Preparation of Rennet. 177 



fourth or digestive stomach of a calf that has not 

 yet ceased to live upon milk, though rennet is also 

 found in the stomachs of puppies and pigs, and the 

 latter have occasionally been used as a source of 

 rennet. The active principle of the rennet is found 

 in the folds of the lining membrane of the stomach 

 of the animal, and is greatest in amount when a full 

 meal is just digested. 



Rennet causes coagulation of the casein by con- 

 tact, and is not itself affected by the change. Its 

 action, however, is greatly modified by various con- 

 ditions of the milk, the most important of which 

 are the temperature and the condition of certain of 

 the ash constituents. Rennet is most active at tem- 

 peratures near the body heat, 98° F. At temper- 

 atures below 80° F., the action of rennet is corre- 

 spondingly slow. On the other hand as the temper- 

 ature is raised above 100° F., the activity of the 

 rennet is at first increased, but after about 130° F. 

 is reached the active principle is rapidly weakened 

 and finally destroyed at about 140° F. 



The activity of rennet is also intimately connected 

 with the lime salts in the ash of the milk, and par- 

 ticularly with their condition as to solubility. The 

 presence of soluble salts of lime renders rennet much 

 more active, while the insoluble salts of lime have 

 a correspondingly retarding effect. Thus, the develop- 

 ment of lactic acid by rendering the lime salts more 

 soluble hastens the action of rennet on the milk. On 

 the other hand, when milk is heated to a tempera- 

 ture above 150° F. a part of the lime salts are made 



