In the production of baked goods, yeast is a key ingredient and serves three primary functions:
Carbon dioxide is generated by the yeast as a result of the breakdown of fermentable sugars in the dough. The evolution of carbon dioxide causes expansion of the dough as it is trapped within the protein matrix of the dough.
This is accomplished by the chemical reaction of yeast produced alcohols and acids on protein of the flour and by the physical stretching of the protein by carbon dioxide gas. This results in the light, airy physical structure associated with yeast leavened products.
of fermentation flavor:
Yeast imparts the characteristic flavor of bread and other yeast leavened products. During dough fermentation, yeast produce many secondary metabolites such as ketones, higher alcohols, organic acids, aldehydes and esters. Some of these, alcohols for example, escape during baking. Others react with each other and with other compounds found in the dough to form new and more complex flavor compounds. These reactions occur primarily in the crust and the resultant flavor diffuses into the crumb of the baked bread.