H. L. Russell

“ To the milk dealer or distributor, bacteria are more or less of a detriment. None of the organisms that find their way into milk, nor the by-products formed by their growth, improve the quality of milk supplies. It is therefore especially desirable from the milk-dealer's point of view that these changes should be held in abeyance as much as possible. Then too, the possibility that milk may serve as a medium for the dissemination of disease-breeding bacteria makes it advisable to protect this food supply from all possible infection from suspicious sources. ”

“ There is, of course, no close and direct relationship between the development of acidity and the growth of bacteria, yet in a general way one follows the other at normal temperatures. Where the temperature is kept rather low, bacterial growth might go on without much acid development, but in the great majority of cases a high degree of acidity means either old milk, in which there has been a long period of incubation, or high temperature, where rapid bacterial growth has been possible. Either of these conditions encourages germ growth and thus impairs the quality of the milk. ”

“ Before one can gain any intelligent conception of the manner in which bacteria affect dairying, it is first necessary to know something of the life history of these organisms in general, how they live, move and react toward their environment. ”

“ It is unnecessary and impracticable to require the same degree of care in handling milk destined to be worked up into factory products as is done, for instance, in sanitary milk supplies, but this fact should not be interpreted to mean that the care of milk for factories is a matter of small consequence. In fact no more important dairy problem exists, and the purer and better the quality of the raw material the better the product will be. ”

“ Products of growth. All bacteria in their development form certain more or less characteristic by-products. With most dairy bacteria, these products are formed from the decomposition of the medium in which the bacteria may happen to live. Such changes are known, collectively, as fermentations, and are characterised by the production of a large amount of by-products, as a result of the development of a relatively small amount of cell-life. The souring of milk, the formation of butyric acid, the making of vinegar from cider, are all examples of fermentative changes. ”

“ Those that are toxic and result in the death of the cell are known as disinfectants; those that merely inhibit, or retard growth are known as antiseptics. All disinfectants must of necessity be antiseptic in their action, but not all antiseptics are disinfectants even when used in strong doses. Disinfectants have no place in dairy work, except to destroy disease bacteria, or preserve milk for analytical purposes. ”

“ The influence of the air on the germ content of the milk is, as a rule, overestimated. If the air is quiet, and free from dust, the amount of germ life in the same is not relatively large. In a creamery or factory, infection from this source ought to be much reduced, for the reason that the floors and wall are, as a rule, quite damp, and hence germ life cannot easily be dislodged. The majority of organisms found under such conditions come from the person of the operators and attendants. Any infection can easily be prevented by having the ripening cream-vats covered with a canvas cloth. ”

“ Growth of bacteria can only occur within certain temperature limits, the extremes of which are designated as the minimum and maximum. Below and above these respective limits, life may be retained in the cell for a time, but actual cell-multiplication is stopped. Somewhere between these two cardinal temperature points, and generally nearer the maximum limit is the most favorable temperature for growth, known as the optimum. ”

“ Numerous non-spore-bearing species remain alive in ice for a prolonged period, and recent experiments with liquid air show that even a temperature of -310° F. for hours does not effectually kill all exposed cells.
Effect of heat. High temperatures, on the other hand, will destroy any form of life, whether in the vegetative or latent stage. The temperature at which the vitality of the cell is lost is known as the thermal death point. This limit is not only dependent upon the nature of the organism, but varies with the time of exposure and the condition in which the heat is applied. ”

“ Milk is so well adapted to the development of bacteria in general, that it is not surprising to find it a suitable medium for the growth of many pathogenic species even at ordinary temperatures. Not infrequently, disease-producing bacteria are able to grow in raw milk in competition with the normal milk bacteria, so that even a slight contamination may suffice to produce infection. ”

“ As the demands of the market change from a high, quick flavor to one that is somewhat milder but of better keeping quality, doubtless pasteurization of the cream will become more and more popular. That such a change is gradually occurring is already evident, although as yet only a small proportion of butter made in this country is now made in this way. Where the cream is unheated, a considerable number of species will be found, and even the addition of a pure culture, if that culture is of the lactic acid-producing species, will to some extent control the type of fermentation that occurs. ”

