Outlines of Dairy Bacteriology Part 3

[Ill.u.s.tration: FIG. 10. Sanitary milk pails designed to diminish the introduction of hairs, scales, dirt, etc., into milk.]

Stocking tested one of these pails (A, Fig. 10) and found that 63 per cent of the dirt and 29 per cent. of the bacteria were prevented from pa.s.sing into the milk. Eckles examined one in which the germ content was found to be 3200 per cc. as against 43200 per cc. in a common open pail. This milk did not sour until it was 64 hours old in the first case while in the latter it curdled in 43 hours.

~Air in barn.~ The atmosphere of the barn where the milking is done may frequently contribute considerable infection. Germ life is incapable of development in the air, but in a dried condition, organisms may retain their vitality for long periods. Anything which contributes to the production of dust in the stable and aids in the stirring up of the same increases the number of organisms to be found in the air (Fig. 11).

Thus, the feeding of dry fodder and the bedding of animals with straw adds greatly to the germ life floating in the air. Dust may vary much in its germ content depending upon its origin. Fraser found the dust from corn meal to contain only about one-sixth to one-eighth as much germ life as that from hay or bran.[36] In time most of these dust particles settle to the floor, but where the herd is kept in the barn, the constant movement of the animals keeps these particles more or less in motion. Much can be done by forethought to lessen the germ content of stables. Feeding dry feed should not be done until after milking.[37] In some of the better sanitary dairies, it is customary to have a special milking room that is arranged with special reference to the elimination of all dust. In this way this source of infection may be quite obviated as the air of a clean, still room is relatively free from bacteria, especially if the floor is moistened. It has often been noted that the milk of stall-fed animals does not keep as well as that milked out of doors, a condition in part attributable to the lessened contamination.

[Ill.u.s.tration: FIG. 11. Effect of contaminated air. The number of spots indicate the colonies that have developed from the bacteria which fell in 30 seconds on the surface of the gelatin plate (3 inches in diameter). This exposure was made at time the cows were fed.]

~Relative importance of different sources of infection.~ It is impossible to measure accurately the influence of the different sources of infection as these are continually subject to modification in each and every case. As a general rule, however, where milk is drawn and handled without any special care, the utensils and the animal contribute the larger proportion of dirt and bacteria that find their way into milk.

Where the manner of milking and handling is designed to exclude the largest number of organisms possible, the bacteria appearing in the fore milk make up the major portion remaining. By putting into practice the various suggestions that have been made with reference to diminis.h.i.+ng the bacterial content of milk, it is possible to greatly reduce the number of organisms found therein, and at the same time materially improve the keeping quality of the milk. Backhaus[38] estimates that the germ life in milk can be easily reduced to one-two thousandth of its original number by using care in milking. He reports a series of experiments covering two years in which milk was secured that averaged less than 10,000 bacteria per cc., while that secured under ordinary conditions averaged over 500,000.

[Ill.u.s.tration: FIG. 12. Bacterial content of milk handled in ordinary way. Each spot represents a colony growing on gelatin plate. Compare with Fig. 13, where same quant.i.ty of milk is used in making culture.

Over 15,000 bacteria per cc. in this milk.]

Fig. 13 gives an ill.u.s.tration as to what care in milking will do in the way of eliminating bacteria. Fig. 12 shows a gelatin plate seeded with the same quant.i.ty of milk that was used in making the culture indicated by Fig. 13. The first plate was inoculated with milk drawn under good conditions, the germ content of which was found to be 15,500 bacteria per cc., while the sample secured under as nearly aseptic conditions as possible (Fig. 13) contained only 330 organisms in the same volume.

[Ill.u.s.tration: FIG. 13. Bacterial content of milk drawn with care.

Diminished germ content is shown by smaller number of colonies (330 bacteria per cc.). Compare this culture with that shown in Fig. 12.]

~"Sanitary" or "certified" milk.~ Within recent years there has been more or less generally introduced into many cities, the custom of supplying high grade milk that has been handled in a way so as to diminish its germ content as much as possible. Milk of this character is frequently known as "sanitary," "hygienic" or "certified," the last term being used in connection with a certification from veterinary authorities or boards of health as to the freedom of animals from contagious disease.

Frequently a numerical bacterial standard is exacted as a pre-requisite to the recommendation of the board of examining physicians. Thus, the Pediatric Society of Philadelphia requires all children's milk that receives its recommendation to have not more than 10,000 bacteria per cc. Such a standard has its value in the scrupulous cleanliness that must prevail in order to secure these results. This in itself is practically a guarantee of the absence of those bacteria liable to produce trouble in children. The number of organisms found in such milks is surprisingly low when compared with ordinary milk. Naturally, there is considerable fluctuation from day to day, and occasionally the germ content is increased to a high figure without any apparent reason. The average results though, show a greatly reduced number of organisms. De Schweinitz[39] found in a Was.h.i.+ngton dairy in 113 examinations extending throughout a year, an average of 6,485 bacteria per cc. The daily a.n.a.lyses made of the Walker-Gordon supply sold in Philadelphia for an entire year, showed that the milk almost always contained less than 5,000 bacteria per cc. and on 120 days out of the year the germ content was 1,000 organisms per cc. or less.

