FERMENTS. 471 



way either one of the optical isomers, by selecting the proper organism. 

 Emil Fischer, 1 who early recognized the significance of stereo-chemical 

 influences in biological processes, studied the alcoholic fermentation from 

 this point of view. He found that, of two isomers, yeast will ferment only 

 one; in fact, the following: 



Ferments Does not Ferment 2 

 d-Glucose Z-Glucose 



d-Mannose Z-Mannose 



d-Galactose Z-Galactose 



d-Fructose Z-Fructose 



The configuration of these compounds has been explained by the 

 researches of Emil Fischer, 3 and it is possible from the structural formulae 

 at hand to determine the influence of the configuration upon the attack 

 by yeasts. The formulae of the four fermentable sugars are as follows: 



It is clear from these formulae that d-fructose, d-glucose, and d-mannose 

 closely resemble one another in their stereo-structure. The OH and 

 H groups are arranged alike in three of the four asymmetric carbon atoms. 

 It is interesting to note that these three sugars also possess close chemical 

 relations, as is indicated by many of their transformations, and especially 

 by the fact that they go over into one another, simply on heating with 

 alkali. The d-galactose, in its configuration, does not stand so close to the 

 sugars mentioned. The same also applies to its behavior. It is more 

 slowly fermented; in fact, some varieties of yeast, such as Saccharomyces 

 apiculatus and productivis, do not act upon it at all. All of the other 

 known aldose and ketose sugars remain unacted upon by yeasts. The 



1 Z. physiol. Chem. 26, 60 (1898-99). 



3 Ber. 23, 382, 2620; 25, 1259; 27, 2031; 27, 2985; 27, 3479; 28, 1429; 27, 2035 

 (1894). 



3 Compare Lecture II, p. 17. 



