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.
Yeast uses sugars by breaking them down into carbon dioxide and water. The yeast needs lots of oxygen in order to complete this type of fermentation.
In a bread dough, oxygen supply is limited and the yeast can only achieve partial fermentation and instead of carbon dioxide and water being given off, carbon dioxide and alcohol are produced. This is called 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.