yeast and making bread

• how is yeast used to make bread?

  • bread dough made by mixing yeast with flour, water and bit of sugar
  • dough left in a warm place to rise
  • enzymes break down carbohydrates in the flour into sugar
  • yeast then uses these sugars in aerobic respiration
  • oxygen runs out - fermentation takes place and CO2 and ethanol produced
  • carbon dioxide produced is trapped and bubbles in the dough
  • these gas pockets expand - dough starts to rise
  • dough is baked in oven - yeast continues to ferment until temperature of dough rises enough to kill yeast - any alcohol produced during anaerobic respiration is boiled away
  • as yeast dies - bread stops rising - pockets left in bread where CO2 is trapped
• what is the practical for the respiration rate of yeast?

  • Mix together some sugar, yeast and distilled water - add mixture to test tube
  • create anaerobic conditions by adding a layer of oil to top of yeast mixture - prevents oxygen from the air getting into the mixture
  • attach a bung with a tube leading to a second test tube of water
  • place tube containing yeast mixture in water bath at certain temperature
  • leave tube to warm up and count number of bubbles produced at given period of time
  • calculate rate of CO2 production by dividing number of bubbles produced by time taken for them to be produced in seconds - gives indication of respiration rate
  • repeat the experiment with the water bath set at different temperatures
  • respiration is controlled by enzymes - as temperature increases so should the rate of respiration