Topic 9 - Yeast and Making Bread

Question 1

how is yeast used in making bread?

Answer 1

1. bread dough is made by mixing yeast with flour, water and sugar
2. the dough is left to rise
3. enzymes break down the carbohydrates in the flour into sugars
4. yeast uses these sugars in aerobic respiration, producing CO₂
5. yeast switches to anaerobic respiration when oxygen runs out. this is known as fermentation and produces CO₂ and ethanol
6. CO₂ produced is trapped in bubbles, which start to expand, so the dough begins to rise
7. the dough is baked in an oven, where the yeast continues to ferment until the temperature is too high and the yeast dies. alcohol produced is boiled away
8. when yeast dies the bread stops rising

Question 2

how would you investigate how the respiration rate of yeast changes under different conditions?

Answer 2

1. mix some sugar, yeast and distilled water in a test tube. add this test tube to a water bath
2. create anaerobic conditions by adding a layer of oil so oxygen can’t get in
3. attach a bung with a tube leading to a second test tube of water
4. leave the tube for a bit
5. count how many bubbles are produced in a given time
6. divide the number of bubbles produced by the time taken to get a rate of CO₂ production
7. repeat at different tempreatures