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

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

Bitter fermentation of milk. Bitterness in milk may be due to bacteria that enter the milk after it is drawn from the cow, or it may be caused by the feed consumed by the animal. It has been previously shown that certain specific substances contained in the food may be absorbed and reappear in the milk. If the animal eats ragweed, lupines, or other plants containing bitter substances, the milk is likely to have a bitter taste, which will be noticeable at the time the milk is drawn. The milk of cows at certain advanced stages of lactation may show a bitter taste, due to a change in the ash constituents of the milk in which the lime salts are largely replaced by salts of sodium.

There are many bacteria that will impart to milk a bitter taste. Milk that has undergone the sweet-curdling fermentation is likely to be bitter, as is the ease with pasteurized milk. Some of the acid-forming bacteria are able to develop a bitter principle, the milk retaining a pleasant odor and having the normal amount of acid, while the taste is intensely bitter. One of the authors (H) found in the case of a Wisconsin brick cheese factory, that the usual acid organism was almost wholly replaced by a bitter type.

Storage of milk at very low temperatures is conducive to the appearance of a bitter taste in milk, the explanation in this case being that the acid-forming bacteria are unable to grow at a low temperature, while some of the putrefactive forms can multiply and develop these astringent or bitter by-products.

Miscellaneous fermentations of milk. There are a number of other abnormal fermentations in milk that occur so rarely as to be of but little economic importance. Some, as the colored milks, are however, quite striking, and on this account have had much attention directed to them in the past. There are bacteria that are able to produce various colored substances, such as red, yellow, and blue. In case milk becomes seeded with large numbers of any of these kinds, it is very likely to be colored by the growth. Red milk may be due to bacteria, but more frequently is caused by the actual presence of blood in the milk, due to a wound in the udder, or the effect of a severe case of inflammation of this gland. Such a condition may be readily distinguished by allowing the milk to stand for a short time, in which case, if due to blood, the red corpuscles will soon settle to the bottom of the container, while bacterial troubles producing a red coloration are more evident on the surface.

It is also claimed that certain bacteria may impart a soapy taste or turnip flavor to milk.

Cycle of fermentations in milk. If a sample of milk is allowed to stand, it will undergo a certain sequence of fermentations that well illustrates the principle that one type of organisms is dependent on some other type to furnish suitable conditions for its development. This cycle of changes that normally occurs in milk is as follows: (1) The bacteria that come from the interior of the udder are the first to develop, but usually the change they produce is not evident.

(2) Of the types that gain admission, subsequent to the milking, the acid-producing species are able to adjust themselves most perfectly to the conditions that obtain in milk. Within a few hours they greatly predominate and soon the milk curdles under the production of acid. Their growth, however, is soon stopped by the accumulation of their own by-products.

(3) The semi-solid curdled milk, on account of its acid reaction then becomes a favorable medium for the growth of molds; a prevalent form, known as Oidium lactis usually develops as a white velvety layer. The molds in their growth form alkaline by-products, which tend to neutralize the acid reaction, so that in the course of two to three weeks, if the layer of the milk is not too deep (an inch or less), the chemical reaction of the milk becomes neutral or alkaline.

(4) The putrefactive bacteria which found their way into milk when it was first drawn, and which have remained dormant in the sour milk, now find favorable conditions for growth. As a result of their activity, the milk soon undergoes a putrid decomposition, which is marked by offensive odors.

If the milk is placed under such conditions as will exclude the growth of the mold, such as where the air is excluded from the surface, the sour milk will remain in that condition for an indefinite period, since the putrefactive bacteria are inhibited in their development by the acid, in a manner comparable to the preservation of pickles in vinegar, or the keeping of silage because of the acid that is produced as a result of the changes that the plant tissue undergoes when excluded from the air. The preservative effect of acids is of much importance in the case of certain dairy products (see Chapter VIII).