“ The atmosphere of the barn where the milking is done may frequently contribute considerable infection. Germ life is incapable of development in the air, but in a dried condition, organisms may retain their vitality for long periods. Anything which contributes to the production of dust in the stable and aids in the stirring up of the same increases the number of organisms to be found in the air (Fig. 11) . Thus, the feeding of dry fodder and the bedding of animals with straw adds greatly to the germ life floating in the air. Dust may vary much in its germ content depending upon its origin. ”

“ Milk is of course seeded with a considerable variety of organisms at first. The liquefying and inert species are the most abundant, the distinctively lactic acid class occurring sparsely, if at all. As milk increases in age, germ growth begins to occur. More or less development of all types go on, but soon the lactic species gain the ascendency, owing to their being better suited to this environment; they soon outstrip all other species, with the result that normal curdling generally supervenes. The growth of this type is largely conditioned by the presence of the milk sugar. ”

“ The basis for the peculiar flavor or taste which ripened cream-butter possesses is due, in large part, to the formation of certain decomposition products formed by various bacteria. Aroma is a quality often confounded with flavor, but this is produced by volatile products only, which appeal to the sense of smell rather than taste. Generally a good flavor is accompanied by a desirable aroma, but the origin of the two qualities is not necessarily dependent on the same organisms. ”

“ No factor of contamination is so susceptible of improvement as that which relates to the reduction in filth and dirt which gains access during and immediately subsequent to the milking. The care which is taken to keep the coat of the animal clean by grooming lessens very much the grosser portion of such contamination, but with a dry, hairy coat, fine scales and dust particles must of necessity be dislodged. [28] Ordinarily the patron thinks all evidence of such dirt is removed if the milk is strained, but this process only lessens the difficulty; it does not overcome it. ”

“ Some species, however, and some yeasts as well possess the peculiar property of taking the oxygen which they need from organic compounds [Pg 7] such as sugar, etc., and are therefore able to live and grow under conditions where the atmospheric air is excluded. These are known as anaerobic. While some species grow strictly under one condition or the other, and hence are obligate aerobes or anaerobes, others possess the ability of growing under either condition and are known as facultative or optional forms. The great majority of milk bacteria are either obligate or facultative aerobes. ”

“ It is important to select the best possible milk for pasteurizing, for if the milk has not been milked under clean conditions, it is likely to be rich in the spore-bearing bacteria. Old milk, or milk that has not been kept at a low temperature, is much richer in germ-life than perfectly fresh or thoroughly chilled milk.
The true standard for selecting milk for pasteurization should be to determine the actual number of bacterial spores that are able to resist the heating process, but this method is impracticable under commercial conditions. ”

“ With many varieties of cheese, especially some of the foreign types, the presence of mold on the exterior is not regarded as detrimental; in fact a limited development is much desired. In hard rennet cheese as cheddar or Swiss, the market demands a product free from mold, although it should be said that this condition is imposed by the desire to secure a good-looking cheese rather than any injury in flavor that the mold causes. Mold spores are so widely distributed that, if proper temperature and moisture conditions prevail, these spores will always develop. ”

“ Considerable difference of opinion has existed in the minds of medical men as to the relative digestibility of raw and heated milks. A considerable amount of experimental work has been done by making artificial digestion experiments with enzyms, also digestion experiments with animals, and in a few cases with children. The results obtained by different investigators are quite contradictory, although the preponderance of evidence seems to be in favor of the view that heating does impair the digestibility of milk, especially if the temperature attains the sterilizing point. ”

“ Water in contact with the soil surface takes up many impurities, and is of necessity rich in microbes. As the rain water percolates into the soil, it loses [Pg 12] its germ content, so that the normal ground water, like the deeper soil layers, contains practically no bacterial life. Springs therefore are relatively deficient in germ life, except as they become infected with soil organisms, as the water issues from the soil. Water may serve to disseminate certain infectious diseases as typhoid fever and cholera among human beings, and a number of animal maladies. ”

