Last reviewed 1 Oct 2026 · 8 min read
Why safe storage matters
Food grains are harvested once or twice a year but eaten daily, so they must be stored safely for months. Storage losses (insects, rodents, moulds, respiration, spillage) are a major part of post-harvest losses. Studies by Indian institutions (for example the 2015 ICAR–CIPHET study) report total post-harvest losses of cereals of the order of a few per cent (roughly 4–6 %), with storage a smaller share of this — the figures differ by crop and study, so check the source. Losses are in quantity (weight) and quality (nutrition, appearance, safety) — a small weight loss may still make grain unfit for consumption (mycotoxins).
The stored-grain ecosystem
Stored grain is a living system in which grain (respiring), microorganisms, insects, mites, rodents, moisture, temperature and air interact. The grain mass is a poor heat conductor, so heat produced locally builds up (hot spots).
Key controls: moisture, temperature, oxygen (air), and time, plus sanitation and protection from pests.
Causes of spoilage
1. Moisture
- High grain moisture (above the safe limit) permits mould growth and insect multiplication.
- Sources: inadequate drying, rain leaks, rising damp from the floor, condensation, moisture migration.
2. Temperature and respiration
Respiration of grain, moulds and insects: (about 2,870 kJ per mole of glucose, i.e., about 15.9 kJ per gram of carbohydrate consumed), which releases water and heat — a self-accelerating cycle (heating).
In a lot of 100 tonnes of grain, a 1 % loss of dry matter (1 tonne, i.e., g) by respiration releases about kJ = about 15.9 GJ of heat and produces water of about g (the ratio g water per g of glucose) = about 600 kg. This water raises the moisture of the 100 t lot by about 0.6 percentage points and drives further spoilage — heating accelerates.
| Temperature | Effect |
|---|---|
| Below about 15 °C | Insects inactive or very slow; mould growth very slow — cool storage prolongs life |
| 25–35 °C | Optimum for insects and moulds |
| Above about 40–45 °C | Heating; grain damage, killing of germ; thermophilic fungi |
- Every 5–10 °C reduction in grain temperature roughly halves insect development and mould growth.
- Heating stages: initial (fungi grow at 25–30 °C, grain feels warm/ musty) → hot spots (45–55 °C, caking) → severe (60–70 °C, charring, spontaneous heating, possible fire in extreme cases).
3. Moulds (fungi) and mycotoxins
| Group | Examples | Effect |
|---|---|---|
| Field fungi | Fusarium, Alternaria, Cladosporium | Need high moisture (above about 20–25 % w.b.); start in the field; toxins (deoxynivalenol/vomitoxin, zearalenone) |
| Storage fungi | Aspergillus (A. flavus, A. glaucus, A. candidus), Penicillium | Grow at 13–18 % moisture (ERH of about 70–85 %); cause heating, discoloration, caking, loss of germination, off-flavours |
| Mycotoxins | Aflatoxin (A. flavus/parasiticus; maize, groundnut, oilseeds, spices), ochratoxin A, fumonisin | Carcinogenic and hepatotoxic; regulated limits (set under the food safety standards — check the current FSSAI limits) |
Prevention: dry quickly to a safe moisture; clean; avoid damage and rain wetting; store dry and cool; avoid moisture migration.
4. Insects
| Category | Insects | Notes |
|---|---|---|
| Primary (internal feeders) | Rice weevil (Sitophilus oryzae), lesser grain borer (Rhyzopertha dominica), Angoumois grain moth (Sitotroga cerealella), pulse beetle (Callosobruchus) | Attack whole, sound grain; larvae develop inside the kernel; the key damage |
| Secondary (external feeders) | Red flour beetle (Tribolium castaneum), saw-toothed grain beetle (Oryzaephilus surinamensis), flat grain beetle (Cryptolestes), Khapra beetle (Trogoderma granarium), Indian meal moth (Plodia) | Feed on broken grain, dust and flour; Khapra beetle is a quarantine pest that can attack whole grain too |
- Damage: weight loss, quality loss (frass, webbing, odour, heating), contamination (insect fragments); insect heating and moisture release leads to mould.
- Conditions: 25–35 °C and 12–15 % moisture favour build-up; breeding stops below about 15 °C.
5. Mites
Tiny arachnids (Acarus siro, etc.) — grow at higher humidity (above about 65–70 % RH) and carry moulds, contaminate the grain and flour.
6. Rodents and birds
- Rats (Rattus rattus, Bandicota bengalensis) and mice (Mus musculus) — eat and spoil far more than they eat, contaminate with urine and faeces, gnaw bags, and spread disease; sparrows, pigeons damage open storage.
- Control: rodent-proof construction (steel/metal sheets to a height of about 1 m on doors; wire mesh on vents), sanitation, traps and baits (rodenticides with care), and proofing the gaps.
7. Other causes
Physical damage (cracks, breakage) making grain vulnerable, weather (rain, flood), contamination with foreign matter and chemicals, poor structures and handling, transit losses.