Role of Bacteria in Yoghurt Production

Section 1 of 6Why use bacteria in food production?

Useful chemistry

• Useful chemistry Bacteria can make complex molecules;
  • in yoghurt making they release enzymes that change the milk.

Rapid reproduction

• Rapid reproduction Bacteria divide very quickly, so a small starting culture gives a large amount of product in a short time.

The organism

• The organism Yoghurt production relies on a particular bacterium, Lactobacillus.

Section 2 of 6Making yoghurt

1

Sterilise all the equipment to kill unwanted microorganisms and remove any chemical contamination.

2

Pasteurise the milk by heating it to about 85-95 °C, which kills other bacteria that could compete with Lactobacillus or spoil the flavour.

3

Cool the milk to about 40-45 °C, the temperature at which Lactobacillus enzymes work well, and add the bacteria.

4

Incubate the mixture at this temperature for several hours while the bacteria ferment the milk sugar.

5

Stir and cool the yoghurt to about 5 °C to stop the bacteria acting any further.

6

Add flavourings, colourings or fruit if required, then package the product.

Section 3 of 6What the bacteria do

1Feed on lactose

• Feed on lactose Lactobacillus uses lactose, the sugar in milk, as its food.

2Lactic acid forms

• Lactic acid forms Fermenting lactose releases lactic acid as a product.

3pH falls

• pH falls The milk becomes more acidic, which sours the taste and makes the milk proteins clump and thicken.

4Preservation

• Preservation The low pH stops many harmful microorganisms from growing, so yoghurt keeps longer than fresh milk.

Section 4 of 6Why each control matters

Control	Reason
Sterile equipment	Prevents contamination that would compete for lactose or change the taste
Pasteurisation	Kills unwanted bacteria without needing to boil the milk
40-45 °C incubation	Optimum range for the bacterial enzymes; too hot would denature them, too cold would slow them
Cooling to 5 °C	Halts the bacteria so the yoghurt does not become too sour

Section 5 of 6Key terms

Term	Meaning
Lactobacillus	The bacterium that ferments lactose into lactic acid in yoghurt making.
Lactose	The sugar found in milk.
Pasteurisation	Heating milk to kill harmful or unwanted microorganisms.
Incubation	Keeping a culture at a controlled temperature so microorganisms can grow.
Lactic acid	The acid produced by the bacteria that sours and thickens the milk.

Section 6 of 6Exam technique

1

Saying the milk is pasteurised to kill the Lactobacillus — it is heated before the bacteria are added, to kill unwanted ones.

2

Naming the acid as citric or ethanoic — it is lactic acid.

3

Saying the bacteria produce alcohol and carbon dioxide — that is yeast; Lactobacillus makes lactic acid.

4

Forgetting that low pH also acts as a preservative.