Fermentations of Milk. 57 



ter and cheese making. These forms come from dirt and 

 manure. They are present in milk in greater or less 

 numbers, depending upon the degree of cleanliness used 

 in producing and handling milk. If they are numerous 

 in a sample of milk, the curd will be filled with gas-holes, 

 instead of having the homogeneous appearance of the 

 curd from good milk. The curd may even be so filled 

 with gas as to float on the surface of the whey. If these 

 forms are few in number and the desirable ones predomi- 

 nate, the gas holes will be less abundant. These bacteria 

 form no spores, and hence are not found in pasteurized 

 milk. They grow at somewhat higher temperatures 

 (95-105 F.) than the desirable lactic bacteria. 



Abnormal fermentations of milk. The acid fermen- 

 tation of milk by reason of its common occurrence is 

 looked upon as a normal change. Various other types of 

 fermentations that are quite different from the acid fer- 

 mentation and hence are called abnormal fermentations 

 or milk faults, appear now and then in milk. 



Sweet curdling of milk. Milk may curdle and yet 

 the taste be perfectly sweet. It is evident that the curd- 

 ling in this case must be due to some other factor than 

 the lactic acid. As is well known sweet milk may be 

 curdled by the use of rennet which is prepared from the 

 fourth stomach of the young calf. The rennet used in 

 cheese making comes from this source. Rennet will 

 curdle several times its weight of milk in a short time. 

 Substances such as rennet are called enzymes. All the 

 digestion in the alimentary tracts of man and animals is 

 carried on by enzymes in the saliva, the gastric juice, the 

 pancreatic juice, and the intestinal fluids. 



Many bacteria form enzymes of various kinds. The 



