High-sugar foods such as jams inhibit bacterial growth but may still support mould growth because:[Fill in the blank]

Question

BlackTom AI · Accepted Answer

Different microbial groups respond differently to reduced water activity, so high-sugar foods can stop bacteria but still allow moulds to grow.

1) Relevant knowledge points and concepts
- Water activity (aw) measures the availability of free water in a food for microbial growth; high solute (sugar) lowers aw by binding water.
- Microorganisms have minimum aw requirements: many bacteria need relatively high aw, yeasts tolerate lower aw, and moulds (especially xerophilic moulds) tolerate the lowest aw.
- Osmotic stress from high sugar inhibits organisms that cannot adapt to low-aw conditions.

2) Step-by-step reasoning
- Adding a lot of sugar (as in jams) increases solute concentration and decreases free water (aw).
- Microbes exposed to low aw experience osmotic stress; only those with adaptations (osmoprotectants, cell-wall properties) can maintain cellular water and grow.
- Compare typical thresholds: many common bacteria require aw ≥ ~0.90–0.95 to grow; many yeasts can grow down to ~0.85–0.88; many moulds can grow at aw ≈ 0.80 and some xerophilic moulds down to ~0.65–0.75.
- Therefore in high-sugar foods where aw is low, bacteria are inhibited but moulds (and some osmophilic yeasts) can still grow.

3) Calculations
- No numeric calculation is required; the argument is based on comparing aw thresholds.

4) Why the correct answer is correct
- Option C directly states the biologically accurate principle: different microbial groups have different sensitivities to reduced water activity, explaining why jams inhibit bacteria yet allow mould growth.

5) Why other options are incorrect
- A: False — sugar can be a nutrient for many microbes, including bacteria; the issue in jams is water availability, not lack of nutrient.
- B: False — moulds are affected by intrinsic factors (pH, aw, nutrients); they simply tolerate lower aw better.
- D: False — many bacteria are anaerobic or facultative anaerobes; oxygen requirement is not the reason bacteria are inhibited in sugary foods.

6) Conclusion
- The correct choice is C. This matches the provided answer: "C. Different microbial groups respond differently to reduced water activity."

High-sugar foods such as jams inhibit bacterial growth but may still support mould growth because:[Fill in the blank]单项选择题

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