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

AND

E. G. HASTINGS

Professor of Agricultural Bacteriology
University of Wisconsin

TENTH EDITION

MADISON, WISCONSIN
H. L. RUSSELL
1914

Copyright 1914
BY
H. L. RUSSELL and E. G. HASTINGS

PREFACE TO THE TENTH EDITION.

This text was originally the outgrowth of a series of lectures on the subject of dairy bacteriology to practical students in the winter Dairy Course in the University of Wisconsin. The importance of bacteriology in dairy processes has now come to be so widely recognized that no student of dairying regards his training as complete until he has had the fundamental principles of this subject.

The aim of this volume is not to furnish an exhaustive treatise of the subject, but an outline and sufficient detail to enable the general student of dairying to obtain as comprehensive an idea of the bacteria and their effects on milk and other dairy products as may be possible without the aid of laboratory practice. When possible the dairy student is urged to secure a laboratory knowledge of these organisms, but lacking this, the student and general reader should secure a general survey of the field of bacteriology in relation to dairying.

In this, the tenth edition, the effort has been made to include all of the recent developments of the subject. Especially is this true in regard to the subject of market milk, a phase of dairying that has gained greatly in importance in the last few years. The changes in the methods of handling market milk have been marked. The results of these changes in influencing the quality of milk offered to the consumer are fully discussed.

H. L. R.
E. G. H.

CONTENTS

CHAPTER I.

STRUCTURE, GROWTH AND DISTRIBUTION.

Relation of bacteriology to dairying. The arts which have been developed by mankind have been the outgrowth of experience. Man first learned by doing, how to perform these various activities, and a scientific knowledge of the underlying principles which govern these processes was later developed.

The art of dairying has been practiced from time immemorial, but a correct understanding of the fundamental principles on which the practice of dairying rests is of recent origin. In working out these principles, chemistry has been of great service, but in later years, bacteriology has also been most successfully applied to the problems of modern dairying. Indeed, it may be said that the science of dairying, as related to the problems of dairy manufacture is, in large degree, dependent upon an understanding of bacteriological principles. It is therefore essential that the student of dairying, even though he is concerned in large measure with the practical aspects of the subject, should acquire as complete an understanding of these principles as possible.

While bacteriology is concerned primarily with the activities of those microscopic forms of plant life known as the bacteria, yet the general principles governing the life of this particular class of organisms are sufficiently similar to those governing the molds and other types of microscopic life that affect milk and its products to make it possible to include all of these types in a general consideration of the subject.

Nature of bacteria. The vegetable kingdom to which the bacteria belong consists of plants of the most varying size and nature. Those of most common acquaintance are the green plants varying in size from those not visible to the naked eye to the largest trees. Another class of plants known as fungi or fungous plants do not contain chlorophyll, the green coloring matter, but are usually colorless and, as a rule, of small size; among them are included such forms as the mushrooms, smuts, rusts and mildews, as well as the molds and yeasts. The bacteria are closely allied to this latter class. When first discovered they were thought to be animals because of the ability of some forms to move about in liquids.

The bacteria, like other kinds of living organisms, possess a definite form and shape. They are the simplest in structure of all the plants, the individual organism consisting of a single cell. The larger and more highly organized forms of life are made up of many microscopic cells, and the life of the individual consists of the work of all the cells. The bacteria are very comparable to the single cells of the higher plants and animals, but in the case of the bacteria the single cell is able to exist apart from all other cells and to carry out all of its life processes including reproduction.

Forms of bacteria. With the multicellular organisms much variation in form is possible, but with these single-celled organisms the possible variation in form is greatly limited. Three well marked types occur among the bacteria: the round or coccus form (plural cocci); the rod-shaped or bacillus (plural bacilli); and the twisted or spirillum type (plural spirilla). Most organisms of special significance in dairying belong to the coccus or bacillus group.

Size of bacteria. The bacteria, as a class, are among the smallest of living objects. None of them are individually visible to the naked eye, and they can be so seen only when clumps or masses are formed in the process of growth.

Fig. 1.—Forms of Bacteria.
A, coccus; B, bacillus; C, spirillum.

While there is considerable relative variation in size, yet in actual dimensions, this difference is so small as to make careful microscopic determinations necessary. An average diameter may be taken as about one thirty-thousandth of an inch, while the length varies naturally several fold, depending upon whether the type under observation is a coccus or a bacillus.

It is very difficult to conceive of the minuteness of the bacteria; the following may give some idea of their size. In a drop of cream ready for churning may be found as many as 10,000,000 and in a piece of fresh cheese as large as a cherry there may be as many living bacteria as there are people on our earth. While the bacteria are very minute, the effect which they exert in milk and other dairy products is great on account of their enormous numbers.

Manner of growth. The cells of which all plants and animals consist increase in numbers by the division of each cell into two cells through the formation of a division wall across the cell. The new cells divide and the plant or animal continues to grow. The same cell division occurs in the bacteria but since the bacteria are single celled, division of the cells means an increase in numbers rather than growth as in the higher forms of life.

Fig. 2.—Division of Bacteria.
The bacteria increase in numbers by the division of each cell into two cells. (After Novy.)

In the case of those bacteria that have a greater length than diameter, the new wall is formed at right angles to the long axis of the cell. As soon as the division is complete each cell is a complete individual, capable of carrying on all of its life processes. The cells may, however, cohere and thus form distinctive groupings that may serve to identify certain types. Some of the cocci form long chains and the term streptococcus is applied to such. Other groupings may be similar to a bale of twine or they may be massed in clusters with no regularity distinguishable.

Spores. Just as ordinary plants form resistant structures, known as seeds, capable of retaining vitality under conditions unfavorable for growth thereby perpetuating the species, so with certain of the bacteria, definite structures, known as spores, that are analogous in some respects to the seeds of the higher plants, are produced within the mother cell. The spores are exceedingly resistant to the influence of an unfavorable environment, such as heat, cold, drying, and even chemical agents. It is this property of the spores which makes it so difficult to destroy the bacterial life in the process of sterilizing milk. The property of spore-formation is fortunately confined to a comparatively small number of different species of bacilli.

Movement. Many of the bacteria are provided with vibratory organs of locomotion, known as cilia (singular cilium) which are variously distributed on the surface of the cell. By the movement of these relatively long, thread-like appendages the individual cell is able to move in liquids. It must be remembered, when these moving cells are observed under the microscope, that their apparent rate of movement is magnified relatively as much as their size.

Conditions for growth. All kinds of living things need certain conditions for growth such as food, moisture, air and a favorable temperature. The bacteria prefer as food such organic matter as milk, meat, and vegetable infusions. Those living on dead organic matter are known as saprophytes, while those which are capable of thriving in the tissues of the living plant or animal are known as parasites. Certain of the parasitic forms are capable of causing disease in plants and animals. In the first group are embraced most of the bacteria that are able to develop in milk or its products, such as those forms concerned in the spoiling of milk or its fermentation. It is true that milk may contain disease-producing bacteria coming either from a diseased animal or from a diseased human being. It is also true that some of such harmful forms are able to grow in milk, such as the organisms causing typhoid fever and diphtheria.

Food. The bacteria like all other plants must have their food in solution. Where they apparently live on solids, such as meats, fruits, etc., they dissolve the food substances before utilizing the same. If the solutions are highly concentrated, as in the case of syrups, preserves and condensed milk, the bacteria cannot readily grow, although all of the necessary food ingredients are present. When such concentrated solutions are diluted, bacterial growth will take place and the solutions will spoil.

Fig. 3.—Photomicrograph of Lactic Acid Bacteria.
Each cell is an individual organism, magnified 1250 diameters.

Generally speaking the bacteria grow best in a neutral or slightly alkaline solution rather than in acid liquids.

Temperature. One of the most important conditions influencing the rate of growth of bacteria is the temperature. Each form has a minimum temperature below which growth can not take place; also a maximum above which growth is again impossible. For the majority of species the minimum temperature ranges from 40 to 45° F. the maximum from 105 to 110° F. Growth takes place most rapidly at the optimum temperature, which, for each species, lies close to the maximum temperature at which growth can occur. Most of the bacteria of importance in the dairy grow well at from 70 to 100° F.

There are forms that can grow below the freezing point of water when they are in solutions that do not freeze at this temperature. There are still other bacteria that can grow at 140° F. a temperature that is quickly fatal to most forms. These are of importance in the dairy since they limit the temperatures at which milk can be stored for long periods of time.

Air supply. Living organisms, both plant and animal, require air or oxygen for the combustion of their food and for the production of energy. Most bacteria use, as do the green plants and animals, the free oxygen of the air for their respiration. Such organisms are called aerobic or air-living. A much smaller group possess the power of taking oxygen from organic compounds such as sugar and the like and therefore are able to live under conditions where air is excluded. These are called anaerobic bacteria. A large number of bacteria are able to live either in the presence or in the absence of free oxygen. Most of the bacteria of importance in the dairy are of this nature.

Rate of growth. When there is an abundant supply of food and when the temperature conditions are favorable, the bacteria increase in numbers with astounding rapidity. It has been determined by actual experiment that the process of cell division under favorable conditions takes place in a few moments. Barber has shown that one of the forms of bacteria constantly found in milk will divide in 17 minutes at 98° F. and that a single organism kept at this temperature for ten hours would increase to 1,240,000,000. If the temperature is reduced to 50° F., the time required for division is increased to several hours. The explanation for the rapid spoiling of milk that is not well cooled is thus apparent. The initial rapid rate of increase cannot be maintained for any length of time as the conditions become more and more unfavorable as growth continues, due to the accumulation of the by-products of the cell activity. Thus, the growth of acid-forming organisms in milk becomes checked by the formation of acid from the fermentation of the sugar.

Detrimental effect of external conditions. Environmental conditions of a detrimental character are constantly at work tending to repress the activity of bacteria or to destroy them. These act more readily on the vegetating cells than on the more resistant spores. It is of the utmost importance that those engaged in dairy work be familiar with these antagonistic forces since it is constantly necessary to repress or to kill outright the bacteria in milk and other dairy products. In many lines of dairy work it is likewise important to be familiar with the conditions favorable for bacterial growth.

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 growing condition. Numerous non-spore-bearing species remain alive in ice for a prolonged period, and experiments with liquid air show that even a temperature of-310° F. maintained for hours does not 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 spore stage. The temperature at which the vitality of the cell is lost is known as the thermal death point. This limit is dependent not only upon the nature of the organism, but upon 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 growing organisms, the thermal death point of most species ranges from 130° to 140° F. for ten minutes. When spores are present, resistance is greatly increased, some forms being able to withstand steam at 212° F. from one to three hours. In the sterilization of milk, it is often necessary to heat for several hours, where a single exposure is made, to destroy the resistant spores, that seem to be more abundant under summer than winter conditions. Steam under pressure is a much more effective agent, as the temperature is thus raised considerably beyond 212° F. An exposure of twenty minutes, at a temperature of 230° to 240° F. will kill all spores. Where heat is used in a dry state, it is much less effective, a baking temperature of 260° to 300° F. for an hour being necessary to kill spores. This condition is of the utmost importance in the destruction of bacteria in the dairy and creamery.

Effect of drying. The spore-bearing bacteria withstand effects of desiccation without serious injury, and many of the non-spore-producing types retain their vitality for some months. The bacteria found in the air are practically all derived from the soil, and exist in the air in a dried condition, in which they are able to remain alive for considerable periods of time. In a dried condition, active cell growth is not possible, but when other conditions, such as moisture and food supply are present, resumption of growth quickly begins. This property is also of importance in the dairy as in the preparation of dry starters for creameries and cheese factories.

