Role of Bacteria in Yoghurt Production (Edexcel IGCSE Biology)
Revision Note
Bacteria in Food Production
Microorganisms can be used by humans to produce foods and other useful substances
As well as fungi (e.g. yeast used to make bread), bacteria are also used in the production of certain foods
Bacteria are useful because they are capable of producing complex molecules (e.g. certain bacteria added to milk produce enzymes that turn the milk into yoghurt)
They are also useful because they reproduce rapidly, meaning the amount of chemicals they can produce can also rapidly increase
Using bacteria to make yoghurt
Yoghurt is made in a process that relies on the presence of a specific type of bacterium – in this case, Lactobacillus
First, all equipment is sterilised to kill other, unwanted bacteria and to prevent chemical contamination
Milk is then pasteurised (heated) at 85-95°C to kill other, unwanted bacteria
Contamination with other bacteria could slow production of the yoghurt by competing with the Lactobacillus for the lactose in the milk
It could also spoil the taste of the yoghurt
The milk is then cooled to 40-45°C and Lactobacillus bacteria is added
The mixture is incubated at this temperature for several hours, while the Lactobacillus bacteria digest milk proteins and ferment (digest) the sugar (i.e. the lactose) in the milk
The Lactobacillus bacteria convert the lactose into lactic acid and this increased acidity sours and thickens the milk to form yoghurt
This lowering of the pH also helps to prevent the growth of other microorganisms that may be harmful, so acts as a preservative
This means the yoghurt can be kept for a longer time (compared to fresh milk)
The yoghurt is then stirred and cooled to 5°C to halt the action of the Lactobacillus bacteria
Flavourings, colourants and fruit may be added before packaging
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