Question

Which group of bacteria is most commonly responsible for souring raw milk?

• A. Thermoduric bacteria
• B. Psychrotrophic bacteria
• C. Anaerobic spore forming bacteria
• D. Lactic acid bacteria (LAB)

Hint:

Souring = Acid production = Lactose fermentation. This metabolic pathway is the hallmark of Lactic Acid Bacteria (LAB).

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
The souring of raw milk is primarily caused by the fermentation of milk sugar (lactose) into organic acids, which lowers the pH of the milk.

Step 2: Detailed Explanation:
- Lactic Acid Bacteria (LAB): This group (including genera such as Lactococcus, Streptococcus, and Lactobacillus) ferment lactose to produce lactic acid as their main metabolic end-product.
- The accumulation of lactic acid decreases the pH of milk, leading to a sour taste and eventually causing the coagulation of casein proteins (clotting).
- Thermoduric bacteria survive heat treatment but are not the primary cause of rapid souring in raw milk.
- Psychrotrophic bacteria cause spoilage (often proteolysis and lipolysis) during cold storage, while anaerobic spore-formers are associated with gas defects in cheese.

Step 3: Final Answer
The primary group of bacteria responsible for the souring of raw milk is Lactic acid bacteria (LAB).