FEK.ME^'TATIOX OEGAXISMS 105 



sterilizer. After sterilizatioa add 5 cc. of 95 per cent alcohol to 

 flask No. 1 ; to flask Xo. 2, enough dextrose to make the sugar 

 content up to four per cent. Add to each of the three flasks 

 about 3 cc. of a fresh, active yeast culture and fit them with 

 fermentation stoppers. Incubate at 25° C. "Weigh the flasks 

 ever}- two days until the weight is constant, and estimate the 

 grams of carbon dioxide given off. When the fleisks reach con- 

 stant weight, attach a condenser and distill off the alcohol from 

 each separately. Determine its specific gravity either by a pyc- 

 nometer or by a hydrometer. From this compute the percentage 

 and the exact amount of alcohol formed from the sugar. Deter- 

 mine the amount of sugar remaining in the flask by the method 

 given above. 



Write the result, giving formulae with balanced equations. 



Exercise 147. The Fermentation of Bread Dough 



A dough of flour, water, and yeast will rise ff kept in a 

 warm place. The flour contains starch and a small amount of 

 diastase, an enzyme which forms sugars from starch. The yeast 

 ferments the sugars thus formed into alcohol and carbon dioxide. 

 The bubbles of gaseous carbon dioxide give the dough a porous 

 structure which is not destroyed when baked. The porous struc- 

 ture makes the bread easier to masticate and hence easier to digest. 



A dough of flour and water onl}- wiU rise if kept in a warm 

 place. The action is due to the production of carbon dioxide 

 by organisms occurring naturally in the flour and water. But the 

 rising will be much more rapid and certain if at the beginning a 

 quantity of yeast (leaven) is incorporated with the dough. The 

 leaven may be a piece of dough kept from a previous baking, or 

 it may be a yeast cake. The commercial yeast cake contains an 

 admixture of more or less bacteria, which are sometimes respon- 

 sible for flavors in the bread. Relatively pure cultures of yeast 

 are used by bakers who wish a rapid rising with little danger of 

 irregularities due to bacterial action. 



1. Soak a. cake of compressed yeast for an hour in distilled 

 water and examine the organisms it contains. Both yeasts and 



