Multiple Choice: You company receives several complaints about your fluid milk becoming 'chunky' after 7 days at refrigeration temperatures (well before your 'best by date'). What is the most likely cause of this microbial spoilage?

Question

Multiple Choice: You company receives several complaints about your fluid milk becoming 'chunky' after 7 days at refrigeration temperatures (well before your 'best by date'). What is the most likely cause of this microbial spoilage?

BlackTom AI · Accepted Answer

Psychrotolerant bacteria with proteolytic activity

Cold-tolerant (psychrotolerant or psychrotrophic) bacteria are capable of growing at refrigeration temperatures (around 4°C). Milk is rich in proteins (caseins) that, when cleaved by proteolytic enzymes (proteases), lose their native structure and can aggregate into visible curds or “chunks.” Many common psychrotolerant spoilage organisms (e.g., Pseudomonas spp.) produce extracellular proteases that degrade casein and can be heat-stable, so even pasteurized milk can show proteolytic spoilage during refrigerated storage.

Step-by-step reasoning:
1. Complaint: “chunky” milk after 7 days at refrigeration — spoilage occurred at low temperature, well before the best-by date. That implicates microbes that can act at cold temperatures.
2. Identify likely mechanism: chunks indicate protein coagulation or gelation rather than sugar fermentation or pectin breakdown. Proteolysis (protein cleavage) of casein causes loss of solubility and aggregation → chunky appearance.
3. Match organism/enzymatic activity: psychrotolerant bacteria with proteolytic activity fit both the low-temperature growth and protein-degrading mechanism.
4. Additional note: some psychrotolerant bacteria produce proteases that are heat-resistant and remain active after pasteurization, causing spoilage during refrigerated storage.

Why other choices are incorrect:
- Thermophilic bacteria with pectinase activity: thermophiles require high temperatures to grow and pectinase degrades pectin (a plant polysaccharide), not milk protein; not relevant to refrigerated milk.
- Psychrotolerant bacteria with pectinase activity: psychrotolerant fits the temperature, but pectinase targets pectin (not present in milk), so it would not produce “chunky” milk.
- Thermophilic bacteria with protease activity: although proteases cause protein breakdown, thermophiles won’t grow or be active at refrigeration temperatures to cause spoilage within 7 days.

Conclusion: The most likely cause is psychrotolerant bacteria with proteolytic activity, which matches the provided correct answer.

Multiple Choice: You company receives several complaints about your fluid milk becoming 'chunky' after 7 days at refrigeration temperatures (well before your 'best by date'). What is the most likely cause of this microbial spoilage?单项选择题

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