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Yeast makes dough rise — Chemistry, 4–6 years

Tiny living yeast can change sugars in dough and make a gas. Trapped bubbles push the dough up, so bread becomes light and full of little holes.

What yeast does

Yeast is made of tiny living cells that are too small to see without special equipment. In warm, damp dough, yeast uses some sugar and makes carbon dioxide gas. The dough holds the gas in soft bubbles, making it swell and rise.

Why bakers use it

Flat dough makes a heavy, close loaf, so people wanted bread that was softer and easier to eat. Long ago, bakers noticed that dough left in a warm place became larger and lighter. They learned that yeast was making gas inside it, not simply adding air from outside.

Watching dough rise

Mix flour, water, a little yeast and a little sugar with an adult. Mark the dough’s height on the bowl, then leave it warm for about an hour. If it rises from 5 centimetres to 8 centimetres, yeast has made gas and the dough has trapped bubbles inside.

The reasonable mistake

It is natural to think the dough grows because more flour has appeared, or because it has soaked up water. The bowl has not received more dough from outside. Yeast has made an invisible gas, and many bubbles together take up more room.

Bread and other foods

Bakers use yeast for bread, rolls and pizza dough. The bubbles make the baked food lighter and give it its holey texture. Some doughs use baking powder instead, which also makes gas, but it works by a different chemical process.

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