CAUSES OF RIPENING CHANGES 373 



digests cheese proteins, and this continues the work 

 long- after the bacteria themselves disappear. The 

 existence of such enzyms from such a source has not 

 been satisfactorily proved yet. The weight of evi- 

 dence up to the present time appears to indicate that 

 the chief, if not the only, work of the lactic acid bac- 

 teria is completed when milk-sugar has been changed 

 into lactic acid. 



We may ask here. What justification have we for 

 the germ theory in general? (i) It has been shown 

 that various germs found in cheese have the power 

 to cause in milk-casein and paracasein changes much 

 like those observed in cheese. (2) Cheese-curd, 

 treated with germicides, fails to ripen. (3) Milk, 

 sterilized and made into sterile cheese, does not ripen. 

 Apparently, there is no ripening, at least nothing 

 like complete ripening, when there are no micro- 

 organisms in cheese. The relations of certain 

 micro-organisms to certain kinds of cheese, espe- 

 cially of the soft type, have been satisfactorily worked 

 out, but the relations to hard types of cheese, 

 like the cheddar, are far from being satisfactorily 

 known. " 



In going over the results of investigations that bear 

 on the subject of cheddar cheese-ripening, we have 

 seen (i) that lactic acid bacteria do an important and 

 necessary work in changing milk-sugar into lactic 

 acid, which reacts with calcium salts in the milk, form- 

 ing neutral calcium lactate and acid calcium phos- 

 phate. (2) We have seen that, in the presence of the 

 acid medium thus furnished by the action of lactic acid 

 bacteria, the peptic enzym contained in rennet is able 

 to bring about quite extensive chemical changes in the 

 protein of the curd or green cheese, forming such 