Effect of light. Bright sunlight exerts a markedly injurious effect on bacterial life, both in a spore and in a growing condition. Where the direct sunlight strikes, more or less complete disinfection results in the course of a few hours, the effect being produced by the chemical or violet rays, and not by the heat or red rays of the spectrum. This action, however, does not penetrate opaque objects, and is therefore confined to the surface. In diffused light, the effect is much lessened, although it is exerted to some extent. Sunlight exerts a beneficial effect on the general health and well-being of animal life, and is a matter of importance to be taken into consideration in the erection of buildings for animals as well as for people.

Effect of chemicals. A great many chemical substances exert a more or less powerful toxic action on various kinds of life. Many of these are of great service in destroying bacteria or holding them 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 large amounts. Disinfectants have no place in dairy work, except to destroy disease-producing bacteria, or to preserve milk for analytical purposes. The so-called chemical preservatives used to "keep" milk depend for their effect on the inhibition of bacterial growth. In this country, most states prohibit the use of these substances in milk. Their only function in the dairy should be to check fermentative and putrefactive processes outside of milk and so keep the air free from taints.

Products of growth. All bacteria, as a result of their growth in food substances, form more or less characteristic compounds that are known as by-products. The changes brought about are those of decomposition and are collectively known as fermentations; they are characterized by the production of a large amount of by-products as the result of the development of a relatively small amount of cell life. The souring of milk, the rotting of eggs, the spoiling of meats, the making of vinegar from cider are examples of fermentations caused by different bacteria.

If the substances decomposed contain but little sugar, as do animal tissues, the conditions are favorable for the growth of the putrefactive bacteria, and foul-smelling gases are formed. When sugars are present, as in milk, the environmental conditions are most favorable for the acid-forming bacteria that do not as a rule produce offensive odors.

Many of the bacteria form substances known as enzymes which are able to produce certain decomposition changes in the absence of the living cells, and it is by virtue of these enzymes that the organisms are able to break down such enormous quantities of organic matter. Most of these enzymes 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 conditions where the activity of the cell is wholly suspended as in a saturated ether or chloroform atmosphere. The production of enzymes is not confined to bacteria, but they are found throughout the animal and plant world, especially in those processes that are concerned in digestion. Rennet, used in cheese making, is an example of an animal enzyme.

Distribution of bacteria. As bacteria possess greater powers of resistance than almost any other form of life, they are found very widely distributed over the surface of the earth. In soil they are abundant, because of the fact that all of the conditions necessary for growth are here best satisfied. They are, however, distributed with reference to the layers of the soil; the soil proper, i.e., that turned over by the plow, is extremely rich in them on account of the abundance of organic matter. But at the depth of a few feet they decrease rapidly in numbers, and in the deeper layers, from six to ten feet, or more, they are normally not present, because of the lack of proper food supply and oxygen. The fertility of the soil is closely associated with their presence.

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 from the soil 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 generally contains 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 bacteria. 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 contaminated with soil organisms, as the water issues from the ground. Wells vary in their germ content, depending upon manner of construction, ease of contamination at surface, etc. Wells are too frequently insufficiently protected from surface leachings, and consequently may contain all kinds of organisms found in the surface soil. Typhoid fever is very frequently disseminated in this way, as is cholera 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 readily infected. This is particularly true with reference to the intestinal tract, and in the undigested residue of the food, bacterial activity is at a maximum. The result is that fecal matter of all kinds contains enormous numbers of organisms so that the pollution of any food medium, such as milk, with such material is sure to introduce elements that seriously affect its quality.

CHAPTER II.

METHODS OF STUDYING BACTERIA.

Necessity of artificial cultivation. The bacteria are so extremely small, that it is impossible to study individual germs separately without the aid of powerful microscopes. 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 the resulting mass of cells would be visible to the unaided eye. This is done by growing the bacteria on various kinds of nutrient media that are prepared for the purpose, but inasmuch as bacteria are so universally distributed, it becomes an impossibility to cultivate any special form alone, unless the medium in which they are grown is first freed from all pre-existing forms of germ life.

Food materials. Many kinds of food substances are used for the cultivation of bacteria in the laboratory. In fact, bacteria will grow on almost any organic substance, whether it is solid or liquid, 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 culture media may be either permanently solid, like potatoes and coagulated egg, or they may retain their solid properties only at certain temperatures, like gelatin or agar. The latter two, which were devised by Robert Koch, are of utmost importance in bacteriological research, for their use permits the separation of the different forms of bacteria that may happen to be in any mixture. Gelatin is advantageously used, 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 80° F. Agar, a gelatinous product derived from a Japanese seaweed, has a much higher melting point, and is used especially with those organisms whose optimum temperature for growth is above the melting point of gelatin.

Besides these solid culture 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.

Sterilization. The various ingredients that are used in the preparation of culture media are not free from micro-organisms, hence the media would soon spoil if they were not destroyed, and the media subsequently protected from contamination from the air, etc. The process of rendering the media free from living micro-organisms is known as sterilization. It may be accomplished in a number of ways, but most often is done by the use of heat. For culture material, which is always organic in character, moist heat is employed. The various culture media, in appropriate containers, are subjected to a thorough steaming in a steam cooker. This destroys all of the vegetating cells but not the resistant spores that may be present. The media are then stored, for twenty-four hours, at temperatures favorable for the germination of the spores and are then again heated. Three such applications on successive days are usually sufficient to free the media from all living germs, since between the heating periods the spores germinate and the resulting vegetative cells are more easily destroyed. The sterile media will keep for an indefinite period in a moist place.

The media are usually placed in glass containers which may be sterilized before use by heating them in an oven, it being possible to thus secure a much higher temperature than with streaming steam. All glass or metal articles may be sterilized by the use of dry heat but for organic media, to avoid burning, moist heat must be used.

All kinds of materials may be sterilized by treatment with steam under pressure. An exposure for a few moments at 250° F., a temperature attained with 15 pounds steam pressure, will destroy all kinds of bacteria and their spores. This method of sterilization is used in the canning of meats and vegetables and in the preparation of evaporated milk. To avoid contamination of the media after sterilization, the flasks and tubes are, after being filled, stoppered with plugs of cotton-wool, which effectually filter out all bacteria and mold spores from the air, and yet allow the air to pass freely in and out of the containers.

Methods of determining the number of bacteria. The method of determining the number and kinds of bacteria in any substance can be illustrated by the process as applied to milk. For this purpose the method of procedure is as follows: Sterile gelatin in glass tubes is melted and then cooled until it is barely warm. To this melted gelatin a definite quantity of milk is added. The medium is gently shaken, so as to thoroughly mix the milk and gelatine, and the mixture then poured into a sterile, flat, glass dish, and quickly covered, where it is allowed to cool 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 spots are called colonies, and are composed of an almost infinite number of individual cells, the result of the continued growth of a single organism that was in the drop of milk and which was firmly held in place when the gelatin solidified. The number of these colonies represents approximately the number of living bacteria that were present in the amount of milk added to the tube of gelatin. If the plate is not too thickly sown with the bacteria, 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.

Fig. 4.—Plate Culture.
Each of the dots is a colony that has been formed by the growth of an organism embedded in the solid culture-medium. By counting the colonies, the number of living bacteria in the amount of milk added to the culture is determined.

In order to make sure that the number of colonies is not so numerous as to prevent counting and further study of their characteristics, a series of plate cultures is usually made in which varying amounts of milk are added to the tubes of gelatine. This is attained by adding a definite amount of the milk or other substance to be examined to a measured amount of sterile water, e.g., one cubic centimeter of milk to ninety-nine cubic centimeters of water. One cubic centimeter of this mixture may be used for the inoculation of the plate culture. This dilution may be carried on to any desired extent; in the examination of many dairy products, it is necessary to use very minute quantities of material, often only one one-millionth of a cubic centimeter.

To study further the peculiarities of the different bacteria, small portions of the individual colonies are transferred to tubes of sterile culture-media. In order to do this the colony is touched with a piece of platinum wire; the minute amount of growth that adheres to the wire is sufficient to seed the tube of fresh culture-medium. The inoculating needle must always be sterilized before use by passing it through a gas flame.

A culture thus obtained is called a pure culture since it contains but a single kind of an organism, as the colony is the result of the growth of a single cell. These cultures then serve as a basis for continued study, and must be planted and grown upon the different kinds of media that are obtainable. In this way the slightest 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.

Fig. 5.—Different Kinds of Bacteria Growing in Gelatin.
A, meager growth, no liquefaction or surface growth; B, profuse surface growth, radiating filaments from the growth below the surface; C, a rapid liquefying form; D, a gas producer that grows equally well in the presence or absence of air; E, form that grows only in the absence of air, an anaerob.

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 in a pure state.

The microscope in bacterial investigations. 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 high magnifying powers (600 to 800 diameters), combined with sharp definition.

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 colored 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 made in different ways, 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 experimental animals, like rabbits, guinea pigs and white mice.

The methods of the bacteriologist in his laboratory are in their effect not dissimilar to those which the farmer employs in securing his crop of pure-bred grain. The laboratory farmer kills the weed seeds in his culture field by the application of heat. His field, which is embraced in his culture dish, has been fertilized and prepared by the addition of certain favorable ingredients. When he has garnered his crop, he maintains its purity by keeping his selected seed, the pure culture, free from all contamination. The dairyman, even though he may not expect to carry on the detailed operations of the laboratory, will understand the reason for the directions which he is often required to follow much better if he knows how the simple operations of the laboratory are carried out. For a fuller knowledge of these matters, the reader is referred to the special texts on bacteriology.

CHAPTER III.

CONTAMINATION OF MILK.

Spoiling of milk. Materials of animal origin are peculiarly prone to undergo changes, rendering them unfit for use, and of these, milk is exceedingly susceptible to such changes. This is due to the fact that the composition of milk is especially adapted to bacterial growth, and that the opportunity for entrance of such organisms is likewise such as to permit of abundant contamination. The consequence is that milk readily undergoes fermentative changes, due to the development of one or another type of micro-organism.

Milk, a suitable bacterial food. While milk is designed by nature for the nourishment of mammalian life, it is, curiously enough, equally well adapted to the growth of these lowest forms of vegetable life. The nutritive substances required by bacteria are here sufficiently dilute to make possible rapid growth.

Milk also contains all the necessary chemical substances to make a suitable bacterial food supply. Of the nitrogenous compounds, albumen is in a readily assimilable form. Casein, the principal nitrogenous constituent of milk, exists in an insoluble condition, and cannot be directly utilized, until it is acted upon by digesting enzymes. The fat in milk does not readily decompose, and while there are a few bacteria capable of splitting this substance, the majority of organisms are unable to utilize it. Milk sugar, on the other hand, is an excellent food for most species.

Fig. 6.—Fat Globules and Bacteria.
Note the relative size of the fat globules of milk and the lactic acid bacteria.

Sources of contamination. Inasmuch as milk is especially exposed to the inroads of bacterial growth, and because of the fact that much of the contamination can easily be prevented, it is highly important that the milk producer and dealer should be thoroughly cognizant of the various sources of contamination. The different factors concerned in contamination may be grouped as follows: the interior of the udder; utensils, including all apparatus with which the milk is brought in contact subsequent to withdrawal from the animal; infection coming from the animal herself, from the milker, and the surrounding air.

Condition of milk when secreted. Immediately after withdrawal from the udder, milk always contains bacteria, yet in the secreting cells of the udder of a healthy cow, germ life does not seem to be present. 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.

A study of the structure of the udder shows the manner in which such infection occurs.

