196 BACTERIA AND OTHER MICROORGANISMS IN CHEESE. 



finally be reached. This softening is due to a change in the chemical 

 nature of the casein by which it changes from an insoluble into more 

 or less soluble products. The changes in the casein during the 

 ripening are, in general, similar to those that occur when the casein 

 is digested under the action of digestive juices, so that they are 

 frequently spoken of as a digestion of the curd. During the ripening 

 the insoluble casein is converted into a series of simpler chemical 

 bodies, peptones, proteases, etc., and as these become partly dissolved 

 the hard texture of the cheese becomes softer. Not only are the 

 changes similar to those of digestion, but they are produced by 

 enzymes similar to, though probably not identical with, the enzymes 

 in the digestive juices. The ripening of a cheese is thus a prediges- 

 tion which renders the cheese more easily digested when eaten. 



The enzymes that produce this ripening of the cheese come frorn 

 three quite different sources. One enzyme with this power of 

 digesting curd is in the original milk itself. This is the galactas'e 

 already mentioned. Another is added to the milk with the rennet ; , 

 for rennet is made from the stomach of a mammal, and will always 

 contain some of the pepsin from the stomach. The latter is an 

 enzyme with strong digestive power and is sure to be present in 

 some quantity in the rennet, and hence in the cheese after the addition 

 of rennet. These two enzymes doubtless continue to act upon the 

 casein during the ripening, and are responsible for a certain portion 

 of the digestive changes that are taking place. But there is also a 

 third source of enzyme that, in some cheeses, is more important than 

 the others. As already noticed, certain microorganisms have the 

 power of secreting enzymes, and some of them, growing in or on the 

 ripening cheese, develop enzymes which contribute largely to the 

 ripening. In some of the soft cheeses this is certainly the chief 

 source of the enzymes. 



Flavors. The production of flavors is of no less importance 

 than the chemical digestion of the cheese. At the present time, 

 however, there is a very profound ignorance concerning the real 

 source and cause of cheese flavors. They are without doubt the 

 products of decomposition. They appear in the cheese only 

 toward the end of the ripening process, and are regarded generally 



