How yeast makes dough rise — Chemistry, 7–10 years
Yeast is a living microorganism used in bread-making. It uses sugar and releases carbon dioxide gas; bubbles of that gas become trapped in the dough and make it expand.
Yeast releases gas in dough
Yeast feeds on some sugar in warm, damp dough. It changes the sugar and releases carbon dioxide gas, whose bubbles push the stretchy dough outward. It also makes small amounts of other substances that affect bread’s smell and taste.
Why did bakers wait?
Before people understood yeast, they noticed that some dough became light and bubbly if it was left in a warm place. The practical problem was how to make dense flour paste into softer bread. Studying yeast explained why time, warmth and sugar can change the dough.
A bottle and a balloon
1. Put warm water, a teaspoon of sugar and a teaspoon of dried yeast in a bottle. 2. Stretch a balloon over the neck and wait 15 minutes. 3. The balloon begins to inflate because yeast releases carbon dioxide. In cold water the change is slower; very hot water can damage the yeast.
The dough does not rise because air is pumped in
It is reasonable to think that kneading simply mixes air into the dough, since holes are visible later. But yeast makes new carbon dioxide bubbles inside the dough while it rests. Kneading spreads the yeast and helps the dough become stretchy enough to hold those bubbles.
Bread, pizza and other foods
Bakers use yeast fermentation to make bread and pizza lighter, with a softer texture and a distinctive smell. Similar processes help make some drinks and foods, such as yoghurt or fermented vegetables, though different microorganisms and conditions are involved. Temperature and time must be controlled.
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