Structure of the udder. The udder is composed of secreting tissue (gland cells) that is supported by fibrous connective tissue. The milk is elaborated in these cells and is discharged into microscopic cavities, from whence it flows through the numerous channels (milk sinuses) that ramify through the substance of the udder, until finally it is conveyed into the milk cistern, a common receptacle holding about one half pint that is located just above the teat. This cavity is connected with the outside by a direct opening (milk duct) through the teat. During the process of milking, the milk is elaborated rapidly in the gland cells, and their contents upon rupture of the milk cells, flow down into the cistern. The normal contraction of the muscles at the lower opening of the outer duct prevents the milk from passing out except when pressure is applied, as in milking. The inner walls of the milk duct and cistern are always more or less moist, and therefore afford a suitable place for bacteria to develop, if infection once occurs, and conditions are favorable for growth.

Manner of invasion. Two possible sources of invasion of the udder by bacteria may exist. If bacteria are present in the circulating blood, there is the possibility of organisms passing directly through the tissues into the milk-secreting cells. The other alternative is the possible direct contamination from the outside by organisms passing up through the milk duct, and so spreading through the open channels in the udder.

Fig. 7.—Sectional View of Udder.
Teat with milk duct connecting the exterior with the milk cistern. Milk sinuses which conduct the milk from the secreting tissue to the milk cistern. (After Moore & Ward.)

Number of bacteria in fore-milk. If a bacteriological examination is made of the milk drawn from each teat at different periods during the milking process, it will be found that the fore-milk, i.e., the first few streams, contains, as a rule, many more organisms per cubic centimeter than that removed later. Not infrequently thousands of organisms per cubic centimeter may be found in the first streams while the middle milk, or strippings, will contain much smaller numbers.

Distribution and nature of bacteria in udder. If the udder itself is carefully examined as to its bacterial content, it appears that the majority of organisms found is confined to the lower portion of this organ, in the teat, milk-cistern and large milk-ducts; while bacteria occur in contact with the secreting tissue, they are relatively less abundant. This would seem to indicate that the more probable mode of infection is through the open teat.

While there is no constant type of bacteria found in the fore-milk, yet it is noteworthy that nearly all observers agree that the organisms most commonly found are not usually the acid-producing, or gas-generating type, so abundant on the skin or hairy coat of the udder and which predominate in ordinary milks. Coccus forms, belonging to both liquefying and non-liquefying types are most generally present. Many of these produce acid slowly and in small quantities.

The bacteria coming from the interior of the udder are of small practical significance since they do not grow rapidly at the temperatures at which milk is stored. If the milk is protected from contamination from other sources, the bacteria from the udder will ultimately cause it to spoil, but under ordinary conditions other forms are present in such greater numbers, and grow so much more rapidly in milk, that the udder forms have small opportunity to exert any effect.

It is interesting to note that the bacteria found in the udder are similar to those that seem to be most abundant in such glandular tissues as the liver and spleen. This fact increases the probability that these comparatively inert coccus forms of the udder may originate directly from the blood stream. The organisms that normally are found in the udder exert no harmful effects on the gland. It might be thought that due to the presence of abundant food and a favorable temperature that growth would be abundant, but such is not the case. At times the udder may be invaded by forms that are not held in check by the natural factors and an inflammation of the udder is likely to result.

Germicidal property of milk. It has been claimed that freshly drawn milk, like other body fluids, possesses germicidal properties, i.e., the power of destroying bacteria with which it may be brought in contact. If milk is carefully examined bacteriologically, hour by hour, after it is withdrawn from the udder, it will generally be found that there is at first not only no increase in number of organisms during a longer or shorter period when it is kept at temperatures varying from 40° to 70° F., but that an actual reduction not infrequently takes place. When cultures of bacteria, such as B. prodigiosus, a red organism, lactic acid organisms, and even the yellow, liquefying coccus, so commonly found in the fore-milk, are artificially introduced into the udder, it has been found that no growth occurs and that in the course of a few days the introduced organisms actually disappear. Whether this failure to colonize can be regarded as evidence of a germicidal property or not is questionable. In fact, this question is a matter of but little practical importance in the handling of milk since, under the best of conditions, the keeping quality of the milk is not materially enhanced. It may be of importance in inhibiting growth in the udder.

Rejection of fore-milk. The fact that the fore-milk contains per cubic centimeter so much more germ life than the remainder of the milk has led some to advocate its rejection when a sanitary milk supply is under consideration. While from a purely quantitative point of view, this custom may be considered advantageous, in practice, however, it is hardly worth while since it is not at all certain that the rejection will have any effect on the keeping quality or healthfulness of milk. This is especially true if the ends of the teats are thoroughly cleaned before milking. It is true that the fore-milk is relatively deficient in fat so that the loss of butter fat occasioned by the rejection of the first few streams is comparatively slight.

Contamination from utensils. One of the most important phases of contamination is that which comes from the utensils used to hold the milk from the time it is drawn until it is utilized. Not only is this important because it is a leading factor in the infection of milk, but because much improvement can be secured with but little trouble, and it is especially necessary that the dairy student should be made familiar with the various conditions that obtain. Pails and cans used to hold milk may be apparently clean to the eye, and yet contribute materially to the germ content of the milk placed in them. Not only does much depend upon their condition, but it is equally important to take into consideration their manner of construction. Dairy utensils should be simple in construction, rather than complex. They should be made so that they can be readily and easily cleaned, or otherwise the cleaning process is apt to be neglected.

Of first importance are those utensils that are used to collect the milk and in which it is handled while on the farm. The warm milk is first received in pails, and unless these are scrupulously cleaned, an important initial contamination then occurs. As ordinarily washed, the process falls far short of ridding the utensils of the bacterial life that is adherent to the inner surface of the pail. Then, too, all angles or crevices afford an excellent hiding place for bacteria, and it is very important to see that all seams are well soldered. Round corners and angles flushed with solder greatly facilitate thorough cleaning of utensils. Tin utensils are recognized as most satisfactory.

Shipping cans are likely to serve as greater infecting agents than pails for they are subject to more wear and tear and are harder to clean. As long as the surface is bright and smooth, it may be easily cleaned, but large utensils, such as cans, are likely to become dented and rusty in spots on the inner side. The storage of milk in such utensils results in its rapid deterioration. The action of rennet has been found to be greatly retarded where milk comes in contact with a rusty iron surface. It is also probable that some of the abnormal flavors in butter are due to the action of acid cream on iron or copper surfaces from which the tin has been worn. It is equally important that attention be paid to the care of strainers, coolers, and the small utensils. Cloth strainers are more or less of a hotbed for bacterial growth, for unless they are boiled, and then dried quickly and thoroughly, germ growth will continue apace in them, as long as they contain any moisture.

Milking machines and farm separators. The introduction of these special types of dairy machinery in the handling of milk on the farm has materially complicated the question of the care of milk. Both of these types of apparatus are much more complicated than the usual milk utensil; consequently, the danger of imperfect cleaning is thereby increased. This is still further accentuated by the fact that cleansing of utensils on the farm can never be done so well as at the factory or milk depot where steam is available. The milking machine may be easily kept in a comparatively germ-free condition, but unless this is done, it contributes its quota of germ life to the milk.

The farm separator is more widely used than the milking machine and in actual practice the grossest carelessness prevails in the matter of its care. Frequently it is not taken apart and thoroughly cleansed, but is rinsed out by passing water through the machine. It is impossible by such a treatment to remove the slime that collects on the wall of the bowl; the machine remains moist and bacterial growth can go on. Such a machine represents a most important source of contamination of milk and cream and it is probable that the widespread introduction of the hand separator has contributed more to lower the quality of cream delivered at the factory than any other single factor.

Contamination from factory by-products. The custom of returning factory by-products in the same set of cans that is used to bring fresh milk is a prominent cause of bad milk. Whey and skim milk are rich in bacterial life, and not infrequently are so handled as to become a foul, fermenting mass. If the cans used to transport this material are not scrupulously cleaned on the farm, transfer of harmful bacteria to the milk is made possible. In this way the carelessness of a single patron may be the means of seeding the whole factory supply. This custom is not only liable to produce a poor quality of milk, but it is more or less of a menace to all the patrons of a factory, inasmuch as the opportunity always obtains that disease-producing organisms may thus be introduced into the supply. Not infrequently is tuberculosis thus spread through the medium of factory by-products.

Fig. 8.—Whey Disposal.
Whey barrels at a Wisconsin Swiss cheese factory. Each patron's share is placed in a barrel which is so situated that it is impossible to empty it completely; thus it is not cleaned during the season.

The manufacture of Swiss cheese presents a striking example of the disregard which factory operators show toward the employment of bacteriological principles. In these factories, the custom is widely practiced of apportioning the patrons' allotment of whey into individual barrels which are supposed to be emptied each day. As these barrels are, however, rarely ever cleaned from the beginning to the end of the season, they become very foul, and the whey placed in them from day to day highly polluted. It is this material which is taken back to the farms in the same set of cans that is used for the fresh milk. When one recalls that the very best type of milk is essential for the making of a prime quality of Swiss cheese, and that to secure such, the maker insists that the patron bring the product to the factory twice daily, the before mentioned practice appears somewhat inconsistent.

Treatment of factory by-products. To overcome the danger of infecting milk from factory by-products with either undesirable fermentative organisms, or disease-producing bacteria, the most feasible process is to destroy these organisms by the application of heat. In Denmark, some portions of Germany, and in some of the states in this country, laws exist which require the heating of all skim milk before it is returned to the farm. This is done by the direct use of exhaust steam, or running the product through heaters.

The treatment of whey in cheese factory practice is especially important since the warm whey must be stored for a number of hours before it is returned to the farms. Even under the best of conditions the whey is certain to be in an advanced state of fermentation when placed in the milk cans, and it only needs the infection of the whey tank with harmful bacteria to cause great loss on account of the injury of the product by these bacteria. Among Canadian factories the custom of heating the whey as it passes from the cheese vat to whey tank has been introduced, and where ever adopted has been retained, because, it has resulted in such an improvement of the cheese that the gain was much greater than the cost, which is estimated at not over fifty cents per ton of cheese. The whey is heated not to exceed 155° F.; the hot whey serves to scald the whey tank and as the mass of whey is usually quite large, it does not cool to a point where bacterial growth can take place for a number of hours. The whey is thus quite sweet when returned to the farm and has greater feeding value. The heating also prevents the creaming of the whey in the tank and thus avoids the soiling of the cans with grease which is most difficult to remove.

Where compulsory legislation is in force it is generally required that these by-products be heated to a temperature of at least 176° F. This is done so as to destroy effectually the organisms of tuberculosis, and especially to permit of the utilization of the so-called Storch test,[1] which enables a person to determine quickly whether milk or whey has been heated or not.

[1] Storch (40 Rept. 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 enzyme known as peroxidase which has the property of decomposing hydrogen peroxid. If milk is heated to 176° F., (80° C.) or above, this enzyme is destroyed, so that the above reaction no longer takes place. If potassium iodide 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.

Cleaning utensils. Various processes are applied to dairy utensils to cleanse them. In removing visible dirt and foreign matter, much of the bacterial life is mechanically eliminated, but most of the cleaning processes fail to destroy the germ life in these utensils.

In rinsing, washing, or even scalding, the water is not applied at a sufficiently high temperature to destroy effectively the bacteria. These processes are primarily used for the removal of dirt and other matter. To facilitate such removal, washing powders of various kinds are frequently employed; some of these possess considerable disinfecting action. All utensils after cleansing should be thoroughly rinsed in clean, hot water. Even where no further treatment is given, a careful cleaning may so reduce the germ content on the inner surface of utensil as to render contamination therefrom relatively unimportant. Most of the contamination in a well cleaned utensil comes from the cracks and angles, which permit of the collection of the dirt. If these are properly attended to, thorough cleaning and rinsing alone will accomplish much.

