Measuring the pH of milk

Introduction

This experiment looks at the changes in pH of milk as it goes sour. Milk is contaminated by bacteria even before it leaves the udder, although at this point they are harmless and few in number. Processing and handling activities can introduce further microorganisms.

The two main groups of bacteria in milk are lactic acid bacteria (LAB) and coliforms. Lactic acid bacteria is the normal bacteria present in milk and dairy products. Coliforms are the main reason for milk spoilage.

This experiment demonstrates the changes in the pH of milk over a several day period (depending on room temperature).
It is suitable for ages 13 upwards.

Equipment required

• DrDAQ data logger connected to a PC
• One pH probe (DrDAQ Part no DD011)
• A bottle or container of milk

Experiment setup

Allow the milk to reach room temperature without cooking and re-pasteurising it. Plug the pH probe into the DrDAQ BNC socket and insert the probe into the container of milk. Then run the PicoLog software.

Set PicoLog to take 1 sample per minute and a maximum sample of 10,000.

Carrying out the experiment

Start PicoLog Recording the data and display the graph. Try and keep the experiment in a stable environment as the extremes in room temperature may give false readings.

Questions and discussion of results

1.Did you see a reduction of pH shortly after starting the experiment?

2.What causes the reduction in the pH of milk?

3.Does the speed of change in pH stay constant over the whole experiment period?

4.Calculate the extent of change in pH obtained in the process.

5. What would be the results of incubating UHT milk over the same time period?

Further study

Try repeating the experiment using unpasteurised or ultra heat treated milk.

Try keeping the container of milk at different temperatures (in the fridge or an incubator).