THE CHEMISTRY OF CHEESE MAKIXG 89 



casein and albumin, are alike in general composition, 

 but differ materially in their physical properties. 

 The proteids are the compounds to which special 

 attention is given in human and animal nutrition, for 

 they impart characteristic value to foods and perform 

 functional processes which other nutrients are not 

 capable of doing. The reader is referred to works on 

 nutrition and the feeding of animals for the func- 

 tions of protein and its value as a nutrient. The 

 milk proteids, particularly casein, undergo numerous 

 clianges during cheese making, and the process con- 

 cerns itself largely with the correct handling of these 

 compounds. 



83. Casein. — In fresh milk the casein is in nearly 

 a soluble condition and is one of the solids of the 

 milk serum, ^^'ith the formation of lactic acid, the 

 casein is changed from a semi-soluble to an insoluble 

 condition, and the lactic acid unites with the casein, 

 forming a new and insoluble compound. When 

 sufficient acid is developed, the precipitation of the 

 casein is complete. This is commonly spoken of as 

 curdling of the milk. The addition of dilute acids 

 to fresh milk will produce the same result, coagu- 

 lation, and in cheese making the object of the fer- 

 ment action is to produce acid to change the casein 

 from a soluble to an insoluble condition. Dilute 

 acids do not coagulate or precipitate the milk 

 albumin. 



84. Albumin. — When milk is allowed to sour and 

 curdle, it separates into a coagulated mass containing 



