Milk and Its Products. 27 I 



in such relatively small quantities that they will not be dis- 

 cussed here. 



Casein is the chief protein of milk and exists there in a col- 

 loidal state and not in perfect solution. It can be separated from 

 the milk by the addition of an acid or by the action of the enzyme, 

 rennin, which is contained in rennet. In the souring of milk, 

 during which process acid is developed, the casein is precipitated. 

 The casein formed in this way probably consists of calcium-free 

 casein, for it is generally held that casein exists in milk in com- 

 bination with calcium. AVith rennin, however, the calcium-casein 

 is split into two compounds, para-casein and whey protein. The 

 para-casein in the presence of the soluble calcium salts of the 

 milk precipitates out, while the whey protein remains in solution. 

 In the absence of calcium salts rennin will not curdle milk. This 

 enzyme acts best at 35 C. and is destroyed at 70 C. It is found 

 in the stomachs of all mammals, while enzymes possessing similar 

 properties have also been found in birds, fishes, many plants, and 

 in the products formed by the action of certain bacteria. 



Mere boiling of milk, unless continued for a considerable time, 

 does not coagulate the casein. Casein is the only protein of 

 cow's milk which contains phosphorus in its molecule. 



Milk albumin differs in some of its physical properties from 

 blood albumin. It is in complete solution in milk but coagulates 

 and precipitates when heated to 72 C. It is not coagulated by 

 rennin or by most acids. It differs from casein in composition 

 and contains about twice as much sulphur and no phosphorus. 

 In colostrum milk, albumin largely predominates, so that the 

 milk coagulates on heating. 



Milk sugar. The sugar contained in milk is known as lactose. 

 It occurs in the milk of all animals, but is not present in plants, 

 and consequently does not exist in the food of the dairy cow. 

 It is prepared by evaporating the whey, left after cheese making, 

 to a small bulk, from which lactose will crystallize out in large 

 crystals. It possesses a faint sweet taste, about one-tenth that 



