184 ELEMENTARY AGRICULTURAL CHEMISTRY 



When milk fat becomes rancid the chief changes are the 

 hydrolysis of a portion of the fat into free acids and glycerol. 

 Thus the butyric acid glyceride may decompose in this way : 



C3lI,(C,HA)3 + 3H,0 = C3H,(OH)3 + 3HC,H,0,. 



The glycerol sometimes oxidises to acrolein, C3II4O, or acrylic 

 acid, C3H4O2. The free volatile acids give rise to the charac- 

 teristic odour of rancid butter. 



The Albuminoids. — Various views are held as to the 

 character of the albuminoids present in milk, some investi- 

 gators believing that there are only two, or, at the most, three, 

 while others declare the existence of five or more. Two, 

 casein and albumin, undoubtedly occur, and are the most 

 important. 



Casein is by far the most abundant. This substance is a 

 white, amorphous body, tasteless and odourless, insoluble in 

 water, alcohol, or ether, but soluble in dilute alkalies, and in 

 solutions of carbonates or phosphates. It is insoluble in 

 dilute, but dissolves in strong, acids. 



Its composition is : 



Per cent. 



Carbon . . . . . . . . 53-30 



Hydrogen 7*07 



Oxygen 22-03 



Nitrogen . . . . . . . . 15"91 



Phosphorus 0-87 



Sulphur 0-82 



Casein can be coagulated in two ways — by the addition of an 

 acid, or by the action of an enzyme contained in rennet. With 

 dilute acids the casein is coagulated unchanged and the curd 

 is almost free from calcium compounds. With rennet, how- 

 ever, the casein is split into two compounds, one of which 

 unites with the calcium salts (chiefly phosphate) present in 

 the milk and forms a curd which entangles the fat ; the other 

 remains in solution in the whey, but can be coagulated by 

 heating to 95° or 100° C. In the absence of calcium salts 



