Required Practical: Decay (AQA GCSE Biology): Revision Note
Exam code: 8461
RP10: Decay in milk
Aim: to investigate the effect of temperature on the rate of decay of fresh milk by measuring pH change
Bacteria that are present in milk carry out anaerobic respiration, which produces lactic acid and lowers the pH of the milk
Process:
Decide on the range of temperatures to be investigated, considering the lesson time available
Use an indicator to monitor the rate of decay of milk
Method
Place 20 cm³ of fresh milk into three beakers. Decide the three (or more) temperatures to be investigated and write these onto the sides of the beakers (e.g. 10°C, 20°C, 30°C).
Use universal indicator paper or solution, or a pH meter to determine the pH of the milk for each beaker.
Cover each beaker with clingfilm and place into a water bath at the required temperature.
Measure the pH of the milk in the three beakers after 24, 48 and 72 hours. Results to be recorded in a table.
Example results
0 hours | 24 hours | 48 hours | 72 hours | |
|---|---|---|---|---|
10°C | 6.5 | 6.3 | 6.2 | 5.9 |
20°C | 6.5 | 6.0 | 5.4 | 4.7 |
30°C | 6.5 | 5.0 | 4.7 | 4.7 |
Decay in milk: analysis
Process:
calculate the rate of decay of fresh milk incubated at three different temperatures
draw a graph of the results
The rate of decay would be expected to be affected as follows:
As temperature increases the rate of decay will increase (up to a point)
The rate of anaerobic respiration in bacteria increases with temperature due to increased kinetic energy of molecules in the milk, and increased bacterial enzyme activity
Over time the rate of decay will decrease
The bacteria eventually run out of sugars to respire, at which point the pH of the milk sample will remain the same
Worked Example
A student is investigating rate changes in the decay of milk. They incubated milk at three different temperatures and recorded the pH every 24 hours. Their results table is shown below.
0 hours | 24 hours | 48 hours | 72 hours | |
|---|---|---|---|---|
10°C | 6.5 | 6.3 | 6.2 | 5.9 |
20°C | 6.5 | 6.0 | 5.4 | 4.7 |
30°C | 6.5 | 5.0 | 4.7 | 4.7 |
Calculate the rate of change in pH of the milk in each 24 hour period for each of the three temperatures.
Draw a graph of the results.
Answer:
To calculate the rate of change we first need to work out how much the pH value changed over each 24 hour period. This is calculated by finding the difference between the current pH value and the previous pH value.
(Note: no change at 0 hours as no time has passed so the pH has not changed)
Change in pH:
Temperature | 24 hours | 48 hours | 72 hours |
|---|---|---|---|
10°C | 6.5 − 6.3 = 0.2 | 6.3 − 6.2 = 0.1 | 6.2 − 5.9 = 0.3 |
20°C | 6.5 − 6.0 = 0.5 | 6.0 − 5.4 = 0.6 | 5.4 − 4.7 = 0.7 |
30°C | 6.5 − 5.0 = 1.5 | 5.0 − 4.7 = 0.3 | 4.7 − 4.7 = 0 |
You can now calculate the rate of change for each 24 hour period by dividing each change in pH by the time taken for this change to occur:
Rate of change = change in value ÷ change in time
Rate of change in pH:
Temperature | 24 hours | 48 hours | 72 hours |
|---|---|---|---|
10°C | 0.2 ÷ 24 = 0.0083 | 0.1 ÷ 24 = 0.0042 | 0.3 ÷ 24 = 0.013 |
20°C | 0.5 ÷ 24 = 0.021 | 0.6 ÷ 24 = 0.025 | 0.7 ÷ 24 = 0.029 |
30°C | 1.5 ÷ 24 = 0.063 | 0.3 ÷ 24 = 0.013 | 0 ÷ 24 = 0 |
Examiner Tips and Tricks
Graph tips:
Ensure your graph takes up at least half the space you are given
Ensure axes scales go up in suitable multiples
Ensure axes have labels and units
Ensure data points are plotted accurately
Independent variable on x-axis
Dependent variable on y-axis
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