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that occur in milk. The fact that on standing at ordinary tempera- 
tures milk gradually turns sour and finally curdles has been known 
ever since milk was first used as a food by man. In early times the 
acid of milk was supposed to be acetic acid, the same as is present 
in vinegar, and as has already been pointed out this acid does, accord- 
ing to Bechamp ( 1 ) , occur in even freshly drawn milk in small quan- 
tities. The substance really responsible for the souring of milk, how- 
ever, viz, lactic acid, was first discovered in milk by Scheele in 1780. 
The new acid was also studied by Berzelius, and its composition 
definitely established through the work of Mitscherlich and Liebig in 
1832. Its chemical constitution and its relation to other varieties of 
lactic acid, occurring in nature or the products of chemical synthesis, 
were first established by the labors of Strecker, Erlenmeyer, and 
lYislecenus. 
In 1847, Blondeau (2) discovered micro-organisms in sour milk, 
but attached to these no particular significance so far as the souring of 
milk is concerned. It remained for Pasteur (3), in 1857, to definitely 
and conclusively show as one of the results of his classic investigations 
on fermentation that the souring of milk is really a kind of fermenta- 
tion, which is accomplished by a peculiar kind of micro-organism, to 
which he gave the name of levure-^actique (lactic yeast). His first 
communication on this subject was read to the Scientific Society of 
Lille, August, 1857, and afterwards to the French Academy in No- 
vember, 1857. Since then our knowledge of the lactic acid fermenta- 
tion has been considerably extended through the labors of Pasteur’s 
students, and still later through the work of other bacteriologists and 
chemists. For example, Boutroux (4) in 1878, in continuing the in- 
vestigations of Pasteur on the souring of milk, arrived at the conclu- 
sion that the lactic acid ferment and the m}^coderma aceti, which is 
concerned in the transformation of alcohol into acetic acid in vinegar 
making, are identical, but that these vary in function, depending on 
their general environment and the composition of the liquid in which 
they grow. This communication also contains a description of the 
lactic ferment and an enumeration of its morphological characteris- 
tics, which are beyond the scope of the present communication. He 
observed that the organism grew best in a nutrient medium con- 
taining, besides albuminous matter, invert-sugar or glucose. He also 
found that under these conditions the liquid can attain a maximum 
acidity of 1.5 per cent lactic acid. Larger amounts of acid than this 
checked the life and growth of the organism, and hence if it is desired 
to convert all of the sugar into lactic acid the acid must be neutralized 
with chalk or zinc carbonate as fast as formed. Under proper con- 
ditions the lactic acid organism employed by Boutroux produces lactic 
acid only. 
