CHEMISTRY OF COWS' MILK 9 



viz., casein, albumin, globulin and galactase. Globulin 

 and galactase are present in so small quantities that 

 we can properly regard casein and albumin, quantita- 

 tively, as being essentially the nitrogen compounds of 

 milk. 



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 casein lactate). 



(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 proteid 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, of 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 definite 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- 



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