Describe how Lactobacillus is used to produce yoghurt from milk.

Pearson Edexcel International GCSE Biology (4BI1) — 5.1 Food production · Describe · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Yoghurt is a widely consumed dairy product made using microorganisms. The process involves specific conditions to ensure the correct fermentation of milk.

Model answer (4 marks)

Lactobacillus is a bacterium that ferments the lactose in milk into lactic acid.
The lactic acid produced lowers the pH of the milk, making it more acidic.
The acidic environment causes the milk proteins to denature and coagulate.
The coagulated proteins give yoghurt its semi‑solid, creamy texture.

Examiner tips

Use the exact terms "Lactobacillus", "ferment", "lactose", "lactic acid", "pH", "denature", "coagulate". Show the sequence: bacterium → lactose → lactic acid → lower pH → protein coagulation. Keep the answer concise, 4 points, no extra wording.

Common mistakes

Confusing Lactobacillus with yeast or fungus. Failing to mention the lowering of pH or the role of lactic acid. Describing protein denaturation as “hardening” instead of “coagulation”.

Mark scheme (4 marks)

Lactobacillus is a bacterium (not a fungus or yeast) used to ferment milk
Lactobacillus converts / ferments lactose (sugar in milk) into lactic acid
The lactic acid lowers the pH / makes the mixture more acidic
The lower pH causes the milk proteins to denature / coagulate / solidify, forming the semi-solid texture of yoghurt

Key terms in this question

Lactobacillus