that in
+emphasizing the need of closer control of the ripening process, greater
+attention has been paid to the carrying out of the details. In the
+hands of the better operators, the differences in flavor of butter made
+with a culture or a natural starter are not marked,[168] but in the
+hands of those who fail to make a good product under ordinary
+conditions, an improvement is often secured where a commercial culture
+is used.
+
+~Pasteurization as applied to butter-making.~ This process, as applied to
+butter making, is often confounded with the treatment of milk and cream
+for direct consumption. It is unfortunate that the same term is used in
+connection with the two methods, for they have but little in common
+except in the use of heat to destroy the germ life of the milk. In
+pasteurizing cream for butter-making, it is not necessary to observe the
+stringent precautions that are to be noted in the preservation of milk;
+for the addition of a rapidly developing starter controls at once the
+fermentative changes that subsequently occur. Then again, the physical
+requirement as to the production of a cooked taste is not so stringent
+in butter-making. While a cooked taste is imparted to milk or even cream
+at about 158 deg. F., it is possible to make butter that shows no
+permanent cooked taste from cream that has been raised as high as 185
+deg. or even 195 deg. F. This is due to the fact that the fat does not
+readily take up those substances that give to scalded milk its peculiar
+flavor.
+
+Unless care is taken in the manipulation of the heated cream, the grain
+or body of the butter may be injured. This tendency can be overcome if
+the ripened cream is chilled to 48 deg. F. for about two hours before
+churning. It is also essential that the heated cream should be quickly
+and thoroughly chilled after being pasteurized.
+
+The Danes, who were the first to employ pasteurization in butter-making,
+used, in the beginning, a temperature ranging from 158 deg. to 167 deg.
+F., but owing to the prevalence of such diseases as tuberculosis and
+foot-and-mouth disease, it became necessary to treat all of the skim
+milk that was returned from the creameries. For this purpose the skim
+milk is heated to a temperature of 176 deg. F., it having been more
+recently determined that this degree of heat is sufficient to destroy the
+seeds of disease. With the use of this higher temperature the capacity of
+the pasteurizing apparatus is considerably reduced, but the higher
+temperature is rendered necessary by the prevailing conditions as to
+disease.
+
+When the system was first introduced in Denmark, two methods of
+procedure were followed: the whole milk was heated to a sufficiently
+high temperature to thoroughly pasteurize it before it was separated, or
+it was separated first, and the cream pasteurized afterwards. In the
+latter case, it is necessary to heat the skim milk after separation to
+destroy the disease organisms, but this can be quickly done by the use
+of steam directly. Much more care must be used in heating the cream in
+order to prevent injury to the grain of the butter. In spite of the
+extra trouble of heating the cream and skim milk separately, this method
+has practically supplanted the single heating. With the continual spread
+of tuberculosis in America the heating of skim milk separately is
+beginning to be introduced.[169]
+
+~Use of starters in pasteurized and unpasteurized cream.~ In order to
+secure the beneficial results presumably attributable to the use of a
+starter, natural as well as a pure culture, it should be employed in
+cream in which the bacteria have first been killed out by
+pasteurization. This is certainly the most logical and scientific method
+and is the way in which the process has been developed in Denmark.
+
+Here in this country, the use of pure cultures has been quite rapidly
+extended, but the system of heating the cream has been used in only a
+slight measure. The increased labor and expense incurred in pasteurizing
+the cream has naturally militated somewhat against the wide-spread use
+of the process, but doubtless the main factor has been the inability to
+secure as high a flavor where the cream was heated as in the unheated
+product. As the demands of the market change from a high, quick flavor
+to one that is somewhat milder but of better keeping quality, doubtless
+pasteurization of the cream will become more and more popular. That such
+a change is gradually occurring is already evident, although as yet only
+a small proportion of butter made in this country is now made in this
+way. Where the cream is unheated, a considerable number of species will
+be found, and even the addition of a pure culture, if that culture is of
+the lactic acid-producing species, will to some extent control the type
+of fermentation that occurs. Such would not be the case with a culture
+composed of the casein-digesting type of bacteria. Only those forms
+could thus be used which are especially well suited to development in
+raw cream. For this reason the pure culture ferments that are generally
+employed in creamery practice are organisms of the lactic acid type,
+able to grow rapidly in cream and produce a pure cream flavor in the
+butter.
+
+~Purity of commercial starters.~ Naturally the butter maker is forced to
+rely on the laboratory for his commercial starter, and the question will
+often arise as to the purity and vigor of the various ferments employed.
+As there is no way for the factory operator to ascertain the actual
+condition of the starter, except by using the same, the greatest care
+should be taken by the manufacturer to insure the absolute purity of the
+seed used.
+
+A bacteriological examination of the various cultures which have been
+placed on the market not infrequently reveals an impure condition. In
+several cases the writer has found a not inconsiderable number of
+liquefying bacteria mixed with the selected organism. Molds not
+infrequently are found in cultures put up in the dry form. Doubtless the
+effect of these accidental contaminations is considerably less in the
+case of a starter composed of a distinctively lactic acid-producing
+organism than with a form which is less capable of thriving vigorously
+in milk, and it should be said that these impurities can frequently be
+eliminated by continued propagation.
+
+The virility and vigor of the starter is also a fluctuating factor,
+dependent in part at least, upon the conditions under which the organism
+is grown. In some cases the germ is cultivated in solutions in which
+acid cannot be formed in abundance. Where the conditions permit of the
+formation of acid, as would be the case if sugar was present with a
+lactic acid-producing species, the vitality of the culture is often
+impaired by the action of the gradually accumulating acid. Some
+manufacturers attempt to minimize this deleterious condition by adding
+carbonate of lime which unites with the acid that is formed.
+
+~Propagation of starters for cream-ripening.~ The preparation and
+propagation of a starter for cream-ripening is a process involving
+considerable bacteriological knowledge, whether the starter is of
+domestic origin or prepared from a pure-culture ferment. In any event,
+it is necessary that the starter should be handled in a way so as to
+prevent the introduction of foreign bacteria as far as possible. It
+should be remembered at all times that the starter is a live thing and
+must be handled throughout its entire history in a way so as to retain
+its vitality and vigor unimpaired. The following points should be taken
+into consideration in growing the starter and transferring it from day
+to day:
+
+1. If a commercial starter is used, see that it is fresh and that the
+seal has not been broken. If the culture is too old, the larger part of
+the organisms may have died out before it is transferred, in which case
+the effect of its addition to the sterilized milk would be of little
+value.
+
+When the commercial ferment is received, it should be stored in the
+refrigerator pending its use so as to retard as much as possible the
+changes that naturally go on in the culture liquid. Be careful that the
+bottle is not exposed to the influence of direct sunlight for in a
+transparent medium the organisms may be readily killed by the
+disinfecting action of the sun's rays.
+
+2. If a home-made starter is employed, use the greatest possible care in
+selecting the milk that is to be used as a basis for the starter.
+
+3. For the propagation and perpetuation of the starter from day to day,
+it is necessary that the same should be grown in milk that is as
+germ-free as it is possible to secure it. For this purpose sterilize
+some fresh skim-milk in a covered can that has previously been well
+steamed. This can be done easily by setting cans containing skim-milk in
+a vat filled with water and heating the same to 180 deg. F. or above for
+one-half hour or more. Steam should not be introduced directly. This
+process destroys all but a few of the most resistant spore-bearing
+organisms. This will give a cooked flavor to the milk, but will not
+affect the cream to which the starter is added. Dairy supply houses are
+now introducing the use of starter cans that are specially made for this
+purpose.
+
+4. After the heated milk is cooled down to about 70 deg. or 80 deg. F.,
+it can be inoculated with the desired culture. Sometimes it is desirable
+to "build up" the starter by propagating it first in a smaller volume of
+milk, and then after this has developed, adding it to a larger amount.
+
+This method is of particular value where a large amount of starter is
+needed for the cream-ripening.
+
+5. After the milk has been inoculated, it should be kept at a
+temperature that is suitable for the rapid development of the contained
+bacteria, 65 deg.-75 deg. F., which temperature should be kept as uniform
+as possible.
+
+This can best be done by setting the covered can in a vat filled with
+warm water. The starter cans are often arranged so that temperature can
+be controlled by circulating water.
+
+6. The starter should not be too thoroughly curdled when it is needed
+for use, but should be well soured and only partially curdled for it is
+difficult to break up thoroughly the curd particles if the starter is
+completely curdled. If these curd masses are added to ripening cream,
+white specks may appear in the butter.
+
+7. The vigor of the starter is in all probability stronger when the milk
+is on the point of curdling than it is after the curd has been formed
+some time. The continued formation of lactic acid kills many of the
+bacteria and thus weakens the fermentative action. It is therefore
+highly important that the acidity of the starter should be closely
+watched.
+
+8. Do not refrigerate the starter when it has reached the proper stage
+of development, as this retards the bacterial growth in the same manner
+as cold weather checks the growth of grain. It is preferable to dilute
+the starter, if it cannot be used when ready, with sufficient freshly
+sterilized sweet milk to hold the acidity at the proper point and thus
+keep the bacteria in the starter in a condition which will favor
+vigorous growth.
+
+9. The starter should be propagated from day to day by adding a small
+quantity to a new lot of freshly prepared milk. For this purpose two
+propagating cans should be provided so that one starter may be in use
+while the other is being prepared.
+
+~How long should a starter be propagated?~ No hard-and-fast rule can be
+given for this, for it depends largely upon how carefully the starter is
+handled during its propagation. If the starter is grown in sterilized
+milk kept in steamed vessels and is handled with sterile dippers, it is
+possible to maintain it in a state of relative purity for a considerable
+period of time; if, however, no especial care is given, it will soon
+become infected by the air, and the retention of its purity will depend
+more upon the ability of the contained organism to choke out foreign
+growths than upon any other factor. Experience seems to indicate that
+pure-culture starters "run out" sooner than domestic starters. While it
+is possible, by bacteriological methods, to determine with accuracy the
+actual condition of a starter as to its germ content, still such methods
+are inapplicable in creamery practice. Here the maker must rely largely
+upon the general appearance of the starter as determined by taste and
+smell. The supply houses that deal in cultures of this class generally
+expect to supply a new culture at least every month.
+
+~Bacteria in butter.~ As ripened cream is necessarily rich in bacteria, it
+follows that butter will also contain germ life in varying amounts, but
+as butter-fat is not well adapted for bacterial food, the number of
+germs in butter is usually less than in ripened cream.
+
+Sweet-cream butter is naturally poorer in germ life than that made from
+ripened cream. Grotenfelt reports in sweet-cream butter, the so-called
+"Paris butter," only a few bacteria while in acid cream butter the germ
+content runs from scores to hundreds of thousands.
+
+~Effect of bacteria in wash water.~ An important factor in contamination
+may be the wash water that is used. Much carelessness often prevails
+regarding the location and drainage of the creamery well, and if same
+becomes polluted with organic matter, bacterial growth goes on apace.
