214 BACTERIA AND OTHER MICROORGANISMS IN CHEESE 
ordinarily have any power of changing casein to peptone. They 
produce lactic acid which curdles the milk, after which they ap- 
parently cease to act uponit at all. Hence, it would not seem that 
they could digest cheese. But if the acid which they produce is 
neutralized by the presence of some alkaline, like carbonate of 
soda, the bacteria continue to grow, and eventually produce the 
peptonization of the casein. Moreover, the grade of the cheeses is 
very closely dependent upon the growth of lactic bacteria, and 
cheese from which lactic acid bacteria are excluded by aseptic 
milking will not ripen normally, while they would do so if the acid 
germs were present. All of these facts together led to the conclu- 
sion that it is this peptonizing power of the lactic acid bacteria, 
under certain conditions, which is responsible for the chemical 
changes that take place in the ripening cheese. This conclusion, 
however, has not been very generally accepted, for while the lac- 
tic acid bacteria, under these conditions, do produce a certain 
amount of peptonization of the casein, the action is extremely slow 
and not very complete; and it has not seemed to most students that 
the phenomenon in question is sufficiently explained by this slow 
action of the lactic acid bacteria. 
Flavor Production by Bacteria.—Apparently the flavors must 
be due to bacterial action. Cheeses ripened in chloroform vapor, 
which allows the enzymes to act, but prevents bacteria from grow- 
ing, though they ripen, do not develop flavors and these must be 
due to some other cause than enzyme action. That they are the 
end-product of chemical decomposition seems to be extremely 
probable. In many cases they are associated with ammonia; and 
ammonia, as is well known, is one of the final products of proteid 
destruction. The only known agency that commonly produces 
the complete destruction of proteids is bacterial, and, while the 
matter has never been put to any satisfactory test, the most prob- 
able explanation seems to be that these cheese flavors are the 
result of bacterial decomposition. 
Against this view, however, has been urged the fact that in the 
