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

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

Fermented drinks from milk. Within the last few years a great deal of attention has been directed toward the preparation of various kinds of drinks from milk. The use of such beverages has rapidly increased. Butter milk is one which meets with the greatest approval. The true butter milk from cream that has been soured by the desirable acid-forming bacteria has a mild agreeable acid taste, wholly free from any sharpness that is often noted in butter milk made from cream in which considerable numbers of the undesirable acid-forming bacteria have grown. Butter milk made from pasteurized cream soured with pure cultures will have good keeping qualities and is a most healthful drink for all classes of people, even for young children.

Butter milk is also prepared by allowing milk to sour and then breaking up the curd by stirring. If the type of fermentation is controlled as may be done (see Chapter VII), such a form of fermented milk is a most desirable drink. It is probably as healthful and has all the therapeutic properties that are ascribed to other forms of fermented milks such as the Bulgarian "Yoghurt."

This type of fermented milk is produced by an acid-forming organism that can form large amounts of acid, 2.0 or 3.0 per cent. The casein is dissolved to some extent and the remainder so changed, that it will remain in suspension for a long time in a finely divided form, after the curd has been broken up. Such milk is sold under various names at home and abroad. One of the authors (H) has found such organisms in practically all milks examined. If raw milk is kept warm (98° to 100° F.) in a stoppered bottle which is filled full, the acidity will be found to increase slowly from day to day, reaching a maximum in ten to fourteen days. If the milk is then examined, it will be found to contain large numbers of an acid-forming organism very different in appearance from the bacteria causing the rapid souring of milk at ordinary temperatures. This organism is very similar if not identical with the one found in the Bulgarian milk to which the name B. Bulgaricus has been given. The use of the milk fermented by this organism has spread rapidly because it is claimed by certain European bacteriologists that it has a favorable effect on the health of people, especially those suffering from intestinal troubles. It is not at all certain that ordinary sour milk or butter milk will not have the same effect; in fact in many of the fermented milks sold in Europe, B. Bulgaricus has not been found, but only the ordinary lactic acid bacteria.

Several alcoholic drinks made from milk, such as kefir and koumiss, have been originated among the nomadic tribes of Western Asia. Kefir is prepared from cow's milk by adding the kefir ferment in the form of grains which contain a number of kinds of bacteria and a yeast. The acid-forming bacteria impart a sour taste to the fermented milk, while the yeast forms carbon dioxide and about two per cent of alcohol. If the milk is allowed to ferment in stoppered bottles, the resulting product will be an acid effervescing drink, which is claimed to be more easily digested than sweet milk. This drink is used frequently in the treatment of invalids but it is improbable that it is more easily digested than ordinary soured milk or butter milk. The grains are removed from the fermented milk, and are then added to a quantity of fresh milk, or they may be dried and kept for future use. When needed again, they are soaked in water, then added to the milk.

Koumiss is made in Russia from mare's milk and has much the same composition as kefir. In America and Europe it is made from cow's milk, by adding cane sugar and compressed yeast. The yeast ferments the cane sugar while the acid-forming bacteria ferment the milk sugar. There is thus obtained a drink that is similar in composition to the real koumiss, in which both the acid and the alcohol come from the fermentation of the milk sugar. In koumiss and kefir the curd is very finely divided and will remain in suspension for a long time as with butter milk.

Determination of the cause of taints in milk. It is often of the greatest importance to be able to locate the cause of abnormal odors or tastes in milk, since methods for overcoming the trouble can be intelligently applied only when the actual cause is known. An abnormal condition may be caused either by the direct absorption of odors before or after the milk is drawn from the animal, or it may be due to bacteria. If the milk appears bad-flavored when first drawn, and if such taint becomes less pronounced as the milk becomes older, it is likely that the trouble is due to some characteristic of the feed. Certain feeds, like green rye, rape, cabbage, and certain of the root crops, like turnips, impart a strong odor to milk, if the same are fed shortly before milking. If the tainted condition appears only some time after the milk is drawn, it may be due to the direct absorption of taints from the surroundings in which the milk is kept, or it may be caused by bacteria. These causes can often be differentiated, by noting whether the taint tends to increase in intensity with age. If such is the case, it is likely that the cause is of germ origin, but if the reverse is true, it cannot be ascribed with certainty to bacteria and recourse must be had to other methods, such as the transfer of a small quantity of the tainted milk to a sample of perfectly fresh milk, or preferably to some milk that has been heated to the boiling point and then cooled. In the case of an odor due to direct physical absorption, it will not appear in the inoculated sample, since the small amount transferred is not sufficient to be noted. If it is due to living organisms, the inoculation of the smallest quantity into a fresh sample is likely to reproduce the same change as originally noted.

Tests for the bacteriological condition of milk. Within certain limits milk can be indirectly examined as to its bacterial content without any special equipment. Milk when drawn from the cow has an apparent acidity ranging from 0.16 to 0.18 per cent. By the use of any of the methods of determining acidity in milk, much can be told concerning the number of bacteria in the milk, and hence concerning its keeping quality. Milk that has an acidity of over 0.2 per cent is certain to contain many bacteria, and consequently will keep poorly. Such milk is of low value for market milk, but may not be objectionable for butter or cheese making. If the acidity is below 0.2 per cent, but little can be told as to the numbers of bacteria, since any increase in acid is always preceded by an enormous increase in the numbers of acid-forming bacteria.

A more important test than the acid test, from the standpoint of the butter and cheese maker, and even the milk dealer, is the fermentation test. In its simplest form, it consists in placing a sample of the milk to be tested in a warm place and noting the time required to curdle and the type of curd formed. In this country the fermentation test has been largely supplanted by the Wisconsin curd test which possesses the advantage of detecting the presence of bacteria harmful in cheese making, especially the gas forming bacteria.

The curd test is helpful in detecting the source of an abnormal condition in a milk supply coming from diverse sources. The milk furnished by each patron can be tested separately and the trouble located, perhaps in an individual herd; the offending herd determined, the test may then be used on the milk of individual cows. The cheese maker and the milk dealer should be able not only to detect which of the patrons furnish him poor milk, but he should be able to give the patron definite instructions how to avoid the sources of such trouble. This information can be given only when the source is positively known.