Pre‑packaged refrigerated deli meats often have a longer shelf life than freshly sliced meats because:[Fill in the blank]

Question

BlackTom AI · Accepted Answer

Modified-atmosphere packaging (MAP) works by replacing the air around the meat with gases (for example, elevated CO2 and N2 with reduced O2), which changes the extrinsic environmental conditions and inhibits growth of aerobic spoilage organisms and oxidation reactions.

1) Relevant knowledge points and concepts
- Microbial growth depends on intrinsic factors (pH, water activity, nutrients) and extrinsic factors (temperature, atmosphere, humidity).
- MAP is an extrinsic control: lowering oxygen and increasing CO2 slows aerobic bacteria and molds and reduces oxidative rancidity.
- Refrigeration slows but does not kill microbes; some pathogens (e.g., Listeria) can grow slowly at refrigeration temperatures.
- Cutting or slicing increases surface area and can introduce contamination from equipment, shortening shelf life.

2) Step‑by‑step reasoning
- Pre‑packaged deli meats are sealed under controlled atmospheres soon after processing; the gas mix reduces available O2 and raises CO2.
- Reduced O2 limits aerobic spoilage organisms (e.g., Pseudomonas) and oxidative chemical changes, so spoilage is delayed.
- Freshly sliced meats at a deli counter are exposed to ambient air and possible cross‑contamination from slicers and hands, increasing microbial load and oxygen exposure, which shortens shelf life.
- Refrigeration alone only reduces growth rate; without atmosphere control the aerobic flora still grow faster than when O2 is limited.

3) Calculations
- No numerical calculations are required for this conceptual question.

4) Why the correct answer is correct
- Option A correctly identifies that MAP alters extrinsic conditions (the atmosphere) to limit microbial growth and oxidation, directly explaining the longer shelf life.

5) Why other options are incorrect
- B: Processing does not remove the nutrients microbes need in meats; nutrients remain abundant.
- C: Refrigeration alone does not eliminate microorganisms; it only slows their growth.
- D: Packaging cannot prevent growth of all spoilage/pathogenic organisms (some are anaerobic or psychrotrophic and can still grow).

6) Conclusion
- Therefore, the correct answer is A: Modified atmosphere packaging alters extrinsic conditions to limit microbial growth, which matches the provided answer.

Pre‑packaged refrigerated deli meats often have a longer shelf life than freshly sliced meats because:[Fill in the blank]单项选择题

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