Yeast: biofuels and breadSpec 21.2.1, 21.2.2
Yeast is a single-celled fungus. When there is no oxygen it respires anaerobically, breaking down glucose to make ethanol (alcohol) and carbon dioxide. This is called fermentation.
glucose → alcohol (ethanol) + carbon dioxide
Supplement (what this means)
Supplement: only for the Extended papers (2 and 4). Core students can skip it. What the labels meanC₆H₁₂O₆ → 2C₂H₅OH + 2CO₂
Biofuels
Sugar from plants such as sugar cane, or sugar made by breaking down starch from maize, is fed to yeast in a closed tank with no oxygen. The yeast respires anaerobically and makes ethanol. The ethanol is separated and used as a biofuel, for example mixed with petrol to fuel vehicles. The plants can be regrown, so it is a renewable fuel.
Bread-making
- Yeast is mixed with flour, sugar and water to make dough, and the dough is kept warm.
- The yeast respires anaerobically and makes carbon dioxide (and ethanol).
- The carbon dioxide forms bubbles that are trapped in the dough, which makes the dough rise.
- When the bread is baked, the gas bubbles expand, the yeast is killed and the dough sets with air spaces. The ethanol evaporates.
Common mistake:
It is the carbon dioxide that makes bread rise, not the ethanol. Write that the yeast respires anaerobically, not just that it 'ferments'.