By the end of this lesson
- Describe the role of yeast in bread making.
- Explain the conditions needed for dough to rise.
- Link yeast respiration to the word equation for anaerobic respiration.
BIG QUESTION
Why is bread full of holes?
The holes are bubbles of carbon dioxide made by yeast respiration and trapped by the dough.
Making dough
Flour, water, sugar and yeastA single-celled fungus used in baking and brewing; it respires aerobically and then, once oxygen runs out, anaerobically to produce ethanol and carbon dioxide. are mixed and kneaded to make dough. Enzymes in flour break some starch into sugars that yeast can use.
Kneading develops a stretchy protein network (gluten) that can trap gas bubbles.
Yeast respiration makes the dough rise
Yeast first respires aerobically, then mainly anaerobic respirationRespiration without oxygen, in the cytoplasm; it releases much less energy per glucose molecule than aerobic respiration. once oxygen in the dough is used up: glucose → ethanol + carbon dioxide.
The carbon dioxide forms bubbles trapped in the dough, so the dough rises (proves). This is done in a warm place because yeast enzymes work faster at a warm, not hot, temperature.
Baking
During baking, high temperatures kill the yeast and stop respiration. The gas bubbles expand, then the dough sets around them, leaving the holes in bread.
The small amount of ethanolThe alcohol produced by anaerobic respiration in yeast and plant cells (alcoholic fermentation). produced evaporates during baking.
Stages of bread making
Link each stage to what the yeast is doing.
| Stage | What happens | Role of yeast |
|---|---|---|
| Mixing and kneading | Ingredients combined; gluten network forms. | Yeast mixed through dough. |
| Proving (warm) | Dough rises. | Anaerobic respiration releases CO₂ bubbles. |
| Baking | Bread sets; ethanol evaporates. | Yeast killed; respiration stops. |
Language that earns precision
- yeast
- A single-celled fungus used in baking and brewing.
- anaerobic respiration
- Respiration without oxygen; in yeast it produces ethanol and carbon dioxide.
- proving
- Leaving dough in a warm place to rise.
Common misconceptions
Claim: Bread rises because yeast produces oxygen.
Correction: It produces carbon dioxide.
Claim: Bread contains a lot of alcohol.
Correction: The ethanol evaporates during baking.
Claim: The hotter the proving temperature, the faster the rise.
Correction: Above the optimum, yeast enzymes denature and the yeast may be killed.
Match the answer to the command word
Name the gas (carbon dioxide) and the process (anaerobic respiration) explicitly.
Use the word equation glucose → ethanol + carbon dioxide.
Recall, apply, then justify
Explain why bread dough rises when left in a warm place.
Hint
Respiration, gas, enzymes.
Suggested marking points
- Yeast respires (anaerobically).
- Carbon dioxide is produced and trapped as bubbles in the dough.
- Warmth increases yeast enzyme activity, so respiration is faster.
State why bread contains no significant ethanol even though yeast produces it.
Hint
What happens in the oven?
Suggested marking points
- Ethanol evaporates during baking.
Explain why dough made with yeast that has been boiled and cooled does not rise.
Planning hint
Enzymes and living cells.
What a strong design includes
- Boiling kills the yeast cells.
- Yeast enzymes are denatured.
- Respiration cannot occur.
- No carbon dioxide is produced.
- So no gas bubbles form to make the dough rise.
- Cooling does not reverse denaturation.