+Melick[170] has made some interesting studies on using pasteurized and
+sterilized well waters for washing. He found a direct relation to exist
+between the bacterial content of the wash water and the keeping quality
+of the butter. Some creameries have tried filtered water but under
+ordinary conditions a filter, unless it is tended to with great
+regularity, becomes a source of infection rather than otherwise.
+
+~Changes in germ content.~ The bacteria that are incorporated with the
+butter as it first "comes" undergo a slight increase for the first few
+days. The duration of this period of increase is dependent largely upon
+the condition of the butter. If the buttermilk is well worked out of the
+butter, the increase is slight and lasts for a few days only, while the
+presence of so nutritious a medium as buttermilk affords conditions much
+more favorable for the continued growth of the organisms.
+
+While there may be many varieties in butter when it is fresh, they are
+very soon reduced in kind as well as number. The lactic acid group of
+organisms disappear quite rapidly; the spore-bearing species remaining
+for a somewhat longer time. Butter examined after it is several months
+old is often found to be almost free from germs.
+
+In the manufacture of butter there is much that is dependent upon the
+mechanical processes of churning, washing, salting and working the
+product. These processes do not involve any bacteriological principles
+other than those that are incident to cleanliness. The cream, if ripened
+properly, will contain such enormous numbers of favorable forms that the
+access of the few organisms that are derived from the churn, the air, or
+the water in washing will have little effect, unless the conditions are
+abnormal.
+
+
+BACTERIAL DEFECTS IN BUTTER.
+
+~Rancid change in butter.~ Fresh butter has a peculiar aroma that is very
+desirable and one that enhances the market price, if it can be retained;
+but this delicate flavor is more or less evanescent, soon disappearing,
+even in the best makes. While a good butter loses with age some of the
+peculiar aroma that it possesses when first made, yet a gilt-edged
+product should retain its good keeping qualities for some length of
+time. All butters, however, sooner or later undergo a change that
+renders them worthless for table use. This change is usually a rancidity
+that is observed in all stale products of this class. The cause of this
+rancid condition in butter was at first attributed to the formation of
+butyric acid, but it is now recognized that other changes also enter
+in.[171] Light and especially air also exert a marked effect on the
+flavor of butter. Where butter is kept in small packages it is much more
+prone to develop off flavors than when packed in large tubs. From the
+carefully executed experiments of Jensen it appears that some of the
+molds as well as certain species of bacteria are able to incite these
+changes. These organisms are common in the air and water and it
+therefore readily follows that inoculation occurs.
+
+Practically, rancidity is held in check by storing butter at low
+temperatures where germ growth is quite suspended.
+
+~Lack of flavor.~ Often this may be due to improper handling of the cream
+in not allowing it to ripen far enough, but sometimes it is impossible
+to produce a high flavor. The lack of flavor in this case is due to the
+absence of the proper flavor-producing organisms. This condition can
+usually be overcome by the addition of a proper starter.
+
+~Putrid butter.~ This specific butter trouble has been observed in
+Denmark, where it has been studied by Jensen.[172] Butter affected by it
+rapidly acquires a peculiar putrid odor that ruins it for table use.
+Sometimes, this flavor may be developed in the cream previous to
+churning.
+
+Jensen found the trouble to be due to several different putrefactive
+bacteria. One form which he called _Bacillus foetidus lactis_, a close
+ally of the common feces bacillus, produced this rotten odor and taste
+in milk in a very short time. Fortunately, this organism was easily
+killed by a comparatively low heat, so that pasteurization of the cream
+and use of a culture starter quickly eliminated the trouble, where it
+was tried.
+
+~Turnip-flavored butter.~ Butter sometimes acquires a peculiar flavor
+recalling the order of turnips, rutabagas, and other root crops. Often
+this trouble is due to feeding, there being in several of these crops,
+aromatic substances that pass directly into the milk, but in some
+instances the trouble arises from bacteria that are able to produce
+decomposition products,[173] the odor and taste of which strongly
+recalls these vegetables.
+
+~"Cowy" butter.~ Frequently there is to be noted in milk a peculiar odor
+that resembles that of the cow stable. Usually this defect in milk has
+been ascribed to the absorption of impure gases by the milk as it cools,
+although the gases and odors naturally present in fresh milk have this
+peculiar property that is demonstrable by certain methods of aeration.
+Occasionally it is transmitted to butter, and recently Pammel[174] has
+isolated from butter a bacillus that produced in milk the same peculiar
+odor so commonly present in stables.
+
+~Lardy and tallowy butter.~ The presence of this unpleasant taste in
+butter may be due to a variety of causes. In some instances, improper
+food seems to be the source of the trouble; then again, butter exposed
+to direct sunlight bleaches in color and develops a lardy flavor.[175]
+In addition to these, cases have been found in which the defect has been
+traced to the action of bacteria. Storch[176] has described a
+lactic-acid form in a sample of tallowy butter that was able to produce
+this disagreeable odor.
+
+~Oily butter.~ Jensen has isolated one of the causes of the dreaded oily
+butter that is reported quite frequently in Denmark. The specific
+organism that he found belongs to the sour-milk bacteria. In twenty-four
+hours it curdles milk, the curd being solid like that of ordinary sour
+milk. There is produced, however, in addition to this, an unpleasant
+odor and taste resembling that of machine oil, a peculiarity that is
+transmitted directly to butter made from affected cream.
+
+~Bitter butter.~ Now and then butter develops a bitter taste that may be
+due to a variety of different bacterial forms. In most cases, the bitter
+flavor in the butter is derived primarily from the bacteria present in
+the cream or milk. Several of the fermentations of this character in
+milk are also to be found in butter. In addition to these defects
+produced by a biological cause, bitter flavors in butter are sometimes
+produced by the milk being impregnated with volatile, bitter substances
+derived from weeds.
+
+~Moldy butter.~ This defect is perhaps the most serious because most
+common. It is produced by the development of a number of different
+varieties of molds. The trouble appears most frequently in packed butter
+on the outside of the mass of butter in contact with the tub. Mold
+spores are so widely disseminated that if proper conditions are given
+for their germination, they are almost sure to develop. In some cases
+the mold is due to the growth of the ordinary bread mold, _Penicillium
+glaucum_; in other cases a black mold develops, due often to
+_Cladosporium butyri_. Not infrequently trouble of this character is
+associated with the use of parchment wrappers. The difficulty can easily
+be held in check by soaking the parchment linings and the tubs in a
+strong brine, or paraffining the inside of the tub.
+
+~Fishy butter.~ Considerable trouble has been experienced in Australian
+butter exported to Europe in which a fishy flavor developed. It was
+noted that the production of this defect seemed to be dependent upon the
+storage temperature at which the butter was kept. When the butter was
+refrigerated at 15 deg. F. no further difficulty was experienced. It is
+claimed that the cause of this condition is due to the formation of
+trimethylamine (herring brine odor) due to the growth of the mold fungus
+_Oidium lactis_, developing in combination with the lactic-acid
+bacteria.
+
+A fishy taste is sometimes noted in canned butter. Rogers[177] has
+determined that this flavor is caused by yeasts (_Torula_) which produce
+fat-splitting enzyms capable of producing this undesirable change.
+
+FOOTNOTES:
+
+[153] Conn and Esten, Cent. f. Bakt., II Abt., 1901, 7:746.
+
+[154] Tiemann, Milch Zeit., 23:701.
+
+[155] Milch Zeit., 1889, p. 7; 1894, p. 624; 1895, p. 383.
+
+[156] Dean, Ont. Agr. Coll., 1897, p. 66.
+
+[157] Storch, Nogle, Unders. over Floed. Syrning, 1890.
+
+[158] Conn, 6 Storrs Expt. Stat., 1893, p. 66.
+
+[159] Conn, 9 Storrs Expt. Stat., 1896, p. 17.
+
+[160] Weigmann, Milch Zeit., 1891, p. 793
+
+[161] McDonnell, ue. Milchsaeure Bakterien (Diss. Kiel, 1899), p. 43.
+
+[162] Storch, Milch Zeit., 1890, p. 304.
+
+[163] Conn, 9 Storrs Expt. Stat., 1896, p. 25.
+
+[164] Milch Zeit., 1891, p. 122; 1894, p. 284; 1895, p. 56; 1896, p.
+163.
+
+[165] McKay, Bull. 32, Iowa Expt. Stat., p. 47
+
+[166] Weigmann, Landw. Woch. f. Schl. Hol., No. 2, 1890.
+
+[167] Weigmann, Cent. f. Bakt., II Abt., 3:497, 1897.
+
+[168] At the National Creamery Buttermakers' Association for 1901, 193
+out of 240 exhibitors used starters. Of those that employed starters,
+nearly one-half used commercial cultures. There was practically no
+difference in the average score of the two classes of starters, but
+those using starters ranked nearly two points higher in flavor than
+those that did not.
+
+[169] Russell, Bull. 143, Wis. Expt. Stat., Feb. 1907.
+
+[170] Melick, Bull. 138, Kansas Expt. Stat., June 1906.
+
+[171] Reinmann, Cent. f. Bakt., 1900, 6:131; Jensen, Landw. Jahr. d.
+Schweiz, 1901.
+
+[172] Jensen, Cent. f. Bakt., 1891, 11:409.
+
+[173] Jensen, Milch Zeit., 1892, 6, Nos. 5 and 6.
+
+[174] Pammel, Bull. 21, Iowa Expt. Stat., p. 803.
+
+[175] Fischer, Hyg. Rund., 5:573.
+
+[176] Storch, 18 Rept. Danish Agric. Expt. Stat., 1890.
+
+[177] Rogers Bull. 57, B. A. I. U. S. Dept Agric., 1904.
+
+
+
+
+CHAPTER VIII.
+
+BACTERIA IN CHEESE.
+
+
+The art of cheese-making, like all other phases of dairying, has been
+developed mainly as a result of empirical methods. Within the last
+decade or so, the subject has received more attention from the
+scientific point of view and the underlying causes determined to some
+extent. Since the subject has been investigated from the bacteriological
+point of view, much light has been thrown on the cause of many changes
+that were heretofore inexplicable. Our knowledge, as yet, is quite
+meager, but enough has already been determined to indicate that the
+whole industry is largely based on the phenomena of ferment action, and
+that the application of bacteriological principles and ideas is sure to
+yield more than ordinary results, in explaining, in a rational way, the
+reasons underlying many of the processes to be observed in this
+industry.
+
+The problem of good milk is a vital one in any phase of dairy activity,
+but it is pre-eminently so in cheese-making, for the ability to make a
+first-class product depends to a large extent on the quality of the raw
+material. Cheese contains so large a proportion of nitrogenous
+constituents that it is admirably suited, as a food medium, to the
+development of bacteria; much better, in fact, than butter.
+
+
+INFLUENCE OF BACTERIA IN NORMAL CHEESE PROCESSES.
