Yeast and bread making

Use of biological resources · Food production: micro-organisms and fish farming · note 1 of 4

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Yeast and bread makingSpec 5.5

In short

Yeast is a single-celled fungus used to make bread because it respires anaerobically, breaking down glucose into ethanol and carbon dioxide. In warm dough, bubbles of carbon dioxide are trapped and make the dough rise. Baking kills the yeast, expands the gas bubbles so the bread is light, and evaporates the ethanol.

Yeast is a single-celled fungus. It is used to make bread because of the way it respires.

When yeast has no oxygen, it respires anaerobically. This is also called fermentation. It breaks down sugar (glucose) to make ethanol and carbon dioxide, and releases energy.

glucose → ethanol + carbon dioxide

Making bread

Flour, water, sugar and yeast are mixed to make a dough. The dough is kneaded.
The dough is left in a warm place. The yeast respires the sugar and releases carbon dioxide.
The bubbles of carbon dioxide are trapped in the dough, so the dough rises.
The dough is baked. The heat kills the yeast, the gas bubbles expand to make the bread light and full of holes, and the ethanol evaporates.

Exam tip:

It is the carbon dioxide that makes the dough rise, not the ethanol. Keep the warm temperature in your answer: it is the optimum range for the yeast enzymes.

Written and checked against the Edexcel IGCSE Science Double Award (4SD0) specification · Updated October 2026

Frequently asked questions

What is anaerobic respiration in yeast?

Anaerobic respiration in yeast is respiration without oxygen, also called fermentation. Yeast breaks down glucose to make ethanol and carbon dioxide, releasing energy: glucose → ethanol + carbon dioxide. Bread making uses this, because the carbon dioxide bubbles trapped in the dough make it rise, and the ethanol evaporates during baking.

How is yoghurt made using bacteria?

Yoghurt is made by adding Lactobacillus bacteria to milk that has been pasteurised and cooled, then incubating it at about 40–45 °C. The bacteria turn the milk sugar lactose into lactic acid. The acid lowers the pH, making the milk proteins clump so the milk thickens, and stops many other micro-organisms growing.

Why are conditions controlled in an industrial fermenter?

Conditions are controlled so the micro-organisms grow and make the product as quickly as possible. Aseptic precautions stop unwanted microbes competing or spoiling the product. Nutrients supply materials for growth, the optimum temperature and pH let enzymes work fastest, sterile air allows aerobic respiration, and stirring spreads nutrients, oxygen and heat evenly.