CONSTITUENTS OF MILK 141 



milk. Even in butter and cheese, the fat-globules 

 of the milk preserve their individuality to a large 

 extent. 



MILK-CASEIN 



Milk-casein is of special importance in connection 

 with cheese-making because the conversion of milk 

 into cheese is dependent upon the peculiar proper- 

 ties of casein. This constituent of milk, in an im- 

 pure and changed form, is most commonly familiar 

 as the solid, white substance, called curd, which forms 

 in milk when it sours. It is also familiar as a prom- 

 inent constituent of separator-slime, . and in this form 

 is not materially changed from the condition in which 

 it exists in milk. 



Composition of milk-casein. Casein, as it exists 

 in milk, is a very complex chemical compound, be- 

 longing to a general class of nitrogen-containing com- 

 pounds known as protein, and to a special subdivision 

 called phosphoproteins. Its elementary composition is 

 about as follows. 



Carbon 53-OO per cent 



Oxygen 22.70 per cent 



Nitrogen I5-7O P er cent 



Hydrogen 7.00 per cent 



Phosphorus 0.85 per cent 



Sulphur 0.75 per cent 



The presence of phosphorus in casein is one of 

 its distinguishing chemical features, but in what 

 particular form of combination the phosphorus ex- 

 ists is not known at present. Casein in milk does 