To exert an actual germ-destroying effect on the bacterial content of the utensil, resort must be had to boiling or steaming. To treat utensils so as to render them wholly germ-free would be impractical under ordinary commercial conditions, as it would consume too much time, although with proper apparatus, this process is not impossible, but it is well within the limits of practicability in factory treatment to apply steam for a short period of time. Where cans, pails and such utensils, are steamed for a minute or so after being thoroughly cleaned, the germ content is greatly reduced. In a series of tests by Harrison, the germ content of a set of cans cleaned in an ordinary way was 442,000 bacteria per cubic centimeter in 100 cubic centimeters of wash water; in a set washed in tepid water and then scalded—the best farm practice—it was 54,000 per cubic centimeter, while in cans carefully washed and then steamed for 5 minutes, it was reduced to 880 per cubic centimeter. It would not be worth while to institute measures that would accomplish the destruction of this small residual content.

The use of steam, therefore, is of great service in eliminating bacterial life in all utensils. In apparatus of at all complicated design, it is absolutely necessary. Of course, ordinarily, steam can be applied only at the factory, as the farm does not usually afford facilities for its easy generation. This fact has led in some cases to the adoption of the method of cleaning and sterilizing the cans at the factory rather than to await their arrival at the farm. This custom is most frequently followed in milk supply plants.

It is also very important in cleaning dairy utensils to see that they are rapidly and thoroughly dried after being washed and steamed. As pointed out above, the short period of steaming that can be followed in practice does not kill all the bacteria. If moisture is retained, conditions permit of the growth of the undestroyed organisms. Tests made on glass milk bottles showed that considerable growth occurred in the condensation water even after quite thorough sterilization. Some of the devices used for the sterilization of such utensils as milk cans are so arranged that, after steam has been introduced, hot air is passed into the can until it is thoroughly dried. Other utensils such as cloth strainers become sources of contamination unless the articles are thoroughly and quickly dried after cleaning.

In a general way, it may be said that whenever a utensil is so constructed and in such a condition that every portion of its surface can be reached by a cloth or a brush, it can be kept in a sanitary condition. But whenever any portion cannot be thus reached, whether it is an angle or a seam in a pail or can, the interior of the separator bowl, or in the pipes used for conducting milk, contamination is certain to result from such places, unless extreme care is taken to destroy the bacteria therein by steaming.

Contamination from the animal. In the process of milking, the bacterial content of the milk is materially increased. In part this comes from the utensils into which the milk is drawn, but the animal herself, the milker, as well as the surrounding air, also contribute to a varying extent. Of these factors, the one fraught by far with the most consequence, is the influence of the animal herself. It is a popular belief that the organisms found in milk are derived from the feed and water which the animal consumes, but under normal conditions, the bacteria consumed in food pass through the intestinal canal and do not appear in the circulation. It must not be assumed, however, that the character of feed and water supply is of no moment. Stock should be given pure and wholesome water 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 the ration fed, the softer are the fecal discharges, producing a condition 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.

Fig. 9.—Bacteria on Hairs.
Each colony on the hair represents one or more bacteria that were adherent to the hair when it was placed on the surface of the solid culture-medium.

The animal 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 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. All of this material contains organisms that are more or less undesirable in milk. Bacteria concerned in gassy fermentations and those capable of producing obnoxious taints are particularly common, so that this type of pollution is especially undesirable in milk.

Amount of dirt in milk. When one remembers that the larger part of fresh manure is of such a nature that it does not appear as sediment, the presence of evident filth in milk must bespeak careless methods of handling.

The sediment or dirt test is used quite extensively to ascertain the amount of dirt milk may contain. By means of a cotton filter, the insoluble residue is removed and is made evident upon a layer of absorbent cotton. Milk that would show with difficulty any evidence of dirt upon ordinary examination reveals such defects very readily in this test.

Exclusion of dirt. It is better to keep bacteria out of milk, so far as practicable, rather than to attempt to remove them after they have once gained entrance. As is usual, prevention of trouble is much more easily accomplished than removing the difficulty after it once occurs.

Fig. 10.—Dirt from Milk.
The dirt adherent to each of the filters was obtained from one pint of milk. The milks tested were produced on different farms.

Much reduction as to the amount of dirt that finds its way into milk may be accomplished by improved stable environment. The fouling of the udder and flanks comes from wading in dirty water, muddy yards, and from improper type of stalls. Barnyards are often a disgrace through the accumulation of manure and seepage. Cows wading in such mire cannot but accumulate mud and filth to a material degree on the teats and udder. Greater care as to drainage of the barnyard and the paving of same with gravel, cinders, etc., will permit of its being kept clean, and so prevent the fouling of animals. But more important than the yard is the stall which the animal occupies in the stable. The essential feature is to have a stall of such construction as to keep the animal out of her own manure when she lies down. To accomplish this, it is necessary to have a manure drop behind the stall proper so that the feces and urine are kept out of the bed of the stall as much as possible.

Fig. 11.—The Model Stall.
A stall of this type keeps the animals clean, and thus aids greatly in producing good milk.

Most of the stalls widely advertised in the farm press seek to accomplish this in one way or another, usually by some arrangement by which the cow is forced back when standing and drawn forward on lying down. In Fig. 11 a type of stall is illustrated that accomplishes this most successfully; the essential feature being a 2×3-inch wood strip nailed to the stall floor immediately in front of the hind feet of the animal when in a standing position. When the animal lies down, she crowds forward to avoid lying on this strip, and thus is out of contact with the manure, except such as is carried onto the bedding by the hind feet. By the use of this stall it is possible to keep the animals free from all accumulations of manure.

Effort should be made to prevent fouling of the animals rather than in cleaning them after once soiled. It is very evident that where the cattle come to the milker with muddy udders, they will not be so cleaned before milking as to prevent a large amount of such dirt from entering the milk. However, when all that can be done towards keeping the cows clean has been accomplished, a small amount of grooming will greatly reduce the contamination coming from them.

The kind of bedding used in the stalls may have a marked influence on the contamination coming from the animal. If the straw is dusty, partially rotten and moldy, the bacteria and molds adhere to the coat of the animal and are thus introduced into the milk. In the case of cattle on pasture, no visible evidences of dirt are usually present but the hair is covered with the dust coming from the soil. There is very good reason to believe that the quality of milk is influenced by the type of pasture on which the cows graze, due to the difference in the types of bacteria in the surface soil. The milk from animals on low land is more likely to show undesirable fermentations than that from those grazing on higher lands. This is not due to the influence of the feed as is often supposed but rather to the dirt from the coat of the animal.

Washing the udder. If a surface is moist, dust and the adherent bacteria cannot be easily dislodged. The air over snow-covered mountains or over oceans is relatively free from bacteria. The udder and flanks of the animals can be carded to remove the loose hairs and the evident dirt; the fine dust can now be removed by wiping with a clean damp cloth just before the milking process. The actual washing and wiping of the udder and flanks still further reduces the contamination coming from the animal; experiments show a reduction of fully three-fourths of total contamination. Clipping the udder and flanks also aids in keeping the animal clean.

It is often asserted that the treatment of the animals in these ways reduces the yield of milk. It is certain that such an effect will persist for only a short time and there is reason to believe that grooming increases the yield.

Fig. 12.—Sanitary Milk Pails.
The small opening is very efficient in keeping the dirt out of milk.

Sanitary milk pails. The entrance of organisms into the milk can be greatly reduced by lessening the area of the milk pail exposed to the dust shower. To accomplish this purpose a number of so-called sanitary or hygienic milk pails have been devised. In some cases, these are the regular type of pail provided with a cover having a small opening through which the milk is received. In other cases, a strainer is interposed so as to remove more effectually the coarse particles. While pails of this type are successful in the removal of a large part of the dirt, and consequently reduce materially the bacterial content of the milk, yet they must be of simple construction, so that they can be kept in a clean condition in order to adapt them for general practical use. The use of such a utensil increases materially the keeping quality of the milk.

Fig. 13.—Sanitary Milk Pails.
The Stadtmueller pail and the Truman pail, two of the most practical of the small-topped pails.

Stocking has shown that under ordinary barn conditions, the use of small-topped pails reduced the number of bacteria 95 per cent; with dirty cows the reduction in bacteria amounted to 97 per cent. A six-inch opening presents only one-fourth as large an exposure as a twelve inch, so that the reduction in bacterial content is greater than the lessening in the size of the openings of the pails. The ordinary pail receives dust not only from the udder, but also from the flank which is usually a more important source of contamination than the udder itself, while the small-topped pail receives only that from the udder.

Fig. 14.—use of Sanitary Milk Pails.
The open pail is fully exposed to the falling dust while the hooded pail excludes much of the dust and dirt coming from the animal.

Milking machines. Where the milk is removed from the udder by machine methods, instead of by hand, it is possible to eliminate nearly all external contamination from the animal and her surroundings. The only opportunity for infection is then through the leakage of air around the teat cups. Care should be taken to see that the teats are in a clean condition before applying the suction cups. The main problem in the use of a milking machine is to keep the apparatus in an aseptic condition. Immersion of the teat cups and the rubber connections in lime water, brine solution, or other mild antiseptics, prevents bacterial development. Hastings has found that milk having a germ content of less than 10,000 bacteria per cubic centimeter may be produced by the use of a properly handled milking machine.

Contamination from the milker. While the milker is a small factor in comparison with the animal in the matter of contamination, yet he can not be neglected, as it is within his power to affect profoundly the quality of the milk. His personal habits as to cleanliness and his appreciation of the precautions necessary in the production of clean milk have much to do with the contamination of the milk. The milking should be done with dry hands, although a little vaseline may be used with effect. The hands should be washed before milking as milk is certain to come in contact with them to some extent. The milking should be done with the whole hand rather than stripping between the thumb and finger; the clothing should be covered with clean overalls and jumper, or at least a clean apron should be worn during the milking. If these are of white material, more frequent laundering is likely to result.

Contamination from air. It is difficult to disassociate the contamination arising from the condition of the air from that derived directly from the animal. Barn operations of various kinds result in the production of dust, particularly where dry forage, such as hay or straw, is handled. Where manure is given an opportunity to dry, dust is readily produced, and such material is particularly replete with bacterial life. Some kinds of dust, such as that originating from ground grains, or shavings that may be used for bedding, contain a small amount of bacterial life in comparison with the dust from hay, or other dry fodder. In a dried condition, the slightest movement is apt to dislodge these fine particles, and they float in the air for considerable periods of time. If milk is drawn and exposed to the air of the barn during the feeding operations, it is subject to the dust shower that is present. Where the storage can is allowed to stand in the stable during the milking, even though it is covered with a strainer, this accumulation of microscopic particles is added to the milk, as they readily pass the meshes of the finest strainer.

Fig. 15.—contamination From the Air.
This culture plate, three inches in diameter, was exposed for 30 seconds in the barn during feeding of dry fodder. A 12-inch pail exposes over 18 times the surface of this plate.

Removal of dirt after introduction. The more primitive method of improving the quality of milk, so far as its dirt content is concerned, is to attempt to remove the grosser particles of contamination after entrance. In the case of straining, the method is usually applied at the time of milking, but in the case of filtering and clarifying, it is carried out at the milk station, in an effort to improve the appearance of milk and overcome the influence of careless methods of the producer. By the use of strainers, either metallic or cloth, it is possible to remove particles of hair, undissolved dirt and manure, but it must be remembered that these grosser visible particles of pollution are not really the cause of the troubles which may ensue in improperly handled milk. The bacteria which are adherent to these foreign particles are in large measure washed off in the process of straining, and pass through the meshes of the finest strainer. The main service, therefore, of straining is to improve the appearance of the milk, and it has no effect on the quality in any way.

