Effect of Temperature on Alcoholic Fermentation in Yeast

The rate of alcoholic fermentation is directly proportional to the temperature of the yeast solution, hence as the temperature increases (to a certain limit) so does the rate of fermentation.

Background

During anaerobic conditions high levels of NADH develop, leaving a shortage of NAD+. Low levels of NAD+ slow the rate of glycolysis. Fermentation restores NAD+ levels while producing alcohol and CO2.

During aerobic respiration glucose is broken down into water and carbon dioxide.
C6H12O6 + O2 =====> 6H2O + 6CO2
Under ideal conditions most eukaryotic cells produce 36 ATP molecules from one molecule of glucose.

During fermentation baker's yeast breaks glucose into ethyl alcohol and carbon dioxide.
C6H12O6 =====> 2C2H5OH + 2CO2
This process only yields 2 ATP per glucose molecule.

Research Question: How does variation in temperature affect alcoholic fermentation of glucose in yeast?

Hypothesis: The rate of alcoholic fermentation is directly proportional to the temperature of the yeast solution, hence as the temperature increases (to a certain limit) so does the rate of fermentation.

Variables

Dependant variables

• The rate of fermentation determined by the amount of foam (gas bubbles) produced and the color of the TES-TAPE

Independent variables

• The temperature of the two test tubes Q and S contained in each of the two beakers; one beaker is filled with water and held at 40°C, while the other beaker is filled with water and held at room temperature.

Controlled variables

The control variables need to be constant in order to get valid and accurate results.

They are:

• The concentration of yeast added into each test tube (7% yeast solution dissolved in 200ml of water which are used to fill half of each tube)*
• The concentration of sugar added into each test tube (5% glucose solution in 200ml of water which are used to fill half of each tube)*
• The time intervals of 5 minutes between the recording of the volume of bubbles and measuring the percentage of glucose in the solution.

Material

• TES-TAPE (tape whose color changes in the presence of glucose; the color change indicates the concentration of glucose present in the solution)
• large beakers
• water bath with a 40°C temperature
• Markers
• Test tubes with their caps
• 7% yeast solution (2 yeast packages in 200 mL water)

• *Excess of both yeast and sugar solutions are obtained and discarded 5% glucose solution (10 grams glucose in 200 mL water)

Controlling the Variables

Controlling the temperature is a crucial factor for obtaining the intended results. It is important that the temperature of one of the tubes be significantly higher than the other (the tube in the hot water bath)

The mixing of the yeast and the glucose in all 4 tubes should occur in a very short span of time. This is because at mixing the reaction is taking place and therefore if there is a large gap of time between the mixing of the reactants in each tube, the final results would be invalid.

The time interval of 5 minutes should be respected. If more time were allocated to the tubes, more glucose could be reacting with the yeast and hence the recorded results would be inaccurate.

The amount of yeast and sugar added to each tube are in this case dictated by the volume of the mixture of the two solutions added. Changing the volumes of yeast and sugar added to each tube can affect the rate of the reaction. It is crucial to measure correctly the volumes of the reactants being added to each tube.

Procedure

1. Puncture each tube cap with small holes
2. Mix 2 packets of dry yeast in 200 mL of distilled water.
3. Mix 10 grams glucose in 200 mL of distilled water.
4. Heat water bath and one of the beakers both filled with water to 40°C
5. Fill the other beaker with water and allow it to maintain room temperature
6. Fill each tube halfway with sugar solution
7. Fill the rest of each tube with yeast solution
8. Take a small piece of TES-TAPE and measure the amount of glucose present in the solutions
9. Seal all the tubes with the caps
10. No bubbles should be visible in the tubes yet
11. Invert the tubes and mark two tubes Q and the other two tubes S
12. Place one beaker in the hot water bath and allow the other to maintain room temperature
13. In each beaker place one Q tube and one S tube
14. At 5 minute intervals proceed with the following:
    • Remove the Q tubes from the water
    • Measure the level of gas bubbles (include any foam as part of the bubbles)
    • Return the Q tubes to the beakers
    • take the S tubes out of the water
    • turn the tubes upright and remove the caps
    • Measure the amount of glucose in each test tube using TES-TAPE
    • Return the tubes to the beakers
15. after 20 minutes stop repeating step 13
16. Draw a graph of the concentration of glucose in each test tube against time and another of the volume of bubbles against time. Comment on the graphs and write down your conclusions.