Decay

• The aim of the experiment is to investigate the effect of temperature on the rate of decay of fresh milk by measuring pH change.
• The equipment required for the experiment includes full fat milk or single cream, sodium carbonate solution, 5% lipase solution, 250 cm 3 beakers, boiling tubes, boiling tube rack, marker pen, thermometer, syringes, calibrated pipette, stopwatch, and pH indicator.
• To prepare for the experiment, write down a hypothesis of the effect of temperature on the rate of decay of milk.
• Fill half of a beaker with hot water (60 ℃ or below) from the kettle for a water bath.
• Use a syringe to transfer 5 cm 3 of lipase solution into a boiling tube and label as ‘lipase’.
• Add 5 drops of Cresol red into another boiling tube and label as ‘milk’.
• Use a calibrated pipette to transfer 5 cm 3 milk into the ‘milk’ tube.
• Use another calibrated pipette to transfer 7 cm 3 sodium carbonate solution to the ‘milk’ tube, which should make a purple solution.
• Place a thermometer into the ‘milk’ tube.
• Place both boiling tubes into the water bath.
• Allow time for the solutions in the boiling tubes to reach the same temperature as the water bath.
• Use another pipette to transfer 1 cm 3 of lipase from the ‘lipase’ tube into the ‘milk’ tube and start timing immediately.
• Record the time required for the colour change to yellow in a table such as below.
• Repeat steps 2-11 at the same temperature twice and take a mean value.
• Repeat steps 2-11 using a range of different temperatures of water baths (a range from 20 ℃ - 60 ℃ ).
• Plot a graph of time taken against temperature.
• The colour change at the end point may be difficult to judge.