Lesson 4 of 5

Fermentation

Goal: Describe fermentation by yeast and its uses.

Puzzles for this lesson

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A bowl of bread dough that has risen high above the rim

Yeast can respire without oxygen. This kind of anaerobic respiration is called fermentation.

glucose → ethanol + carbon dioxideA little energy is released.

Swipe to meet yeast and see two uses of fermentation.

A drawing of oval yeast cells, hugely magnified

Yeast

A microorganism: a fungus made of single cells. This drawing shows them hugely magnified.

A loaf of bread cut open to show the holes inside

Making bread

Bubbles of carbon dioxide make the dough rise. The holes in a loaf are where the bubbles were.

A glass jar of grape juice fermenting, with froth on top

Brewing

Yeast ferments the sugar in grape juice or in barley. The ethanol is the alcohol in wine and beer.

  • Ethanol and carbon dioxide are the products. Ethanol is an alcohol. Carbon dioxide is a gas.
  • Warmth speeds fermentation up. That is why dough is left to rise in a warm place. Too much heat kills the yeast.
  • What about the ethanol in bread? Nearly all of it evaporates in the hot oven.

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Key words in this lesson

Respiration A chemical reaction in cells that releases energy from glucose

Anaerobic respiration Respiration without oxygen

Glucose The sugar that cells break down in respiration

Oxygen The gas that aerobic respiration uses up

Carbon dioxide The gas made in aerobic respiration and in fermentation

Fermentation Anaerobic respiration in yeast

Yeast A fungus made of single cells, used to make bread and beer

Ethanol The alcohol made in fermentation

Microorganism A living thing too small to see without a microscope

Product A substance that is made in a chemical reaction

For teachers: AQA 3.9.3 · Pearson Edexcel 1.5 in part