238 THE CHEMISTBT OF THE FABM 



Cheese.— Manufacture. This substance is prepared 

 by the action of rennet on milk. Eennet is made by 

 extracting the fourth stomach of the calf with water 

 containing 5 per cent., or more, of common salt. Its 

 power of coagulating milk is due to the presence of an 

 enzyme called rennin, which doubtless plays a similar 

 part in the ordinary process of digestion in the calf's 

 stomach. Eennet solidifies the milk by separating the 

 casein from solution,^ the fat globules are separated at 

 the same time, being entangled in the curd formed. 

 The action of rennet is very slow in the case of cold 

 milk, it acts most speedily at a temperature of lOS*^ 

 Fahr. ; above this point the action rapidly declines, 

 and ceases at about 130^ Fahr. The rapidity of curd- 

 ling is in a direct proportion to the quantity of rennet 

 added. Slightly sour milk is more quickly curdled by 

 rennet than sweet milk, but the presence of an acid 

 (lactic acid) is not essential to the curdling. 



The composition of cheese depends greatly upon 

 that of the milk from which it is made; rich cheese 

 is made from new milk, cream being sometimes 

 added to the milk for the production of the richest 

 sorts ; poorer kinds of cheese are made from milk 

 wholly or partially skimmed. 



As cheese is usually made in the morning from a 

 mixture of the evening and morning milk, one-half 

 of the milk has already become sufficiently old to 



' According to modern investigators the curdling of milk by an acid 

 consists in the precipitation of the unaltered albuminoid, caseinogen, 

 rriginally present in the milk ; rennet, on the other hand, splits up 

 the caseinogen, precipitating the greater part of it as casein in com- 

 bination with a small quantity of a calcium salt, while a small part, 

 termed lactalbumin, or caseose, remains in solution in the whey. 