From a practical point of view, the improvement in quality of sanitary milk, in comparison with the ordinary product is seen in the enhanced keeping quality. During the Paris Exposition in 1900, milk and cream from several such dairies in the United States were s.h.i.+pped to Paris, arriving in good condition after 15 to 18 days transit. When milk has been handled in such a way, it is evident that it is much better suited to serve as a food supply than where it has undergone the fermentative changes incident to the development of myriads of organisms.

~Application of foregoing precautions to all milk producers.~ Milk is so susceptible to bacterial changes that it is necessary to protect it from invasion, if its original purity is to be maintained, and yet, from a practical point of view, the use to which it is destined has much to do with the care necessary to take in handling. The effect of the bacterial contamination of milk depends largely upon the way in which the product is used. To the milk-man engaged in the distribution of milk for direct consumption, all bacterial life is more or less of a detriment, while to the b.u.t.ter-maker and cheese-maker some forms are a direct necessity. It is unnecessary and impracticable to require the same degree of care in handling milk destined to be worked up into factory products as is done, for instance, in sanitary milk supplies, but this fact should not be interpreted to mean that the care of milk for factories is a matter of small consequence. In fact no more important dairy problem exists, and the purer and better the quality of the raw material the better the product will be. Particularly is this true with reference to cheese-making.

Dairymen have learned many lessons in the severe school of experience, but it is earnestly to be hoped that future conditions will not be summed up in the words of the eminent German dairy scientist, Prof.

Fleischmann, when he says that "all the results of scientific investigation which have found such great practical application in the treatment of disease, in disinfection, and in the preservation of various products, are almost entirely ignored in milking."

~Growth of bacteria in milk.~ Milk is so well suited as a medium for the development of germ life that it might be expected that all microorganisms would develop rapidly therein, and yet, as a matter of fact, growth does not begin at once, even though the milk may be richly seeded. At ordinary temperatures, such as 70 F., no appreciable increase is to be noted for a period of 6-9 hours; at lower temperatures (54 F.) this period is prolonged to 30-40 hours or even longer. After this period has elapsed, active growth begins and continues more or less rapidly until after curdling.

The cause of this suspended development is attributed to the germicidal properties inherent to the milk.[40]

Milk is of course seeded with a considerable variety of organisms at first. The liquefying and inert species are the most abundant, the distinctively lactic acid cla.s.s occurring spa.r.s.ely, if at all. As milk increases in age, germ growth begins to occur. More or less development of all types go on, but soon the lactic species gain the ascendency, owing to their being better suited to this environment; they soon outstrip all other species, with the result that normal curdling generally supervenes. The growth of this type is largely conditioned by the presence of the milk sugar. If the sugar is removed from milk by dialysis, the liquid undergoes putrefactive changes due to the fact that the putrefactive bacteria are able to grow if no acid is produced.

~Relation of temperature to growth.~ When growth does once begin in milk, the temperature at which it is stored exerts the most profound effect on the rate of development. When milk is not artificially cooled, it retains its heat for some hours, and consequently the conditions become very favorable for the rapid multiplication of the contained organisms, as is shown in following results obtained by Freudenreich[41]:

_No. of bacteria per cc. in milk kept at different temperatures._

77 F. 95 F.

5 hrs. after milking 10,000 30,000 8 " " " 25,000 12,000,000 12 " " " 46,000 35,280,000 26 " " " 5,700,000 50,000,000

[Ill.u.s.tration: FIG. 14. Effect of cooling milk on the growth of bacteria.]

Conn[42] is inclined to regard temperature of more significance in determining the keeping quality than the original infection of the milk itself. Milk which curdled in 18 hours at 98 F., did not curdle in 48 hours at 70, and often did not change in two weeks, if the temperature was kept at 50 F.

Where kept for a considerable period at this low temperature, the milk becomes filled with bacteria of the undesirable putrefactive type, the lactic group being unable to form acid in any appreciable amounts.

Running well water can be used for cooling, if it is possible to secure it at a temperature of 48-50 F. The use of ice, of course, gives better results, and in summer is greatly to be desired. The influence of these lowered temperatures makes it possible to s.h.i.+p milk long distances[43] by rail for city supplies, if the temperature is kept low during transit.

