What makes bread dough rise
Alcoholic fermentation of glucose · Fermentation · Redox · Exothermic
Every slice of bread is full of holes blown by yeast exhaling carbon dioxide, and the dough briefly contains alcohol.
Reactants approach
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ΔH = -100 kJ · exothermic, releases heat
ΔH per mol glucose; catalysed by yeast enzymes.
Meet the molecules
- Reactantdissolved in water
C6H12O6
glucose
- Productdissolved in water
C2H5OH
ethanol
- Productgas
CO2
carbon dioxide
What you notice
Mix flour, water and a spoonful of yeast, leave the dough somewhere warm, and an hour later it has roughly doubled in size. Yeast are single-celled fungi. With little oxygen inside the dough, they break down sugars by fermentation, releasing carbon dioxide and ethanol. Gluten, the stretchy protein network formed when wheat flour is kneaded, traps the CO2 in thousands of small pockets that inflate like balloons. In the oven the gas expands further, the ethanol evaporates, and the starch and proteins set around the holes you see in a slice of bread.
Try it
Stir a teaspoon of dried yeast and a teaspoon of sugar into a bottle of warm (not hot) water, stretch a balloon over the neck, and watch it inflate over an hour.
Step by step
- 1
Starch becomes sugar
Enzymes from the flour and the yeast cut starch into glucose and other small sugars that yeast cells can take in.
- 2
Glycolysis splits glucose
Inside each cell, a ten-step pathway splits one six-carbon glucose into two three-carbon pyruvate molecules, earning the cell a small amount of ATP.
- 3
Carbon dioxide leaves
Each pyruvate loses one carbon as CO2, leaving acetaldehyde, CH3CHO. This CO2 is what inflates the dough.
- 4
Ethanol forms
Acetaldehyde is reduced to ethanol by NADH. This regenerates NAD+, which glycolysis needs to keep running without oxygen.