Once the bread is mixed it is then left to rise (ferment).
As fermentation takes place the dough slowly changes from a rough dense mass lacking extensibility and with poor gas holding properties, into a smooth, extensible dough with good gas holding properties.
The yeast cells grow, the gluten protein pieces stick together to form networks, and alcohol and carbon dioxide are formed from the breakdown of carbohydrates (starch, sugars) that are found naturally in the flour.
The yeast uses sugars in much the same way as we do, i.e. it breaks sugar down into carbon dioxide and water. Enzymes present in yeast and flour also help to speed up this reaction.
When there is plenty of oxygen present the following reaction occurs:
The energy which is released is used by the yeast for growth and activity.
In a bread dough where the oxygen supply is limited, the yeast can only partially breakdown the sugar. Alcohol and carbon dioxide are produced in this process known as alcoholic fermentation.
The carbon dioxide produced in these reactions causes the dough to rise (ferment or prove), and the alcohol produced mostly evaporates from the dough during the baking process.
During fermentation each yeast cell forms a centre around which carbon dioxide bubbles form. Thousands of tiny bubbles, each surrounded by a thin film of gluten form cells inside the dough piece.
The increase in dough size occurs as these cells fill with gas.