By the end of this lesson
- Describe the stages of yoghurt production.
- Explain the role of Lactobacillus.
- Explain why each step, including pasteurisation and incubation, is needed.
BIG QUESTION
How do bacteria turn milk into yoghurt?
Fermentation produces lactic acid, and the acid changes the shape of milk proteins so the milk sets.
Preparing the milk
Milk is pasteurisedHeated briefly, for example to about 85–95 °C for a short time, to kill most of the microorganisms already in milk that could spoil yoghurt or cause disease. by heating (for example about 85–95 °C for a short time) to kill most existing microorganisms that could spoil the yoghurt or cause disease, then cooled to about 40–45 °C.
Cooling is needed so that the added bacteria are not killed.
Fermentation by Lactobacillus
A starter culture of LactobacillusA bacterium used to make yoghurt, which respires anaerobically, converting lactose into lactic acid. (often with other lactic acid bacteria) is added. The milk is incubated at around 40–45 °C for several hours.
The bacteria ferment the milk sugar lactoseThe sugar found in milk., producing lactic acidThe specification name for the product of anaerobic respiration in muscle (present in cells mainly as lactate), and the acid made when Lactobacillus ferments lactose in milk.. The falling pH causes milk proteins to coagulate and thicken, and gives the sour taste. Low pH also inhibits many spoilage microorganisms.
Finishing
The yoghurt is cooled to slow bacterial activity, and flavourings or fruit may be added. It is packaged in sterile containers and kept refrigerated.
Aseptic technique throughout prevents contamination by unwanted microorganisms.
Steps in yoghurt production
Learn why each step matters.
| Step | Conditions | Reason |
|---|---|---|
| Pasteurisation | Heat milk, e.g. 85–95 °C. | Kill unwanted microorganisms. |
| Cooling | To about 40–45 °C. | So added bacteria are not killed. |
| Add starter culture | Lactobacillus. | Ferments lactose. |
| Incubation | About 40–45 °C for hours. | Optimum for bacterial enzymes; lactic acid builds up. |
| Cooling and storage | Refrigeration. | Slows further fermentation and spoilage. |
Language that earns precision
- Lactobacillus
- A genus of bacteria that ferment sugars to lactic acid.
- lactic acid
- The acid produced by Lactobacillus fermentation of lactose.
- pasteurisation
- Heat treatment that kills most microorganisms in a food.
- incubation
- Keeping a culture at a controlled temperature for growth.
Common misconceptions
Claim: Lactobacillus produces ethanol like yeast.
Correction: It produces lactic acid.
Claim: Pasteurisation sterilises milk completely.
Correction: It kills most, not all, microorganisms.
Claim: Yoghurt thickens because bacteria add protein.
Correction: Lactic acid lowers pH, causing existing milk proteins to coagulate.
Match the answer to the command word
Give steps in order and include a reason for each temperature.
Link lactic acid → lower pH → proteins coagulate → thick texture; and low pH → fewer spoilage microbes.
Recall, apply, then justify
Explain the role of Lactobacillus in yoghurt production.
Hint
Sugar, acid, pH effect.
Suggested marking points
- Lactobacillus ferments lactose (milk sugar).
- It produces lactic acid, lowering the pH.
- Low pH makes milk proteins coagulate / thicken (and gives sour taste).
Explain why milk is cooled to about 45 °C before the bacteria are added.
Hint
Too hot and too cool.
Suggested marking points
- Higher temperatures would kill the bacteria / denature their enzymes.
- About 45 °C is near the optimum for bacterial growth/fermentation.
Design an investigation into the effect of incubation temperature on the rate of yoghurt production.
Planning hint
Measure pH change over time.
What a strong design includes
- Use the same volume of pasteurised milk in each container.
- Add the same volume of the same starter culture.
- Incubate at a range of temperatures, e.g. 25, 35, 45, 55 °C.
- Measure pH at regular intervals using a pH probe.
- Time how long each takes to reach a set pH (e.g. 4.6).
- Repeat at each temperature.
- Use aseptic technique to prevent contamination.