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of Bacillus acidi lactici with Bacillus aerogenes. It has been ob- 
served 'by tliis author that in the ordinary souring of milk lactic 
acid is produced chiefly by Streptococcus lacticus and Bacillus aero- 
genes, and further, that the former organism predominates in approx- 
imate proportion to the purity of the milk. Conn (11) also has 
shown that 95 to 100 per cent of all organisms in sour milk are of 
the Bacillus lactis acidi type (Streptococcus lacticus). 
It is now known that lactose (sugar of milk) is not directly fer- 
mentable, but must first be converted into the simpler sugars glucose 
and galactose. It has been shown by a number of investigators that 
many yeasts and bacteria produce an enzyme which is capable of 
effecting this hydrolysis, and Hirschfeld (12) has shown that in the 
souring of milk the lactic acid bacteria accomplish the inversion of 
lactose, and that this change takes place most rapidly in the first 
thirteen to twenty-four hours after the introduction of the organisms 
into the milk. The relative amounts of lactose inverted being: First 
day, 0.16; second day, 0.23; third day, 0.29. Finally in this con- 
nection it has been shown by Buchner and Meisenheimer (13) and 
independently by Herzog (14), that an enzyme can be extracted from 
certain of the lactic acid forming bacteria, which in the absence of 
organisms is able to transform lactose and cane sugar into lactic acid. 
Buchner and Meisenheimer extracted the ferment from Bacillus Del- 
bruecki (Leichmann), whereas in his experiments Herzog employed 
the Bacterium acidi lactici. On the other hand Beyerinck (8) is of 
the opinion that the production of lactic acid by the lactic acid 
bacteria is not a mere enzymic function but a catabolic process. 
The hinds of lactic acid produced in milk by the lactic acid bacteria . — 
As is well known, 4 isomers of lactic acid exist. Three of these are 
stereo-isomers of alpha-oxy-propionic acid or ethylidine lactic acid, 
the chemical structure of which is represented by the formula 
CH3.CHOH.COOH. 
This compound, as indicated by the above formula, contains one 
asymmetric carbon atom, viz, the central one, and hence two optically 
active forms of lactic acid and one optically inactive form composed of 
equimolecular proportions of the two active varieties are possible and 
all three are known to exist. Aqueous solutions of one of these 
forms of lactic acid rotates the plane of polarization to the riglit, the 
second active variety rotates it to the left, and the third inactive 
form has no effect on the plane of polarization. Hence the first acid 
is called dextrolactic acid, and is designated as the d-lactic acid or 
simply d-acid; the second is called lsevo-lactic acid and is designated 
as 1-lactic acid or 1-acid; and the third lactic acid of the above formula 
is called inactive or racemoid lactic acid and is designated (d + 1) 
lactic acid or sometimes r-acid. 