+
+In the manufacture of cheddar cheese bacteria exert a marked influence
+in the initial stages of the process. To produce the proper texture that
+characterizes cheddar cheese, it is necessary to develop a certain
+amount of acid which acts upon the casein. This acidity is measured by
+the development of the lactic-acid bacteria that normally abound in the
+milk; or, as the cheese-maker expresses it, the milk is "ripened" to the
+proper point. The action of the rennet, which is added to precipitate
+the casein of the milk, is markedly affected by the amount of acid
+present, as well as the temperature. Hence it is desirable to have a
+standard amount of acidity as well as a standard temperature for
+coagulation, so as to unify conditions. It frequently happens that the
+milk is abnormal with reference to its bacterial content, on account of
+the absence of the proper lactic bacteria, or the presence of forms
+capable of producing fermentative changes of an undesirable character.
+In such cases the maker attempts to overcome the effect of the unwelcome
+bacteria by adding a "starter;" or he must vary his method of
+manufacture to some extent to meet these new conditions.
+
+~Use of starters.~ A starter may be employed to hasten the ripening of
+milk that is extremely sweet, so as to curtail the time necessary to get
+the cheese to press; or it may be used to overcome the effect of
+abnormal conditions.
+
+The starter that is employed is generally one of domestic origin, and is
+usually taken from skim milk that has been allowed to ferment and sour
+under carefully controlled conditions. Of course much depends upon the
+quality of the starter, and in a natural starter there is always the
+possibility that it may not be perfectly pure.
+
+Within recent years the attempt has been made to control the effect of
+the starter more thoroughly by using pure cultures of some desirable
+lactic-acid form.[178] This has rendered the making of cheese not only
+more uniform, but has aided in repressing abnormal fermentations
+particularly those that are characterized by the production of gas.
+
+Recently, pure cultures of Adametz's _B. nobilis_, a digesting organism
+that is claimed to be the cause of the breaking down of the casein and
+also of the peculiar aroma of Emmenthaler cheese, has been placed on the
+market under the name _Tyrogen_. It is claimed that the use of this
+starter, which is added directly to the milk and also rubbed on the
+surface of the cheese, results in the improvement of the curds, assists
+in the development of the proper holes, imparts a favorable aroma and
+hastens ripening.[179]
+
+Campbell[180] states that the discoloration of cheese in England, which
+is due to the formation of white spots that are produced by the
+bleaching of the coloring matter in the cheese, may be overcome by the
+use of lactic-acid starters.
+
+The use of stringy or slimy whey has been advocated in Holland for some
+years as a means of overcoming the tendency toward gas formation in Edam
+cheese which is made from practically sweet milk. This fermentation, the
+essential feature of which is produced by a culture of _Streptococcus
+Hollandicus_,[181] develops acid in a marked degree, thereby inhibiting
+the production of gas.
+
+The use of masses of moldy bread in directing the fermentation of
+Roquefort cheese is another illustration of the empirical development of
+starters, although in this instance it is added after the curds have
+been prepared for the press.
+
+~Pasteurizing milk for cheese-making.~ If it were possible to use properly
+pasteurized milk in cheese-making, then practically all abnormal
+conditions could be controlled by the use of properly selected starters.
+Numerous attempts have been made to perfect this system with reference
+to cheddar cheese, but so far they have been attended with imperfect
+success. The reason for this is that in pasteurizing milk, the soluble
+lime salts are precipitated by the action of heat, and under these
+conditions rennet extract does not curdle the casein in a normal manner.
+This condition can be restored, in part at least, by the addition of
+soluble lime salts, such as calcium chlorid; but in our experience,
+desirable results were not obtained where heated milks to which this
+calcium solution had been added were made into cheddar cheese.
+Considerable experience has been gained in the use of heated milks in
+the manufacture of certain types of foreign cheese. Klein[182] finds
+that Brick cheese can be successfully made even where the milk is heated
+as high as 185 deg. F. An increased weight is secured by the addition of
+the coagulated albumin and also increased moisture.
+
+~Bacteria in rennet.~ In the use of natural rennets, such as are
+frequently employed in the making of Swiss cheese, considerable numbers
+of bacteria are added to the milk. Although these rennets are preserved
+in salt, alcohol or boric acid, they are never free from bacteria.
+Adametz[183] found ten different species and from 640,000 to 900,000
+bacteria per cc. in natural rennets. Freudenreich has shown that rennet
+extract solutions can be used in Swiss cheese-making quite as well as
+natural rennets; but to secure the best results, a small quantity of
+pure lactic ferment must be added to simulate the conditions that
+prevail when natural rennets are soaked in whey, which, it must be
+remembered, is a fluid rich in bacterial life.
+
+Where rennet extract or tablets are used, as is generally the case in
+cheddar making, the number of bacteria added is so infinitesimal as to
+be negligible.
+
+~Development of acid.~ In the manufacture of cheddar cheese, the
+development of acid exerts an important influence on the character of
+the product. This is brought about by holding the curds at temperatures
+favorable to the growth of the bacteria in the same. Under these
+conditions the lactic-acid organisms, which usually predominate, develop
+very rapidly, producing thereby considerable quantities of acid which
+change materially the texture of the curds. The lactic acid acts upon
+the casein in solutions containing salt, causing it to dissolve to some
+extent, thus forming the initial compounds of digestion.[184] This
+solution of the casein is expressed physically by the "stringing" of the
+curds on a hot iron. This causes the curds to mat, producing a close,
+solid body, free from mechanical holes. Still further, the development
+of this acid is necessary for the digestive activity of the pepsin in
+the rennet extract.
+
+In some varieties of cheese, as the Swiss, acid is not developed and the
+character of the cheese is much different from that of cheddar. In all
+such varieties, a great deal more trouble is experienced from the
+production of "gassy" curds, because the development of the
+gas-producing bacteria is held in check by the rapid growth of the
+lactic acid-producing species.
+
+~Bacteria in green cheese.~ The conditions under which cheese is made
+permit of the development of bacteria throughout the entire process. The
+cooking or heating of curds to expel the excessive moisture is never so
+high as to be fatal to germ life; on the contrary, the acidity of the
+curd and whey is continually increased by the development of bacteria in
+the same.
+
+The body of green cheese fresh from the press is, to a considerable
+extent, dependent upon the acid produced in the curds. If the curds are
+put to press in a relatively sweet condition the texture is open and
+porous. The curd particles do not mat closely together and "mechanical
+holes," rough and irregular in outline, occur. Very often, at relatively
+high temperatures, such cheese begin to "huff," soon after being taken
+from the press, a condition due to the development of gas, produced by
+gas-generating bacteria acting on the sugar in the curd. This gas finds
+its way readily into these ragged holes, greatly distending them, as in
+Fig. 30.
+
+[Illustration: FIG. 30. _L_, a sweet curd cheese direct from the press.
+"Mechanical" holes due to lack of acid development; _P_, same cheese
+four days later, mechanical holes distended by development of gas.]
+
+~Physical changes in ripening cheese.~ When a green cheese is taken from
+the press, the curd is tough, firm, but elastic. It has no value as a
+food product for immediate use, because it lacks a desirable flavor and
+is not readily digestible. It is nothing but precipitated casein and
+fat. In a short time, a deep-seated change occurs. Physically this
+change is demonstrated in the modification that the curd undergoes.
+Gradually it breaks down and becomes plastic, the elastic, tough curd
+being changed into a softened mass. This change in texture of the cheese
+is also accompanied by a marked change in flavor. The green cheese has
+no distinctively cheese flavor, but in course of time, with the gradual
+change of texture, the peculiar flavor incident to ripe cheese is
+developed.
+
+The characteristic texture and flavor are susceptible of considerable
+modification that is induced not only by variation in methods of
+manufacture, but by the conditions under which the cheese are cured. The
+amount of moisture incorporated with the curd materially affects the
+physical appearance of the cheese, and the rate of change in the same.
+The ripening temperature, likewise the moisture content of the
+surrounding air, also exerts a marked influence on the physical
+properties of the cheese. To some extent the action of these forces is
+purely physical, as in the gradual loss by drying, but in other respects
+they are associated with chemical transformations.
+
+~Chemical changes in ripening cheese.~ Coincident with the physical
+breaking down of the curd comes a change in the chemical nature of the
+casein. The hitherto insoluble casein is gradually transformed into
+soluble nitrogenous substances (_caseone_ of Duclaux, or _caseogluten_
+of Weigmann). This chemical phenomenon is a breaking-down process that
+is analogous to the peptonization of proteids, although in addition to
+the peptones and albumoses characteristic of peptic digestion,
+amido-acids and ammonia are to be found. The quantity of these lower
+products increases with the age of the cheese.
+
+The chemical reaction of cheese is normally acid to phenolphthalein,
+although there is generally no free acid, as shown by Congo red, the
+lactic acid being converted into salts as fast as formed. In very old
+cheese, undergoing putrefactive changes, especially on the outside, an
+alkaline reaction may be present, due to the formation of free ammonia.
+
+The changes that occur in a ripening cheese are for the most part
+confined to the proteids. According to most investigators the fat
+remains practically unchanged, although the researches of Weigmann and
+Backe[185] show that fatty acids are formed from the fat. In the green
+cheese considerable milk-sugar is present, but, as a result of the
+fermentation that occurs, this is rapidly converted into acid products.
+
+~Bacterial flora of cheese.~ It might naturally be expected that the green
+cheese, fresh from the press, would contain practically the same kind of
+bacteria that are in the milk, but a study of cheese shows a peculiar
+change in the character of the flora. In the first place, fresh cottage
+cheese, made by the coagulation of the casein through the action of
+acid, has a more diversified flora than cheese made with rennet, for the
+reason, as given by Lafar,[186] that the fermentative process is farther
+advanced.
+
+When different varieties of cheese are made from milk in the same
+locality, the germ content of even the ripened product has a marked
+similarity, as is illustrated by Adametz's work[187] on Emmenthaler or
+Swiss hard cheese, and Schweitzer Hauskaese, a soft variety. Of the nine
+species of bacilli and cocci found in mature Emmenthaler, eight of them
+were also present in ripened Hauskaese.
+
+Different investigators have studied the bacterial flora of various
+kinds of cheese, but as yet little comparative systematic work has been
+done. Freudenreich[188] has determined the character and number of
+bacteria in Emmenthaler cheese, and Russell[189] the same for cheddar
+cheese. The same general law has also been noted in Canadian[190] and
+English[191] cheese. At first a marked decrease in numbers is usually
+noted, lasting for a day or two. This is followed by an enormous
+increase, caused by the rapid growth of the lactic-acid type. The
+development may reach scores of millions and often over a hundred
+million organisms per gram. Synchronous with this increase, the
+peptonizing and gas-producing bacteria gradually disappear. This rapid
+development, which lasts only for a few weeks, is followed by a general
+decline.
