Drying 500 kg of green peas against a customer specification
A vegetable dehydration unit in Kota receives 500 kg of fresh green peas at 78% moisture (wet basis) and must dry them to 10% moisture for a snack manufacturer. The batch is to be run in a tray dryer with a fresh capacity of 250 kg per load. Under pressure to finish before the truck leaves, the operator proposes to (i) raise the inlet air temperature from 65 °C to 90 °C for the whole cycle, (ii) load 400 kg per tray set instead of 250 kg, and (iii) skip blanching 'because the peas are already fresh'. A trial run at 90 °C produced peas that looked dry outside, but after three days in a sealed bag the lot smelled musty and tested at 15.8% moisture. The customer's specification is 10% moisture maximum and a six-month shelf life in laminated pouches.
1. Calculate the mass of dried product and the water to be removed when 500 kg of peas at 78% moisture is dried to 10% moisture. Show the steps and state the number of dryer loads at 250 kg per load.
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Step 1 - dry solids (unchanged during drying): 500 kg × (100 − 78)/100 = 500 × 0.22 = 110 kg solids.
Step 2 - final mass at 10% moisture: solids are 90% of the dried product, so 110 / 0.90 = 122.2 kg dried peas.
Step 3 - water removed: 500 − 122.2 = 377.8 kg water, i.e. about 75.6% of the incoming weight.
Step 4 - loads: 500 / 250 = 2 loads, each giving about 61.1 kg dried product and removing about 188.9 kg water.
Interpretation: the figures show why the operator cannot judge dryness by look or by weight alone - nearly four-fifths of the load is water that must leave through a limited tray surface. A 250 kg load at about 15 kg/m² needs roughly 17 m² of tray area, and loading 400 kg on the same area cuts the air path and slows drying rather than speeding it. Drying to 10% rather than 0% is deliberate: 10% moisture gives a water activity low enough to stop mould growth, while further drying only adds cost and destroys texture.
2. Evaluate each of the operator's three proposals and explain the quality defects seen in the trial run.
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(i) 90 °C for the whole cycle - reject. Rapid surface evaporation seals the outside (case hardening) while the centre stays wet. That is exactly what produced peas that felt dry but read 15.8% moisture after three days, when internal moisture migrated outward. High temperature also destroys chlorophyll (dull colour), thiamine and ascorbic acid, and gives cooked, hay-like off-flavours. Recommended: two-stage drying - 70 °C with good air velocity down to about 30–40% moisture, a tempering/equalising rest of a few hours, then 55–60 °C to finish at 10%.
(ii) 400 kg on a 250 kg dryer - reject. Overloading reduces air velocity and temperature uniformity across trays, so the top dries and the bottom sours; it also increases the risk of uneven moisture between trays.
(iii) Skipping blanching - reject. Blanching for about 2–3 minutes at 95 °C inactivates peroxidase and lipoxygenase, prevents enzymatic browning and off-flavour development, fixes colour and reduces surface microbial load. Without it, the musty, hay-like smell in the sealed bag is expected. The three changes together account for the whole defect pattern.
3. What would you do with the 15.8% moisture trial lot, and what tests would you run before releasing the two production loads?
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The trial lot must not be blended with good stock: a small quantity of high-moisture peas raises the moisture of the whole bin and seeds mould. Either re-dry the lot alone to below 10% and re-inspect it, or reject it. Re-drying is technically acceptable if colour, odour and texture are still within specification, but it must be QC-approved and labelled as re-dried, since it will be harder and darker than a first-pass product.
Release tests before the two production loads leave:
- Moisture by oven method at 105 °C to constant weight (not the hand-held meter alone); specification ≤10%, with every tray sampled, top and bottom.
- Water activity (target below 0.60) as a shelf-life predictor.
- Sensory: colour, odour, absence of mustiness, rehydration behaviour.
- Microbiological: total plate count, yeast and mould, coliforms - the musty trial suggests this is essential.
- Seal integrity and headspace of the laminated pouches, plus a sample stored at 37 °C for four weeks to confirm six months' shelf claim.
Only after all five are within specification should the loads be dispatched, with a batch record showing the drying curve, temperatures and loading.