112. Buttermilk is the product left after removal of the fat from cream by churning. It has about the same amount of nutrients as skim milk. The casein is in a slightly modified form due to the development of lactic acid during the ripening of the cream, and on this account buttermilk is more easily digested and assimilated by many individuals than milk in other forms. The development of the acid generally reduces the number of species of other than the lactic organisms, and these are increased.
113. Goat's Milk is somewhat richer in solids than cow's milk, containing about one per cent more proteids, a little more fat, and less sugar. When used as a substitute for human or cow's milk, it generally needs to be slightly diluted, depending, however, upon the composition of the individual sample.
114. Koumiss is a fermented beverage made from milk by the use of yeast to secure alcoholic fermentation. Koumiss contains about one per cent each of lactic acid and alcohol, and the casein and other nutrients are somewhat modified by the fermentation changes. Koumiss is generally considered a non-alcoholic beverage possessing both food and dietetic value.
115. Prepared Milks.—Various preparations are made to resemble milk in general composition. These are mechanical mixtures of sugar, fats, and proteids. Milk sugar, casein, or malted proteids are generally the materials employed in their preparation. Often the dried and pulverized solids of skim milk are used. Many of the prepared milks are deficient in fat. While they are not equal to cow's milk, their use is often made necessary from force of circumstances.
116. Human Milk is not as rich in solid matter as cow's milk. It contains about the same amount of fat, one per cent more sugar, and one per cent less proteids. In human milk nearly one half of the protein is in the form of albumins, while in cow's milk there is about one fifth in this form. The fat globules are much smaller than those of cow's milk. In infant feeding it is often necessary to modify cow's milk by the addition of water, cream, and milk sugar, so as to make it more nearly resemble in composition human milk.
Fig. 25.—Apparatus Used in Testing Milk.
1, pipette; 2, lactometer; 3, acid measure; 4, centrifuge; 5, test bottle.
117. Adulteration of Milk.—Milk is not as extensively adulterated as it was before the passage and enforcement of the numerous state and municipal laws regulating its inspection and sale. The most frequent forms of adulteration are addition of water and removal of cream. These are readily detected from the specific gravity and fat content of the milk. The specific gravity of milk is determined by means of the lactometer, an instrument which sinks to a definite point in pure milk. In watered milk it sinks to greater depth, depending upon the amount of water added. The fat content of milk is readily and accurately determined by the Babcock test, in which the fat is separated by centrifugal action. For the detection of adulterated milk the student is referred to Chapter VI, "Chemistry of Dairying," by Snyder.