Yoghurt & Cheese Making IGCSE Biology 0610

Yoghurt and Cheese Making: The Role of Bacteria

Biology editorial review is supported by the named review panel. Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.

Key concept

Yoghurt is made by adding Lactobacillus bacteria to warm milk. The bacteria ferment lactose (milk sugar) into lactic acid, which lowers the pH, causing milk proteins to coagulate and thicken. Cheese involves similar acidification plus the addition of rennet to form curds.

Yoghurt and cheese questions test understanding of bacterial fermentation and the role of specific enzymes. Students must describe the process step by step, naming the bacterium, the substrate, the product and the effect on the milk.

Yoghurt production

The steps in yoghurt making are:

1. Milk is pasteurised (heated to about 72 degrees C for 15 seconds) to kill any harmful bacteria present, then cooled to about 40-45 degrees C.

2. A starter culture of Lactobacillus bacteria is added to the warm milk.

3. The bacteria ferment lactose (the sugar in milk) into lactic acid through anaerobic respiration.

4. The lactic acid lowers the pH of the milk. This causes the milk protein (casein) to coagulate (denature and clump together), thickening the liquid into yoghurt.

5. The yoghurt is cooled to stop fermentation and may have flavourings or fruit added.

Why conditions matter

The milk is pasteurised first to remove competing bacteria that could spoil the product or cause disease. It is then cooled to the optimum temperature for Lactobacillus (about 40-45 degrees C). At this temperature, the bacterial enzymes work efficiently, and fermentation is rapid.

If the temperature is too high, the bacteria are killed. If too low, fermentation is slow and other bacteria may grow. Maintaining sterile conditions throughout prevents contamination by unwanted microorganisms.

Cheese production

Cheese making begins similarly to yoghurt: milk is pasteurised and Lactobacillus bacteria are added to produce lactic acid. Then rennet is added. Rennet contains the enzyme chymosin (rennin), which causes further coagulation of milk proteins, forming solid curds (protein and fat) and liquid whey (water and dissolved substances).

The curds are separated from the whey, pressed into moulds, salted and left to mature (ripen). Different bacterial or fungal cultures, maturation times and conditions produce different types of cheese. Blue cheese, for example, is ripened with Penicillium mould.

Try a focused question (3 marks)

Describe the role of Lactobacillus bacteria in yoghurt production.

Show the answer and marking guidance

Lactobacillus bacteria ferment lactose (milk sugar) into lactic acid.
The lactic acid lowers the pH of the milk.
The lower pH causes milk proteins (casein) to coagulate, thickening the milk into yoghurt.

Original Cambridge-style practice written for this site. It is not an official Cambridge past-paper question.

Common mix-ups to avoid

Check these points against your own answer before moving on.

Saying yeast is used to make yoghurt (Lactobacillus bacteria are used).
Forgetting to mention lactic acid as the product of bacterial fermentation.
Not explaining why pasteurisation is done before adding the starter culture.
Confusing curds (solid) with whey (liquid) in cheese making.