+
+In the ripening of cheese a question arises as to whether the process
+goes on throughout the entire mass of cheese, or whether it is more
+active at or near the surface. In the case of many of the soft cheese,
+such as Brie and limburger, bacterial and mold development is
+exceedingly active on the exterior, and the enzyms secreted by these
+organisms diffuse toward the interior. That such a condition occurs in
+the hard type of cheese made with rennet is extremely improbable. Most
+observers agree that in this type of cheese the ripening progresses
+throughout the entire mass, although Adametz opposes this view and
+considers that in Emmenthaler cheese the development of the specific
+aroma-producing organism occurs in the superficial layers. Jensen has
+shown, however, that the greatest amount of soluble nitrogenous products
+are to be found in the innermost part of the cheese, a condition that is
+not reconcilable with the view that the most active ripening is on the
+exterior.[192]
+
+The course of development of bacteria in cheddar cheese is materially
+influenced by the ripening temperature. In cheese ripened at relatively
+low temperatures (50 deg.-55 deg. F.),[193] a high germ content is
+maintained for a much longer period of time than at higher temperatures.
+Under these conditions the lactic-acid type continues in the ascendancy
+as usual. In cheese cured at high temperatures (80 deg.-86 deg. F.) the
+number of organisms is greatly diminished, and they fail to persist in
+appreciable numbers for as long a time as in cheese cured at temperatures
+more frequently employed.
+
+~Influence of temperature on curing.~ Temperature exerts a most potent
+influence on the quality of the cheese, as determined not only by the
+rate of ripening but the nature of the process itself. Much of the poor
+quality of cheese is attributable to the effect of improper curing
+conditions. Probably in the initial stage of this industry cheese were
+allowed to ripen without any sort of control, with the inevitable result
+that during the summer months the temperature generally fluctuated so
+much as to impair seriously the quality. The effect of high temperatures
+(70 deg. F. and above) is to produce a rapid curing, and, therefore, a
+short lived cheese; also a sharp, strong flavor, and generally a more or
+less open texture. Unless the cheese is made from the best quality of
+milk, it is very apt to undergo abnormal fermentations, more especially
+those of a gassy character.
+
+[Illustration: FIG. 31. Influence of curing temperature on texture of
+cheese. Upper row ripened eight months at 60 deg. F.; lower row at 40
+deg. F.]
+
+Where cheese is ripened at low temperatures, ranging from 50 deg. F. down
+to nearly the freezing temperatures, it is found that the quality is
+greatly improved.[194] Such cheese are thoroughly broken down from a
+physical point of view even though they may not show such a high per
+cent of soluble nitrogenous products. They have an excellent texture,
+generally solid and firm, free from all tendency to openness; and,
+moreover, their flavor is clean and entirely devoid of the sharp,
+undesirable tang that so frequently appears in old cheese. The keeping
+quality of such cheese is much superior to the ordinary product. The
+introduction of this new system of cheese-curing promises much from a
+practical point of view, and undoubtedly a more complete study of the
+subject from a scientific point of view will aid materially in
+unraveling some of the problems as to flavor production.
+
+~Theories of cheese curing.~ Within the last few years considerable study
+has been given the subject of cheese curing or ripening, in order to
+explain how this physical and chemical transformation is brought about.
+
+Much of the misconception that has arisen relative to the cause of
+cheese ripening comes from a confusion of terms. In the ordinary use of
+the word, ripening or curing of cheese is intended to signify the sum
+total of all the changes that result in converting the green product as
+it comes from the press into the edible substance that is known as cured
+cheese. As previously shown, the most marked chemical transformation
+that occurs is that which has to do with the peptonization or breaking
+down of the casein. It is true that under ordinary conditions this
+decomposition process is also accompanied with the formation of certain
+flavor-producing substances, more or less aromatic in character; but it
+by no means follows that these two processes are necessarily due to the
+same cause. The majority of investigators have failed to consider these
+two questions of casein decomposition and flavor as independent, or at
+least as not necessarily related. They are undoubtedly closely bound
+together, but it will be shown later that the problems are quite
+different and possibly susceptible of more thorough understanding when
+considered separately.
+
+In the earlier theories of cheese ripening it was thought to be purely a
+chemical change, but, with the growth of bacteriological science,
+evidence was forthcoming that seemed to indicate that the activity of
+organisms entered into the problem. Schaffer[195] showed that if milk
+was boiled and made into cheese, the casein failed to break down.
+Adametz[196] added to green cheese various disinfectants, as creolin and
+thymol, and found that this practically stopped the curing process. From
+these experiments he drew the conclusion that bacteria must be the cause
+of the change, because these organisms were killed; but when it is
+considered that such treatment would also destroy the activity of enzyms
+as well as vital ferments, it is evident that these experiments were
+quite indecisive.
+
+A determination of the nature of the by-products found in maturing
+cheese indicates that the general character of the ripening change is a
+peptonization or digestion of the casein.
+
+Until recently the most widely accepted views relating to the cause of
+this change have been those which ascribed the transformation to the
+activity of micro-organisms, although concerning the nature of these
+organisms there has been no unanimity of opinion. The overwhelming
+development of bacteria in all cheeses naturally gave support to this
+view; and such experiments as detailed above strengthened the idea that
+the casein transformation could not occur where these ferment organisms
+were destroyed.
+
+The very nature of the changes produced in the casein signified that to
+take part in this process any organism must possess the property of
+dissolving the proteid molecule, casein, and forming therefrom
+by-products that are most generally found in other digestive or
+peptonizing changes of this class.
+
+~Digestive bacterial theory.~ The first theory propounded was that of
+Duclaux,[197] who in 1887 advanced the idea that this change was due to
+that type of bacteria which is able to liquefy gelatin, peptonize milk,
+and cause a hydrolytic change in proteids. To this widely-spread group
+that he found in cheese, he gave the generic name _Tyrothrix_ (cheese
+hairs). According to him, these organisms do not function directly as
+ripening agents, but they secrete an enzym or unorganized ferment to
+which he applies the name _casease_. This ferment acts upon the casein
+of milk, converting it into a soluble product known as _caseone_. These
+organisms are found in normal milk, and if they function as casein
+transformers, one would naturally expect them to be present, at least
+frequently, if not predominating in the ripening cheese; but such is not
+the case. In typical cheddar or Swiss cheese, they rapidly disappear (p.
+168), although in the moister, softer varieties, they persist for
+considerable periods of time. According to Freudenreich, even where
+these organisms are added in large numbers to the curd, they soon
+perish, an observation that is not regarded as correct by the later
+adherents to the digestive bacterial theory, as Adametz and Winkler.
+
+Duclaux's experiments were made with liquid media for isolation
+purposes, and his work, therefore, cannot be regarded as satisfactory as
+that carried out with more modern technical methods. Recently this
+theory has been revived by Adametz,[198] who claims to have found in
+Emmenthaler cheese a digesting species, one of the Tyrothrix type, which
+is capable of peptonizing the casein and at the same time producing the
+characteristic flavor of this class of cheese. This organism, called by
+him _Bacillus nobilis_, the Edelpilz of Emmenthaler cheese, has been
+subjected to comparative experiments, and in the cheese made with pure
+cultures of this germ better results are claimed to have been secured.
+Sufficient experiments have not as yet been reported by other
+investigators to warrant the acceptance of the claims made relative to
+the effect of this organism.
+
+~Lactic-acid bacterial theory.~ It has already been shown that the
+lactic-acid bacteria seems to find in the green cheese the optimum
+conditions of development; that they increase enormously in numbers for
+a short period, and then finally decline. This marked development,
+coincident with the breaking down of the casein, has led to the view
+which has been so ably expounded by Freudenreich[199] that this type of
+bacterial action is concerned in the ripening of cheese. This group of
+bacteria is, under ordinary conditions, unable to liquefy gelatin, or
+digest milk, or, in fact, to exert, under ordinary conditions, any
+proteolytic or peptonizing properties. This has been the stumbling-block
+to the acceptance of this hypothesis, as an explanation of the breaking
+down of the casein. Freudenreich has recently carried on experiments
+which he believes solve the problem. By growing cultures of these
+organisms in milk, to which sterile, freshly precipitated chalk had been
+added, he was able to prolong the development of bacteria for a
+considerable period of time, and as a result finds that an appreciable
+part of the casein is digested; but this action is so slow compared with
+what normally occurs in a cheese, that exception may well be taken to
+this type of experiment alone. Weigmann[200] inclines to the view that
+the lactic-acid bacteria are not the true cause of the peptonizing
+process, but that their development prepares the soil, as it were, for
+those forms that are more directly concerned in the peptonizing process.
+This they do by developing an acid substratum that renders possible the
+more luxuriant growth of the aroma-producing species. According to
+Gorini,[201] certain of the Tyrothrix forms function at high
+temperatures as lactic acid producing bacteria, while at lower
+temperatures they act as peptonizers. On this basis he seeks to
+reconcile the discrepancies that appear in the experiments of other
+investigators.
+
+~Digestive milk enzym theory.~ In 1897 Babcock and the writer[202] showed
+that milk underwent digestive changes spontaneously when bacterial
+activity was suspended by the addition of such anaesthetics as ether,
+chloroform and benzol. The chemical nature of the by-products produced
+by this auto-digestion of milk resembles quite closely those found in
+ripened cheese, except that ammonia is not produced as is the case in
+old cheese. The cause of the decomposition of the casein, they found to
+be due to the action of a milk enzym which is inherent to the milk
+itself. This digestive ferment may be separated from fresh milk by
+concentrating centrifuge slime extracts by the usual physiological
+reagents. This ferment, called by them _galactase_, on account of its
+origin in milk, is a proteolytic enzym of the tryptic type. Its activity
+is destroyed by strong chemicals such as formaldehyde, corrosive
+sublimate, also when heated to 175 deg. F. or above. When such extracts
+are added to boiled milk, the digestive process is started anew, and the
+by-products produced are very similar to those noted in a normal cheese.
+
+Jensen[203] has also shown that the addition of pancreatic extracts to
+cheese accelerated the formation of soluble nitrogenous products.
+
+The action of galactase in milk and cheese has been confirmed by
+Freudenreich[204] and Jensen,[205] as well as by American investigators,
+and this enzym is now generally accepted as one of the factors concerned
+in the decomposition of the casein. Freudenreich believes it is able to
+change casein into albumose and peptones, but that the lactic-acid
+bacteria are chiefly responsible for the further decomposition of the
+nitrogen to amid form.
+
+Failure before to recognize the presence of galactase in milk is
+attributable to the fact that all attempts to secure sterile milk had
+been made by heating the same, in which case galactase was necessarily
+destroyed. A brief exposure at 176 deg. F. is sufficient to destroy its
+activity, and even an exposure at lower temperatures weakens its action
+considerably, especially if the reaction of the medium is acid. This
+undoubtedly explains the contradictory results obtained in the ripening
+of cheese from pasteurized milk, such cheese occasionally breaking down
+in an abnormal manner.
+
+The results mentioned on page 172, in which cheese failed to ripen when
+treated with disinfectants,--experiments which were supposed at that
+time to be the foundation of the bacterial theory of casein
+digestion--are now explicable on an entirely different basis. In these
+cases the casein was not peptonized, because these strong disinfectants
+destroyed the activity of the enzyms as well as the bacteria.
