Bread & Beer Production IGCSE Biology 0610

Bread and Beer Production: Yeast and Anaerobic Respiration

Biology editorial review is supported by the named review panel. Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.

Key concept

Yeast carries out anaerobic respiration (fermentation), converting glucose into ethanol and carbon dioxide. In bread making, the CO2 causes the dough to rise. In beer making, the ethanol is the desired product and CO2 gives the fizz.

Bread and beer questions test understanding of anaerobic respiration in yeast. Students must write the word equation, explain the role of each product, and describe the conditions needed for fermentation.

Anaerobic respiration in yeast

Yeast is a single-celled fungus that can respire both aerobically and anaerobically. In the absence of oxygen, yeast carries out anaerobic respiration (also called fermentation):

glucose → ethanol + carbon dioxide

C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂

This process releases a small amount of energy compared to aerobic respiration because the glucose is only partially broken down. The ethanol still contains chemical energy.

Bread making

In bread making, yeast is mixed with flour (which contains starch), water and sugar. The yeast produces enzymes that break down some starch into glucose. The yeast then ferments the glucose, producing carbon dioxide and ethanol.

The carbon dioxide gas forms bubbles in the dough, causing it to rise and giving bread its light, spongy texture. The dough is then baked at a high temperature, which kills the yeast, evaporates the ethanol, and sets the structure. The small amount of ethanol produced evaporates during baking.

Beer making

In beer production, barley grains are allowed to germinate (malting), which activates enzymes that break down starch into maltose (a sugar). The germination is then stopped by heating. The maltose is dissolved in water and hops are added for flavour.

Yeast is added to the liquid, and fermentation occurs in sealed vessels. The yeast converts the sugars into ethanol (the alcohol in beer) and carbon dioxide (which provides the fizz or carbonation). The conditions are kept anaerobic by sealing the vessel, and at a suitable temperature (around 25-30 degrees C) for optimum yeast enzyme activity.

Conditions for fermentation

Successful fermentation requires: a sugar substrate (glucose or maltose), yeast as the microorganism, anaerobic conditions (absence of oxygen), and a suitable temperature (warm but not too hot, typically 25-35 degrees C).

If the temperature is too low, yeast enzymes work slowly and fermentation is slow. If it is too high, the yeast enzymes denature and fermentation stops. If oxygen is present, yeast respires aerobically, producing carbon dioxide and water instead of ethanol.

Try a focused question (3 marks)

Explain why bread dough rises when yeast is added.

Show the answer and marking guidance

Yeast carries out anaerobic respiration (fermentation), using sugars in the dough.
Carbon dioxide gas is produced as a product of fermentation.
The carbon dioxide forms bubbles that become trapped in the dough, causing it to rise and expand.

Original Cambridge-style practice written for this site. It is not an official Cambridge past-paper question.

Common mix-ups to avoid

Check these points against your own answer before moving on.

Saying yeast produces oxygen (it produces carbon dioxide and ethanol).
Writing the aerobic respiration equation instead of the anaerobic one.
Forgetting that the ethanol evaporates during baking, so bread does not contain alcohol.
Not mentioning that anaerobic conditions (absence of oxygen) are needed for ethanol production.