~Mixing night and morning milk.~ Not infrequently it happens when old milk is mixed with new, that the course of the fermentative changes is more rapid than would have been the case if the two milks had been kept apart. Thus, adding the cooled night milk to the warm morning milk sometimes produces more rapid changes in both. The explanation for this often imperfectly understood phenomenon is that germ growth may have gone on in the cooled milk, and when this material is added to the warmer, but bacteria-poor, fresh milk, the temperature of the whole ma.s.s is raised to a point suitable for the more rapid growth of all bacteria than would have occurred if the older milk had been kept chilled.

~Number of bacteria in milk.~ The number of organisms found in milk depends upon (1) the original amount of contamination, (2) the age of the milk, and (3) the temperature at which it has been held. These factors all fluctuate greatly in different cases; consequently, the germ life is subject to exceedingly wide variations. Here in America, milk reaches the consumer with less bacteria than in Europe, although it may often be older. This is due largely to the more wide-spread use of ice for chilling the milk _en route_ to market. Examinations have been made of various supplies with the following results: Sedgwick and Batchelder found in 57 tests of Boston milk from 30,000-4,220,000 per cc. Jordan and Heineman found 30% of samples of Chicago milk to range from 100,000 to 1,000,000 while nearly one half were from 1-20,000,000 per cc. The germ content of city milks increase rapidly in the summer months.

Park[44] found 250,000 organisms per cc. in winter, about 1,000,000 in cool weather and 5,000,000 per cc. in hot summer weather. Knox and Ba.s.sett in Baltimore report 1,500,000 in spring and nearly 4,500,000 in summer. Eckles[45] studied milk under factory conditions. He finds from 1,000,000 to 5,000,000 per cc. in winter, and in summer from 15-30 millions.

~Bacterial standards for city supplies.~ It would be very desirable to have a hygienic standard for city milk supplies, as there is a b.u.t.ter fat and milk-solid test, but the wide spread variation in germ content and the impracticability of utilizing ordinary bacterial determinations (on account of time required) makes the selection of such a standard difficult. Some hold, as Park, that such a standard is feasible. The New York City Milk commission has set a standard of 30,000 bacteria per cc.

for their certified milk and 100,000 per cc. for inspected milk.

Rochester, N. Y. has attempted the enforcement of such a standard (limit, 100,000 per cc.) with good results it is claimed while Boston has placed the legal limit at 500,000 per cc. Quant.i.tative standards would seem more applicable to "certified" or sanitary supplies than to general city supplies, where the wide range in conditions lead to such enormous variations that the bacterial standard seems too refined a method for practical routine inspection.

~Other tests.~ Any test to be of much service must be capable of being quickly applied. The writer believes for city milk inspectors that the acid test would serve a very useful purpose. This test measures the acidity of the milk. There is, of course, no close and direct relations.h.i.+p between the development of acidity and the growth of bacteria, yet in a general way one follows the other at normal temperatures. Where the temperature is kept rather low, bacterial growth might go on without much acid development, but in the great majority of cases a high degree of acidity means either old milk, in which there has been a long period of incubation, or high temperature, where rapid bacterial growth has been possible. Either of these conditions encourages germ growth and thus impairs the quality of the milk.

The rapid determination of acidity may be made in an approximate manner so as to serve as a test at the weigh-can or intake. The test is best made by the use of the well known alkaline tablet which is composed of a solid alkali, and the indicator, phenolphthalein. The tablets are dissolved in water, one to each ounce used. A number of white cups are filled with the proper quant.i.ty of the solution necessary to neutralize say, 0.2 per cent. lactic acid. Then, as the milk is delivered, the proper quant.i.ty is taken from each can to which is added the tablet solution. A retention of the pink color shows that there is not sufficient acid in the milk to neutralize the alkali used; a disappearance of color indicates an excess of acid. The standard selected is of course arbitrarily chosen. In our experience, 0.2 per cent. acidity (figured as lactic), has proven a satisfactory point. With carefully handled milk the acidity ought to be reduced to about 0.15 per cent. The acidity of the milk may be abnormally reduced if milk is kept in rusty cans, owing to action of acid on the metal.

[Ill.u.s.tration: FIG. 15. Apparatus used in making rapid acid test. A definite quant.i.ty of the alkali solution and indicator is placed in the white tea cup. To this is added the quant.i.ty of milk by means of the cartridge measure which would just be neutralized if the acidity was 0.2 per cent. A retention of the pink color shows a low acid milk; its disappearance, a high acid milk.]

