Technology - Post-Harvest Processing and Food Engineering
Scope of the Technology domain
This domain covers post-harvest technology and food engineering: drying and aeration, cleaning and grading, milling and size reduction, material handling, storage, and the mass and energy balances that connect them.
Moisture content: wet basis versus dry basis
Wet basis (wb) puts water over total wet mass; dry basis (db) puts water over dry matter only, with db = wb/(100 − wb) × 100. Drying problems are safest when anchored on dry matter, which is conserved: dry matter = initial mass × (100 − Mi)/100 = final mass × (100 − Mf)/100, hence final mass = initial mass × (100 − Mi)/(100 − Mf).
Drying behaviour
Grain shows a short constant-rate period (free surface water) followed by a falling-rate period governed by internal moisture diffusion. Thin-layer drying is fitted as MR = exp(−k t^n) with MR = (M − Me)/(M0 − Me). Equilibrium moisture content Me depends on air temperature and relative humidity through the sorption isotherm, which is why humid aeration air can re-wet dry grain. Airflow, bed depth and air temperature together fix the moisture gradient through the bed.
Worked example
1000 kg paddy at 22% wb is dried to 14% wb. Dry matter = 1000 × 0.78 = 780 kg and stays constant. Final mass = 780 ÷ 0.86 = 907 kg. Water removed = 1000 − 907 = 93 kg. Moisture fell 8 percentage points, yet mass fell only about 9.3% - percentages and masses are not interchangeable.
Traps that cost marks
- Subtracting percentage points instead of converting through dry matter.
- Setting dry matter as 86% of the initial mass rather than of the final mass.
- Applying milling outturn to grain at its actual moisture when the trade quotes a standard moisture basis.
- Confusing size-reduction laws: Rittinger (new surface created), Kick (deformation), Bond (crushing work index, W = 10Wi(1/√P − 1/√F)).