212 ORGANIC AGRICULTURAL CHEMISTRY 



Caseinogen is not soluble but is held in colloidal suspension, 

 and it is this colloidal suspended protein together with the 

 emulsified fat already mentioned which give milk its white 

 opaque color. 



Casein or Curd. — When milk sours, due to the lactic acid 

 fermentation of the milk sugar, or when sweet milk is acidified 

 carefully with dilute acetic acid, the caseinogen is converted 

 into wholly insoluble casein. The casein separates as a floccu- 

 lent mass, or curd, which is the basis for the manufacture of 

 cheese. 



This conversion of colloidal caseinogen into insoluble casein 

 is also effected by a milk-coagulating enzyme or enzymes known 

 as re7inin. In the animal body gastric rennin in the gastric 

 juice and pancreatic rennin in the pancreatic juice produce this 

 change. Commercially, the enzymes obtained from animals 

 are used under the name of rennet. 



The exact nature of the chemical changes that occur in this 

 coagulation is not fully established, nor is it determined whether 

 the enzyme coagulation is exactly the same as that produced by 

 acids. The generally accepted view is as follows : The enzyme 

 splits the caseinogen into a truly soluble form of protein and 

 perhaps also yields a peptone. The soluble protein then unites 

 with calcium, from calcium salts present in the milk, yielding a 

 calcium salt of the protein which is the casein or curd. 



Lactalbumin. — After the separation of the casein from milk 

 the filtrate is known as whey. This contains in solution the 

 other two proteins lactalbumin and lacto globulin. On heating 

 the whey the albumin coagulates just as egg albumin does, 

 and may then be filtered off. The lactoglobuHn does not coagu- 

 late with heat and remains in the final filtrate. Its amount, 

 however, is very small and we need not consider it further. 

 The albumin obtained as above agrees in properties with other 

 albumins, e.g. egg albumin and blood serum albumin, giving the 

 characteristic protein reactions. 



Inorganic Constituents, Salts. — The salt constituents con- 

 sist almost entirely of salts of inorganic acids. Only one organic 



