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 from 
three quite different sources. One enzyme with this power of 
digesting curd is in the original milk itself. This is the galactase 
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 
