Bacteria & Making Yogurt

This lesson covers:

  1. How microorganisms can be used in food production 
  2. How bacteria are used to make yoghurt

The role of microorganisms in food production


Microorganisms, such as bacteria and yeast, can be used in food production in a variety of ways.


  1. Bacteria are used to ferment dairy products, such as yoghurt and cheese, and also to produce vinegar.
  2. Yeasts are used to bake bread and brew beer.
  3. Some microorganisms can be used to produce food additives, such as flavourings.

Yoghurt is a product of milk fermentation by the bacterium Lactobacillus


Diagram showing the fermentation process of milk to yoghurt by Lactobacillus bacterium.

Lactobacillus uses the lactose in milk as its substrate for anaerobic respiration, converting it into lactic acid and releasing energy in the process. The resulting lactic acid is what contributes to the distinctive taste and texture of yoghurt.

How yoghurt is made


  1. All equipment is sterilised to kill unwanted microorganisms and prevent contamination.
  2. The milk is pasteurised at 72°C for 15 seconds to kill any harmful microorganisms.
  3. The milk is cooled and Lactobacillus bacteria is added.
  4. The mixture is incubated at 40-45°C for several hours. During this time the Lactobacillus bacteria ferment the lactose in the milk.
  5. The Lactobacillus bacteria convert the lactose into lactic acid, which sours and thickens the milk, forming yoghurt.
  6. The yoghurt is then stirred and cooled - halting the action of the Lactobacillus bacteria.
  7. Flavourings, colorants, and fruit may be added before packaging.

What is yoghurt made from?

Soybeans

Milk

Rice

Wheat

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Which type of respiration does Lactobacillus utilize in the process of making yogurt?

Aerobic respiration

Anaerobic respiration

Photosynthesis

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What is the purpose of sterilising equipment in the process of making yoghurt?

(Select all that apply)

To kill unwanted microorganisms

To prevent chemical contamination

To cool the milk

To pasteurise the milk

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At what temperature is the milk pasteurised in the process of making yoghurt?

55°C

40°C

72°C

100°C

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In yoghurt production, what is the substrate used by Lactobacillus for anaerobic respiration?

Lactose

Fructose

Maltose

Sucrose

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