4.1.7Use of Biological Resources
Role of Bacteria in Yoghurt Production — Edexcel International GCSE Science Double Award (Modular) - Biology Unit 2 Revision Notes & Topical Questions
5 min read 6 parts Lesson 8 of 15
Part 1 of 4Why 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.
Part 2 of 4Making 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.
Part 3 of 4What 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.
Part 4 of 4Why 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 |
Key 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. |
Avoid these mistakes
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.
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