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

RP10: Decay in milk

Aim:

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

  • Bacteria in the milk release enzymes (lipase) that break down the lipids (fats) in the milk into fatty acids and glycerol; the fatty acids lower the pH of the milk

Method:

1. Decide the three (or more) temperatures to be investigated and write these onto the sides of a set of beakers

   • E.g. 10 °C, 20 °C, 30 °C

2. Place 20 cm³ of fresh milk into each beaker

3. Use universal indicator paper or solution, or a pH meter to determine the pH of the milk for each beaker; record the result

4. Cover each beaker with clingfilm and place into a water bath at the required temperature.

5. Measure and record the pH of the milk in each beaker again after

   • 24 hours

   • 48 hours

   • 72 hours

• Control variables: keep the volume of milk, the type of milk, how fresh the milk is and the time of day the pH is measured the same in every beaker

• Do not give temperature (the independent variable) or the pH readings (the dependent variable) as control variables

Decay in milk: expected results

• The rate of decay should be affected as follows:

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

    • At higher temperatures the bacteria's enzymes are more active (the molecules have more kinetic energy and collide more often) and the bacteria divide faster, so the lipids are broken down more quickly, fatty acids are produced more quickly and the pH falls more quickly

  • Over time the rate of decay will decrease

    • Eventually all the fat (lipid) in the milk has been broken down, so no more fatty acids are made and the pH stops falling