Method
When milk is stored at room temperature it goes sour as bacteria in the milk start the decay process and use enzymes to produce acidic molecules. However this is a slow process so we will use the enzyme lipase to model this process and it is not strictly decay.
- Label a boiling tube ‘lipase’ and add 5 cm3 of the lipase solution.
- Label another boiling tube ‘milk’ and add five drops of the Cresol red solution.
- Use a calibrated dropping pipette to add 5 cm3 of milk to the ‘milk’ boiling tube.
- Use another pipette to add 7 cm3 of sodium carbonate solution to the ‘milk’ boiling tube. The solution should be purple.
- Put a thermometer into the ‘milk’ boiling tube.
- Set up a water bath to your first chosen temperature.
- Put both boiling tubes into the water bath. Wait until the contents reach the same temperature as the water bath.
- Use another dropping pipette to transfer 1 cm3 of lipase from the ‘lipase’ tube to the ‘milk’ tube. Immediately start the stop clock.
- Stir the contents of the ‘milk’ boiling tube until the solution turns yellow.
- Record the time taken for the colour to change to yellow, in seconds.
- Then repeat the investigation for different temperatures of water bath.