CHEMISTRY OF COWS' MILK 9 



viz., casein, albumin and globulin. Globulin is present 

 in so small quantities that we can properly regard 

 casein and albumin as being essentially the nitrogen 

 compounds of milk, when we consider them quanti- 

 tatively. 



Milk-Casein is the most important nitrogen com- 

 pound in milk, because, (ist) it is the one present in 

 largest quantity; (2d) its presence makes it possible 

 to convert milk into cheese; and (3d) it has a high 

 value as food. Milk-casein is most familiar to us in 

 the form of the solid, white substance called curd, 

 which forms in milk when it sours, though, strictly 

 speaking, this well-known, white substance is not milk- 

 casein, but a closely related compound. 



(1) Composition of milk-casein. Casein is a very 

 complex chemical compound, containing the elements 

 carbon, oxygen, hydrogen, nitrogen, sulphur, and phos- 

 phorus. In milk the protein molecule of casein is com- 

 bined with calcium, or some calcium compound, and 

 hence the proper chemical name of milk-casein is 

 calcium casein. It exists in milk, not in solution, but 

 in the form of extremely minute, solid, gelatinous par- 

 ticles in suspension. The slime found in the bowl of 

 centrifugal . separators consists, to a considerable ex- 

 tent, in milk-casein. 



(2) Action of acids upon milk-casein. When milk 

 sours in the ordinary way, the lactic acid formed acts 

 upon the calcium casein, two chemical changes taking 

 place. First, the lactic acid combines with the cal- 

 cium of the calcium casein, forming calcium- free ca- 

 sein, or simply casein set free from its combination 

 with calcium. When more lactic acid forms, the sec- 