+
+Another important factor in the breaking down of the casein is the
+_pepsin_ in the rennet extract. The digestive influence of this agent
+was first demonstrated for cheddar cheese by Babcock, Russell and
+Vivian,[206] and simultaneously, although independently, by Jensen[207]
+in Emmenthaler cheese. In this digestive action, only albumoses and
+higher peptones are produced. The activity of pepsin does not become
+manifest until there is about 0.3 per cent. acid which is approximately
+the amount developed in the cheddar process. These two factors
+undoubtedly account for by far the larger proportion of the changes in
+the casein; and yet, the formation of ammonia in well ripened cheese is
+not accounted for by these factors. This by-product is the main end
+product of proteid digestion by the liquefying bacteria but their
+apparent infrequency in cheese makes it difficult to understand how they
+can function prominently in the change, unless the small quantity of
+digestive enzyms excreted by them in their growth in milk is capable of
+continuing its action until a cumulative effect is obtained. Although
+much light has been thrown on this question by the researches of the
+last few years, the matter is far from being satisfactorily settled at
+the present time and the subject needs much more critical work. If
+liquefying bacteria abound in the milk, doubtless they exert some
+action, but the role of bacteria is doubtless much greater in the
+production of flavor than in the decomposition of the curd.
+
+~Conditions determining quality.~ In determining the quality of cheese,
+several factors are to be taken into consideration. First and foremost
+is the flavor, which determines more than anything else the value of the
+product. This should be mild and pleasant, although with age the
+intensity of the same generally increases but at no time should it have
+any bitter, sour, or otherwise undesirable taste or aroma. Texture
+registers more accurately the physical nature of the ripening. The
+cheese should not be curdy and harsh, but should yield quite readily to
+pressure under the thumb, becoming on manipulation waxy and plastic
+instead of crumbly or mealy. Body refers to the openness or closeness of
+the curd particles, a close, compact mass being most desirable. The
+color of cheese should be even, not wavy, streaked or bleached.
+
+For a cheese to possess all of these characteristics in an optimum
+degree is to be perfect in every respect--a condition that is rarely
+reached.
+
+So many factors influence this condition that the problem of making a
+perfect cheese becomes exceedingly difficult. Not only must the quality
+of the milk--the raw material to be used in the manufacture--be
+perfectly satisfactory, but the factory management while the curds are
+in the vat demands great skill and careful attention; and finally, the
+long period of curing in which variation in temperature or moisture
+conditions may seriously affect the quality,--all of these stages, more
+or less critical, must be successfully gone through, before the product
+reaches its highest state of development.
+
+It is of course true that many phases of this complex series of
+processes have no direct relation to bacteria, yet it frequently happens
+that the result attained is influenced at some preceding stage by the
+action of bacteria in one way or another. Thus the influence of the
+acidity developed in the curds is felt throughout the whole life of the
+cheese, an over-development of lactic-acid bacteria producing a sour
+condition that leaves its impress not only on flavor but texture. An
+insufficient development of acid fails to soften the curd-particles so
+as to permit of close matting, the consequence being that the body of
+the cheese remains loose and open, a condition favorable to the
+development of gas-generating organisms.
+
+~Production of flavor.~ The importance of flavor as determining the
+quality of cheese makes it imperative that the nature of the substances
+that confer on cheese its peculiar aromatic qualities and taste be
+thoroughly understood. It is to be regretted that the results obtained
+so far are not more satisfactory, for improvement in technique is hardly
+to be expected until the reason for the process is thoroughly
+understood.
+
+The view that is most generally accepted is that this most important
+phase of cheese curing is dependent upon bacterial activity, but the
+organisms that are concerned in this process have not as yet been
+satisfactorily determined. In a number of cases, different species of
+bacteria have been separated from milk and cheese that have the power of
+producing aromatic compounds that resemble, in some cases, the peculiar
+flavors and odors that characterize some of the foreign kinds of cheese;
+but an introduction of these into curd has not resulted in the
+production of the peculiar variety, even though the methods of
+manufacture and curing were closely followed. The similarity in germ
+content in different varieties of cheese made in the same locality has
+perhaps a bearing on this question of flavor as related to bacteria. Of
+the nine different species of bacteria found in Emmenthaler cheese by
+Adametz, eight of them were also present in ripened Hauskaese. If
+specific flavors are solely the result of specific bacterial action, it
+might naturally be expected that the character of the flora would
+differ.
+
+Some suggestive experiments were made by Babcock and Russell on the
+question of flavor as related to bacterial growth, by changing the
+nature of the environment in cheese by washing the curds on the racks
+with warm water. In this way the sugar and most of the ash were removed.
+Under such conditions the character of the bacterial flora was
+materially modified. While the liquefying type of bacteria was very
+sparse in normal cheddar, they developed luxuriantly in the washed
+cheese. The flavor at the same time was markedly affected. The control
+cheddar was of good quality, while that made from the washed curds was
+decidedly off, and in the course of ripening became vile. It may be
+these two results are simply coincidences, but other data[208] bear out
+the view that the flavor was to some extent related to the nature of the
+bacteria developing in the cheese. This was strengthened materially by
+adding different sugars to washed curds, in which case it was found that
+the flavor was much improved, while the more normal lactic-acid type of
+bacteria again became predominant.
+
+~Ripening of moldy cheese.~ In a number of foreign cheeses, the peculiar
+flavor obtained is in part due to the action of various fungi which grow
+in the cheese, and there produce certain by-products that flavor the
+cheese. Among the most important of these are the Roquefort cheese of
+France, Stilton of England, and Gorgonzola of Italy.
+
+Roquefort cheese is made from goat's or cow's milk, and in order to
+introduce the desired mold, which is the ordinary bread-mold,
+_Penicillium glaucum_, carefully-prepared moldy bread-crumbs are added
+to the curd.
+
+At ordinary temperatures this organism develops too rapidly, so that the
+cheese to ripen properly must be kept at a low temperature. The town of
+Roquefort is situated in a limestone country, in a region full of
+caves, and it is in these natural caves that most of the ripening is
+done. These caverns are always very moist and have a temperature ranging
+from 35 deg. to 44 deg. F., so that the growth of the fungus is retarded
+considerably. The spread of the mold throughout the ripening mass is
+also assisted in a mechanical way. The partially-matured cheese are run
+through a machine that pricks them full of small holes. These slender
+canals allow the mold organism to penetrate the whole mass more
+thoroughly, the moldy straw matting upon which the ripening cheese are
+placed helping to furnish an abundant seeding of the desired germ.
+
+When new factories are constructed it is of advantage to introduce this
+necessary germ in quantities, and the practice is sometimes followed of
+rubbing the walls and cellars of the new location with material taken
+from the old established factory. In this custom, developed in purely an
+empirical manner, is to be seen a striking illustration of a
+bacteriological process crudely carried out.
+
+In the Stilton cheese, one of the highly prized moldy cheeses of
+England, the desired mold fungus is introduced into the green cheese by
+exchanging plugs taken with a cheese trier from a ripe Stilton.
+
+~Ripening of soft cheese.~ The type of ripening which takes place in the
+soft cheeses is materially different from that which occurs in the hard
+type. The peptonizing action does not go on uniformly throughout the
+cheese, but is hastened by the development of molds and bacteria on the
+outside that exert a solvent action on the casein. For this reason, soft
+cheeses are usually made up in small sizes, so that this action may be
+hastened. The organisms that take part in this process are those that
+are able to form enzyms (similar in their action to trypsin, galactase,
+etc.), and these soluble ferments gradually diffuse from the outside
+through the cheese.
+
+Most of these peptonizing bacteria are hindered in their growth by the
+presence of lactic acid, so that in many cases the appearance of the
+digesting organisms on the surface is delayed until the acidity of the
+mass is reduced to the proper point by the development of other
+organisms, principally molds, which prefer an acid substratum for their
+growth.
+
+In Brie cheese a blue coating of mold develops on the surface. In the
+course of a few weeks, a white felting appears which later changes to
+red. This slimy coat below the mold layer is made up of diverse species
+of bacteria and fungi that are able to grow after the acid is reduced by
+the blue mold. The organisms in the red slimy coat act upon the casein,
+producing an alkaline reaction that is unfavorable to the growth of the
+blue mold. Two sets of organisms are, therefore essential in the
+ripening process, one preparing the soil for the ferment that later
+produces the requisite ripening changes. As ordinarily carried on, the
+process is an empirical one, and if the red coat does not develop as
+expected, the maker resorts to all kinds of devices to bring out the
+desired ferment. The appearance of the right form is dependent, however,
+upon the proper reaction of the cheese, and if this is not suitable, the
+wished-for growth will not appear.
+
+
+INFLUENCE OF BACTERIA IN ABNORMAL CHEESE PROCESSES.
+
+The reason why cheese is more subject to abnormal fermentation than
+butter is because its high nitrogen content favors the continued
+development of bacteria for some time after it is made. It must be
+borne in mind, in considering the more important of these changes, that
+not all defective conditions in cheese are attributable to the influence
+of living organisms. Troubles frequently arise from errors in
+manufacturing details, as too prolonged cooking of curds, too high
+heating, or the development of insufficient or too much acid. Then
+again, the production of undesirable flavors or impairment in texture
+may arise from imperfect curing conditions.
+
+Our knowledge regarding the exact nature of these indefinite faults is
+as yet too inadequate to enable many of these undesirable conditions to
+be traced to their proper source; but in many cases the taints observed
+in a factory are due to the abnormal development of certain bacteria,
+capable of evolving unpleasant or even putrid odors. Most of them are
+seeded in the milk before it comes to the factory and are due to
+careless manipulation of the milk while it is still on the farm. Others
+gain access to the milk in the factory, owing to unclean conditions of
+one sort or another. Sometimes the cheese-maker is able to overcome
+these taints by vigorous treatment, but often they pass on into the
+cheese, only to detract from the market value of the product. Most
+frequently these "off" flavors appear in cheese that are cured at too
+high temperatures, say above 65 deg. F.
+
+~"Gassy" fermentations in cheese.~ One of the worst and at the same time
+most common troubles in cheese-making is where the cheese undergoes a
+fermentation marked by the evolution of gas. The presence of gas is
+recognized by the appearance either of spherical or lens-shaped holes of
+various sizes in the green cheese; often they appear in the curd before
+it is put to press. Usually in this condition the curds look as if they
+had been punctured with a pin, and are known as "pin holey" curds. Where
+the gas holes are larger, they are known as "Swiss holes" from their
+resemblance to the normal holes in the Swiss product. If the development
+of gas is abundant, these holes are restricted in size. Often the
+formation of gas may be so intense as to cause the curds to float on the
+surface of the whey before they are removed. Such curds are known as
+"floaters" or "bloaters."
+
+If "gassy" curds are put to press, the abnormal fermentation may
+continue. The further production of gas causes the green cheese to
+"huff" or swell, until it may be considerably distorted as in Fig. 33.
+In such cases the texture of the cheese is greatly injured, and the
+flavor is generally impaired.
+
+[Illustration: FIG. 33. Cheese made from gassy milk.]
