PROPERTIES OF MILK 13 
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 .g%, 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 
