Bread is one of the world’s great staple foods, made by mixing flour, most often wheat flour, with water, yeast or another leavening agent, and salt, then baking the resulting dough. Fermentation by yeast produces carbon dioxide that aerates the dough, while baking heat sets the structure through starch gelatinization and gluten coagulation and creates the crust through Maillard browning and caramelization. The result is a food with a soft, open crumb and a flavoursome crust, supplying energy, protein, fibre and B vitamins. Breads vary enormously around the world, from leavened loaves and rolls to flatbreads, each shaped by local grains and fermentation traditions.