+
+Such abnormal changes may occur at any season of the year, but the
+trouble is most common in summer, especially in the latter part.
+
+This defect is less likely to occur in cheese that is well cheddared
+than in sweet curd cheese. When acidity is produced, these gassy
+fermentations are checked, and in good cheddar the body is so close and
+firm as not readily to permit of gaseous changes.
+
+In Swiss cheese, which is essentially a sweet curd cheese, these
+fermentations are very troublesome. Where large holes are formed in
+abundance (blaehen), the trouble reaches its maximum. If the gas holes
+are very numerous and therefore small it is called a "nissler."
+Sometimes the normal "eyes" are even wanting when it is said to be
+"blind" or a "glaesler."
+
+[Illustration: FIG. 34. Block Swiss cheese showing "gassy"
+fermentation.]
+
+One method of procedure which is likely to cause trouble in Swiss
+factories is often produced by the use of sour, fermented whey in which
+to soak the natural rennets. Freudenreich and Steinegger[209] have shown
+that a much more uniform quality of cheese can be made with rennet
+extract if it is prepared with a starter made from a pure lactic
+ferment.
+
+The cause of the difficulty has long been charged to various sources,
+such as a lack of aeration, improper feeding, retention of animal gases,
+etc., but in all these cases it was nothing more than a surmise. Very
+often the milk does not betray any visible symptom of fermentation when
+received, and the trouble is not to be recognized until the process of
+cheese-making is well advanced.
+
+Studies from a biological standpoint have, however, thrown much light on
+this troublesome problem; and it is now known that the formation of gas,
+either in the curd or after it has been put to press, is due entirely to
+the breaking down of certain elements, such as the sugar of milk, due to
+the influence of various living germs. This trouble is, then, a type
+fermentation, and is, therefore, much more widely distributed than it
+would be if it was caused by a single specific organism. These
+gas-producing organisms are to be found, sparingly at least, in almost
+all milks, but are normally held in check by the ordinary lactic
+species. Among them are a large number of the bacteria, although yeasts
+and allied germs are often present and are likewise able to set up
+fermentative changes of this sort. In these cases the milk-sugar is
+decomposed in such a way as to give off CO_{2} and H, and in some cases,
+alcohol. Russell and Hastings[210] found a lactose-splitting yeast in a
+severe outbreak of gassy cheese in a Swiss factory. In this case the gas
+did not develop until the cheese were a few weeks old. In severe cases
+the cheese actually cracked to pieces.
+
+According to Guillebeau, a close relation exists between those germs
+that are able to produce an infectious inflammation (mastitis) in the
+udder of the cow and some forms capable of gas evolution.
+
+If pure cultures of these gas-producing bacteria are added to perfectly
+sweet milk, it is possible to artificially produce the conditions in
+cheese that so frequently appear in practice.
+
+~Treatment of "pin-holey" curds.~ When this type of fermentation appears
+during the manufacture of the cheese, the maker can control it in part
+within certain limits. These methods of treatment are, as a rule, purely
+mechanical, as when the curds are piled and turned, and subsequently
+ground in a curd mill. After the gas has been forced out, the curds are
+then put to press and the whole mats into a compact mass.
+
+Another method of treatment based upon bacteriological principles is the
+addition of a starter to induce the formation of acid. Where acid is
+developed as a result of the growth of the lactic-acid bacteria, the
+gas-producing species do not readily thrive. Another reason why acid
+aids in repressing the development of gas is that the curd particles are
+partially softened or digested by the action of the acid. This causes
+them to mat together more closely, and there is not left in the cheese
+the irregular mechanical openings in which the developing gas may find
+lodgment.
+
+Another method that is also useful with these curds is to employ salt.
+This represses gaseous fermentations, and the use of more salt than
+usual in making the cheese will very often restrain the production of
+gas. Tendency to form gas in Edam cheese is controlled by the addition
+of a starter prepared from slimy whey (lange wei) which is caused by the
+development of an acid-forming organism.
+
+Some have recommended the custom of washing the curds to remove the whey
+and the gas-producing bacteria contained therein. Care must be taken not
+to carry this too far, for the removal of the sugar permits
+taint-producing organisms to thrive.[211]
+
+The temperature at which the cheese is cured also materially affects the
+development of gas. At high curing temperatures, gas-producing organisms
+develop rapidly; therefore more trouble is experienced in summer than at
+other seasons.
+
+If milks which are prone to undergo "gassy" development are excluded
+from the general supply, it would be possible to eliminate the source of
+the entire trouble. To aid in the early recognition of such milks that
+are not apparently affected when brought to the factory, fermentation or
+curd tests (p. 76) are of great value. The use of this test in the hands
+of the factory operator often enables him to detect the exact source of
+the trouble, which may frequently be confined to the milk delivered by a
+single patron.
+
+~"Fruity" or "sweet" flavor.~ Not infrequently the product of a factory
+may acquire during the process of ripening what is known as a "sweet" or
+"fruity" flavor. This flavor resembles the odor of fermented fruit or
+the bouquet of certain kinds of wine. It has been noted in widely
+different sections of the country and its presence bears no relation to
+the other qualities of the cheese. The cause of this trouble has
+recently been traced[212] to the presence of various kinds of yeasts.
+Ordinarily yeasts are rarely present in good cheese, but in cheese
+affected with this trouble they abound. The addition of starters made
+from yeast cultures resulted in the production of the undesirable
+condition.
+
+~Mottled cheese.~ The color of cheese is sometimes cut to that extent that
+the cheese presents a wavy or mottled appearance. This condition is apt
+to appear if the ripening temperature is somewhat high, or larger
+quantities of rennet used than usual. The cause of the defect is
+obscure, but it has been demonstrated that the same is communicable if a
+starter is made by grating some of this mottled cheese into milk. The
+bacteriology of the trouble has not yet been worked out, but the defect
+is undoubtedly due to an organism that is able to grow in the ripening
+cheese. It has been claimed that the use of a pure lactic ferment as a
+starter enables one to overcome this defect.
+
+~Bitter cheese.~ Bitter flavors are sometimes developed in cheese
+especially where the ripening process is carried on at a low temperature
+in the presence of an excess of moisture for a considerable length of
+time.
+
+Guillebeau[213] isolated several forms from Emmenthaler cheese which he
+connected with udder inflammation that were able to produce a bitter
+substance in cheese.
+
+Von Freudenreich[214] has described a new form _Micrococcus casei amari_
+(micrococcus of bitter cheese) that was found in a sample of bitter
+cheese. This germ is closely related to Conn's micrococcus of bitter
+milk. It develops lactic acid rapidly, coagulating the milk and
+producing an intensely bitter taste in the course of one to three days.
+When milk infected with this organism is made into cheese, there is
+formed in a few days a decomposition product that imparts a marked
+bitter flavor to the cheese.
+
+Harrison[215] has recently found a yeast that grows in the milk and also
+in the cheese which produces an undesirable bitter change.
+
+It is peculiar that some of the organisms that are able to produce
+bitter products in milk do not retain this property when the milk is
+worked up into cheese.
+
+~Putrid or rotten cheese.~ Sometimes cheese undergoes a putrefactive
+decomposition in which the texture is profoundly modified and various
+foul smelling gases are evolved. These often begin on the exterior as
+small circumscribed spots that slowly extend into the cheese, changing
+the casein into a soft slimy mass. Then, again, the interior of the
+cheese undergoes this slimy decomposition. The soft varieties are more
+prone toward this fermentation than the hard, although the firm cheeses
+are by no means exempt from the trouble. The "Verlaufen" or "running" of
+limburger cheese is a fermentation allied to this. It is where the
+inside of the cheese breaks down into a soft semi-fluid mass. In severe
+cases, the rind may even be ruptured, in which case the whole interior
+of the cheese flows out as a thick slimy mass, having sometimes a putrid
+odor. The conditions favoring this putrid decomposition are usually
+associated with an excess of moisture, and an abnormally low ripening
+temperature.
+
+~Rusty spot.~ This name is applied to the development of small
+yellowish-red or orange spots that are formed sometimes throughout the
+whole mass of cheddar cheese. A close inspection shows the colored
+points to be located along the edges of the curd particles. According to
+Harding,[216] this trouble is most common in spring and fall. The cause
+of the difficulty has been traced by Connell[217] to the development of
+a chromogenic bacterium, _Bacillus rudensis_. The organism can be most
+readily isolated on a potato surface rather than with the usual
+isolating media, agar or gelatin.
+
+~Other pigment changes.~ Occasionally, with the hard type of cheese, but
+more frequently with the softer foreign varieties, various abnormal
+conditions arise that are marked by the production of different pigments
+in or on the cheese. More frequently these are merely superficial and
+affect only the outer layers of the cheese. Generally they are
+attributable to the development of certain chromogenic organisms
+(bacteria, molds and yeasts), although occasionally due to other causes,
+as in the case of a blue discoloration sometimes noted in foreign cheese
+made in copper kettles.[218]
+
+De Vries[219] has described a blue condition that is found in Edam
+cheese. It appears first as a small blue spot on the inside, increasing
+rapidly in size until the whole mass is affected. This defect he was
+able to show was produced by a pigment-forming organism, _B.
+cyaneo-fuscus_. By the use of slimy whey (lange wei) this abnormal
+change was controlled.
+
+~Moldy cheese.~ With many varieties of cheese, especially some of the
+foreign types, the presence of mold on the exterior is not regarded as
+detrimental; in fact a limited development is much desired. In hard
+rennet cheese as cheddar or Swiss, the market demands a product free
+from mold, although it should be said that this condition is imposed by
+the desire to secure a good-looking cheese rather than any injury in
+flavor that the mold causes. Mold spores are so widely distributed that,
+if proper temperature and moisture conditions prevail, these spores will
+always develop. At temperatures in the neighborhood of 40 deg. F. and
+below, mold growth is exceedingly slow, and often fructification does
+not occur, the only evidence of the mold being the white, felt-like
+covering that is made up of the vegetating filaments. The use of
+paraffin has been suggested as a means of overcoming this growth, the
+cheese being dipped at an early stage into melted paraffin. Recent
+experiments have shown that "off" flavors sometimes develop where cheese
+are paraffined directly from the press. If paraffin is too hard, it has
+a tendency to crack and separate from the rind, thus allowing molds to
+develop beneath the paraffin coat, where the conditions are ideal as to
+moisture, for evaporation is excluded and the air consequently
+saturated. The use of formalin (2% solution) has been suggested as a
+wash for the outside of the cheese. This substance or sulfur is also
+applied in a gaseous form. Double bandaging is also resorted to as a
+means of making the cheese more presentable through the removal of the
+outer bandage.
+
+The nature of these molds has not been thoroughly studied as yet. The
+ordinary blue-green bread mold, _Penicillium glaucum_, is most
+frequently found, but there are numerous other forms that appear,
+especially at low temperatures.
