THE COMPOSITION OF MILK 5 



serum. They retain their form and individuality on 

 account of surface tension, which is the pressure that 

 is exerted on the surface of the globules and is equal 

 on all sides, hence the spherical form of the globules. 



5. Casein. Average milk contains about 3 per 

 cent of casein, which in fresh milk is practically in a 

 soluble condition, but in sour milk is precipitated as 

 curd. The per cent of casein in milk is quite con- 

 stant, ranging from 2.8 to 3.5 per cent. As a general 

 rule, normal milk contains less casein than fat. In 

 a pure state casein is a grayish white powder. 

 Casein takes a very important part in cheese making 

 and other dairy operations. Its chemical and physi- 

 cal properties and the changes which it undergoes 

 will be considered in other chapters. 



6. Albumin. Average milk contains about one 

 half of one per cent of albumin, which is nearly iden- 

 tical with egg albumin, or the " white " of the egg. 

 Albumin and casein have about the same general 

 composition, but different properties ; they belong 

 to the class of bodies called proteids, and are very 

 complex in composition, differing from fat and sugar 

 by containing the element nitrogen, which is not 

 found in the fats and sugars. When fresh milk is 

 boiled, the coagulum which forms on the surface is 

 albumin. The amount of albumin in milk is quite 

 constant and ranges from one half to three quarters 

 of a per cent. 



7. Milk Sugar. Lactose, or milk sugar, is present 

 in milk to the extent of about 5 per cent. When 



