367 
Quite recently Beyerinck (21) has again discussed the lactic fer- 
mentation of milk. He has found that temperature and oxygen 
pressure determine the nature of the autofermentation of milk. At 
temperatures below 40° C. the fermentation brought about by B. 
coli is replaced by a butyric acid fermentation, which, after lasting 
some time, is succeeded by a lactic acid fermentation. In good milk, 
even at 40° C., at which temperature gas-producing bacteria develop 
most rapidly, no gas is produced. This fact therefore forms the 
basis of a dairy test for judging of the purity of milk. He .recog- 
nizes three forms of lactic acid fermentation depending on the tem- 
perature. At very low temperatures there occurs the slimy lactic acid 
fermentation, which, according to this author, is due to the smaller 
cell walls of the organism. At medium temperatures the common 
lactic acid fermentation predominates, this being caused by the lacto- 
coccus and at higher temperatures the lactic acid fermentation 
caused by the lactobacillus. Methods for isolating these organisms 
from milk are given, and also their morphological characteristics 
and their zymotic reactions. He, like other observers, has found the 
lactic acid ferment to be very variable. 
Reference has already been made to the fact that the lactic acid 
fermentation of milk is used commercially in the manufacture of 
lactic acid. The lactic acid fermentation of milk is also turned to 
practical account in the manufacture of cheese. It has been shown 
by Epstein (22) that the ripening of cheese is due largely to the 
action of organisms which induce the lactic acid fermentation. 
Each particular kind of cheese is produced by the agency of special 
organisms which act chemically by means of enzymes and give rise 
to the peculiar odor and flavor of the cheese. These organisms are 
chosen both with regard to their power to induce the lactic acid fer- 
mentation and also with regard to the peculiar kind of cheese desired. 
Similar views regarding the ripening of cheese are held by Von 
Freudenreich (23). According to this author the lactic acid bacteria 
play the preponderating if not the exclusive role in the ripening of 
Emmenthaler cheese.- Similarly Boekhaut and de Vries (24) have 
shown that cheese which does not contain the lactic acid bacteria 
does not ripen. On the other hand Chodat and Hofman-Bang (25) 
are of the opinion that the importance of the lactic acid bacteria in 
the ripening of cheese has been overestimated, and attribute the 
greater number of the changes occurring in this process to another 
organism — namely, tyrothrix. 
In this connection it is interesting to note that Van Slyke (26) 
found that when only rennet is allowed to act on milk no cheese 
flavor is developed. 