+
+~Poisonous cheese.~ Cases of acute poisoning arising from the ingestion of
+cheese are reported from time to time. Vaughan has succeeded in showing
+that this condition is due to the formation of a highly poisonous
+alkaloid which he has isolated, and which he calls _tyrotoxicon_.[220]
+This poisonous ptomaine has also been demonstrated in milk and other
+milk products, and is undoubtedly due to the development of various
+putrefactive bacteria that find their way into the milk. It seems quite
+probable that the development of these toxic organisms can also go on
+in the cheese after it is taken from the press.
+
+~Prevention or cheese defects.~ The defective conditions previously
+referred to can rarely be overcome in cheese so as to improve the
+affected product, for they only become manifest in most cases during the
+later stages of the curing process. The only remedy against future loss
+is to recognize the conditions that are apt to prevail during the
+occurrence of an outbreak and see that the cheese are handled in such a
+way as to prevent a recurrence of the difficulty.
+
+Many abnormal and undesirable results are incident to the manufacture of
+the product, such as "sour" or "mealy" cheese, conditions due to the
+development of too much acid in the milk or too high a "cook." These are
+under the direct control of the maker and for them he alone is
+responsible. The development of taints due to the growth of unwelcome
+bacteria that have gained access to the milk while it is yet on the farm
+are generally beyond the control of the cheese maker, unless they are so
+pronounced as to appear during the handling of the curds. If this does
+occur he is sometimes able, through the intervention of a starter or by
+varying some detail in making, to handle the milk in such a way as to
+minimize the trouble, but rarely is he able to eliminate it entirely.
+
+One of the most strenuous duties which the maker must perform at all
+times is to point out to his patrons the absolute necessity of their
+handling the milk in such a way as to prevent the introduction of
+organisms of a baleful type.
+
+FOOTNOTES:
+
+[178] Russell, 13 Rept. Wis. Expt. Stat., 1896, p. 112; Campbell, Trans.
+High. & Agr. Soc. Scotland, 5 ser., 1898, 10:181.
+
+[179] Winkler, Milch Zeit. (Hildesheim), Nov. 24, 1900.
+
+[180] Campbell, No. Brit., Agric., May 12, 1897.
+
+[181] Weigmann, Milch Zeit., No. 50, 1889.
+
+[182] Klein, Milch Zeit. (Hildesheim), No. 17, 1900.
+
+[183] Adametz, Landw. Jahr., 18:256.
+
+[184] Van Slyke and Hart, Bull. 214, N. Y. Expt. Stat., July 1902.
+
+[185] Milch Zeit., 1898, No. 49.
+
+[186] Lafar, Technical Mycology, p. 216.
+
+[187] Adametz, Landw. Jahr., 18:228.
+
+[188] Freudenreich, Landw. Jahr. d. Schweiz, 4:17; 5:16.
+
+[189] Russell, 13 Rept. Wis. Expt. Stat., 1896, p. 95.
+
+[190] Harrison and Connell, Rev. gen. du Lait, Nos. 4, 5, 6, 7 and 8,
+1903-04.
+
+[191] Lloyd, Bath and West of Eng. Soc. Rept., 1892, 2:180.
+
+[192] Freudenreich, Landw. Jahr. d. Schweiz, 1900; Adametz, Oest. Molk.
+Zeit., 1899, No. 7.
+
+[193] Russell, 14 Wis. Expt. Stat., 1897, p. 203. Harrison and Connell,
+Rev. gen. du Lait Nos. 4, etc., 1903-04.
+
+[194] Babcock and Russell, 18 Rept. Wis. Expt. Stat., 1901. Dean,
+Harrison and Harcourt, Bull. 121, Ont. Agr'l. Coll., June 1902.
+
+[195] Schaffer, Milch Zeit., 1889, p. 146.
+
+[196] Adametz, Landw. Jahr., 18:261.
+
+[197] Duclaux, Le Lait, p. 213.
+
+[198] Adametz, Oest. Molk. Zeit., 1900, Nos. 16-18.
+
+[199] Freudenreich, Landw. Jahr. d. Schweiz, 1897, p. 85.
+
+[200] Weigmann, Cent. f. Bakt., II Abt., 1898, 4:593; also 1899, 5:630.
+
+[201] Gorini, Abs. in Expt. Stat. Rec., 11:388.
+
+[202] Babcock and Russell, 14 Rept. Wis. Expt. Stat., 1897, p. 161.
+
+[203] Jensen, Cent. f. Bakt., II Abt., 3:752.
+
+[204] Freudenreich, Cent. f. Bakt., II Abt., 1900, 6:332.
+
+[205] Jensen, Ibid., 1900, 6:734.
+
+[206] 17 Rept. Wis. Expt. Stat., 1900, p. 102.
+
+[207] Jensen, Landw. Jahr. d. Schweiz, 1900.
+
+[208] Babcock and Russell, 18 Rept. Wis. Expt. Stat., 1901.
+
+[209] Cent. f. Bakt. 1899, p. 14.
+
+[210] Bull. 128, Wis. Expt. Stat., Sept. 1905.
+
+[211] Babcock and Russell, 18 Rept. Wis. Expt. Stat., 1901.
+
+[212] Harding, Rogers and Smith, Bull. 183, N. Y. (Geneva) Expt. Stat.,
+Dec., 1900.
+
+[213] Guillebeau, Landw. Jahr., 1890, p. 27.
+
+[214] Freudenreich, Fueehl. Landw. Ztg., 43:361.
+
+[215] Harrison, Bull. 123 Ont. Agr'l. Coll., May, 1902.
+
+[216] Bull. 183, N. Y. (Geneva) Expt. Stat., Dec. 1900.
+
+[217] Connell, Bull. Canadian Dept. of Agr., 1897.
+
+[218] Schmoeger, Milch Zeit., 1883, p. 483.
+
+[219] De Vries, Milch Zeit., 1888, pp. 861, 885.
+
+[220] Zeit. f. physiol. Chemie, 10:146.
+
+
+
+
+INDEX.
+
+
+Acid, effect of, on churning, 137;
+  in butter-making, 138.
+
+Acid test, 52.
+
+Aeration of milk, 59.
+
+Aerobic bacteria, 7.
+
+Alcoholic fermentation in milk, 72.
+
+Anaerobic bacteria, 7.
+
+Animal, influence of, on milk infection, 34.
+
+Animal odor, 56.
+
+Anthrax, 94.
+
+Antiseptics, 9, 88.
+
+Aroma, of butter, 140.
+
+
+Bacillus: definition of, 2.
+  _acidi lactici_, 64;
+  _cyaneo-fuscus_, 188;
+  _cyanogenus_, 74;
+  _foetidus lactis_, 157;
+  _lactis aerogenes_, 65;
+  _lactis erythrogenes_, 74;
+  _lactis saponacei_, 67;
+  _lactis viscosus_, 71;
+  _nobilis_, 162, 174;
+  _prodigiosus_, 74;
+  _rudensis_, 188;
+  _synxanthus_, 75;
+  _tuberculosis_, 84.
+
+Bacteria:
+  on hairs, 35;
+  kinds in milk, 63;
+  in barn air, 42;
+  in milk pails, 27;
+  in butter, 154;
+  classification of, 4;
+  in cheese, 160;
+  culture of, 17;
+  in cream, 128;
+  discovery of, 1;
+  external conditions affecting, 8;
+  form of, 2;
+  in butter, 142;
+  in butter-making, 127;
+  in centrifuge slime, 39;
+  In fore milk, 28;
+  in rennet, 163;
+  In separator slime, 39;
+  manure, 37;
+  number of, in milk, 50.
+  Distribution of:
+    milk of American cities, 50;
+    European cities, 50;
+    in relation to cheese, 168.
+  Of disease:
+    anthrax, 94;
+    cholera, 98;
+    diphtheria, 99;
+    lockjaw, 94;
+    toxic, 100;
+    tuberculosis, 84;
+    typhoid fever, 98.
+  Methods of study of:
+    culture, 15;
+    culture media, 13;
+    isolation, 14.
+
+Bitter butter, 158;
+  cheese, 189;
+  milk, 72.
+
+Bloody milk, 74.
+
+Blue cheese, 191;
+  milk, 74.
+
+Bovine tuberculosis, 84.
+
+Brie cheese, 182.
+
+Butter:
+  bacteria in, 154;
+  bitter, 158;
+  "cowy," 157;
+  fishy, 159;
+  lardy, 157;
+  moldy, 158;
+  mottled, 156;
+  oily, 158;
+  putrid, 156;
+  rancid, 155;
+  tallowy, 157;
+  turnip flavor in, 157.
+  Making:
+    aroma, 140;
+    flavor in, 140;
+    pure culture, 143;
+    in ripening of cream, 136.
+
+Butyric acid fermentation, 69.
+
+By-products of factory, methods of preserving, 25.
+
+
+Casease, 68.
+
+Caseone, 68.
+
+Centrifugal force, cleaning milk by, 38.
+
+Cheese:
+  bacterial flora of, 168;
+  bitter, 189;
+  blue, 187;
+  Brie, 182;
+  Edam, 72, 162;
+  Emmenthaler, 185;
+  flavor of, 179;
+  gassy fermentations in, 183;
+  Gorgonzola, 180;
+  molds on, 191;
+  mottled, 189;
+  "nissler," 185;
+  poisonous, 192;
+  putrid, 190;
+  ripening of moldy, 180;
+  ripening of soft, 181;
+  Roquefort, 180;
+  rusty spot in, 188;
+  Stilton, 180;
+  Swiss, 185.
+  Making and curing:
+    chemical changes in curing, 166;
+    influence of temperature on curing, 169;
+    influence of rennet, 177;
+    physical changes in curing, 165;
+    prevention of defects, 193;
+    starters in, 161;
+    temperature in relation to bacterial influence, 169.
+  Theories of curing:
+    digestive, 173;
+    galactase, 175, 177;
+    lactic acid, 174.
+
+Chemical changes in cheese-ripening, 166.
+
+Chemical disinfectants in milk:
+  bleaching powder, 81;
+  corrosive sublimate, 81;
+  formalin, 80;
+  sulfur, 80;
+  whitewash, 81;
+  vitriol, 81.
+
+Chemical preservatives, 80.
+
+Children, milk for, 45.
+
+Cholera in milk, 98.
+
+Classification by separator, 38.
+
+Coccus, definition of, 2.
+
+Cold, influence on bacteria, 8, 48.
+
+Contamination of milk through disease germs, 95, 191.
+
+Covered milk pails, 41.
+
+Cream, bacterial changes in, 135;
+  mechanical causes for bacteria in, 135;
+  pasteurized, 113;
+  restoration of consistency of pasteurized, 132.
+  Ripening of, 136;
+    advantage of pure cultures in, 144;
+    by natural starters, 142;
+    characteristics of pure cultures in, 145;
+    objections to pure cultures in, 146;
+    principles of pure cultures in, 143;
+    propagation of pure cultures, 151;
+    purity of commercial starters, 150;
+    home-made starters in, 146.
+
+Creaming methods, 134.
+
+Curd test, 76.
+
+
+Dairy utensils a source of contamination, 21.
