Required practical activity 10: investigate the effect of temperature on…

The idea

Decay is the breakdown of biological material by microorganisms such as bacteria and fungi. These microorganisms respire and release enzymes that digest the milk's lactose into acids, so the pH of fresh milk falls over time. In this practical you set up several samples of fresh milk, each kept at a different controlled temperature, and measure pH at regular intervals. A faster fall in pH indicates a faster rate of decay. You should use a pH meter or universal indicator solution and record readings at fixed times, for example every 24 hours. Plot pH against time for each temperature and compare the gradients. The temperature that produces the steepest fall in pH shows the fastest decay, usually around the optimum for the microorganisms involved.

What earns the marks

• State that microorganisms in the milk break down lactose and produce acid, causing pH to fall.
• Describe setting up several milk samples at different temperatures while keeping other variables such as volume of milk and starting pH the same.
• Measure pH at regular, stated time intervals using a pH meter or universal indicator and record results in a suitable table.
• Calculate or compare the rate of decay from the change in pH divided by the time taken, or from the gradient of a pH-time graph.
• Identify the temperature giving the fastest pH fall as the temperature with the fastest rate of decay.
• Explain that very high temperatures may kill or denature the microorganisms' enzymes, so decay can slow again.

Common mistakes and how to put them right

• Recording only a single pH reading at the end instead of taking repeated readings over time.

  Put it right by measuring pH at regular intervals so a rate can be calculated.

• Changing several variables at once, such as using different milk volumes at each temperature.

  Put it right by keeping all variables except temperature constant.

• Assuming the highest temperature always gives the fastest decay.

  Put it right by explaining that enzymes and microorganisms have an optimum temperature and may be destroyed above it.

Exam tips

• Name the independent variable as temperature and the dependent variable as pH, and state at least two control variables.
• When describing rate, use the change in pH divided by the time taken, and include the units.
• Link your conclusion back to enzyme activity and microorganism respiration rather than simply saying 'it decayed faster'.