The primary function of a bread starter is to act as a natural leavening agent. The starter contains yeast and lactic acid bacteria. As these microorganisms feed on the sugars in the flour, they produce carbon dioxide gas, which causes the dough to rise. Another function of the starter is to add flavor to the rolls. Finally, a starter contributes to the development of a more open crumb structure and a thicker, more artisanal crust.