Decay

• If milk is left at room temperature it goes sour. This is because bacteria in the milk use enzymes to produce acidic molecules.
• Decay is very slow so it's hard to observe in a lesson, so in the 'decay' practical we actually model decay by using the enzyme lipase.
• • PART ONE TO WATER BATH
  • Firstly we label a test tube 'lipase'
  • Then use a pipette to add 5cm^3 of lipase solution to the test tube.
  • Label another test tube milk and add 5 drops of cresol red
  • Now add 5cm^3 of milk and 7cm^3 of sodium carbonate solution to the milk test tube
  • (The solution should be purple as sodium carbonate is alkaline and cresol red is purple in alkaline)
  • Add a thermometer to the milk test tube
• • PART TWO
  • Now place both test tubes in a water bath at room temperature
  • Wait until the temperature of the solutions is the same as the water
  • Use a pipette to transfer 1cm^3 of lipase solution to the milk
  • Stir + start a timer
  • Eventually the enzyme lipase will break down fat molecules in the milk
  • This releases fatty acids
  • The milk solution will become acid and cresol red will become yellow. stop the timer.
  • repeat the experiment at various temperatures and record the time taken.
• It is very important to use a clean test tube as lipase from the previous experiment could affect results
• It can be hard to decide the exact point to stop the timer, reduce the effect of this by sharing data with other groups to calculate a mean.
• A typical graph will have a slow reaction at low temperature, a fast reaction at the optimum temperature, and will no longer work at too hot temperatures due to the denaturing of enzymes.