In the manufacture of yoghurt, bacteria turn lactose into lactic acid. The acid denatures or spoils the milk protein and sets it solid. In the process, the milk starts to taste sour and its pH drops. At the same time, the bacteria use up oxygen.
How do the pH and oxygen levels change over time? Is there a pattern in how they change as the bacteria turn the milk sour. Using sensors, linked to the computer, you can monitor and record these changes.

What you need
Flask, cotton wool, live yoghurt, milk, water bath, clamps and stands, interface, pH buffer solution, pH sensor, oxygen probe, oxygen sensor.

Setting up
Place 200 cm3 milk and 10 cm3 yoghurt into the beaker. Set the water bath to 35°C. Connect the oxygen sensor to the first socket on the interface. Connect the pH sensor to the second socket.
Place the pH and oxygen probes in the yoghurt and milk mixture.
You may be able to adjust the oxygen sensor to read 21% oxygen in room air. You may also be able to calibrate the pH sensor to read correctly in pH buffer solution.
Set up your software so that it will record for up to 10 hours. Start recording.


  • Which graph is which?
  • What does the graph tell you about the change in pH during fermentation?
  • Why does the pH change?
  • What does the graph tell you about the change in oxygen level during fermentation?
  • Why does the oxygen level change?
  • How do the graphs change with respect to each other? Is there a pattern here?

Leave a Reply

Your email address will not be published. Required fields are marked *