In this deck, you will learn about the required practical for AQA GCSE Biology that investigates the effect of temperature on the rate of decay. We will explore how to use milk, the enzyme lipase, and a pH indicator to measure how quickly decay happens at different temperatures. You will learn about the variables you need to control, the method you should follow, and how to interpret the results.

Decay is the breakdown of complex materials into simpler ones by microorganisms or enzymes. In this practical, we are looking at the decay of milk. Milk contains fat, which is a type of lipid. We use an enzyme called lipase to speed up the breakdown of this fat. When lipase breaks down the fat, it produces fatty acids. As their name suggests, these fatty acids are acidic, which means they lower the pH of the solution.

To see this pH change happen, we need a pH indicator. A common choice is phenolphthalein. To start the experiment, we make the milk solution slightly alkaline, for example by adding sodium carbonate. In this alkaline solution, phenolphthalein is a bright pink colour. As the lipase breaks down the fat and produces fatty acids, the solution becomes acidic. This causes the phenolphthalein to lose its colour and become colourless. The time it takes for this to happen is our measurement of the rate of decay.