Edexcel IGCSE Biology · Spec 5.6
Investigating Yeast Fermentation
Covers plan a reproducible yeast fermentation investigation, Identify independent, dependent and control variables and Improve reliability and control practical risks.
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Investigating Yeast Fermentation
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Explained
Investigating yeast fermentation, and planning it so the results mean something
Yeast is a single celled fungus. Given sugar and no oxygen it respires anaerobically, converting glucose into ethanol and carbon dioxide and releasing a small amount of energy.
The carbon dioxide is what makes this measurable. It is a gas, so it can be collected and its volume recorded, or counted as bubbles, or followed as a loss of mass as it escapes. That is why fermentation appears as a practical while most respiration does not.
Deciding what to measure
Collect the gas in a gas syringe and record the volume at fixed intervals. This gives a number you can plot and a rate you can calculate, and it is the most reliable of the three methods.
Counting bubbles through a delivery tube is quicker to set up but less trustworthy, because bubbles vary in size and a fast stream is hard to count accurately.
Measuring mass loss on a balance works because the escaping gas takes mass with it. It suits a slow reaction and a sensitive balance.
Whichever you choose, the rate is the volume, or the mass change, divided by the time. Comparing final volumes alone is not comparing rates unless every run lasted the same length of time.
The variables
Choose one factor to change. Temperature is the usual choice, and sugar concentration, pH and sugar type are the alternatives.
Measure the volume of carbon dioxide produced in a set time, which is the dependent variable.
Keep everything else the same: the mass or volume of yeast, the volume and concentration of sugar solution, the pH, the total volume of liquid, the apparatus, and the length of time you run each test.
A layer of oil on the surface is worth adding. It keeps oxygen out, so the yeast respires anaerobically throughout, and without it some of the carbon dioxide produced comes from aerobic respiration instead.
What the temperature results show, and why
The rate rises with temperature to an optimum of around 30 to 40 degrees Celsius, then falls sharply.
The rise happens because the yeast's enzymes and their substrates gain kinetic energy, so they collide more often. The fall happens because the enzymes denature: their active sites change shape and the substrate no longer fits.
Both halves of that curve are about enzymes, not about the yeast being comfortable or uncomfortable. The falling half is permanent, so a sample heated to 70 degrees and cooled back to 30 does not recover.
What the mark scheme accepts and rejects
An Edexcel International GCSE Biology mark scheme takes a fermentation question in a different context, making yoghurt, and its reasoning transfers directly to this practical.
It awards three marks from five points: that the milk is heated to between 70 and 95 degrees Celsius or pasteurised, that this kills any microorganisms or bacteria, that it is then cooled and kept at 40 to 45 degrees, that this allows the added bacteria to survive, and that it provides the optimum temperature for the enzymes, or for respiration, or for fermentation.
Two things in that are worth carrying into a yeast investigation.
The first is that the mark scheme treats optimum temperature for enzymes, for respiration and for fermentation as equivalent. Fermentation is enzyme controlled, so a temperature explanation about fermentation is an explanation about enzymes, and either wording earns the mark.
The second is the shape of the reasoning: heat high enough to destroy, then cool to the working temperature. It is the same physical fact that limits this practical from the other direction. There is a temperature above which the organism's enzymes are finished, and heating past it does not speed the reaction up, it ends it.
The same mark scheme elsewhere describes yeast raising bread, and credits that the enzymes in the yeast convert starch in the flour to sugars before the yeast can respire at all. Yeast cannot respire starch. If your investigation uses anything other than a simple sugar, that conversion step has to happen first, and it will slow the start of the reaction.
Reliability and safety
Repeat each temperature at least three times and calculate a mean. Identify any anomalous result and exclude it from the mean rather than quietly averaging it in.
Use a water bath rather than a Bunsen flame, so the temperature is stable and can be read. Allow the mixture to reach the water bath temperature before starting the timer, because a flask placed into a warm bath is not yet at that temperature.
Check the bung and tubing are airtight before each run. A leak loses gas that was produced, and it looks exactly like a slow reaction rate.
Yeast and sugar are low hazard, but hot water baths can scald, and a sealed flask producing gas can build up pressure, so the bung must not be forced in tightly. Wear eye protection and stand containers away from the edge of the bench.
Spec 5.6
What you need to know
- Plan a reproducible yeast fermentation investigation
- Identify independent, dependent and control variables
- Improve reliability and control practical risks
Active recall
Quick check
Answer each question before opening the answer.
What should a gas syringe measure in this investigation?
The volume of carbon dioxide produced over time
Name two variables that must be controlled when testing temperature
Any two of yeast amount, glucose amount or concentration, pH, apparatus and timing
How should reliability be improved?
Repeat each temperature and calculate a mean rate
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