Edexcel IGCSE Biology 复习笔记 5.1.5 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

Industrial fermentation

• Fermenters are containers used to grow (‘culture’) microorganisms like bacteria and fungi in large amounts
• These can then be used for brewing beer, making yoghurt and mycoprotein and other processes not involving food, like producing genetically modified bacteria and moulds that produce antibiotics (e.g. penicillin)
• The advantage of using a fermenter is that conditions can be carefully controlled to produce large quantities of exactly the right type of microorganism

A diagram of an industrial fermenter used to produce large quantities of microorganisms

Controlling Conditions in an Industrial Fermenter Table

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