Why do we knead dough?

The answer

Kneading develops gluten, the elastic protein network that traps the carbon dioxide gas yeast produces. Without enough gluten, the gas escapes and the loaf turns out dense. Hand kneading often takes around ten minutes, and a mixer usually less. The windowpane test, stretching a small piece until it turns thin enough to let light through without tearing, shows when the dough is ready. Long rests and gentle folds can build gluten too, which is why no-knead recipes still work.