Question

Which microorganism is primarily responsible for spoilage of refrigerated milk?

• A. Bacillus subtilis
• B. Pseudomonas spp.
• C. Aspergillus niger
• D. Saccharomyces cerevisiae

Hint:

Refrigeration does NOT stop all bacteria — psychrotrophs like Pseudomonas still grow.

Answer 1

The Correct Option is B

Concept: Refrigerated milk spoilage is primarily caused by psychrotrophic microorganisms, which are capable of growing at low temperatures (around 4°C). Among these, Pseudomonas species are the most dominant and significant spoilage organisms.

Step 1: Understanding psychrotrophic bacteria.
Psychrotrophs are microorganisms that:
• Grow at refrigeration temperatures
• Produce enzymes that degrade proteins and fats These enzymes remain active even after pasteurization.

Step 2: Role of Pseudomonas spp.
Pseudomonas species:
• Are aerobic Gram-negative bacteria
• Produce proteases and lipases
• Cause off-flavors, bitterness, and spoilage in milk

Step 3: Evaluating other options.
• Bacillus subtilis — Spore-forming bacteria, more relevant in heat-resistant contamination.
• Pseudomonas spp. — Dominant spoilage organism in refrigerated milk. Correct.
• Aspergillus niger — Fungus, not typical in refrigerated milk spoilage.
• Saccharomyces cerevisiae — Yeast, mainly used in fermentation, not milk spoilage.

Step 4: Why Pseudomonas dominates?
Because:
• It grows rapidly at low temperatures
• It produces heat-stable enzymes
• It survives and multiplies during storage Final Conclusion:
The primary microorganism responsible for spoilage of refrigerated milk is Pseudomonas spp., hence option (2).