Yeast

• A microorganism that respires aerobically when oxygen is present

• It breaks down sugar into carbon dioxide and water

• It is used in baking where it creates bubbles of carbon dioxide that makes dough rise

Bread dough

• Made with yeast, flour water and a bit of sugar

• Dough is left in a warm place to rise, due to the yeast

• Enzymes break down the carbohydrates in the flour into sugars

• When oxygen runs out, yeast changes to anaerobic respiration

  • this is fermentation, where it produces carbon dioxide and alcohol (ethanol)

• Carbon dioxide is trapped in the bubbles of the dough

• These pockets of gas expand and the dough rises

• When the dough is baked in an oven, the yeast continues to ferment until the temperature of the dough is high enough to kill the yeast

• Alcohol produced in anaerobic respiration is boiled away

• As yeast dies, bread stops rising, but pockets are left in the bread where the CO2 was trapped

Respiration of yeast practical

• To find how the rate of CO2 production by yeast during anaerobic respiration changes under different conditions

Effect of changing temperature experiment:

1. Mix sugar, yeast and distilled water

2. Add the mixture into a test tube

3. To get an anaerobic condition, add a layer of oil to the top of the yeast mixture, this prevents oxygen from the air getting into the mixture

4. Attach a bung with a tube leading to a second test tube of water

5. Place the tube containing the yeast mixture in a water bath at a certain temperature

6. Leave the tube to warm up a bit and then count how many bubbles are produced in a period of time

7. Calculate rate of CO2 production by dividing the number of bubbles produced by the time taken in seconds

   1. this gives a respiration rate

Respiration is controlled by enzymes, so as temperature increases so should the rate of respiration (until optimum temperature)

This experiment can also be done with limewater instead of water, where you see how fast it takes for it to turn cloudy