The Fermenting Jam Jar
A student left an open jar of homemade strawberry jam (pH 3.4, water activity about 0.85, 55% sugar) on a kitchen counter for ten days. The surface developed a fuzzy grey-green layer and a faint musty smell, while the jam below remained unchanged. She then sealed a second identical jar and stored it in the fridge; after ten days it showed no growth. She asks whether the surface growth is dangerous and whether the jam can be saved by scraping off the top layer. A classmate claims 'mould only grows where there is no sugar, so this must be a bacterium.'
1. Identify the most likely group of organisms responsible for the surface growth and explain, using the environmental conditions, why growth occurred on the surface but not deeper in the jam.
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The fuzzy, filamentous grey-green colony with a musty odour is characteristic of filamentous fungi (moulds), not bacteria - the classmate is wrong. Moulds tolerate low pH (3.4 is well within their range) and reduced water activity far better than most bacteria; the high sugar concentration actually selects for osmotolerant/xerotolerant moulds rather than excluding them.
Why only at the surface:
- Moulds are strict aerobes. Oxygen diffuses slowly into a viscous, sugary matrix, so the only well-oxygenated zone is the air-exposed surface.
- The open jar also allowed airborne spore contamination; the sealed refrigerated jar was protected and cold (slower metabolism).
- Deeper jam lacks O₂ and is also more acidic/anaerobic, so bacterial or fungal growth is suppressed there.
So the surface layer is a fungal biofilm, not a bacterial bloom.
2. The student plans to scrape off the visible mould and eat the rest. Evaluate this plan and justify your recommendation biologically.
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Reject the plan. Visible mould is only the reproductive/aerial part; the vegetative mycelium penetrates the substrate well beyond the visible colony, and moulds can secrete mycotoxins (e.g. patulin-type compounds are associated with mouldy fruit products) that diffuse into the surrounding jam. The jam is soft and moist, so toxins and hyphae spread further than in a firm food.
Key points:
- Scraping removes the fruiting structures but not the embedded hyphae or dissolved toxins.
- Toxins are heat-stable, so re-boiling does not reliably make it safe.
- The musty smell indicates volatile metabolic products, i.e. substantial colonisation.
Recommendation: discard the whole jar. The fridge control shows that refrigeration plus sealing prevents the problem, so the practical lesson is storage, not salvage.
3. Design a simple control experiment the student could run to test whether the fridge jar stayed clean because of temperature or because it was sealed.
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Use a 2×2 design with identical jam jars, all inoculated with the same small amount of airborne spores (or left open briefly to standardise contamination), then:
| Condition | Sealed | Open |
|---|---|---|
| Room temp (~22 °C) | A | B |
| Fridge (~4 °C) | C | D |
- A vs B isolates the effect of sealing at room temperature.
- C vs D isolates the effect of sealing in the cold.
- A vs C isolates the effect of temperature when both are sealed.
Record presence/absence and area of surface growth after 10 days. If C stays clean but D grows, the seal (spore exclusion + O₂ limitation) matters; if A grows but C does not, temperature is decisive. To separate these you need all four jars, because the original comparison changed two variables at once.