1Yeast
Yeast is a single-celled fungus. It can respire without oxygen. This anaerobic respiration in yeast is called fermentation.
yeastfermentation
Lesson 22.4 notes
Fermentation
Anaerobic respiration in yeast: the word summary, the balloon test, bread, brewing, temperature, and the three ways to respire.
2The word summary
Fermentation: glucose → ethanol + carbon dioxide. Ethanol is an alcohol. Like anaerobic respiration in muscles, it releases only a little energy, but enough for the yeast to live on.
ethanol
3Seeing fermentation
Yeast and sugar in warm water, with a balloon over the flask. As the yeast ferments the sugar, the balloon fills with gas. The gas turns limewater cloudy: it is carbon dioxide.
4Bread
Yeast in bread dough ferments the sugars. Bubbles of carbon dioxide are trapped in the dough, so it rises. In the hot oven the yeast is killed and the ethanol evaporates away.
dough
5Brewing
Yeast ferments the sugars in grapes or barley to make wine or beer. The ethanol is the alcohol in the drink; the carbon dioxide makes beer and sparkling wine fizzy.
winebeer
6Temperature
Yeast ferments fastest when warm, at about 30 to 40 °C. Above about 50 °C its enzymes are damaged (denatured) and it dies, so the gas stops. In the cold it works very slowly.
enzymes
7Three ways to respire
Aerobic: glucose + oxygen → carbon dioxide + water, in all living cells. Anaerobic in muscles: glucose → lactic acid. Fermentation in yeast: glucose → ethanol + carbon dioxide.
Explore: Yeast lab
Pick a temperature, add sugar, and collect the gas the yeast gives off for 10 minutes. The results are plotted for you: find the best temperature.
Common mistake: Yeast makes ethanol, not lactic acid
Anaerobic respiration has different products in different organisms. In your muscles it makes lactic acid. In yeast it makes ethanol and carbon dioxide: this is fermentation.
Remember
- Fermentation is anaerobic respiration in yeast: glucose → ethanol + carbon dioxide.
- The carbon dioxide makes bread rise and some drinks fizzy. The ethanol is the alcohol in beer and wine; in bread it evaporates in the oven.
- Yeast respires fastest when warm, at about 30 to 40 °C. Above about 50 °C its enzymes are damaged and it dies.
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