Food Production
Use of Biological Resources
Yeast is a single-celled fungus. In bread-making it is mixed with flour, water and sugar to form a dough, and it respires the sugars.
- Yeast carries out anaerobic respiration (fermentation) in the dough, where oxygen soon runs low:
glucose → ethanol + carbon dioxide (+ energy)
- The carbon dioxide produced is trapped as bubbles in the stretchy dough, making it rise so the baked bread is light and full of holes.
- The small amount of ethanol (and any remaining gas) is driven off during baking in the hot oven, so the finished bread contains essentially no alcohol.
- Warmth speeds the yeast's respiration, which is why dough is left to rise in a warm place; too hot, though, and the yeast's enzymes denature and it stops working.
Exam tip
Describing how yeast makes bread rise as a chain
The four-mark description of how yeast is used to make bread offers six credited links, any four of which score: yeast respires aerobically at first, then anaerobically (fermentation is accepted); the starch in the flour is converted to maltose and then glucose, using the enzymes amylase and maltase (two separate links); respiration releases carbon dioxide; and the gas is trapped as bubbles, so the dough rises and its volume increases. "Yeast makes the dough rise" on its own is only the last link, so start from the starch, name the enzymes and name the gas.
Core practical: anaerobic respiration in yeast under different conditions
The rate of anaerobic respiration in yeast can be measured by how fast it releases carbon dioxide.
- Mix with a glucose solution that has been boiled and cooled (boiling drives off dissolved oxygen so respiration is anaerobic).
- Put the mixture in a tube and seal it; let the carbon dioxide it makes pass through a delivery tube into a counting setup. The CO₂ can be counted as bubbles per minute through limewater, or its effect followed using hydrogencarbonate indicator (which turns from orange/red towards yellow as CO₂ builds up).
- A layer of oil on top of the yeast–glucose mixture keeps air out and confirms the respiration is anaerobic.
- To test the effect of temperature, run identical tubes in water baths at a range of temperatures (for example 10, 20, 30, 40 and 50 °C) and count the bubbles produced per minute in each.
- Keep everything else the same (same volume and concentration of yeast and glucose, same time), and use a control with no yeast to show the CO₂ comes from the yeast.
Result: the rate rises with temperature up to an optimum (around 35–40 °C) as the yeast's enzymes work faster, then falls at higher temperatures as those enzymes denature. The same method can compare different sugars or different glucose concentrations.