Production of clean milk. The problem of clean milk is important, whatever may be the use to which milk may be put. It is important in the manufacture of butter, but owing to the fact that the fat is not readily acted upon by bacteria, it is not so sensitive to bacterial conditions, as when the milk is made into cheese. In this product, the bacterial condition of the milk is a matter of prime importance. In milk destined for direct consumption, the exclusion of the bacteria becomes yet more important. While it is impossible to exclude bacteria so completely that milk will not undergo fermentative changes, yet for domestic consumption it is preferable to have milk with as low bacterial content as can readily be secured. The highest type of market milk, that known as sanitary, or certified, is produced under such extreme conditions of care as to contain the minimum germ content. To accomplish these results requires such stringent control as to increase greatly the cost of the product. Pure, clean milk can be produced at a very slight increase in cost over the regular expense of milk production, if the right kind of attention is given to certain details of a practical character. Improvement in our milk supplies must largely come from this source, for any improvement to be permanent must be made to pay, and it requires considerable education to secure the co-operation of consumers and their willingness to pay for any material increase in the quality of the product.

In the foregoing factors concerned in the contamination of milk, it is of course impossible to measure accurately the influence of the different sources of infection, as these are continually subject to variation in every case. As a rule, the most important factors are those pertaining to the utensils and the condition of the animal herself. If these two factors are brought under reasonable control, the major portion of contamination that ordinarily obtains is done away with. The application of the remedial or preventive measures heretofore mentioned will greatly reduce the germ content of the milk.

Cooling of milk on farm. Bacterial growth is directly related to temperature conditions, and with summer temperatures, such development goes on apace, unless it is checked by early cooling. The larger portion of bacteria that find their way into milk, especially those that are previously in contact with the air, are in a dormant condition, and are therefore not stimulated into immediate growth, unless reasonably high temperatures prevail. In milk, which comes from the animal at blood heat, this growth is greatly stimulated. To counteract this effect, milk should be chilled as soon after milking as possible. If the temperature is immediately lowered to 50° F., or lower, actual cell development is greatly retarded, and the rate of souring, and other fermentative changes thereby diminished. In this country ice is liberally used in accomplishing this result. In Europe, the use of ice is much less common. The employment of such artificial means of refrigeration makes possible the shipment of milk for long distances by rail. New York city now receives milk that is produced in Canada and northeastern Ohio.

Fig. 16.—Effect of Cooling Milk.

Aeration of milk. The custom has been extensively recommended of subjecting milk to the influence of air in the belief that such exposure permits of the interchange of gases that would improve the quality. In practice, this process, known as aeration, is carried on in different ways. 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, falling in drops or tiny streams through the air. Whenever this process is carried on at a temperature lower than that of the milk, it results in more or less rapid cooling.

In earlier times, aeration was generally recommended and practiced, especially in connection with the cheese industry, but carefully controlled experiments fail to show that the process exerts any material influence on the rate of germ development. If it is carried out in an atmosphere more or less charged with bacteria, as in the barn or stable, it is more than likely to add to the bacterial content of the milk. While to some extent odors may be eliminated by the process, the custom is not followed so generally now as it used to be some years ago.

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 absorbing volatile odors. 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 it 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.

Many kinds of feed consumed by the animal produce a more or less pronounced taint or flavor in the milk. With some plants, such as garlic, leeks, turnips, and cabbage, the odor is so pronounced as to render the milk quite unfit for use. In some states along the Atlantic seaboard, wild plants of this character in woodland pastures may be so abundant as to make it impossible to pasture milch animals. The difficulty in such cases is due to absorption of the volatile principles into the circulation of the animal, and if such feed is consumed shortly before milking, the characteristic odors appear in the milk. If consumed immediately after the milk is withdrawn from the animal, sufficient time may elapse so that the peculiar odors are dissipated before the milk is again secreted. The same principle applies in a lesser degree to the use of certain green fodders that are more suitable for feed, such as rape, green rye, or even silage. Silage produces a distinct, but not unpleasant odor in milk, but newly pastured rye often confers so strong an odor as to render the milk unusable.

Where certain drugs are employed in the treatment of animals, such as belladonna, castor oil, sulfur, or turpentine, the peculiar odors may reappear in the milk. Such mineral poisons as arsenic have been known to persist for a period of three weeks before elimination.

On account of the elimination of many drugs, unchanged, from the animal in the milk, the milk of any animal that is receiving medicine should not be used for human food. When such milk is mixed with that of a number of other animals and when it is used by adults, no harm is likely to result, but when the dilution is not great and the milk is used for young children it may affect them through its content of the drug. The feed may not only affect the quality of milk but its value as food. One of the most prominent of American dairymen, who has for many years produced milk especially for children's use, has said that he could feed his cows so as to make ill every child receiving the milk.

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 experiments of one of us (R), it has been 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 quite rapidly.

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 is noted where the same is stored in a refrigerator with certain fruits, meats, fish, etc.

Distinguishing bacterial from other 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 by-products are most evident.

Moreover, if the difficulty is of bacterial origin, it can be frequently produced in another lot of milk (heated or sterilized is preferable) by inoculating the 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.

Factory contamination. As the time element is of importance in the production of troubles due to bacteria, it follows that infection of milk on the farm is fraught with more consequence than factory contamination, as the organisms introduced would have a longer period of development. Nevertheless, the conditions in the factory are by no means to be ignored, as they not infrequently permit the milk to become seeded with highly undesirable types. A much more rigid control can be exercised in the factory, where steam is at hand as an aid in the destruction of organisms. In the cleaning of pumps and pipes, steam is absolutely necessary to keep such apparatus in a sanitary condition.

The water supply of the factory is a matter of prime importance, as water is used so extensively in all factory operations. 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 best be determined by making a curd test (p. 99) 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 cubic centimeter at most. The danger from ice is much less, for the reason that good daily practice does not sanction using ice directly in contact with milk or cream. Then, too, water is largely purified in the process of freezing, although if secured from a polluted source, reliance should not be placed in this method of purification, for even freezing does not destroy all vegetating bacteria.

The ordinary house fly is an important source of contamination in creameries, cheese factories and city milk plants. They are of importance not only in increasing the number of fermentative bacteria in milk but they may serve to contaminate it with disease-producing organisms. The windows of all places where milk is handled, whether on the farm or elsewhere should be screened.

It should be kept in mind in the handling of milk and other dairy products that human food is being prepared and that cleanliness is desirable from every point of view, and that the methods of handling and production should compare with those used in the preparation of foods which like milk cannot be cleaned when once polluted. Desirability, keeping quality, healthfulness and the value of every product made from milk depends upon the extent and amount of contamination.

CHAPTER IV.

INFECTION OF MILK WITH PATHOGENIC BACTERIA.

That the disease-producing, or pathogenic bacteria, are able to infect milk supplies is shown by the fact that numerous epidemics of contagious disease have been directly traced to milk infection. Milk is generally consumed in a raw state, and as a considerable number of this class of organisms are able not only to live but actually grow in milk, which is such an ideal culture-medium for the development of most bacteria, it is not surprising that disease processes should be traced to this source. The organisms in milk capable of causing disease do not alter or change its physical properties sufficiently to enable their presence to be detected by a physical examination.

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 udder. 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 live a saprophytic existence, growing in milk, as in any other nutrient medium, 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 milk supply is tuberculosis, but in addition to this, foot and mouth disease (aphthous fever in children), Malta fever, and acute enteric troubles have also been traced to a similar source of infection.

The most important specific diseases that are disseminated through subsequent infection of the milk are typhoid fever, diphtheria, scarlet fever, and cholera, but, of course, the possibility exists that any disease germ capable of living and thriving in milk may be spread in this way. In addition to these diseases that are caused by the introduction of specific organisms (the causal organism of scarlet fever has not yet been definitely determined), there are a large number of more or less illy defined troubles of an intestinal character that occur especially in infants and young children that are undoubtedly attributable to the activity of micro-organisms 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.

Tuberculosis. This disease is by far the most important bacterial malady that affects man and beast. In man, it assumes a wide variety of phases, ranging from consumption, tuberculosis of the lungs, which is by far the most common type, to scrofulous glands in the neck, cold abscesses, hip-joint, and bone diseases, as well as affection of the bowels. These various manifestations are all produced by the inroads of the specific organism, Bacillus tuberculosis. The bovine, as well as swine, fowls, and other warm-blooded animals, are also affected with similar diseases. In man, the importance of the malady is recognized when it appears that fully one-seventh of the human race die of this scourge. In cattle, the disease is equally widespread, particularly in those countries where live stock has been intensively developed. In the northern countries of Europe, such as Denmark, Germany, England, France, and the Netherlands, as well as in Canada, and this country, this disease has been most widely disseminated. This has been occasioned, in large measure, because of the exceedingly insidious nature of the disease in cattle, thereby permitting interchange of such diseased stock without the disease being recognized. Tuberculosis is found more abundantly in this country in dairy than in beef stock. Dairy cattle are, however, not more susceptible, but the closer environment in which milch cattle are kept, and the fact that there has been greater activity in the matter of introducing improved strains, accounts for the larger percentage of affected animals.

It has been a disputed question for some years whether the organisms producing bovine and human tuberculosis are identical or from the practical standpoint, whether the bovine type of disease is transmitted under natural conditions to man. The bacteriologist can readily detect differences in appearance, in growth of cultures, and in disease-producing properties between the two strains. Of the two, the bovine is much the more virulent when inoculated into experimental animals. In a considerable number of cases, record of accidental infection from cattle to man has been observed. These have occurred in persons making postmortem examination on tuberculous animals, and the tubercular nature of the wound proven by excision and inoculation.

More recently, since the agitation by Robert Koch of Germany, a number of scientific commissions have studied particularly the problem of transmission. It is now estimated that perhaps seven per cent of the tuberculosis in man is of bovine origin. This is almost wholly confined to children. The portions of the body that become diseased, when the infection has resulted from the use of milk, are the glands of the neck and of the abdomen.

Manner of infection in man. In the main, the source of the malady may be traced either to air infection or to the food, if one disregards the comparatively small number of cases of wound infection. Air is frequently a medium by which the germ is transferred from one person to another. The sputum is exceedingly rich in tubercle bacilli and since this material is carelessly distributed by tubercular people, the air of the cities, villages and public buildings will frequently contain tubercle organisms. Some of the organisms in the air find their way into the lungs, there to develop and produce consumption. The organisms in the air may be deposited in the nasal passages and throat, and ultimately find their way into the tissues of the body by penetrating the walls of the throat or of the intestine. It is probable that the tubercle bacilli thus introduced may find their way to the lungs and there develop without leaving any trace of their path.

Food may also possibly serve as a medium of infection. The contamination of solid food from flies and other sources is, of course, a possibility, but tuberculous meat from cattle and swine is much more likely to occur, although it must be said that the processes of preparing such food for use (roasting, frying, and boiling) are sufficient to destroy the vitality of the causal organism. The fact that most food products of this character are now inspected renders this possibility less likely to occur.

Unquestionably, the likelihood of ingesting tubercle organisms is much greater with milk than with any other food supply, as milk is consumed usually in an uncooked state, and as microscopic and physiologic tests indicate that not infrequently milk from tuberculous animals contains these organisms.

Distribution of the disease in animals. As practically any organ of the body may be affected with tuberculosis, it naturally follows that the lesions of this disease are widely distributed. The disease germ is introduced, in the main, through the lymph and not the blood system; consequently, in the initial stages the evidence of tuberculosis is often comparatively slight, and the lesion is restricted in its development. Where such a condition obtains, it is known as "closed," in contradistinction to "open" tuberculosis, where the diseased tissue is more or less broken down and is discharging into the circulation, or elsewhere. Manifestly, the danger of spreading not only in the affected animal itself, but to the outside, is much greater in the case of the open lesion. Especially is this true where the disease is present in the lungs or organs that have an exterior opening so that the material containing the organisms is discharged from the body in the sputum, manure, urine or milk. The intestines themselves are rarely affected, but the lymph glands associated with the intestinal tract are not infrequently involved.

