Fermentations in Milk. 67 



a large number of organisms of this class found in surface 

 waters, soils and in decomposing organic matter. The 

 colon bacillus of the intestinal tract is a germ of this type 



FIG. 16. Cheese made from " gassy " milk. 



that finds its way into milk with manure particles. B. lactis 

 aerogenes, a common inhabitant of milk is also a gas-pro- 

 ducer. Abnormal fermentations of this class occur most 

 frequently in the hot summer months, but are not neces- 

 sarily confined to this season. Wherever carelessness pre- 

 vails in the matter of cleaning utensils, troubles from gassy 

 milks are very apt to occur. 



"Sweet curdling" and digesting: fermentations. Not 



infrequently milk, instead of undergoing spontaneous sour- 

 ing, curdles in a weakly acid or neutral condition, in which 

 state it is said to have undergone " sweet curdling." The 

 coagulation of the milk is caused by the action of enzyms 

 of a rennet type that are formed by the growth of various 

 species of bacteria. Later the whey separates more or less 

 perfectly from the curd, producing a "wheyed off" condi- 

 tion. Generally the coagulum in these cases is soft and 



