Outlines of dairy bacteriology, 10th edition / A concise manual for the use of students in dairying

by H. L. Russell; E. G. Hastings

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 This direct absorption may occur before the milk is withdrawn from the animal, or afterwards if exposed to strong odors.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.