Why Does Yeast Make Bread Rise?
There is a lot of sugar in the dough from which bread is made. Sugar contains three things—carbon, hydrogen and oxygen. When sugar is fermented it amounts practically to burning it. To make good bread from the dough it is necessary to ferment the sugar which is in the ingredients from which it is made. Yeast, which is a simple living plant, has the power to ferment sugar. When sugar ferments, two things are produced. One thing is the formation of carbonic acid gas. A great deal of this carbonic acid gas is caught in the dough in the form of large or small bubbles and some of it escapes into the air. The other part tries to escape into the air also but cannot, and causes the dough to rise, which makes the bread light, as we say. The holes you see in the bread after it is baked are the little pockets where the carbonic acid gas was retained in the dough. These bubbles of gas all through the dough act like a lot of little balloons and lift the dough up with themselves as they try to get to the top and escape into the air.