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diff --git a/27778.txt b/27778.txt new file mode 100644 index 0000000..5a2c377 --- /dev/null +++ b/27778.txt @@ -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. 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