A number of foods also include lactic acid to alter the pH balance or change the flavor. Certain types of bacteria added to milk, for example, produce the acid, which helps create the texture and tartness found in yogurt. Sourdough bread often relies on both yeast and bacteria in the air around the dough's starter for its tart flavor, usually caused by the formation of acid within the bread. Beer and wine sometimes include bacteria that produces this acid, which can help eliminate other, somewhat unpleasant flavors produced during fermentation.