A DAIRY LABORATORY GUIDE 69 



EXPLANATION. COMMERCIAL RENNET 

 AND ITS USE 



. Commercial rennet is a liquid prepared by soak- 

 ing in brine the fourth stomach of the calf. It has 

 under certain conditions the power of precipitating 

 the casein from milk. It is so desirable for this 

 purpose that rennet is used almost exclusively in 

 the manufacture of cheese. One peculiar character- 

 istic of rennet is the fact that it is not destroyed 

 by its use in precipitating casein. If it could be 

 extracted the same rennet could be used over and 

 over again for precipitating the casein in different 

 lots of milk. 



Temperature has a decided effect upon the action 

 of rennet. The colder milk is, the less rapid the 

 rennet action. This is very noticeable at tempera- 

 tures below 80° F. The rennet acts most rapidly 

 at temperatures from ioo° F. to 120° F. At 130° 

 F. the action of the rennet is less active and its 

 action is entirely destroyed somewhere between 

 140° F. and 150° F. The high temperature destroys 

 the rennin, which is the active principle of the 

 rennet. The action of rennet increases as the acid 

 in the milk increases. The action of rennet seems 

 to depend to a great degree upon the solubility of 

 the lime salts of the milk. If an insoluble lime 

 salt is added to the milk, the action of the rennet 

 is retarded, and if a soluble lime salt is added to 

 milk the action of rennet is hastened. If milk is 

 heated to a high temperature the lime salts are 

 supposed to be rendered insoluble and the rennet 

 has no effect on the casein. Freezing does not seem 

 to injure rennet. When it is again melted and 

 given the proper temperature it will act as though 

 it had never been frozen. 



