Outlines of Dairy Bacteriology, 8th edition / A Concise Manual for the Use of Students in Dairying

by H. L. Russell

By far the most important group is that embraced under the second head. It includes not only the true lactic acid types in which no gas is formed, but those species capable of producing gases and various kinds of acids. These organisms are the distinctively milk bacteria, although they do not predominate when the milk is first drawn. Their adaptation to this medium is normally shown, however, by this extremely rapid growth, in which they soon gain the ascendency over all other species present. It is to this lactic acid class that the favorable flavor-producing organisms belong which are concerned in butter-making. They are also indispensable in cheese-making.

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

Direct absorption of taints. A tainted condition in milk may result from the development of bacteria, acting upon various constituents of the milk, and transforming these in such a way as to produce by-products that impair the flavor or appearance of the liquid; or it may be produced by the milk being brought in contact with any odoriferous or aromatic substance, under conditions that permit of the direct absorption of such odors.

This latter class of taints is entirely independent of bacterial action, and is largely attributable to the physical property which milk possesses of being able to absorb volatile odors, the fat in particular, having a great affinity for many of these substances. This direct absorption may occur before the milk is withdrawn from the animal, or afterwards if exposed to strong odors.

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

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

This same principle applies in lesser degree to many green fodders that are more suitable as feed for animals, as silage, green rye, rape, etc. Not infrequently, such fodders as these produce so strong a taint in milk as to render it useless for human use. Troubles from such sources could be entirely obviated by feeding limited quantities of such material immediately after milking. Under such conditions the taint produced is usually eliminated before the next milking. The milk of swill-fed cows is said to possess a peculiar taste, and the urine of animals fed on this food is said to be abnormally acid. Brewers' grains and distillery slops when fed in excess also induce a similar condition in the milk.

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

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

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