CONSTITUENTS OF MILK 145 



that casein unites with definite quantities of acids 

 to form definite, insoluble compounds. Changes 

 similar to those occurring when milk sours in the 

 usual way take place when milk is treated with 

 other acids, such as hydrochloric, acetic, sulphuric, 

 etc. 



Free casein is insoluble in water, and also in very 

 dilute acids at ordinary temperatures. The action 

 of acids on calcium casein and on free casein is 

 hastened by increase of temperature. Less acid is 

 required at higher temperature to precipitate casein. 

 Casein dissolves easily even in quite dilute acids, 

 more easily at higher temperatures, forming soluble 

 compounds which are either combinations of acid 

 with casein or decomposition products of casein, 

 according to the concentration of the acid, the tem- 

 perature and other conditions of treatment. 



Action of alkalis on milk-casein. Casein is acid 

 in character in that it unites easily with fixed 

 alkalis, ammonia and alkaline carbonates, forming 

 salts easily soluble in water. Thus, the curd 

 of sour milk or fresh cheese can be dissolved by 

 treatment with dilute sodium carbonate or am- 

 monia. This fact is made use of in cooking, when 

 tough, insoluble cheese, such as that often made 

 from skim-milk, is rendered more easily soluble by 

 use of baking-soda. An interesting experiment in 

 this connection is to rub in a mortar some pure 

 casein, suspended in water, with some calcium car- 

 bonate. The calcium combines with the casein, and 

 carbon dioxid gas is given off. The soluble com- 

 pounds of casein with alkalis are not curdled by 

 rennet, but are precipitated on treatment with acids. 



