The main raw material of bread is flour. When kneading flour into dough, the baker should add a certain amount of yeast. Yeast is a beneficial microorganism, which can multiply in large quantities under certain conditions. When yeast is kneaded into wet dough, it begins to grow and reproduce. Can yeast secrete various substances? Starch is decomposed into dextrin and further into maltose and glucose. Finally, a large amount of carbon dioxide gas is produced. Carbon dioxide gas is distributed in the gluten of dough, making gluten spongy and porous. After kneading and baking, the carbon dioxide in the dough expands when heated, which makes the bread loose and porous.