Students investigated how temperature affects the decay of milk by measuring its pH over several days. The decay was caused by bacteria digesting lipids in the milk, producing fatty acids.

![Graph showing the decay of milk at three different temperatures: 15 °C, 25 °C, and 35 °C. The y-axis represents the pH of the milk, ranging from 4.0 to 7.0 with gridlines every 0.1 units. The x-axis represents time in days, ranging from 0 to 7 days with gridlines every day. Three curves are shown:

• At 35 °C: Day 0 is at pH 6.7, Day 1 is at pH 6.4, Day 2 is at pH 4.8, Day 3 is at pH 4.5, and it remains at 4.5 through Day 7.
• At 25 °C: Day 0 is at pH 6.7, Day 1 is at pH 6.7, Day 2 is at pH 6.1, Day 3 is at pH 5.2, Day 4 is at pH 4.6, and it reaches 4.5 by Day 5.
• At 15 °C: Day 0 is at pH 6.7, Day 1 is at pH 6.6, Day 2 is at pH 6.4, and then continues to drop gradually to 4.5 by Day 7.](https://assets.mathsgenie.co.uk/aqa-qb/ad278606-ce8c-4343-bfba-60f44c97ecd0.png)