Infection of milk with tubercle bacilli. In a small percentage of cases, the udder itself becomes involved. Where this condition obtains, one or more hard lumps are formed, which slowly increase in size, usually being restricted to one quarter of the udder. Sometimes the affected quarter may develop to an enormous size, producing a hard, painless tumor. Not often does the affected tissue break down into pus; consequently, no abnormal appearance is to be noted in the milk secretion until the disease has made very extended progress, in which case the percentage of fat generally diminishes. Whenever the udder shows physical manifestation of this disease, the milk almost invariably is rich in tubercle bacilli.

Tubercle organisms may also appear in milk of animals in which no physical symptoms of the disease are to be found. This fact has been demonstrated by microscopic and animal experiments, but it is also abundantly confirmed by the frequent contraction of the disease by calves and hogs when fed on factory by-products. This latter class of animals is particularly dangerous, because there is no way in which the danger can be recognized.

Fig. 17.—a Tuberculous Animal.
The animal appears perfectly healthy although she has had the disease for five years.

It has also been proven that milk may become infected through the feces. In coughing up material from the lungs and associated glands, the matter is swallowed, instead of expectorated, as in man. The organisms retain their vitality in the intestine, and are voided in the feces. Under ordinary conditions, the flanks and udder become more or less polluted with such filth, and the evidence is conclusive that infection of milk is not infrequently occasioned in this way. The fact that hogs following tuberculous steers in the feeding lots are very likely to acquire the disease is explained by the presence of tubercle organisms in the manure of such animals.

Fig. 18.—a Tuberculous Animal.
The last stages of generalized tuberculosis. Note the emaciated condition.

It must be kept in mind that many animals may be infected with tubercle bacilli and therefore have tuberculosis in the incipient stages, without their being able to disseminate the disease to others. In the early stages, they are bacillus-carriers without being necessarily dangerous at that particular time, but the possibility always exists, as the disease develops in the system, that the trouble may assume a more formidable character, and that slowly developing chronic lesions may become acute, and "open," in which case, the affected animal becomes a positive menace to the herd. As the time when the lesions change from the "closed" to the "open" type and the animal becomes a source of danger cannot be determined, the only safe way to do is to exclude the milk of all tuberculous animals from the general supply, whether for direct consumption, or for manufacture into dairy products and to look upon every diseased animal as a menace to the herd. This is rendered all the more necessary when the milk is used for the feeding of children, who are relatively more susceptible to intestinal infection than the adult. The early stages of the disease in cattle are, however, so insidious that no reliance can be placed upon the detection of the malady by physical means. Fortunately, in the tuberculin test, a method is at hand, which in a simple, but effective manner, enables the disease to be distinguished in even the early stages, long before recognition is possible in any other way.

Tubercle bacilli in dairy products. When infected milk is used for the preparation of butter and cheese, the organisms inevitably are incorporated in them. In the separation of milk a relatively large part of the tubercle organisms in the milk appear in the cream. In the making of cheese even more of the organisms are held in the curd. In butter and cheese, as in milk, no growth of the organism can take place; however, the vitality of the organism is retained for a considerable number of months. It is not believed that these products are of much importance in the spread of tuberculosis in the human family, since they are not consumed by children to any extent. Cream is to be considered as a means of distribution since it is often used by children.

Treatment of tuberculous milk. It is easily possible to treat milk or factory by-products so as to render them positively safe. The process of pasteurization or sterilization is applicable to whole milk, and when effectively done destroys entirely the vitality of any tubercle bacilli. In making such exposure, care should be taken to prevent the formation of the "scalded layer," as the resistance of the organism toward heat is greatly increased under these conditions. In a closed receptacle, 140° F. for 15 to 20 minutes has been found thoroughly effective in destroying this organism. A momentary exposure at 176° F. is likewise sufficient. This is the method that is almost universally used in Denmark in the manufacture of the finest butter.

In the treatment of factory by-products, heat should also be employed. In Denmark, compulsory pasteurization at not less than 176° F. is required. This treatment prevents not only the dissemination of tuberculosis among hogs and young cattle, but is equally efficacious in preventing the spread of foot and mouth disease.

The per cent of tuberculous milch cows varies widely in different sections of the country, being greatest in the older dairy sections, and in those supplying milk to the cities, on account of the constant buying and selling of animals, thus giving more frequent opportunity of introducing the disease into the herds. Throughout the country at large, probably less than ten per cent of the cows are tuberculous, and it is estimated that at least one per cent of the diseased animals have tuberculous udders. It has been suggested that the dilution of the milk of such animals with that of healthy cows would remove a great part of the danger from milk. In the case where the milk of a large number of herds is mixed, this may be of some importance, but in no case is it safe to assume that dilution of the milk of tuberculous cows is any guarantee of safety.

It has been shown that milk, perfectly normal in appearance, coming from a tuberculous udder could be diluted a million times and still produce the disease on inoculation into experimental animals. In the case of swine, the susceptibility is so great that a single feeding of infected milk, even in a very dilute condition, causes with certainty the production of the disease.

Some observers maintain that the contamination of the milk with the manure of tuberculous animals is of greater hygienic importance, than that coming from diseased udders, since the number of animals having tuberculosis of the lungs and intestines is far greater than those with diseased udders.

Economic aspects of bovine tuberculosis. Not only is this disease invested with much importance because of its inter-relation with the human, but from an economic point of view alone, it is undoubtedly the greatest scourge that affects the dairyman. Its insidiousness makes it exceedingly difficult to recognize. The consequence is that many fine herds become seriously involved before its presence is recognized. In the main, the disease is introduced into a herd by purchase, often by buying in pure-bred stock to improve the quality of the herd. Where the disease has been established in a region for some time, there is also danger that unheated factory by-products, as skim milk and whey, may function in its spread. Where such conditions prevail, the spread of the disease in the creamery district is exceedingly rapid. When once introduced into a herd, the disease sooner or later spreads from the originally affected animal to others in the herd. Close contact, and close confinement in ill ventilated stables facilitate the spread of the disease, and sooner or later, other animals acquire the trouble. This may all occur while all animals appear in a healthy condition.

The symptoms of the disease in the earlier stages are quite indefinite. As the disease progresses, the nutritive functions appear to be disturbed, and sooner or later, the body weight begins to decline, and finally marked emaciation ensues. Accompanying this condition, especially when the disease is in the lungs, is a cough, which is generally aggravated with active exercise. While the run-down condition permits frequently of the detection of the disease in the advanced stages, it is wholly impossible with any accuracy to diagnose the trouble in the incipient stages. It is at this stage that the tuberculin test comes to the aid of the stockman.

Tuberculin test. This test is made by the injecting beneath the skin of the animal a small quantity (about 2 c. c.) of tuberculin, and noting the temperature of the animal, before and after the injection. Tuberculin, a product of the growth of the tubercle bacillus, when injected into the body causes a marked rise in temperature, in the case of an animal affected with the disease, and no such elevation in the case of a healthy animal. The process of preparing tuberculin makes it absolutely free from danger, so far as liability of producing the disease, or in any way injuring the animal, is concerned. Fig. 19 shows the temperature range of both reacting and non-reacting animals. While the test is not absolutely infallible, it is so far superior to any and all other methods of diagnosis that it should take precedence over them.

Miscellaneous diseases. There are a number of diseases that affect both human beings and cattle, the causal organisms of which may be transmitted through the milk. Foot and mouth disease is one wide spread in European countries but which has not yet gained a permanent foothold in this country. The ingestion of the milk, which always contains the causal organism, produces the disease in both humans and cattle. In the human the disease is very similar to that in cattle; it may end in death. Vesicles are produced in the mouth, on the lips, nose and fingers. The causal organism, which has not yet been demonstrated, may occur in butter or cheese. It is easily destroyed by pasteurizing the milk.

Fig. 19.—Temperature Curves.
1, the temperature curve of a healthy animal after injection with tuberculin; 2 and 3, the temperature curves of tuberculous animals after injection with tuberculin. (After Moore.)

Anthrax, actinomycosis (lumpy jaw), rabies, and malta fever are diseases the organisms of which have been found in the milk of affected animals. In case of the first three, while the possibility exists of the infection of human beings by milk, it is improbable that such infection does normally occur. Malta fever is becoming an important disease in portions of southern Europe. It is produced in man by the use of milk of goats suffering from the disease.

The organism causing contagious abortion in cattle is known to be present in the milk of the infected animal at the time of its withdrawal from the udder. It is not probable that the organism is of any sanitary significance as far as man is concerned. It has been shown that the organism is able to produce a disease in guinea pigs on artificial inoculation that is very similar, so far as the lesions are concerned, to tuberculosis. It is also probable that the by-products of creameries and cheese factories may serve to spread the disease from one herd to another.

Inflammation of the udder (garget) is a frequent trouble in every herd. It is marked by the swelling of one or more quarters, by the appearance of fever and changes in the appearance and composition of the milk. The inflammation may be caused by cold or injury, or by the invasion of the udder with pus-forming bacteria. In the first case the trouble is not likely to persist for any length of time, and does not spread to other members of the herd. The milk may be more or less stringy, and may show a slimy flocculent sediment. It cannot be asserted that such milk is harmful to man but it should be rejected on general sanitary grounds, and because it cannot always be differentiated from that coming from an udder in which the inflammation is produced by bacteria.

Inflammation caused by the invasion of the udder with specific bacteria is usually of greater severity, the entire gland often becoming involved. The secretion of milk may cease and the function of the diseased quarters may never be restored. The milk in the less severe cases may not be abnormal in appearance, but with increasing severity, the nature of the milk changes, until it may be a watery liquid. The milk of any animal suffering from any form of garget should be rejected, as it may cause trouble, especially in children. There is some reason to believe that organisms coming from cases of garget have been responsible for the extensive outbreaks of septic sore throat that have occurred in some parts of the country.

The milk of animals suffering from indigestion, diarrhea, abscesses on any part of the body, as from those which have retained the afterbirth should be likewise rejected. In short only the milk of healthy animals should be used for human food; that from any animal suffering from any disease or which is receiving medical treatment should not be so used.

Typhoid fever. The most important disease germ, distributed through the medium of milk, that is unable to produce a diseased condition in the cow is the organism of typhoid fever. This malady is an intestinal affliction of man, and the germ causing the same is found abundantly in the dejecta, both solid and liquid, as well as in the blood in certain stages of the disease. While the causal organism does not leave the body through the expired air, it is found abundantly in both the urine and feces. Therefore, the dejecta, and any articles that may be soiled with the same become a positive menace.

Many different methods of transmitting the contagion exist, such as water, food infected in various ways, contact with infected persons, and through the medium of milk. Milk is not so frequently the cause of dissemination as the other factors, but where milk supplies become contaminated, epidemics of considerable magnitude are wont to occur. The danger from milk is also aggravated by the fact that the typhoid bacillus 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. In a considerable percentage of cases, the disease is not sufficiently severe to cause the patient to take to his bed. These so-called "walking typhoid" cases are particularly dangerous, because they serve to spread the disease organism more widely.

A very considerable proportion of the people that recover from typhoid fever still continue to harbor the typhoid bacillus in their urinary and gall bladders. This condition may obtain for years, and since such individuals are in perfect health and are ignorant of their own condition, and since they give off the organisms more or less constantly, they are often the cause of extensive milk borne epidemics. Such persons are known as "typhoid carriers" and constitute one of the gravest problems the public official has to contend with in his struggle to prevent the spread of typhoid fever.

