Required Practical: Decay (AQA GCSE Biology) : Revision Note

RP10: decay in milk

• Aim: to investigate the effect of temperature on the rate of decay of fresh milk by measuring pH change

  • Bacteria that are present in milk carry out anaerobic respiration, which produces lactic acid and lowers the pH of the milk

• You will:

  • decide on the range of temperatures that you wish to investigate, considering the lesson time available

  • use an indicator to monitor the rate of decay of milk

Decay in milk: analysis

• You will:

  • calculate the rate of decay of fresh milk incubated at three different temperatures

  • draw a graph of the results

• We would expect the rate of decay to be affected as follows:

  • as temperature increases the rate of decay will increase (up to a point)

    • The rate of anaerobic respiration in bacteria increases with temperature due to increased kinetic energy of molecules in the milk, and increased bacterial enzyme activity

  • over time the rate of decay will decrease

    • The bacteria eventually run out of sugars to respire, at which point the pH of the milk sample will remain the same