98 BACTERIOLOGY. 



sibly diffuse into the surrounding- liquid, and cause there the 

 characteristic cleavage of sugar into alcohol. It has been 

 usually held that the sugar was absorbed by the cell and 

 then split into alcohol and carbonic acid. While the yeast 

 decomposes most of the sugar into alcohol and other pro- 

 ducts, it utilizes a small portion for the purpose of building 

 up its own cell contents. Cellulose and glycogen may be 

 formed direct from the sugar, whereas the fats and proteins 

 result indirectly. 



The yeast-plant cannot convert starch, or cane-sugar 

 directly into alcohol. .These substances must first be con- 

 verted into glucose. The diastase present in malt is em- 

 ployed to convert the starch of a grain into sugar. In 

 Japan and China certain moulds which possess a marked 

 diastatic action are employed for this purpose. On the 

 other hand, maltose and cane-sugar are changed by a 

 ferment produced by the yeast-cell, invertin, into glucose 

 which is then converted into alcohol and carbonic acid. 



C 12 H 22 O n + H 2 - C 6 H 12 6 + C 6 H 12 6 . 



Cane-sugar. Dextrose. Levulose. 



C 6 H 12 6 = 2C 2 H 6 + 2C0 2 . 



Invert-sugar. Alcohol. 



As is well known, the yeast is also utilized in the mak- 

 ing of dough. It has been mentioned above that the yeast 

 cell is unable to act directly upon starch. It has been sup- 

 posed that the bacteria present invert the starch and thus 

 enable the yeast to carry on an alcoholic fermentation. As 

 a result of the formation of carbonic acid the mass is 

 distended. 



Acetic acid fermentation. Alcoholic liquors such as wine, 

 beer, cider, etc. , on exposure to air undergo acetic acid fer- 

 mentation. The alcohol is oxidized to acetic acid and the 

 resulting liquid is commonly known as vinegar. The free 



