Solar Drying of Tomato Slices in Humid Weather
A self-help group in Gorakhpur uses a solar cabinet dryer to make dried tomato slices in late July. They load 15 kg of 8 mm slices onto each square metre of tray, close the dryer, and run the exhaust fan only when the inside temperature exceeds 60°C. Ambient conditions are 30–32°C and 85–90% relative humidity, with frequent cloudy spells. After 3 days the outer edges of the slices are hard and dark, the centres are still moist, and white and green mould appears on many slices. The group wants to fix the problem by dipping slices in 2% salt solution and drying for 5 days instead of 3. They have no refrigerator and need a product that can be stored for 4 months for a school-feeding order.
1. Diagnose the two defects - hard dark edges with moist centres, and mould - using drying principles. Which process variables are most likely responsible?
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Hard dark edges with moist centres indicate case hardening. The surface dries too quickly at high temperature and low humidity, forming a dense layer that traps moisture inside. Mould is a secondary result: the centre water activity remains high, and the humid, cloudy conditions keep the cabinet air near saturation, so overall drying is slow.
The key variables are:
- Loading rate: 15 kg/m² is very heavy for tomato slices.
- Slice thickness: 8 mm is thick for a humid-weather dryer.
- Airflow and exhaust: running the fan only above 60°C means humid air is not removed during cloudy periods.
- Air velocity: low forced convection slows moisture removal.
Together these produce a dry outside and a wet inside - ideal for mould.
2. Evaluate the proposed 2% salt dip plus 5 days of drying. Will it reliably give a 4-month shelf-stable product? Justify.
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The salt dip may slow surface mould slightly, but it will not solve the core problem. A 2% dipping solution is far too dilute to lower water activity to a shelf-stable level; it is not a preservation concentration. Five days in 85–90% relative humidity is likely to extend the warm, moist period that mould needs.
A 4-month shelf life requires dried tomato at about 10–12% moisture (water activity below about 0.6) and a moisture-barrier package. Without a dehumidifier or finish dryer, the group cannot reliably reach that in monsoon conditions. Longer drying also increases browning and vitamin loss. The proposal treats a symptom, not the drying constraint.
3. Give a revised drying and packaging plan with at least four quantitative targets.
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- Slice thickness: 5–6 mm.
- Tray load: no more than 6 kg/m².
- Blanch: 1–2 minutes to control enzymes and reduce the initial microbial load.
- Airflow: run exhaust continuously; maintain about 1–2 m/s air velocity across the tray.
- Temperature programme: start at 55–60°C, then raise to 65–70°C.
- Final moisture: ≤12%, checked by weighing; target water activity ≤0.60.
- Finish drying: use a dehumidifier or small electric dryer during cloudy spells.
- Packaging: cool and equilibrate, then pack in metalised or high-barrier film, preferably with an oxygen absorber; store below 30°C.
- Verification: carry out a 4-week accelerated storage test before committing to the school order.