Effect of temperature on the rate of decay
GCSE Biology (8461) · Required practical 10 — method, variables, the marks examiners report students losing.
Investigate the effect of temperature on the rate of decay of fresh milk by measuring the time for its pH to change as it turns sour.
Apparatus
- Fresh milk and a fixed volume of dilute sodium carbonate solution with phenolphthalein indicator (or a pH probe)
- Lipase enzyme (which digests fat in milk to fatty acids)
- Water baths set to a range of temperatures
- Test tubes, pipettes, a thermometer and a stopwatch
Method
- 1Add the same volume of milk, lipase and pink alkaline phenolphthalein solution to a test tube.
- 2Place the tube in a water bath at a set temperature and start the stopwatch.
- 3As the lipase breaks down the fat, fatty acids form and the mixture becomes acidic, so the pink colour disappears (or the pH falls).
- 4Record the time for the indicator to turn colourless (or for the pH to reach a set value).
- 5Repeat at each temperature, keeping the volumes and concentrations the same.
Variables
Independent
Temperature of the water bath
Dependent
Rate of decay (from the time for the pH/colour to change)
Control
- Volume and freshness of the milk
- Volume and concentration of lipase and indicator
- The pH end point used
Results & processing
- Rate of decay = 1 / time for the colour to disappear (or for the pH to change by a set amount).
- As temperature increases the rate increases up to an optimum, because the microbes/enzymes are more active; above the optimum the enzymes denature and the rate falls.
- A graph of rate against temperature peaks at the optimum temperature.
Where students lose marks
Not controlling the volumes of milk and lipase.
Fix: Use the same volumes and concentrations every time so only temperature affects the result.
Judging the colour end point inconsistently.
Fix: Use the same end point (e.g. fully colourless) and, if possible, a colour standard or pH probe to reduce subjectivity.
Saying higher temperature always speeds up decay.
Fix: Only up to the optimum — above it the enzymes denature, so the rate falls again.
Improve the method
- Repeat at each temperature and take a mean time.
- Use a pH probe and data logger instead of judging colour by eye for objective, precise data.
- Test a narrow range of temperatures around the optimum to find it more accurately.
Try it — exam-style
At 30 degrees C the indicator turns colourless in 200 seconds. Calculate the rate of decay in s−1.
The rate of decay increases up to about 40 degrees C and then falls at higher temperatures. Explain why the rate falls.
Questions are written in the style of past AQA papers — never copied from them.
Drill it properly
Stuck on effect of temperature on the rate of decay?
The rate maths and the denaturing turn-around are the marks here — I drill both, and your first lesson is free.