Gluten

Two groups of wheat proteins, glutenins and gliadins, link together when flour is wet and worked. The more a dough is kneaded, the stronger and more elastic the network becomes, which is what lets bread hold the gas from yeast. For tender cakes, muffins and pastry, bakers do the opposite: they mix briefly, use lower-protein flour and add fat and sugar, which limit gluten.

Bread flour has more protein, so it can form more gluten than all-purpose or cake flour. Rye and barley contain related proteins. People with celiac disease must avoid gluten entirely, which is why gluten-free baking relies on other ingredients for structure.