Where outbreaks are caused by milk, they can readily be traced by means of the milk route, as there are always a sufficient number of susceptible persons, so that outbreaks of epidemic proportions develop. In the Stamford, Conn., outbreak in 1895, 386 cases developed on one milk route. In this case it was shown that the carrying cans were thoroughly washed, but were later rinsed out with cold water from a polluted shallow well.

The mode of infection of milk varies, but in general, the original pollution is occasioned by the use of infected water in washing the utensils, or a case of "walking typhoid" or bacillus carrier, who directly infects the milk. In case of sickness in rural families, some member of the household may serve in the dual capacity of nurse and milkmaid, thus establishing the necessary connection. Busey and Kober report twenty-one outbreaks, in which dairy employees also acted in the capacity of nurses. The fact that the urine of a convalescent may retain the typhoid germ in large numbers for some weeks renders the danger from this source in reality greater than from feces, as, naturally, much less care is exercised in the disposition of the urine.

The house fly is now regarded as one of the important means of spreading typhoid fever, indeed it is often called the "typhoid fly." The infectious material deposited in an open vault may serve as a source from which the fly carries the organisms to milk and other foods in the house or elsewhere. The protection of vaults and the screening of every place where human food is handled or prepared is the only protection.

It should be emphasized that in the case of the tubercle organism, no growth ever occurs in milk, but with the typhoid bacillus growth is possible. It thus needs but the contamination of the milk with the smallest particle of material containing them to seed the milk. By the time it is consumed it may contain myriads of the disease-producing organisms.

Diphtheria. This is a highly infectious disease, affecting children primarily and is characterized by the formation of membranous exudates in the throat and air passages, which are teeming with the causal organism, the diphtheria bacillus. This organism is capable of forming highly toxic products, and it is to the effect of these poisons that its fatal result is generally due. The organism is thrown out from the body, in the main, through the mouth, the surroundings of the patient being infected directly from the air, and indirectly, by contact with polluted hands, lips, etc. Thus, the germ deposited from the lips of a case of the disease, on the common drinking cup, slate, lead pencils, toys, and the like, may easily pass from child to child. Not infrequently, the causal organism persists in the throat long after all evidence of membranous growth has subsided, and so the child itself may act as a "bacillus carrier."

Not so many epidemics of diphtheria as of typhoid have been traced to milk, but the evidence is sufficient to indict milk as a disseminator of contagion. In several cases, the diphtheria germ has actually been isolated from infected milk supplies. Actual growth of the diphtheria germ is said to take place in raw milk more rapidly than in sterilized.

Scarlet fever. While the germ of scarlet fever has not yet been isolated, and therefore its life history in relation to milk cannot be depicted so accurately, yet milk-borne epidemics of this disease are sufficiently abundant to leave no doubt but that this food medium may sometimes serve as a means of disseminating such troubles. Infection of the milk doubtless comes in the case of this disease from direct contact with a person suffering from the malady.

Cholera. While this disease is of no practical importance in America, owing to its relative infrequency, yet outbreaks of cholera have been traced to milk, in spite of the fact that the causal organism is more sensitive to the action of acids than most disease-producing bacteria. In several outbreaks in India, milk has been the medium through which the disease was spread. Generally, infection of the milk has been traced to the use of polluted water.

Children's diseases. An exceedingly high mortality exists among infants and young children in the more congested centers, especially during the summer months. In the main, the cause of these troubles is due to intestinal disturbances, and unquestionably, the character of the food enters largely into the problem. As milk constitutes such a large proportion of the diet of the young, and is so susceptible to bacterial invasion, it would appear probable that much of the trouble of this character is due to the condition of this food supply. This is rendered more probable when it is remembered that bottle-fed infants suffer a much higher mortality than breast-fed children, due probably to the fact that the lengthened period between the time the milk is drawn and consumed permits of abundant bacterial growth. Much carelessness also prevails among the poor in cities, relative to the care of utensils used in feeding children. Nursing bottles often serve to infect the milk. Where milk is pasteurized, or properly heated, it has been found that the mortality rate has been greatly reduced, thus indicating that the condition of the milk was directly responsible for the death rate. In fact, the mortality from these indefinite intestinal troubles probably exceeds that from all of the specific infectious diseases combined. Improved care in handling this sensitive food supply will do much to better conditions in this direction.

Ptomaine poisoning. Acute poisoning affecting adults as well as children, not infrequently occurs from the use of foods of various kinds. Cases of poisoning arising from the use of shell fish, canned meats, ice cream, cheese, and other dairy products, are from time to time reported. These troubles are due to the production of toxic compounds, in the main, probably caused by bacterial decompositions. Often such troubles may affect a number of persons, as at banquets and such gatherings, thereby giving the semblance of an epidemic. While such troubles are doubtless to be ascribed to bacterial activity, they are not transmissible from person to person.

In the case of troubles arising from ice cream and such confections, the probable cause is due to the storage of milk or cream under refrigerator conditions, where germ growth can go on in the product, and yet the temperature be sufficiently low to prevent the usual acid fermentations.

CHAPTER V.

FERMENTATIONS OF MILK.

Milk, under normal conditions, is always contaminated with bacteria coming from the most varied sources. If it is produced under clean conditions, the number of bacteria will be small, but in any case, the number of kinds of bacteria that find their way into milk will be large. Many of them find in milk at ordinary temperatures suitable conditions for growth; they use a portion of some of the constituents of the milk as food, producing certain other compounds that are known as "by-products." These by-products impart to milk a taste and odor that is not found in fresh milk. The effect of the action of bacteria may also be made evident by the change in the appearance of the milk. When these various changes become evident to the senses, either by taste, smell or sight, the milk usually is so modified as to be unfit for many ordinary purposes. The preservation of milk, a subject to be treated later, is a study of the ways of preventing or retarding the growth of bacteria in milk, and thus delaying the time when evidences of their action first become apparent.

Each class of bacteria produces more or less specific changes in the milk as a result of their growth. Certain bacteria are of the greatest benefit to the butter and cheese maker, while others are distinctly harmful to the manufacturer of dairy products. The changes produced by the different bacteria are called "fermentations" of milk, each being most commonly named from the most important by-product formed.

Acid fermentation of milk. Fresh milk has a sweet taste and little or no odor, but if it is allowed to stand at ordinary temperatures, it sours; the taste is no longer sweet because the sweetness of the sugar of the milk is masked by the acid produced from the decomposition of a portion of the sugar by the bacteria. The change in odor and taste of milk is apparent long before the appearance is altered and increases in intensity as the acid-fermentation progresses. The first alteration in appearance is most usually one of consistency; the liquid milk is transformed into a semi-solid mass. The terms "curdling" and "sour" are usually synonymous. Milk is, however, often said to be sour as soon as the acid fermentation has progressed to a point where it is evident to taste or smell. This process of souring, or the acid fermentation is so common a change that raw milk which does not show this type of fermentation is looked upon with suspicion, and, usually, justly so. The process in the past was thought to be something inherent in the milk, a natural and inevitable change. It is now known that this is not so, but that it is due to certain kinds of bacteria, and that if these are prevented from getting into milk, it will not sour, but will undergo some other less desirable type of decomposition.

The acid-forming bacteria comprise but a very small part of the total number of organisms that find their way into the milk during its production on the farm, yet in sour milk scarcely any other kinds of bacteria can be found. At ordinary air temperatures, the acid-forming bacteria grow more rapidly in milk than do any other forms, and the acid produced by them renders the milk an unfavorable medium for the growth of other bacteria. This is the reason why milk practically always undergoes the acid fermentation, although it is contaminated with a host of other kinds of bacteria. If a mixture of seeds is sown on low wet ground, certain kinds will grow best; if the same mixture is sown on drier land, other types will find most favorable conditions for growth, and the plants which appeared on the low land will not appear. The same condition is found in milk where the environment is most favorable for the acid-forming bacteria.

Amount of acid formed in milk. In this country the acidity of milk is expressed as so many per cent of lactic acid. A milk that shows an acidity of one per cent should, theoretically, contain one pound of lactic acid in each one hundred pounds of milk. The acid determined does not actually represent lactic acid, as there are other substances in milk which act as acids, with the reagents used in the present methods of determining the acidity of milk. For instance, perfectly fresh milk has an apparent acidity of 0.13 to 0.18 per cent, although no fermentation has occurred. Other acids than lactic are formed in the acid fermentation, but the entire acid content is referred to as lactic when speaking of the acidity of milk. When the developing acidity of milk reaches 0.25 to 0.3 per cent, a sour taste becomes evident and the milk will curdle on heating. When the acidity increases to 0.6 to 0.7 per cent, the milk curdles at ordinary temperatures. The acidity continues, however, to increase until it reaches about 1 per cent, which is the maximum amount that will be produced in milk by the ordinary acid-forming bacteria. Milk contains about 4 per cent of milk sugar, all of which is fermentable. If this were all decomposed by bacteria, the acidity of the milk would actually exceed 4 per cent. It is thus evident that the reason why more acid is not formed in milk is not because of any lack of sugar. The bacteria, like all other kinds of living things, are injured by their own by-products, unless these are constantly removed in some way; in milk the bacteria cannot escape the action of the acid which they themselves have formed, consequently growth ceases. The amount of acid formed is dependent on the kind of bacteria present and on the composition of the milk. Certain bacteria will not produce enough acid to cause the curdling of the milk; still others will form 2 or even 3 per cent. These types, however, do not play any important part in the spontaneous souring of milk.

In milk the acid first formed combines with the ash constituents and the casein to form salts which do not seriously affect the growth of the bacteria. Ultimately, the limit of the ash and casein to take up acid is reached, and free lactic acid which is harmful to bacterial growth appears. If the content of casein and ash constituents is high, a higher degree of acidity will be reached than in a milk with a lower content. If a large part of the volume of the milk is made up of a compound that has no role whatever in the acid fermentation, such as the butter fat in cream, the amount of acid formed per unit volume of milk will be reduced, since in determining the acidity, a definite volume of milk is taken, and the acidity is expressed, as such a per cent of this amount.

Types of acid-forming bacteria. When substances undergo decomposition, it is a common belief that compounds offensive to the odor and taste are formed; but such is not necessarily the case. The products of the decomposition may be as agreeable and as harmless as the compounds decomposed. Whether the decomposition products of any substance are offensive or not is dependent on the kinds of micro-organisms acting on it. There are forms of acid-producing bacteria that change milk in odor, taste, and appearance, yet the sour milk is not offensive in any sense of the word. Other bacteria also sour the milk, but produce offensive odors and a disagreeable taste. Thus, the acid-forming bacteria may be divided into two main groups, which may be designated as desirable and undesirable. This division is of importance to the butter and cheese maker and to the consumer of milk.

Desirable acid-forming bacteria. If milk is produced under clean conditions, it is not likely to have a disagreeable odor or taste at any time, even when it is sour; rather the taste is agreeable like that of good butter milk. The curd is perfectly homogeneous, showing no holes or rents, due to the development of gas, and there is but little tendency for the whey to be expressed from the curd. This type of fermentation is largely produced by the group of bacteria to which has been given the name, Bacillus lactis acidi.

The main by-product of this group of bacteria is lactic acid; small amounts of acetic acid and alcohol, with traces of other compounds, are also formed. The agreeable odor and to some extent the flavor of milk fermented by these bacteria is due to other by-products than lactic acid, for this has no odor and only a sour taste. The acid fermentation of milk is often called the lactic acid fermentation. In reality only the fermentation produced by the desirable group in which lactic acid is the most evident by-product should be thus called.

Fig. 20.—Different Types of Curds.
On the left a solid, homogeneous curd produced by desirable bacteria; on the right, the curd produced by harmful bacteria. Note the gas holes and free whey.

