10 CASEIN. 



whilst an alkaline reaction hinders it. In the case of 

 normal milk, which has an amphoteric reaction, the action 

 is intermediate between these two extremes. The increased 

 action in the case of sour milk is explained by the presence 

 of a larger quantity of soluble salts of lime, which are 

 essential to the action of the rennet. With an alkaline 

 reaction the proportion of these salts is lower, and con- 

 sequently the rennet is less efficacious. Cases have been 

 known in which the imperfect action of rennet resulting in 

 the milk refusing to curdle at all has been traced entirely 

 to the poverty of the milk in soluble lime salts. Old milk, 

 again, takes longer to curdle than when fresh ; and the 

 percentage of water present also plays a part. Milk that 

 has been boiled, or heated to 167 C., is more difficult to 

 curdle, and will not do so at all in some cases owing to the 

 alteration effected in the lime salts by heat. 



Lactalbumen and lactoglobulin are coagulated by boiling, 

 in the same way as albumen and globulin. Casein, on the 

 other hand, forms a skin on the surface of the milk, and 

 does not coagulate below 266 F. in a closed vessel. The 

 formation of the skin is prevented by stirring, or by heating 

 the milk in an enclosed space or one saturated with steam. 



As already mentioned, the decomposition of milk can be 

 effected by other substances, particularly acids and acid 

 salts ; though these are rarely used, since the chief use of 

 casein, in the form of cheese, is as a foodstuff. 



The coagulation of milk furnishes two chief products : 

 the curd, containing a larger or smaller proportion of fat, 

 proteids and lactic acid ; and the whey, containing the 

 soluble salts of the milk, together with curd, lactic acid, 

 milk sugar, etc. Strictly speaking, the whey can be differ- 

 entiated into cheese-milk and whey, the former term being 

 applied to the liquid remaining after the separation of the 

 curd, whilst whey, in the narrower sense, is the residue left 