+
+Diarrhoeal diseases, 100.
+
+Digesting bacteria, 67.
+
+Digestibility of heated milk, 111.
+
+Diphtheria, 99.
+
+Dirt in milk, 34.
+
+Dirt, exclusion of, 36.
+
+Disease germs in milk, 95;
+  effect of heat on, 91;
+  origin of, 83.
+
+Disinfectants, 9:
+  carbolic acid, 81;
+  chloride of lime, 81;
+  corrosive sublimate, 81;
+  formalin, 80;
+  sulfur, 80;
+  vitriol salts, 81;
+  whitewash, 79.
+
+Disinfectants in milk:
+  alkaline salts, 106;
+  boracic acid, 106;
+  formalin, 106;
+  preservaline, 107;
+  salicylic acid, 106.
+
+Domestic pasteurizing apparatus, 119.
+
+Drugs, taints in milk due to, 56.
+
+Drying, effect of, 8.
+
+
+Edam cheese, 72, 162.
+
+Emmenthaler cheese, 185.
+
+Endospores, 3.
+
+Enzyms, 10.
+
+
+Factory by-products, 22;
+  treatment of, 25.
+
+Farrington alkaline tablet, 52.
+
+Fecal bacteria, effect of, on butter, 35.
+
+Fermentation:
+  In cheese: gassy, 183.
+  In milk:
+    alcoholic, 72;
+    bitter, 72;
+    blue, 74;
+    butyric, 69;
+    digesting, 67;
+    gassy, 66;
+    kephir, 72;
+    koumiss, 72;
+    lactic acid, 63;
+    lange-wei, 72;
+    red, 74;
+    ropy, 69;
+    slimy, 69;
+    soapy, 73;
+    souring, 63;
+    sweet curdling, 67;
+    treatment of, 75.
+  Tests, 76;
+    Gerber's, 76;
+    Walther's, 76;
+    Wisconsin curd, 76.
+
+Filtration of milk, 38.
+
+Fishy butter, 159.
+
+Flavor:
+  of butter, 140;
+  of cheese, 179.
+
+Foot and mouth disease, 93.
+
+Fore milk, 28.
+
+Formaldehyde, 80.
+
+Formalin, 80.
+
+Fruity flavor in cheese, 188.
+
+
+Galactase in cheese, 175.
+
+Gassy fermentations:
+  in cheese, 183;
+  in milk, 67;
+  in Swiss cheese, 167.
+
+Glaesler, 185.
+
+Gorgonzola cheese, 180.
+
+Growth of bacteria, essential conditions for, 4;
+  in milk, 46.
+
+
+Hair, bacteria on, 35.
+
+Heat, influence on bacterial growth, 8.
+
+Heated milk:
+  characteristics of, 109;
+  action toward rennet, 112;
+  body, 110;
+  digestibility, 111;
+  fermentative changes, 111;
+  flavor, 110;
+  hydrogen peroxid test in, 23;
+  Storch's test, 23.
+
+Hygienic milk, bacteria in, 45.
+
+
+Infection of milk:
+  animal, 34;
+  dairy utensils, 21;
+  fore milk, 28;
+  milker, 36.
+
+Isolation of bacteria, methods of, 14.
+
+
+Kephir, 72.
+
+Koumiss, 72.
+
+
+Lactic acid:
+  fermentation in milk, 63;
+  theory in cheese-curing, 174.
+
+Lange-wei, 72.
+
+Lardy butter, 157.
+
+Light, action on bacteria, 9.
+
+
+Manure, bacteria in, 33.
+
+Methods:
+  of isolation, 14;
+  culture, 15.
+
+_Micrococcus casei amari_, 189.
+
+Microscope, use of, 17.
+
+Milk:
+  a bacterial food medium, 19;
+  bacteria in, 48.
+  Disease organisms in:
+    anthrax, 94;
+    cholera, 98;
+    diphtheria, 99;
+    foot and mouth disease, 93;
+    poisonous, 101;
+    ptomaines, 101;
+    scarlet fever, 99;
+    tuberculosis, 84;
+    typhoid fever, 98.
+  Contamination, 20:
+    from air, 42;
+    from animal odors, 55;
+    dirt, 34;
+    distinction between bacterial and non-bacterial, 57;
+    fore milk, 28;
+    infection in factory, 59;
+    milker, 36;
+    relative importance of various kinds, 43;
+    utensils, 21.
+
+Milk fermentations:
+  alcoholic, 72;
+  bitter, 72;
+  bloody, 74;
+  blue, 74;
+  butyric acid, 69;
+  gassy, 66, 167;
+  kephir, 72;
+  koumiss, 72;
+  lactic acid, 63;
+  red, 72;
+  ropy, 69;
+  slimy, 69;
+  soapy, 74;
+  souring, 63;
+  sweet curdling, 67;
+  tests for, 76;
+  treatment of, 75;
+  yellow, 75.
+
+Milk, heated:
+  action towards rennet, 112;
+  digestibility, 111;
+  flavor of, 110;
+  fermentative changes in, 111;
+  hydrogen peroxid test, 110.
+
+Milking machines, influence of, on germ content, 37.
+
+Milk preservation:
+  chemical agents in, 106;
+  condensation, 107;
+  freezing, 108;
+  heat, 108;
+  pasteurization, 113;
+  sterilization, 112.
+
+Milk-sugar as bacterial food, 19.
+
+Mold, in butter, 158;
+  in cheese, 191.
+
+Mottled cheese, 189.
+
+
+"Nissler" cheese, 185.
+
+
+Odors, direct absorption of, in milk, 55.
+
+_Oidium lactis_, 159.
+
+Oily butter, 158.
+
+
+Pasteurization of milk;
+  acid test in, 128;
+  bacteriological study of, 124, 126, 149;
+  for butter, 147;
+  for cheese, 162;
+  for direct use, 113;
+  of skim milk, 25;
+  details of, 128;
+  temperature and time limit in, 118.
+
+Pasteurizing apparatus:
+  continuous flow, 122;
+  coolers, 131;
+  Danish, 123;
+  domestic, 119;
+  Farrington, 122;
+  intermittant flow, 121;
+  Miller, 122;
+  Potts, 121;
+  regenerator, 122;
+  Reid, 126;
+  Russell, 121;
+  testing rate of flow, 124.
+
+_Penicillium glaucum_, 159, 180, 190.
+
+Pepsin, 10.
+
+Physical changes in cheese-ripening, 165.
+
+Poisonous bacteria:
+  in cheese, 192;
+  in milk, 100, 101.
+
+Preservaline, 167.
+
+Preservation of milk:
+  by exclusion, 103;
+  chemical agents, 106;
+  condensing, 107;
+  filtration, 38;
+  freezing, 108;
+  pasteurization, 112;
+  physical agents, 107;
+  sterilization, 112.
+
+Ptomaine poisoning, 101.
+
+Pure cultures, 15.
+
+Pure culture starters:
+  advantages of, 144;
+  characteristics of, 145;
+  home-made cultures compared with, 146;
+  propagation of, 151.
+
+Putrid cheese, 190;
+  butter, 156.
+
+
+Rancidity in butter, 155.
+
+Red milk, 74.
+
+Rennet:
+  action in heated milk, 112;
+  bacteria in, 163;
+  influence of, on cheese-ripening, 177.
+
+Restoration of consistency in pasteurized cream, 132.
+
+Ripening of cheese:
+  moldy cheese, 180;
+  soft cheese, 181.
+  Of cream, 136;
+    artificial starters, 143;
+    natural starters, 142;
+    principles of pure culture starters in, 143.
+
+Ropy milk, 69.
+
+Roquefort cheese, 180.
+
+Rusty spot in cheese, 190.
+
+Rusty cans: effect of, on acidity, 53.
+
+
+Sanitary milk, 45, 104.
+
+Sanitary pails, 41.
+
+Scarlet fever in milk, 99.
+
+Separator slime:
+  bacteria in, 39;
+  tubercle bacillus in, 93.
+
+Scalded layer, resistance of bacteria in, 91.
+
+Skim-milk, a distributor of disease, 96.
+
+Slimy milk, 69.
+
+Soapy milk, 74.
+
+Soft cheese, ripening of, 186.
+
+Sources of contamination in milk:
+  barn air, 42;
+  dairy utensils, 21;
+  dirt from animals, 34;
+  factory cans, 25;
+  fore-milk, 28;
+  milker, 36.
+
+Souring of milk, 63.
+
+Spirillum, definition of, 2.
+
+Spores, 3.
+
+Starters:
+  in cheese-making, 161;
+  in butter-making, 142;
+  propagation of, 151;
+  pure cultures in cream-ripening, 143.
+
+Sterilization of milk, 112.
+
+_Streptococcus Hollandicus_, 72, 162.
+
+Stilton cheese, 181.
+
+Storch's test, 23.
+
+Sulfur as a disinfectant, 81.
+
+Sweet curdling milk, 68.
+
+Sweet flavor in cheese, 188.
+
+Swiss cheese, 177;
+  gassy fermentations in, 24, 185.
+
+
+Taints, absorption of, 55.
+
+Taints, bacterial vs. physical, 58.
+
+Taints in milk, absorption of, 55.
+
+Taints, use of starters in overcoming, 79.
+
+Taints in butter:
+  putrid, 156;
+  rancidity, 155;
+  turnip flavor, 157.
+
+Tallowy butter, 157.
+
+Temperature:
+  effect on bacterial development, 6, 48;
+  effect of low, 108;
+  effect of high, 108;
+  and time limit in milk pasteurization, 113.
+
+Tests for milk:
+  fermentation, 76;
+  Storch's, 23;
+  acid, 52.
+
+Theories in cheese-curing:
+  digestive, 171;
+  galactase, 175, 177;
+  lactic acid, 174.
+
+Trypsin, 10.
+
+Tubercle bacillus:
+  in milk, 88;
+  in separator slime, 93;
+  thermal death limits, 117.
+
+Tuberculin test, 86.
+
+Tuberculosis, bovine, 84.
+
+Turnip flavor in butter, 157.
+
+Typhoid fever, 98.
+
+Tyrogen, 162.
+
+Tyrotoxicon, 101, 190.
+
+
+Udder:
+  artificial introduction of bacteria into, 32;
+  milk germ-free in, 19;
+  infection of, 28;
+  washing, 89;
+  tuberculosis in, 87.
+
+
+Viscogen, 132.
+
+
+Water: as a source of infection, 61.
+
+Whey, pollution of vats, 23;
+  method of preserving, 25;
+  treatment of, in vats, 25.
+
+Whitewash, 81.
+
+Wisconsin curd test, 76.
+
+
+Yeasts:
+  alcoholic ferments in milk, 73;
+  fruity flavor in cheese, 186;
+  gassy due to yeasts, 186;
+  in bitter cheese, 189;
+  in canned butter, 159;
+  kephir, 72.
+
+
+
+
+
+
+End of the Project Gutenberg EBook of Outlines of Dairy Bacteriology, 8th
+edition, by H. L. Russell
+
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