PRODUCTION OF YOGHURT
Again, as, with brewing, only the role of the organisms in the production process is specifically required.
It is suggested that students should recall
- that milk contains the sugar lactose;
- that specific lactic acid bacteria ferment lactose to produce lactic acid;
- that the increasing acidity restricts the reproduction and activity of the lactic acid bacteria (though without killing them) and helps prevent growth of contaminant organisms.
Many teachers, especially if wishing to make yoghurt as a practical activity, will examine the overall commercial process with their students
Flow chart for commercial production of stirred yoghurt
Heat milk to 85-95°C for 15-30 minutes
Homogenise the milk
Cool the milk to incubation temperature
Add the bacterial starter
Incubate the mixture at 40-46°C for 4-5 hours
(until the pH has fallen to around 4.3)
Stir the thickened yoghurt and cool it to 5°C
Add fruit, flavourings, colorants (if required)
Pack and despatch the yoghurt, keeping it refrigerated
Presented with such a flow chart in the examination, candidates might be asked questions such as:
Explain how bacteria, apart from those in the starter, are prevented from growing in the milk.
Why is the milk cooled before the starter is added?
How does bacterial activity give yoghurt its sour flavour?