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The fourth isomer of lactic acid has an altogether different chemical 
constitution from the other 3 forms of the acid and is known to 
chemists as beta-oxypropionic acid, hydracrylic acid, or ethylene 
lactic acid. It has the chemical structure represented by the formula 
CH 2 OH.CH 2 .COOH. 
This acid contains no asymmetric carbon atom. It therefore 
exhibits no optical activity and only one form of it is known. This 
is not a product of the lactic acid fermentation and hence does not 
further concern us in this connection. In the changes occurring in 
the fermentation of milk we are concerned with only the first 3 forms 
of the acid, viz, with the optical isomers of alpha-oxypropionic acid. 
Formerly it was generally accepted as pretty well established that the 
lactic acid produced in the souring of milk consisted -mainly if not 
entirely of the r-acid. Indeed ordinary lactic acid, viz, the r-acid, 
was frequently spoken of as fermentation ‘ ‘ Gahrungs lactic acid. 
In 1895, however, it was shown by Gunther and Tliierf elder that the 
lactic acid present in milk which has soured spontaneously does not 
always consist entirely of inactive lactic acid. They showed in fact 
that while the inactive acid was present in naturally sour milk there 
was often a preponderance of the dextro-rotatory acid. Further, the 
Bacillus lactis acidi (Streptococcus lacticus, Kruse), in pure lactose, 
was found invariably to produce the d-acid. 
In this connection Gadamer (16) has observed that commercial 
lactic acid is either inactive or dextro rotatory. 
Quite recently Heinemann (10) has made an exhaustive study of 
the kinds of lactic acid produced in milk by the lactic acid bacteria. 
The following are his conclusions : 
1. Milk naturally soured at room temperature contains chiefly d-acid. Milk soured 
at 37° C. contains chiefly r-acid with 1-acid in excess if allowed to stand several days. 
2. Streptococcus lacticus and Streptococcus pyogenes produce the same kind of 
lactic acid, i. e., d-acid. B. aerogenes from milk (B. acidi lactici) and the ordinary 
laboratory strain of B . (lactis) aerogenes (Escherich) produce the same kind of lactic 
acid, i. e., 1-acid. 
3. The lactic acid produced in naturally-soured milk varies: 
(а) According to the relative numbers of Streptococcus lacticus and B . aerogenes 
present. The higher the number of B. aerogenes the more 1-acid is produced. 
(б) According to the temperature at which the fermentation takes place other con- 
ditions being equal. At 37° 0. relatively more 1-acid is formed than at room tem- 
perature. 
(c) According to the length of time the fermentation has lasted. The longer the time 
the more 1-acid is formed. 
4. In “certified” milk, d-acid. only was present at room temperature for nine days, 
while both d-acid and 1-acid were present in milk of poorer quality after one to four 
days. At 37° C. 1-acid was apparent after six days in “certified” milk and on the 
second day in other milk. It seems as if the purer the milk the longer the excess of 
d-acid persists. , 
