Why does bread dough rise?

A small lump of yeast dough can swell because tiny gas pockets grow inside it. Yeast is a living fungus made of single cells. In suitable dough, it uses sugars and produces carbon dioxide gas.

Gas production alone is not enough to hold the dough up. When wheat flour and water are mixed, proteins can form a stretchy network called gluten. Small gas pockets already present in the dough provide places where carbon dioxide can collect. The network stretches around those expanding pockets.

The drawing shows a piece of dough before and after its pockets enlarge. The empty-looking circles represent gas, not extra flour. Its outer shape becomes bigger because the material has stretched around more gas space.

If the dough cannot retain gas, much of the gas escapes instead of producing lasting rise. If the yeast is inactive, it supplies no new yeast gas in this simplified comparison. The familiar rising loaf begins with two cooperating processes: living cells make gas, and a stretchy material holds it long enough to expand.