The bacteria of this group may enter the milk from the dust coming from the coat of the cow. They are also found in the barn dust and on cultivated plants. Under ordinary farm conditions, the larger part of those found in milk come directly from the utensils. If the milk is drawn under extremely clean conditions and care is taken to sterilize the utensils, but few acid-forming bacteria of any kind will enter the milk; under such conditions most of the acid-forming bacteria will belong to the group in question. They find, however, such favorable conditions for growth in milk that they develop more rapidly than most other types with which milk becomes seeded; consequently under normal conditions, they gain the ascendency and so control the type of fermentation.

The desirable type of acid-forming bacteria do not form spores; hence, are easily killed by heating the milk. They can grow in the presence or in the absence of free oxygen. In the bottom of a can of milk or in the middle of a cheese, there is no air, yet these bacteria grow as well under these conditions, as in milk exposed to the air. The range of temperature for growth varies from 50° to 100° F. but development is most rapid at 90° to 95° F. and about 1 per cent of acid is formed.

Another group of bacteria which may be classed among the desirable acid-forming organisms is constantly found in milk. They have little to do with the ordinary acid fermentation as they grow very slowly at ordinary temperatures. If a sample of raw milk is placed at the temperature of the animal body, the acidity will reach 1 per cent in a few hours. Thereafter the acidity will increase slowly and may reach three per cent or above. The continued increase in acid is due to the growth of long rods of the Bacillus Bulgaricus type, which apparently enter the milk with the fecal matter. The nature of the change produced by them in milk is very similar to that caused by Bact. lactis acidi in that lactic acid is the chief product; no gas is produced and hence the curd is uniform in appearance. Temperatures from 100° to 110° F. favor their development. Organisms belonging to this group are used in the preparation of the fermented milks now so widely sold in the cities.

These desirable, acid-forming bacteria are of the greatest service in every branch of the dairy industry, whether in butter or in cheese making, or in the sale of milk in the city. The dairy industry is dependent upon fermentative activity, as much as the manufacture of beer or wine, and the main basis of this is the acid fermentation of the milk by these desirable types of bacteria.

Although milk contains a large amount of nitrogenous substances (casein and albumen), it does not undergo putrid decomposition, as do meat and eggs, not because it is not fitted for the growth of the bacteria causing that type of change, but because the acid formed in it stops the growth of the putrefactive bacteria. If a sample of milk is placed in a stoppered bottle, it will have much the same taste and odor at the end of several months as at the end of a few days. The acid acts as a preservative, like the vinegar in pickles, or the acid in silage and in sauerkraut. Meat placed in a stoppered bottle which is then filled with milk will be preserved.

The products formed in the decomposition of meat and eggs are not only offensive but may also be injurious to the health of the consumer. Milk that has been fermented by the desirable kinds of acid-forming bacteria is not harmful. It is consumed in a variety of forms (buttermilk, cottage cheese) as a common article of food and its use is rapidly increasing. The preparation of the pure culture buttermilks or artificially soured milks that are now so frequently recommended for digestive troubles rests upon an acid fermentation of this type.

Undesirable acid-forming bacteria. Other types of bacteria capable of forming substances that impart to milk an offensive odor and a disagreeable taste not infrequently appear instead of the desirable group. Instead of producing from the sugar of milk large quantities of lactic acid, these types generate other acids, such as acetic and formic, which impart a sharp taste to the milk. Besides the acids the bacteria of this group form gases from the sugar of the milk. Some produce small amounts of gas; others so much that the curd will be spongy and will float on the surface of the whey. The fermentation caused by them is often called a "gassy fermentation" and is dreaded by butter and cheese makers since the gas is indicative of bad flavors that will appear in the product. Gas may also be produced in other types of fermentations to be discussed later.

This class of bacteria enters the milk with the dust, dirt, and manure, in which materials they are especially abundant. No spores are formed; hence they are easily killed by heating the milk. They grow both in the presence and in the absence of free oxygen. High temperatures favor their growth, most rapid development taking place at 100° to 103° F.

Spontaneous fermentation of milk. The normal souring of milk is due to a mixture of these two groups of bacteria. The relative proportions existing between the two in any sample of milk is dependent on a number of factors, most important of which is the degree of cleanliness exercised in the production of the milk. Where careless conditions obtain under which dust and manure particles find their way into milk, it becomes more abundantly seeded with gas-generating bacteria, and consequently, the type of fermentation is undesirable. If, however, the milk is drawn into clean utensils and care is taken to exclude dirt, the pure lactic acid types are able to control the character of the changes produced, and a clean, pleasant tasting liquid results. It will be seen that things are well arranged by nature; one of the most important food products undergoes a type of decomposition that is not offensive and when produced under clean conditions, the sour milk is as healthful a food as is the fresh product. Thus there is every reason for cleanliness in the production of milk, for cleanliness' sake and because clean milk means better products, and greater returns to everyone, producer and dealer.

There are other kinds of acid-forming bacteria in milk but they are of small importance compared with those just discussed. Some of the bacteria derived from the inside of the udder of the cow form acid, but these forms grow very slowly in milk at ordinary temperatures, and have no influence on the keeping quality.

Fig. 21.—Different Types of Curds.
The flask on the left shows the soft curd produced by the bacteria that curdle the milk without the production of acid. The flask on the right shows the gassy curd formed by butyric acid bacteria in heated milk.

Sweet curdling fermentation of milk. Samples of milk are sometimes found that are curdled, but which do not taste sour, or have the normal odor of sour milk. The curd is usually soft and the taste bitter. It is evident that the curdling cannot be due to the same factors as in the normal souring of milk. Such a change is similar to the action of rennet which is used to curdle the milk in cheese making. This ferment will curdle perfectly sweet milk, producing a curd that looks like that formed in the acid fermentation of milk. The cause of these sweet curdling milks, which appear from time to time, is due to the introduction of certain bacteria which have the power of secreting an enzyme resembling that found in rennet. In such cases the milks curdle prematurely especially when warmed. The curd may gradually disappear, for the bacteria also produce another enzyme that digests the curd, and thus renders it soluble. When this advanced phase becomes evident, it is often called the digestive fermentation of milk. This change is produced largely by putrefactive bacteria of various kinds that find their way into milk with dust and dirt. Many of them are spore formers; hence, are not killed when milk is heated, as in pasteurization, while the acid-formers are destroyed. Pasteurized milk is thus likely to undergo the sweet-curdling fermentation, if it is kept for any length of time. Raw milk rarely undergoes this type of decomposition, since the rennet-forming bacteria under ordinary conditions are unable to develop in competition with the acid-forming bacteria.

Butyric acid fermentation of milk. A fermentation that is much less frequently noted than the two previously discussed is known as the butyric fermentation, since butyric acid is the principal by-product. The causal bacteria cannot compete with the ordinary acid-forming bacteria in raw milk; hence it is most frequently noted in pasteurized milk, since the organisms produce spores and are not killed by the heating. Pasteurized milk under the action of the butyric acid bacteria undergoes a gassy fermentation, developing a pronounced acidity and the disagreeable odor of butyric acid, which resembles that of rancid butter. The butyric acid bacteria are anaerobic, and thus can grow in butter and cheese away from the air.

Slimy or ropy fermentation of milk. A slimy or ropy condition of milk is frequently noted on the farm and in the dairy. Several causes for this abnormal condition exist. Sometimes the milk may be slimy when milked from the cow. This occurs most frequently in the case of inflammation of the udder which may or may not be due to bacteria. The direct cause of the abnormal condition in milk is the presence of fibrin and white corpuscles from the blood which form masses of slimy material; in such cases the trouble does not increase in intensity with age, nor can it be propogated by transference to another sample of fresh milk.

Fig. 22.—Slimy Milk.
It does not mix with water when poured into it.

Another type of slimy milk is produced by the growth of certain types of bacteria which enter the milk after it is drawn from the udder. These may come from various sources. The bacteria concerned belong to two groups: (1) those that grow best in the air and do not form acid; (2) those that grow in the absence of air, throughout the entire mass of milk and which form acid. The slimy condition is noted in the milk only after the milk has been stored for some time; it usually increases with the age of the milk and can be produced in a second sample by transferring a little of the slimy milk to it.

The fermentation produced by the aerobic bacteria is most often met in bottled milk and cream during the warmer times of the year. On account of their relation to oxygen, the growth is confined to the surface of the milk and only the upper layer becomes slimy; thus when the cream is removed, the abnormal condition is noted. The sliminess is due to the mass of bacterial growth rather than to the production of any specific substance in the milk. This trouble may be of considerable economic importance to the dealer, as such abnormal milk is objectionable for ordinary use, but as far as is known, it is incapable of affecting the health of the consumer.

In numerous outbreaks of this trouble the source of contamination has been traced to infection from well water or a stream, as the organisms causing the trouble are found naturally in water. Keeping the milk in a tank in the pump house sometimes permits of troubles of this sort, the water used for cooling giving opportunity for contamination. Cattle wading in a stream sometimes pollute their udders and so indirectly infect the milk. Such outbreaks rarely persist for any considerable length of time as the common acid organisms soon regain the ascendency.

Creameries and cheese factories are sometimes troubled with sliminess in starters. This seems to be due to some change which the ordinary lactic acid bacteria undergo on long propagation rather than to contamination of the starter. There are, however, types of acid-producing bacteria that are able to form specific substances in milk that are slimy in character. Two of these forms of slimy milk are of economic importance. The slimy whey (lange Wei) of Holland is added to milk in the manufacture of Edam cheese, apparently serving the same purpose as the addition of the pure culture starter in cheddar cheese making. In Norway, a sour, slimy milk (taettemjolk) is used as food. It is produced by the addition of some previously fermented milk. This beverage is also used in some of the Norwegian settlements of Wisconsin, the original seed having been brought from Norway, and the bacteria maintained by constant propagation from one sample of milk to another. The milk has the odor and taste of butter milk, but is not especially appetizing in appearance to any one not accustomed to it; it is, however, as harmless to health as is any other form of sour milk. It is not known that any of these forms of slimy milk are distinctly harmful to the quality of butter or cheese.

Alcoholic fermentation of milk. The bacteria as a class are incapable of producing alcohol in appreciable amounts. The alcoholic beverages, beer, wine, and cider, are produced by the growth of yeast, in such sugar containing liquids as fruit juices, extracts of grains, etc. The common types of yeasts are incapable of acting on milk sugar, but they can ferment glucose, maltose, and cane sugar, forming equal amounts of alcohol and carbonic acid gas, which causes the effervescence of fermented and carbonated drinks. There are, however, some types of yeasts found in milk and its products that are able to ferment milk sugar.

All yeasts grow best in an acid medium, hence those fermenting milk sugar find suitable conditions for growth in sour milk or whey. They may at times become of economic importance in the cheese industry, because of the contamination of the milk with large numbers of them. The arrangement of the whey vat is often such that it cannot be completely emptied and cleaned; the sour whey thus presents favorable conditions for the growth of the lactose-fermenting yeasts. The return of the whey to the farm in the milk can that is often imperfectly cleaned may serve to contaminate the milk with the yeast. In the making of Swiss cheese the whey is often so handled as to favor especially the growth of such yeasts, and since this type of cheese is prepared from sweet milk, the competition between the yeast and the acid-forming bacteria is not so sharp as in the making of cheddar cheese. The writers have found several instances where considerable loss was occasioned in the Swiss cheese industry through the development of gassy cheese due to this type of fermentation.

The yeasty or alcoholic fermentation may also be of importance in butter making. In many sections of the country the milk is separated on the farm and the cream is forwarded to the creamery at more or less infrequent intervals. It becomes sour and if it has become contaminated with yeasts, they will find favorable conditions for growth in the acid medium. A large amount of carbon dioxide gas is produced. Cans of gathered cream often foam to such an extent as to run over, and in some cases actual explosions have occurred on account of the great pressure caused by the gas.