Required practical
Investigate the effect of temperature on the rate of decay of fresh milk by measuring pH change
As milk decayWhen plants wither and die. its pH reduces. This is because bacteria present in milk carry out a chemical process to provide them with energy. This process converts lactose sugar in the milk to lactic acid, and producing this acid reduces the pH of the milk.
Aim
To investigate the effect of temperature on the rate of decay of fresh milk by measuring pH change.
Method
- Place 20 cm3 of fresh milk into three beakers
- Decide the three temperatures you will investigate. Write these onto the sides of the beakers. They may be 5, 20 and 35°.
- Use universal indicator paper or solution to determine the pH of the milk in the three beakers
- Cover each beaker in cling film and incubate at the appropriate temperature
- Use universal indicator paper or solution to determine the pH of the milk in the three beakers after 24, 48 and 72 hours
Results
| 0 hours | 24 hours | 48 hours | 72 hours | |
| 5° | 6.5 | 6.4 | 6.4 | 6.0 |
| 20° | 6.5 | 6.1 | 5.5 | 4.8 |
| 35° | 6.5 | 5.1 | 4.8 | 4.8 |
| 5° | |
|---|---|
| 0 hours | 6.5 |
| 24 hours | 6.4 |
| 48 hours | 6.4 |
| 72 hours | 6.0 |
| 20° | |
|---|---|
| 0 hours | 6.5 |
| 24 hours | 6.1 |
| 48 hours | 5.5 |
| 72 hours | 4.8 |
| 35° | |
|---|---|
| 0 hours | 6.5 |
| 24 hours | 5.1 |
| 48 hours | 4.8 |
| 72 hours | 4.8 |
Conclusions
Question
What conclusions can you draw from this data? Use numbers in your answer. [6 marks]
At 5° the pH of the milk reduced the least [1 mark]. This suggests that the least decay has occurred at this temperature [1 mark]. The milk incubated at 20° and 35° both reduced to pH 4.8 [1 mark]. This suggests that the decay after 72 hours is similar at these temperatures [1 mark]. However, the reduction in pH occurs much quicker at 35° [1 mark]. The pH reaches its lowest after 48 hours not 72 hours as is the case with the milk incubated at 20°. These results show the decay of milk occurs quickest at 35° [1 mark] and then stops.
Question
What limitations might there be in drawing these conclusions?
The students did not take repeat readings to look for concordant results. They could be more confident in their conclusions if they took repeat readings.
Question
How would you modify the procedure described above to find out if skimmed milk decayed less quickly than fresh milk?
Use similar quantities of fresh and skimmed milk but keep both samples at 35°. The pH of the milk would need to be measured at the start, and after 24 and 48 hours, but not 72 hours.