[Illustration: FIG. 18.--CURD FROM A TAINTED MILK. LARGE, IRREGULAR HOLES MECHANICAL; SMALL PINHOLES DUE TO GAS.] [Illustration: FIG. 19.--CURD FROM FOUL MILK.] “Milk contaminated by the introduction of dust, dirt, fecal matter, or kept in imperfectly cleaned cans becomes fouled with gas-producing bacteria that break down the milk sugar and so produce gases and usually undesirable odors.... Therefore milks showing the presence of gas or bad odors in any considerable degree are milks that have been more or less polluted with extraneous organisms or carelessly handled, and as a consequence such milks show a type of curd revealed in Figs. 17, 18, and 19. =Whey.=--The residue left from milk in the process of the making of cheese is known as whey. Whey consists of that portion of milk which is not precipitated by the rennet and which separates when the casein of milk is coagulated and sets in the process of cheese making. The whey contains the principal portion of the water in milk, the most of the milk sugar therein, and small quantities of butter and soluble nitrogenous portions (albumin) and solid particles which remain suspended in the solution. It may, therefore, be properly considered as milk from which the greater part of the nitrogenous portions and fat particles has been separated. The value of whey as a food product consists chiefly in the milk sugar which it contains. It is not very largely used for human food but is valued as a food for young domesticated animals, especially pigs and poultry. =Composition of Whey.=--The whey resulting from the manufacture of cheese contains nearly all the foods of the whole milk with the exception of the casein and fat. It is composed of from 6 to 8 percent of solids consisting chiefly of milk sugar, some albumin, a little fat, and about 0.6 percent of mineral matter. =Koumiss.=--Koumiss originated in Asia Minor in the production of a fermented drink from mare’s milk, which is richer in milk sugar than the lactic secretions of most other mammals. By the fermentation of the milk sugar mare’s milk is converted into a fermented beverage containing a small percentage of alcohol. In this country koumiss is made almost exclusively from cow’s milk and by special fermentation at a low temperature. It is a beverage valued especially by convalescents and invalids and frequently is capable of nourishing the body in diseases which affect the digestive organs when other foods fail of assimilation. It is also a cooling and delicious beverage for those in health when properly prepared and stored.