BACTERIA IN CHEESE-MAKING. 255 



species of bacteria have the power of producing exactly those 

 changes in the casein which render it soluble. The peptoniz- 

 ing bacteria (p. 194) certainly have this power, and these have 

 been given the chief role in cheese-ripening. Moreover, it 

 has frequently been shown that cheese in which the growth 

 of bacteria is prevented by antiseptics or by previous steriliza- 

 tion of the milk, will not ripen. Until recently it has not been 

 questioned that cheese-ripening is just as completely a result 

 of bacterial growth as cream-ripening. 



Action of Enzymes. The most recent work has shown, how- 

 ever, that these chemical changes are not produced wholly by 

 bacterial growth, but are in part due to the action of chemical 

 ferments or enzymes. As mentioned on a previous page, Bab- 

 cock and Russell have shown that fresh milk contains an 

 enzyme, which they have named galactase. This galactase is 

 a normal constituent of milk as secreted from the mammary 

 gland, and is certainly not produced by bacteria. Galactase 

 has an action upon casein practically identical with the change 

 which occurs in the ripening of cheese. This has been demon- 

 strated by subjecting milk to the action of ether vapor, which 

 prevents bacterial growth but does not hinder the action of 

 enzymes. When such milk is tested from time to time for 

 soluble products, it is found that the soluble casein increases 

 regularly, and, inasmuch as all bacterial life is checked by the 

 ether, the only explanation is that the change in the casein has 

 been produced by the chemical enzyme. Besides the galactase, 

 there is always a second enzyme in the cheese, derived from 

 the rennet used for curdling the milk. Rennet is derived 

 from the stomach of a young mammal, and can hardly be 

 prepared without mixing with it a considerable quantity of 

 pepsin. Pepsin has also an action upon casein which changes 

 it from an insoluble to a soluble condition. 



Cheese is thus provided with two enzymes capable of affect- 

 ing the solubility of the casein, entirely independent of the 



