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The lactic acid fermentation of milk sugar was also investigated by 
Richet (5), who found that when milk is kept at 40° C. it becomes 
acid and coagulates and finally attains an acidity of 1.6 per cent, 
which amount it never exceeds. He made the further interesting 
observation that if gastric juice be added to milk the casein is coagu- 
lated and finally dissolved, and in less than twenty-four hours the 
milk contains a larger quantity of lactic acid than otherwise would 
have been produced in a weeR, and after four or five days as much as 
4 per cent of lactic acid was formed. He observed that while neither a 
pure solution of lactose nor gastric juice will ferment, if the two be 
mixed fermentation takes place; and that the casein of milk after it 
has been dissolved by gastric juice also ferments, yielding lactic and 
butyric acids, besides other products of fermentation. On the other 
hand, the whey of milk obtained by coagulation with rennin never 
attains an acidity greater than 1.6 per cent of lactic acid, even after 
having been kept for six months. He found that the lactic acid 
fermentation is increased by exposing a large surface of the milk to 
the air. The activity of the ferment increases up to 44° C., remains 
constant between 44 and 52° C., and above 52° C. diminishes in 
activity as the temperature rises. Digestive juices and peptones 
were found to aid lactic fermentation, but leucine and gly cocoll 
were found to have no effect upon the process. 
The general trend of more recent investigations on the subject of 
lactic acid fermentation has been to show that the change of milk 
sugar into lactic acid takes place under the influence, either direct or 
indirect, of a whole series of micro-organisms, whose number has been 
considerably augmented by recent investigations in this field. Marp- 
mann (6), for example, during the summer of 1885 investigated the 
micro-organisms of cow’s milk in the neighborhood of Goettingen 
and detected five seemingly new and different species of organisms 
which more or less strongly induce the lactic acid fermentation in 
solutions of cane sugar and also in milk. 
Leaving out of consideration the levure lactique of Pasteur, the first 
of these organisms whose morphological and biological character- 
istics seem to have been determined with sufficient accuracy is the 
Bacillus acidi lactici of Hueppe (7). It is now known that in addi- 
tion to the Bacillus acidi lactici (Hueppe) the following organisms 
can bring about the lactic acid fermentation, viz, Bacillus aerogenes, 
Bacillus coli. Bacillus lactis acidi (Leichmann and others), Strepto- 
coccus lacticus (Kruse), Streptococcus pyogenes, Pneumonococcus A 
and Pneumonococcus B, Bacillus Delbruecki (Leichmann), Bacillus 
acidificans longissimus (Lafar), etc. 
Beyerinck (8) has also made exhaustive studies of the lactic acid 
ferments employed in the arts. This author applies the name 
Lactobacillus Delbruecki to all species of the lactic-acid ferment 
