THE USEFUL MICROBES 
57 
is always in dough, after it is mixed, enough 
sugar for the yeast to grow in and cause 
fermentation. It begins to feed on the dough 
and thus ferments the sugar, making carbon 
dioxide and alcohol. The alcohol mixes with 
the water in the dough, and the carbon dioxide 
forms into small bubbles. The dough is so 
sticky and heavy that the gas cannot rise out 
of it into the air, as it does from liquids. It 
collects as small bubbles in the midst of the 
dough, and causes the dough to swell out. As 
a result, the bread “rises.” Then, after the 
proper kneading, the dough is put in the oven 
to bake. The heat of baking drives off the 
small amount of alcohol that has been formed 
and makes the bubbles of gas expand so as to 
enlarge the holes. It also hardens the dough 
into the firmness of bread. The holes formed 
by the gas are changed into the countless tiny 
openings in the bread, making it light and easy 
to digest. Yeast also gives the bread its flavor. 
The yeasts used in raising bread have been 
carefully chosen from the many kinds of yeasts 
How does 
bread rise? 
