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