Question

The sweet curdling in pasteurized milk may be due to

• A. Bacillus subtilis
• B. Bacillus polymixa
• C. Bacillus stearothermophilus
• D. Bacillus circulans

Hint:

Sweet curdling is enzymatically driven by proteolytic spore-formers like Bacillus subtilis and Bacillus cereus, occurring at a neutral or near-neutral pH.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Sweet curdling is a milk defect characterized by coagulation without a significant increase in acidity (hence the term "sweet").
It is caused by extracellular, rennet-like proteolytic enzymes produced by bacterial contaminants that survive pasteurization or contaminate milk post-heating.

Step 2: Detailed Explanation:
Spore-forming aerobic bacilli, primarily Bacillus subtilis and Bacillus cereus, survive the pasteurization process due to their heat-resistant endospores.
Upon germination and growth in pasteurized milk, these bacteria produce extracellular proteases that degrade casein.
This enzymatic breakdown mimics the action of chymosin (rennet), destabilizing the micellar structure of casein and resulting in curd formation at near-neutral pH.
Unlike lactic acid bacteria, these spore-formers do not produce large amounts of lactic acid, so the curdling occurs without souring.
Other species like Bacillus stearothermophilus are thermophilic and primarily cause flat-sour spoilage in canned products rather than sweet curdling in fluid pasteurized milk.

Step 3: Final Answer
The correct option is (A).