88 PHYSIOLOGY OF YEASTS 



HCO 



OHCH 



OHCH 



OHCH 



HCOH 



H 2 COH 



d-talose 



This indicates the differences, although very slight, in the molec- 

 ular arrangement of the molecules which has much effect on enzyme 

 action. It is thus that d-glucose and 1-glucose differ from one another 

 only by the inverse position of their molecules. The stereochemical 

 formula of one represents the image of the other as seen in a mirror. 

 This structure is sufficient, however, to render d-glucose fermentable 

 and to repress the fermentability of 1-glucose. The differences between 

 the grouping of d-galactose and d-talose are very slight. So slight are 

 they that d-galactose is fermentable and d-talose is not. 



Among the ketohexoses only d-fructose and levulose are ferment- 

 able. 1 



It is proper to add that the yeasts have a role in the fermentabil- 

 ity of the hexoses. A true electivity has been established for certain 

 sugars. Dubrunfaut has shown that, in a mixture of yeasts, one yeast 

 will attack one hexose while another will decompose another hexose. 



Formula of Alcoholic Fermentation and Secondary Products 



Alcoholic fermentation consists in the transformation of sugars 

 into carbon dioxide and ethyl alcohol. This change is accompanied 

 by a liberation of heat, known for a long time in the fermentation of 

 grape juice, and observed by Buchner in the fermentation induced 

 by yeast juice. Bouffard has established a liberation of 20 to 23 

 calories for each 180 grams of sugar destroyed. A. Brown has ob- 

 tained 21.4 calories. 



Gay-Lussac has represented the alcoholic fermentation by the 

 following simple equation 



C 6 H 12 6 = 2C 2 H 6 O + 2CO 2 



This equation does not take into consideration the heat exchange, 

 for it is very difficult to express by such a simple formula a phenome- 

 non of such complexity. It merely gives a general idea with regard 

 to the change, but does not take into consideration the secondary 

 products which are formed during the fermentation. Pasteur has 

 1 Fischer, E. and Thierfeldsr, H. Berichte, 27, 1894. 



