Milk Decay GCSE Biology Practical
Investigate the effect of temperature on the rate of decay of milk by measuring pH change.
In this required practical, students are to investigate the effect of temperature on the rate of decay of milk. The experiment uses a colour changing indicator to identify the end point of the reaction. Temperature is varied using a water bath so the students handling of laboratory equipment is also put to the test
Equipment (per pair of participants)
• Safety glasses
• Bunsen burner
• 250ml beaker
• 100ml beakers
• 7 x boiling tubes
• Boiling tube rack
• Chinagraph pencil or permanent pen
• 10ml syringes
• Thermometer
• Stopwatch
• Cresol Red indicator in dropping
bottles
• Kettle
• Lipase solution (5%)
• Anhydrous Sodium Carbonate solution
(0.05M)
• CLEAPSS Hazcards: 32, 33, 95A
Method
- Students should put on safety glasses and label two boiling tubes, one as ‘Lipase’, one as ‘Milk’.
- They should then add 5mls of Lipase to the ‘Lipase’ tube.
- 5mls of milk and five drops of Cresol Red indicator is then added to the ‘Milk’ tube.
- 7mls of sodium carbonate solution is then added to the ‘Milk’ tube, the resulting solution should now be purple/pink.
- A thermometer is placed into the ‘Milk’ tube.
- Students should set up a 250ml beaker as a water bath and use the Bunsen burner to heat it until it reaches their chosen temperature.
- Place both the ‘Lipase’ and ‘Milk’ tubes into the water bath and wait until the thermometer in the ‘Milk’ tube reaches your desired temperature.
- Using a fresh dropping pipette, students should add 1ml of the Lipase from the ‘Lipase’ tube into the ‘Milk’ tube and at the same time, start the stopwatch.
- They should stir the contents of the ‘Milk’ tube using the thermometer and wait until the contents turn yellow. Once the yellow colour is present, students stop the stopwatch and record the time taken in seconds.
- Students should now repeat the procedure with their water bath set at (at least) 5 different temperatures
Video produced by @revisechemistrywithmrb practical GCSE Chemistry tutorials by a specialist Chemistry teacher with over 25 years experience teaching Chemistry and Biology.
Technician tips
The milk used must be fresh, full fat/whole milk (source locally).
- The Sodium Carbonate used to make the solution must be made from the Anhydrous salt. 5.2g/L of the salt gives a 0.05mol solution.
- All reagents, including the milk, should be at room temperature at the start of the experiment.
- The practical uses 5% Lipase Solution. One 25g pot of Lipase is used to make 500mls of 5% solution. Lipase is quite dicult to dissolve. We would recommend giving plenty of time for the Lipase to dissolve as much as possible before the lesson using an electronic magnetic stirrer.
To do this practical, you will need
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Investigate the effect of temperature on decay of milk for the GCSE Biology required practical. The experiment involves students using a colour changing indicator to identify the end point of the reaction. Shop our full range of biology equipment for this practical including safety glasses, bunsen burners, beakers and more. Discover more science equipment at Philip Harris developed by our science experts for schools and FE.
