Practical - The decay of milk
As milk its pH reduces. This is because bacteria present in milk carry out anaerobic respiration 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 which three temperatures you will investigate. Write these onto the sides of the beakers. They may be 5, 20 and 35°C.
- 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.
See Also
A milkman adds a very small amount of baking soda to fresh milk.Why does he shift the pH of the fresh milk from 6 to slightly alkaline?Maintain Your Pool with Baking Soda | Arm & HammerThe Importance of Measuring pH in MilkFresh milk has a pH of 6. How do you think the pH will change as it turns into curd?pH will reducepH will increaseIt become same as milkNone of theseExample results
The table below shows the pH of milk after incubation at different temperatures
| Temperature (°C) | 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 limitations might there be in drawing these conclusions?
Question
How would you modify the procedure described above to find out if skimmed milk decayed less quickly than whole milk?
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