MILK AND ITS PRODUCTS 129 



The larger portion of the casein is suspended in milk 

 in an extremely finely divided amorphus condition. It is 

 intimately associated with the insoluble calcium phosphate 

 of milk and possibly held in chemical combination with 

 this. Its study presents many difficulties, which leaves its 

 exact composition still undetermined. 



Casein is easily precipitated by means of rennet extract 

 and dilute acids, but the resulting precipitates are not 

 identically the same. It is not coagulated by heat. 



Albumen. In composition albumen very closely re- 

 sembles casein, differing from this only in not containing 

 sulphur. It is soluble and unaffected by rennet, which 

 causes most of it to pass into the whey in the manufacture 

 of cheese. It is coagulated at a temperature of 170 F. 

 It is in their behavior toward heat and rennet that casein 

 and albumen radically differ. 



Milk Sugar. This sugar, commonly called lactose, has 

 the same chemical composition as cane sugar, differing 

 from it chiefly in possessing only a faint sweetish taste. 

 It readily changes into lactic acid when acted upon by 

 the lactic acid bacteria. This causes the ordinary phenom- 

 enon of milk souring. The maximum amount of acid in 

 milk rarely exceeds .9%, the germs usually being checked 

 or killed before this amount is formed. There is there- 

 fore always a large portion of the sugar left in sour milk. 

 All of the milk sugar is in solution. 



Ash. Most of the ash of milk exists in solution. It 

 is composed of lime, magnesia, potash, soda, phosphoric 

 acid, chlorine, and iron, the soluble lime being the most 

 important constituent. It is upon this that the action of 

 rennet extract is dependent. For when milk is heated 

 to high temperatures the soluble lime is rendered insoluble 

 and rennet will no longer curdle milk. It seems also that 



