Yeast makes dough expand by turning sugars into carbon dioxide gas and ethanol during fermentation. The carbon dioxide becomes trapped in the stretchy dough, forming bubbles that make the dough rise. This process matters because it gives bread its light texture, open crumb, and characteristic flavor.
Without yeast activity, many breads would stay dense and flat.
Yeast cells are living fungi that use enzymes to break down simple sugars for energy when mixed into moist dough. In low oxygen conditions inside dough, yeast performs alcoholic fermentation, producing CO2 and ethanol. Gluten proteins from wheat flour form a flexible network that stretches around the gas bubbles instead of letting them escape.
Warmth speeds yeast metabolism, but too much heat can kill the cells and stop rising.
Key Facts
- Yeast fermentation equation: C6H12O6 -> 2 C2H5OH + 2 CO2 + energy
- Carbon dioxide gas expands dough by filling bubbles trapped in the gluten network.
- Gluten forms when wheat proteins glutenin and gliadin combine with water and are kneaded.
- Yeast works best in warm dough, often about 25 to 35 °C for rising.
- Temperatures above about 50 to 60 °C can kill yeast and stop fermentation.
- During baking, trapped gases expand and dough structure sets as starches gelatinize and proteins coagulate.
Vocabulary
- Yeast
- Yeast is a single-celled fungus that uses sugars for energy and produces carbon dioxide during fermentation.
- Fermentation
- Fermentation is an anaerobic process in which cells release energy from sugar without using oxygen.
- Carbon dioxide
- Carbon dioxide is a gas produced by yeast that forms bubbles and makes dough rise.
- Gluten
- Gluten is a stretchy protein network in wheat dough that traps gas bubbles and gives bread structure.
- Proofing
- Proofing is the resting time when yeast ferments sugars and dough rises before baking.
Common Mistakes to Avoid
- Using water that is too hot, which can kill yeast cells and prevent the dough from rising.
- Thinking yeast creates air from nothing, which is wrong because yeast converts sugar into carbon dioxide gas and ethanol.
- Skipping kneading completely, which can leave the gluten network too weak to trap carbon dioxide bubbles well.
- Adding too much salt directly onto yeast, which can slow yeast activity by drawing water out of cells through osmosis.
Practice Questions
- 1 A dough starts at 1.2 L and rises to 2.0 L after proofing. By how many liters did its volume increase, and what is the percent increase?
- 2 One glucose molecule produces 2 carbon dioxide molecules during fermentation. If yeast ferments 0.50 mol of glucose, how many moles of CO2 are produced?
- 3 A dough made with yeast rises poorly even though sugar is present. Explain two possible biological or structural reasons why the dough did not expand well.