Yeast cells contain a large number of enzymes, which can decompose starch into glucose or convert glucose into alcohol to produce carbon dioxide. In the process of brewing wine, yeast ferments juice or grain to produce alcohol. When making bread, the carbon dioxide produced by yeast will cause many small holes in the dough, which will make the bread swell and soften, and the alcohol contained in the dough will evaporate during steaming and baking.