ENZYMES AND QTHER FERMENTS DIGESTION. I 29 



least three of these changes have been thoroughly studied, and 

 are of great importance in the digestion of foods. 



Rennet or Rennin. It has long been known that a certain 

 product found in the stomachs of young animals and especially 

 in the calf's stomach, has the power of clotting milk rapidly, 

 which property has been applied in the manufacture of cheese. 

 The same substance is found also in the pancreas, and the same 

 or a quite similar enzyme in a number of plants. In fact, this 

 curdling or clotting ferment, like others already described, is 

 quite widely distributed in nature. As occurring in the stom- 

 ach it is mixed with another ferment, which will be described 

 below, known as pepsin. The two substances are apparently 

 quite distinct from each other and may be more or less per- 

 fectly separated. Some chemists are, however, inclined to 

 consider them as essentially similar. 



Rennet acts on the protein substance casein, throwing it 

 into a coagulated or clotted form. The chemistry of the reac- 

 tion is obscure and not thoroughly worked out. The essen- 

 tials of what is known about it will be given later. It is pos- 

 sible to obtain an active extract from the stomach of the calf 

 or young pig which may be kept indefinitely and used for 

 cheese making or other purposes. Formerly in the cheese 

 industry small fragments of the dried calf's stomach, preserved 

 for the purpose, were mixed with the milk and stirred about 

 to induce the coagulation. At the present time a liquid extract 

 is made on the commercial scale by the action of an appro- 

 priate solvent on the cleaned stomach. Glycerol may be used, 

 or water plus a small amount of salicylic acid to prevent putre- 

 faction. In some European countries certain plants have been 

 employed in the place of animal rennet in the cheese industry. 

 Rennet works well in an acid medium and is easily destroyed 

 by alkalies. 



Pepsin. The best known and most thoroughly studied of 

 the proteolytic enzymes is pepsin which has the power of di- 

 gesting coagulated albumin in an acid medium. It may be 

 obtained best from the mucous membrane of the hog's stomach 



10 



