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 laevulose according to the equation : 



Dextrose Laevulose 



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 milky, showing 

 that the gas evolved consists of carbon dioxide. The contents 

 of the flask after fermentation has continued for some time 

 will be found 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 



