Confession - these answers are by Pico technical staff none of whom will admit to having been in a physics or chemistry lesson for a few years. Please check these answers before use. We would be grateful to anyone who can email us with suggestions of better questions and answers!


Here are the results we obtained.

Answers to questions

Q1. Did you see a reduction in pH shortly after starting the experiment?
A1.This partly depends upon the freshness of the milk but there should be little change at this stage.

Q2. What causes the change in pH as the milk turns sour?
A2.The lactose in the milk is converted to lactic acid. As the acid level increases the pH level drops.

Q3. Is the reduction in pH constant over the entire experiment?
A3. No. We can see from our graph the gradient significantly increases half way through for fresh pasteurised milk.

Q4. Calculate the extent of change in pH obtained during the experiment.
A4. This will partly depend upon the students results. For our graph the milk went through a change in pH of 1.95 (6.75 - 4.8).

Q5. What do you think will be the result of incubating UHT milk over the same time period?
A5. This question is there to make the students think of the different ways, and reasons for processing milk. I expect the results to be very stable over the same time period.

Further study

1. Try repeating the experiment using unpasteurised and or Ultra Heat Treated milk.
a1.This question demonstrates the contrast of unpasteurised and UHT milk and the consequences!

2. Try keeping the container of milk at different temperatures (in the fridge or an incubator).
a2. This demonstrates that the growth of bacteria is slowed by refrigeration, and increased by warmth.

Teachers’ notes

This experiment can take several days to carry out. It may make sense to start the experiment on a Friday afternoon and look at the results on a Monday morning.

Credits, comments and further info

This experiment was written by Pico Technology.