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The kinds of lactic acid produced in milk by the lactic acid bac- 
teria . — 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 lactip 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 right, 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 laevo-lactic acid and is desig- 
nated 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. 
The fourth isomer of lactic acid has an altogether different chemical 
constitution from the other three 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 asymetric carbon atom. It therefore ex- 
hibits 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. 
F ormerly 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 Giinther and Thierf elder (15) 
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. Fur- 
ther, 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. 
