Agricultural Process Engineering and Post-Harvest Technology
Agricultural Process Engineering and Post-Harvest Technology covers the steps between harvest and consumption: cleaning, grading, drying, milling, storage and value addition. The exam tests moisture calculations, drying and storage principles, milling out-turn and selection of equipment.
Core rules
- Moisture content wet basis (wb) = mass of water ÷ total mass × 100. Dry basis (db) = mass of water ÷ mass of dry matter × 100.
- During drying only water leaves; dry matter stays constant. Use dry matter as the anchor for weight-change calculations.
- Paddy is normally safe for storage at about 12–14% wb; high moisture encourages mould, heating and insects.
- Cleaning removes foreign matter by sieving, winnowing or density separation; grading sorts by size, density or colour.
- Milling out-turn is the mass of milled rice recovered as a percentage of paddy; it falls with excessive drying, immature grain and poor milling.
- Storage structures must keep grain dry, cool and protected from rodents and insects.
Worked example
1000 kg of paddy at 20% wb is dried to 14% wb. Dry matter = 1000 × 0.80 = 800 kg. At 14% wb, dry matter is 86% of total, so final mass = 800 ÷ 0.86 = 930.2 kg. Water removed = 1000 − 930.2 = 69.8 kg. Reasoning: the dry matter is unchanged, so divide it by the dry-matter fraction at the new moisture. Subtracting 6% of the original mass (60 kg) is wrong because the final mass is smaller.
Traps
Do not mix wet-basis and dry-basis figures in one formula. Do not assume 20% to 14% removes 6% of the initial weight. In milling questions, distinguish paddy, brown rice, milled rice and bran; out-turn is not the same as head-rice recovery. For storage, temperature and moisture interact: damp grain heats even at moderate air temperature.