“ The great majority of bacteria of interest in dairying belong to the saprophytic class; only those species capable of infecting milk through the development of disease in the animal are parasites in the strict sense of the term. Most disease-producing species, as diphtheria or typhoid fever, while parasitic in man lead a saprophytic method of life so far as their relation to milk is concerned. ”

“ The most satisfactory way to detect the presence of the taints more often present is to make a fermentation test of one kind or another. These tests are most frequently used at the factory, to enable the maker to detect the presence of milk that is likely to prove unfit for use, especially in cheese making. They are based upon the principle that if milk is held at a moderately high temperature, the bacteria will develop rapidly. ”

“ In the second group, the bacterial species lives a saprophytic existence, growing in milk, if it happens to find its way therein. In such cases milk indirectly serves as an agent in the dissemination of disease, by giving conditions favorable to the growth of the disease germ.
By far the most important of diseases that may be transmitted directly from animal to man through a diseased milk supply is tuberculosis, but in addition to this, foot and mouth disease (aphthous fever in children) , anthrax and acute enteric troubles have also been traced to a similar source of infection. ”

“ He found the same germ in both water and cheese and by inoculating a culture into pasteurized milk succeeded in producing the undesirable flavor. The danger from ice is much less, for the reason that good dairy practice does not sanction using ice directly in contact with milk or cream. Then, too, ice is largely purified in the process of freezing, although if secured from a polluted source, reliance should not be placed in the method of purification; for even freezing does not destroy all vegetating bacteria. ”

“ The bacteria are so extremely small that it is impossible to study individual germs separately without the aid of first-class microscopes. For this reason, but little advance was made in the knowledge of these lower forms of plant life, until the introduction of culture methods, whereby a single organism could be cultivated and the progeny of this cell increased to such an extent in a short course of time, that they would be visible to the unaided eye. ”

“ The quality of flavor and aroma in butter is, of course, also affected by other conditions, as, for instance, the presence or absence of salt, as well as the inherent qualities of the milk, that are controlled, to some extent at least, by the character of the feed which is consumed by the animal. ”

“ Unless care is taken the flanks and udder become polluted with fecal matter, which upon drying is displaced with every movement of the animal. Every hair or dirt particle so dislodged and finding its way into the milk-pail adds its quota of organisms to the liquid. This can be readily demonstrated by placing cow's hairs collected with care on the moist surface of gelatin culture plates. Almost invariably, bacteria will be found in considerable numbers adhering to such hairs as is indicated in Fig. 9. Dirt particles are even richer in germ life. ”

“ The effect of chilling or rapid cooling on the keeping quality of milk is well known. When the temperature of milk is lowered to the neighborhood of 45° F., the development of bacterial life is so slow as to materially increase the period that milk remains sweet. Within recent years, attempts have been made to preserve milk so that it could be shipped long distances by freezing the product, which in the form of milk-ice could be held for an indefinite period without change. ”

“ The importance of flavor as determining the quality of cheese makes it imperative that the nature of the substances that confer on cheese its peculiar aromatic qualities and taste be thoroughly understood. ”

“ In several cases the writer has found a not inconsiderable number of liquefying bacteria mixed with the selected organism. Molds not infrequently are found in cultures put up in the dry form. Doubtless the effect of these accidental contaminations is considerably less in the case of a starter composed of a distinctively lactic acid-producing organism than with a form which is less capable of thriving vigorously in milk, and it should be said that these impurities can frequently be eliminated by continued propagation. ”

“ The reason why cheese is more subject to abnormal fermentation than butter is because its high nitrogen content favors the continued development of bacteria for some time [Pg 183] after it is made. It must be borne in mind, in considering the more important of these changes, that not all defective conditions in cheese are attributable to the influence of living organisms. Troubles frequently arise from errors in manufacturing details, as too prolonged cooking of curds, too high heating, or the development of insufficient or too much acid. ”

“ The water supply of a factory is also a question of prime importance. When taken from a shallow well, especially if surface drainage from the factory is possible, the water may be contaminated to such an extent as to introduce undesirable bacteria in such numbers that the normal course of fermentation may be changed. The quality of the water, aside from flavor, can be best determined by making a curd test (p. 76) which is done by adding some of the water to boiled milk and incubating the same. ”