VAN SLYKE, L. L., and A. W. BOSWORTH, Composition and properties of the brine-soluble compounds in cheese, Jour. Biol. Chem. 14, pages 231-236, 1913. VAN SLYKE, L. L., and E. B. HART, A study of some of the salts formed by casein and paracasein with acids; their relations to American Cheddar cheese, N. Y. Exp. Sta. Bul. 214, 1902. VAN SLYKE, L. L., and E. B. HART, Casein and paracasein in some of their relations to bases and acids, American Chem. Jour. 33, pages 461-996, 1905. VAN SLYKE, L. L., and E. B. HART, Some of the relations of casein and paracasein to bases and acids, and their application to Cheddar cheese, N. Y. Exp. Sta. Bul. 261, 1905. CHAPTER IV _LACTIC STARTERS_ Acidity in cheese-making arises almost exclusively from the lactic acid produced from the fermentation of milk-sugar (lactose) by bacteria. Hydrochloric acid is used in the Wisconsin[22] process of making pasteurized milk cheese and sometimes for making skimmed-milk curd for baking purposes. It is regularly used in precipitating casein not for food but for manufacturing purposes. +56. Acidifying organisms.+--Many species of bacteria have been shown to possess the power to produce lactic acid by fermenting lactose. In practice, however, the cheese-maker seeks to control this fermentation by the actual introduction of the desired organisms and by the production of conditions which will insure this dominance through natural selection. For this purpose the initial souring for most types of cheeses is produced by some variety of the species originally described by Esten[23] and commonly referred to as _Bacterium lactis-acidi_, but variously named as _B. acidi-lactici_, _Streptococcus lacticus_, _B. guntheri_ by different authors. Organisms of this series dominate all other species in milk which is incubated at 70° F. They produce a smooth solid mass without a sign of gas holes and without the separation of whey from the curd, and develop in milk a maximum acidity of about 0.90 of one per cent when titrated as lactic acid. (For titration see Chapter V.) This species is usually present in small numbers in fresh milk. There are many varieties or strains of the species with differing rates of activity and measurable differences in acid produced but with approximately the same qualitative characters. Most commercial starters for cheese- and butter-making belong to this group of species, although special mixtures with other organisms are prepared for special purposes. In addition to this group, most varieties of cheese contain some members of the colon-aërogenes group. When the milk is in proper condition, the activity of this group should be held in check by the early and rapid development of acid. Free development of members of this group usually shows itself in the presence of gas holes in the curd.