Investigating Fermentation in Yeast

Aim

This experiment investigates anaerobic respiration (fermentation) in yeast by testing for the carbon dioxide gas it produces, and studies how temperature affects the rate of fermentation.

Variables

• Manipulated variable: temperature of the yeast-glucose mixture, set using water baths at about 20°C, 30°C, 40°C and 50°C.
• Responding variable: rate of gas (carbon dioxide) production, measured as the number of bubbles released per minute or the volume of gas collected over a fixed time by water displacement.
• Controlled variables: concentration and volume of the yeast suspension, concentration and volume of the glucose solution, and the time interval over which gas is measured.

Materials and apparatus

• Dried yeast
• Glucose solution
• Warm water (for reactivating the dried yeast)
• Boiling tube or conical flask
• Delivery tube
• Inverted measuring cylinder set up in a water trough (for collecting gas)
• Lime water (to test the gas produced)
• Thermometer
• Water baths at set temperatures
• Stopwatch

Procedure

1. Reactivate the dried yeast by mixing it with a little warm water, then combine it with an equal volume of glucose solution in a boiling tube.
2. Place the boiling tube in a water bath set at the first temperature, such as 20°C, and leave it for a few minutes to reach that temperature.
3. Connect the boiling tube by a delivery tube to an inverted, water-filled measuring cylinder standing in a trough of water.
4. Start the stopwatch and count the number of gas bubbles collected in the measuring cylinder per minute, or record the volume of gas collected over a fixed time, such as 5 minutes.
5. Repeat steps 1 to 4 at each of the other temperatures, keeping the concentration and volume of yeast suspension and glucose solution the same throughout.
6. Record the rate of gas production at each temperature in a table.
7. Test a sample of the gas collected by passing it into fresh lime water, and observe whether the lime water turns milky, confirming the gas is carbon dioxide.

Expected results

The rate of gas production increases with temperature up to an optimum temperature, then decreases sharply at higher temperatures. The table below summarises the typical pattern observed.

Conclusion

Yeast respires anaerobically, fermenting glucose to produce carbon dioxide (and ethanol), and the gas collected turns lime water milky, confirming it is carbon dioxide. The rate of fermentation depends on temperature: it increases as temperature rises towards an optimum around 35-40°C, where the yeast's enzymes work fastest, then falls sharply at higher temperatures because the enzymes are denatured and can no longer catalyse the breakdown of glucose.

Safety

Common mistakes

Paper 3-style questions

Question 1 (hypothesis). A student thinks the rate of fermentation in yeast increases as temperature rises from 20°C towards about 40°C. State a hypothesis and the result that would support it.

Model answer. Hypothesis: as temperature rises from 20°C towards about 40°C, the rate of fermentation increases. It is supported if the number of gas bubbles produced per minute increases as the water-bath temperature is raised towards the optimum.

Question 2 (variables and fair test). State the manipulated and responding variables in this investigation and two variables that must be controlled to make it a fair test.

Model answer. The manipulated variable is the temperature of the yeast-glucose mixture; the responding variable is the rate of gas production (bubbles per minute or volume of gas in a fixed time). Two controlled variables are the concentration and volume of the yeast suspension and of the glucose solution.

Question 3 (tabulation and graph). The student counts bubbles per minute at 20, 30, 40 and 50°C. Suggest how the results should be tabulated and describe the shape of the graph.

Model answer. Results are tabulated with temperature / °C against rate of gas production / bubbles per minute. A graph of rate (y-axis) against temperature (x-axis) rises to a peak at the optimum around 35-40°C and then falls, giving a curve, not a straight line.

Question 4 (inference). At 50°C the rate of gas production is lower than at 40°C. What inference can be made, and why?

Model answer. The inference is that the yeast's enzymes have been denatured by the higher temperature. Above the optimum, heat changes the shape of the active site so the substrate no longer fits, so fewer glucose molecules are broken down and less carbon dioxide is produced.

Source:SRC-DSKP-EN

Frequently asked questions