Quick answer: Post-harvest drying lowers the moisture content of grain, seeds, nuts, herbs, flowers or fruit to a level where moulds, insects and respiration stop causing losses. The main methods are sun drying, ambient-air drying, heated-air drying and low-temperature drying with dehumidified air. Choose by crop, volume, weather and quality: heat dries fastest but can damage seeds, colour and aroma; dehumidified air dries more slowly at low temperature and preserves quality, and works regardless of outdoor humidity. To size any dryer, start from the water to be removed: water (kg) = initial mass × (initial MC − final MC) ÷ (100 − final MC), with moisture on a wet basis.
Quick answer
- Water to remove (wet basis) = M × (m₁ − m₂) ÷ (100 − m₂).
- Example: 10 t of grain from 20 % to 14 % → about 700 kg of water.
- Air at about 60–65 % RH brings most cereal grains towards about 13–14 % moisture.
Drying methods compared
| Method | How it works | Pros | Cons | Typical uses |
|---|---|---|---|---|
| Sun drying | Spread on floors, mats or racks | No energy cost | Weather-dependent, contamination, uneven, slow | Smallholder grain, fruit |
| Ambient-air (in-bin or store) | Fans push outside air through the product | Low cost | Only works when outside air is dry enough; slow | Grain in bins in dry seasons |
| Heated air | Air heated before passing through product | Fast, high capacity | Energy; risk of cracking, seed damage, quality loss | Commercial grain dryers |
| Low-temperature dehumidified air | Dehumidifier dries the air, often at 20–40 °C | Gentle, controlled, independent of weather | Slower than hot-air drying; equipment cost | Seeds, herbs, flowers, nuts, specialty crops, high-value products |
| Desiccant-dried air | Very dry air at low temperature | Very low final moisture | Reactivation energy | Seeds and sensitive products |
The FAO publishes extensive guidance on grain drying and storage in developing and commercial systems (FAO; FAO).
Calculating the water to remove
W = M × (m₁ − m₂) ÷ (100 − m₂)
- W = water to remove (kg)
- M = initial (wet) mass (kg)
- m₁, m₂ = initial and final moisture content, % wet basis
Example: 10,000 kg of maize harvested at 20 % moisture, dried to 14 %: W = 10,000 × (20 − 14) ÷ (100 − 14) ≈ 698 kg of water.
If that has to happen in 48 hours, the dryer must remove about 15 kg/h. A dehumidifier-based system removing 15 kg/h at the drying room's condition needs roughly 360 L/day of capacity at that condition, so more in catalogue terms. Note that moisture on a dry basis gives a different number; see wet vs dry basis.
Equilibrium moisture content: how dry the air must be
A product left long enough in air at a given temperature and RH reaches an equilibrium moisture content (EMC). For drying to continue, the air must have an EMC below your target.
| Air RH (around 25 °C) | Approximate EMC of cereal grains |
|---|---|
| 80 % | about 16–18 % |
| 70 % | about 14–15 % |
| 60–65 % | about 13–14 % |
| 50 % | about 11–12 % |
Values vary by crop and temperature; USDA-ARS has published instruments and methods for measuring grain EMC (USDA-ARS). The practical point: in humid weather, ambient air cannot dry grain to safe storage levels; heated or dehumidified air can.
Safe storage moisture (typical)
| Product | Typical safe storage moisture (wet basis) |
|---|---|
| Maize | about 13–14 % |
| Wheat | about 12–13 % |
| Rice (paddy) | about 13–14 % |
| Seeds for planting | Often lower; follow seed standards |
| Dried herbs, flowers | Product-specific; often aimed by water activity or brittleness |
Store dried products at a steady, moderate humidity so they do not re-absorb water; see warehouse humidity by stored goods.
Why dehumidified-air drying suits high-value crops
- Low temperature protects seed germination, colour, aroma and active compounds.
- Weather-independent: works in humid climates and rainy harvests.
- Closed loop: heat from the dehumidifier warms the drying air; little outside air is needed.
- Repeatable: temperature and RH can be set per product.
A typical drying room recirculates air through the product and a dehumidifier. See drying room setup and our industrial dehumidifiers and desiccant dehumidifiers.
Common mistakes
- Using dry-basis moisture in a wet-basis formula.
- Assuming drying speed equals the theoretical water amount ÷ dehumidifier rating.
- Drying too hot for seeds or aromatic crops.
- Storing dried product in humid air.
- Measuring moisture in only one spot of a batch.
FAQ
Frequently asked questions
What are the main post-harvest drying methods?
Sun drying, ambient-air drying, heated-air drying, and low-temperature drying with dehumidified or desiccant-dried air.
How do I calculate how much water to remove when drying grain?
Water = initial mass × (initial moisture − final moisture) ÷ (100 − final moisture), using wet-basis percentages.
Why use a dehumidifier for crop drying?
It dries at low temperature, independent of outdoor humidity, preserving quality for seeds, herbs, flowers and specialty crops.
What humidity is needed to dry grain to 14 %?
Air at roughly 60–65 % RH or lower at around 25 °C brings most cereal grains towards 13–14 %.
Sources
- FAO post-harvest and grain storage publications (T1838E, X0039E, S1250E).
- USDA-ARS, Design and testing of an instrument to measure equilibrium moisture content of grain.
