Preparation ‘of Rennet 247 
and is less liable to introduce taints and bad flavors, 
than a ferment made from sour whey, and with a little 
care and attention, the fermentation will go on regu- 
larly from day to day. ‘ 
Rennet—The ordinary source of rennet is the 
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 
