CHAPTEE VIII 



THE ALCOHOLIC FERMENTATION OF GRAPE SUGAR 



It has already been shown how by the action of the enzyme 

 invertase, secreted by the yeast cell, ordinary cane sugar takes 

 up the elements of water to form a molecule of dextrose and a 

 molecule of Isevulose according to the equation : — 



C,,H,,0,, + H,0 = C^,,0, + Cfi,,0, 



Dextrose LffiTulose 



and it was shown how this enzyme could readily be extracted 



from the yeast. If yeast is allowed to develop in a solution 



of sugar an entirely different and more profound change 



takes place. This may be demonstrated by the following 



experiment. 



About eight grams of cane sugar are added to about 200 c.c. 



of water in an ordinary half-litre flask, and about 1 c.c. of 



fresh brewer's yeast added. The flask is then placed in an 



incubator at a temperature of 24° C, and after some time an 



effervescence of gas takes place. If a stopper with a bent 



tube is attached to the flask and the tube led below the 



surface of a little lime water, the latter will turn nulky, showing 



that the gas evolved consists of carbon dioxide. The contents 



of the flask after fermentation has continued for some time 



will be foimd to have an alcoholic smell. If the flask is now 



attached to a Liebig's condenser, and placed on a water-bath, 



the alcohol can be distilled over. It is possible more simply 



to demonstrate its presence by attaching a long tube to the 



k2 



