Tray Dryer Capacity for Mango Slices
A cooperative in Malda wants to dry 500 kg of fresh mango slices per day from 82% moisture (wet basis) to 12% moisture (wet basis). It has a tray dryer with 20 trays, each 1.2 m × 1.0 m (1.2 m²), loaded at 4 kg fresh slices per m². Hot air at 65°C and 20% relative humidity enters the dryer; the exhaust air leaves at 55°C and 55% RH. A trial batch took 9 hours, including 1 hour for loading and unloading. The cooperative has one 8-hour shift and wants to dry all 500 kg in that shift. The fan delivers 3,000 m³/h. The manager plans to increase tray loading to 8 kg/m² to fit more fruit and to raise the air temperature to 80°C to shorten drying. He also wants to pack the dried slices hot at 45°C into high-density polyethylene bags immediately after the dryer.
1. Calculate the fresh-fruit capacity per batch and the number of batches required for 500 kg/day. Is the one-shift plan feasible?
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Total tray area = 20 × 1.2 = 24 m². At 4 kg/m², load = 24 × 4 = 96 kg fresh slices per batch. Batches needed = 500 ÷ 96 = 5.2, so 6 batches. At 9 hours per batch including loading and unloading, 6 × 9 = 54 hours, which cannot fit into one 8-hour shift. Even with zero loading time, 6 batches would need 48 hours. The plan is not feasible; the dryer is undersized for the target, or the daily target must be reduced to about 96 kg per batch.
2. Evaluate the manager's plan to double tray loading to 8 kg/m² and raise the air temperature to 80°C. Give one quality and one safety consequence.
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Doubling the loading increases bed depth and reduces the exposed surface area per kg, so drying slows. The outer layer may dry and harden while the centre stays moist - case hardening. Raising the temperature to 80°C increases the drying rate and may shorten drying time, but it also increases browning, ascorbic acid loss and colour damage, and can worsen case hardening. It does not create more tray area, so it cannot remove the six-batch constraint. Quality consequence: darker, scorched slices with lower vitamin C. Safety consequence: case-hardened slices can retain high moisture and support mould after packing if moisture is not equalised.
3. Calculate the water removed per day. What packaging change would you recommend for the dried slices, and why?
Read the worked answer
Dry solids = 500 × 0.18 = 90 kg. Final mass at 12% moisture = 90 ÷ 0.88 = 102.3 kg. Water removed = 500 − 102.3 = 397.7 kg/day. Packaging: do not pack at 45°C in HDPE. HDPE has high moisture and oxygen permeability, and hot packing will cause condensation inside the bag, raising surface moisture. Cool the slices to near ambient, equalise moisture, then pack in a high-barrier laminate such as metallised PET/PE or PET/Al/PE, or use a sealed pouch with an oxygen absorber if needed. Store away from sunlight and humid air.