~Kinds of bacteria in milk.~ The number of bacteria in milk is not of so much consequence as the kinds present. With reference to the number of different species, the more dirt and foreign matter the milk contains, the larger the number of varieties found in the same. While milk may contain forms that are injurious to man, still the great majority of them have no apparent effect on human health. In their effect on milk, the case is much different. Depending upon their action in milk, they may be grouped into three cla.s.ses:

1. Inert group, those producing no visible change in the milk.

2. Sour milk forms, those breaking up the milk sugar with or without the formation of gas.

3. Digesting or peptonizing group, those capable of rendering the casein of milk soluble and more or less completely digested.

A surprisingly large number of bacteria that are found in milk belong to the first cla.s.s. Undoubtedly they affect the chemical characteristics of the milk somewhat, but not to the extent that it becomes physically perceptible. Eckles[46] reports in a creamery supply from 20 to 55 per cent. of entire flora as included in this cla.s.s.

By far the most important group is that embraced under the second head.

It includes not only the true lactic acid types in which no gas is formed, but those species capable of producing gases and various kinds of acids. These organisms are the distinctively milk bacteria, although they do not predominate when the milk is first drawn. Their adaptation to this medium is normally shown, however, by this extremely rapid growth, in which they soon gain the ascendency over all other species present. It is to this lactic acid cla.s.s that the favorable flavor-producing organisms belong which are concerned in b.u.t.ter-making.

They are also indispensable in cheese-making.

The third cla.s.s represents those capable of producing a liquefied or digested condition on gelatin or in milk. They are usually the spore-bearing species which gain access from filth and dirt. Their high powers of resistance due to spores makes it difficult to eradicate this type, although they are materially held in subjection by the lactic bacteria. The number of different kinds that have been found in milk is quite considerable, something over 200 species having been described more or less thoroughly. In all probability, however, many of these forms will be found to be identical when they are subjected to a more critical study.

~Direct absorption of taints.~ A tainted condition in milk may result from the development of bacteria, acting upon various const.i.tuents 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 cla.s.s of taints is entirely independent of bacterial action, and is largely attributable to the physical property which milk possesses of being able to absorb volatile odors, the fat in particular, having a great affinity for many of these substances. This direct absorption may occur before the milk is withdrawn from the animal, or afterwards if exposed to strong odors.

It is not uncommon for the milk of animals advanced in lactation to have a more or less strongly marked odor and taste; sometimes this is apt to be bitter, at other times salty to the taste. It is a defect that is peculiar to individual animals and is liable to recur at approximately the same period in lactation.

The peculiar "cowy" or "animal odor" of fresh milk is an inherent peculiarity that is due to the direct absorption of volatile elements from the animal herself. This condition is very much exaggerated when the animal consumes strong-flavored substances as garlic, leeks, turnips and cabbage. The volatile substances that give to these vegetables their characteristic odor are quickly diffused through the system, and if such foods are consumed some few hours before milking, the odor in the milk will be most p.r.o.nounced. The intensity of such taints is diminished greatly and often wholly disappears, if the milking is not done for some hours (8-12) after such foods are consumed.

This same principle applies in lesser degree to many green fodders that are more suitable as feed for animals, as silage, green rye, rape, etc.

Not infrequently, such fodders as these produce so strong a taint in milk as to render it useless for human use. Troubles from such sources could be entirely obviated by feeding limited quant.i.ties of such material immediately after milking. Under such conditions the taint produced is usually eliminated before the next milking. The milk of swill-fed cows is said to possess a peculiar taste, and the urine of animals fed on this food is said to be abnormally acid. Brewers' grains and distillery slops when fed in excess also induce a similar condition in the milk.

Milk may also acquire other than volatile substances directly from the animal, as in cases where drugs, as belladonna, castor oil, sulfur, turpentine, jalap, croton oil, and many others have been used as medicine. Such mineral poisons as a.r.s.enic have been known to appear eight hours after ingestion, and persist for a period of three weeks before being eliminated.

~Absorption of odors after milking.~ If milk is brought in contact with strong odors after being drawn from the animal, it will absorb them readily, as in the barn, where frequently it is exposed to the odor of manure and other fermenting organic matter.

It has long been a popular belief that milk evolves odors and cannot absorb them so long as it is warmer than the surrounding air, but from experimental evidence, the writer[47] has definitely shown that the direct absorption of odors takes place much more rapidly when the milk is warm than when cold, although under either condition, it absorbs volatile substances with considerable avidity. In this test fresh milk was exposed to an atmosphere impregnated with odors of various essential oils and other odor-bearing substances. Under these conditions, the cooler milk was tainted very much less than the milk at body temperature even where the period of exposure was brief. It is therefore evident that an exposure in the cow barn where the volatile emanations from the animals themselves and their excreta taint the air will often result in the absorption of these odors by the milk to such an extent as to seriously affect the flavor.

The custom of straining the milk in the barn has long been deprecated as inconsistent with proper dairy practice, and in the light of the above experiments, an additional reason is evident why this should not be done.