Practical - The decay of milk - How are levels maintained within an ecosystem? - OCR 21st Century - GCSE Biology (Single Science) Revision - OCR 21st Century

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.

Example 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 conclusions can you draw from this data? Use numbers in your answer. [6 marks]

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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How are levels maintained within an ecosystem? - OCR 21st CenturyPractical - The decay of milk