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I The hinds of lactic acid produced in milk hy the lactic acid hac- 
I tevia . — As is well known, 4 isomers of lactic acid exist. Three of 
these are stereo-isomers of alpha-oxy-propionic acid or ethylidine 
I lactic acid, the chemical structure of which is represented by the 
formula 
CH 3 .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 
j of equimolecular proportions of the two active varieties are possible 
j 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 
i second active variety rotates it to the left, and the third inactive 
form has no effect on the plane of polarization. Hence the first 
I acid is called dextrolactic acid, and is designated as the d-lactic acid 
; or simply d-acid; the second is called Isevo-lactic acid and is desig- 
I 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 
i (d -j- 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 
i lactic acid. It has the chemical structure rej^resented by the formula 
CH30H.CH,.C00H. 
' 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. 
' 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. 
J In 1895, however, it was shown by Gunther and Thierf elder (15) 
j- that the lactic acid present in milk which has soured spontaneously 
|, does not always consist entirely of inactive lactic acid. They showed, 
I in fact, that while the inactive acid was present in naturall}^ sour milk 
I there was often a preponderance of the dextro-rotatory acid. Fur- 
Ip 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. 
