Ip8 BACTERIA AND OTHER MICROORGANISMS IN CHEESE. 
they are necessary to the ripening is proved by the fact that cheeses 
do not ripen normally when they are ripened in chloroform vapor, 
which prevents bacteria growth, but allows the enzyme action to 
continue as usual. Although some of the digestive changes go on as 
usual in these cheeses, the ripening does not become complete, 
and the cheeses never develop either the same final chemical char- 
acter or the flavors of cheeses in which the bacteria have had an 
opportunity for growth. 
Chemical Action of Bacteria. When cheese-ripening was 
first studied, it was believed to be primarily due to the action of 
bacteria. We have already seen that certain kinds of bacteria have 
the power of changing casein to peptone the liquefying type and 
this change in the cheese-ripening was at first supposed to be due to 
the growth of these peptonizing bacteria. But later it became evi- 
dent that the liquefying bacteria are not common in cheeses, espe- 
cially in the better grades. If present at the beginning they rapidly 
decrease in numbers, until they almost or entirely disappear, a fact 
which forced the conclusion that they cannot contribute materially 
to the ripening of cheeses. More recently, it has been claimed that 
certain "acid liquefiers" i.e., peptonizing bacteria that at the same 
time produce acid are intimately connected with the ripening. 
But there does not yet appear to be much evidence for this. 
These facts led to a suggestion that the ripening is due really to 
the lactic acid bacteria. These do not liquefy gelatin and do not 
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 upon it 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 peptoniza- 
tion 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 conclusion that it is 
this peptonizing power of the lactic acid bacteria, under certain con- 
