Last reviewed 1 Oct 2026 · Facts as of 1 Oct 2026 · 9 min read
Hermetic (airtight) storage
Principle
In a sealed container, the respiration of the grain, insects, mites and microbes uses up oxygen and produces CO₂. As the oxygen falls (below about 2–5 %) and CO₂ rises, insects stop feeding and die (or become inactive) and aerobic moulds are suppressed — without chemicals. The method is suited to dry grain, since moist grain in a sealed bag still supports anaerobic fermentation and can spoil.
A sealed 100 kg bag of wheat (bulk density 780 kg/m³, grain volume 0.128 m³) has an intergranular void space of about 40 %:
Void volume m³ litres; oxygen in the air . If respiration reduces the oxygen to 3 %, the remaining oxygen is L — the insects and the grain have consumed about 9 L of oxygen (producing a similar volume of CO₂ initially). A small infestation consumes this within days to weeks, after which the pests die; the bag must stay airtight, since leaks restore oxygen.
Hermetic systems
| System | Description |
|---|---|
| Hermetic bags (e.g., PICS — Purdue Improved Crop Storage triple-layer; Super Grain bags) | Two inner layers of polyethylene (thick, low-permeability) and an outer woven polypropylene bag; 50–100 kg capacity; used for maize, pulses (cowpea), wheat, rice, seeds; can be reused for 2–3 seasons if undamaged |
| Hermetic liners/cocoons | Large PVC/polyethylene sheets forming a sealed cocoon around stacked bags or bulk grain (capacities from 5 to hundreds of tonnes, e.g., GrainPro cocoons); open-air or in warehouses |
| Metal silos (small, galvanised, soldered) | Household metal silos (about 0.2–2 t), very robust and rodent-proof, but need airtight lids |
| Large hermetic silos (steel/RCC with gas-tight design) | Bulk commercial storage with sealed hatches, pressure relief valves, and sometimes added CO₂/N₂ |
| Plastic drums and jerry cans, glass jars | Household/seed |
| Underground hermetic pits | Traditional in dry regions |
Requirements and cautions
- Grain must be dry (at or below the safe moisture), clean, and free of heavy infestation at loading.
- Seal fully: twist and tie the inner bag separately; the outer bag also tied; protect against punctures (rodents, sharp objects); inspect the bags periodically.
- Temperature fluctuations can cause moisture condensation; store in shade/cool.
- No re-opening during storage — each opening restores oxygen; use the whole bag once opened.
- Seed viability: hermetic storage at proper moisture keeps viability well; seed must be dry to the recommended level.
- Limits: cost of bags/cocoons, damage by rats, need for training, and the method suppresses but does not necessarily kill all stages quickly (the adults die, eggs/larvae may be inactive); for large lots, monitor the O₂/CO₂ and temperature.
Benefits: no insecticide residues, long storage (6–12 months or more), protection from moisture and rodents, suited to farmers and cooperatives.
Cover and plinth (CAP) storage
CAP storage is a temporary method of storing bagged food grain in the open (for example during procurement peaks, when godowns are full), used by the Food Corporation of India (FCI) and state agencies.
Features
| Component | Details |
|---|---|
| Plinth | A raised platform of bricks/concrete/stone, above the ground level and surrounding drainage, with a slope to shed water — protects from ground moisture and flooding (plinth height of at least about 0.45 m above the highest flood/ground level in typical specifications; check the current norm) |
| Dunnage | Wooden crates, bamboo mats, or polyethylene sheet layers (and sometimes with an air-gap) placed over the plinth under the bags |
| Stack | Bags in a stable, interlocked pattern, stacked to a limited height (the bag count per stack and the stack height are laid down by the agency); gaps between the stacks (for inspection and fumigation) |
| Cover | Heavy LDPE/HDPE tarpaulins (specified thickness/gsm, UV-stabilised), laid over the stack and overlapped; fixed with ropes and weights to resist wind; the cover must reach the ground edge and the plinth (and have ventilation arrangements per specification) |
| Fumigation | Gas-tight sheet cover during fumigation |
| Drainage and fencing | Storm drains around the plinth; security fencing; lighting; fire-fighting equipment |
- Use: short-to-medium duration (e.g., a few months to under a year — limited by the agency's rules); frequent inspection, early liquidation (first in, first out); bags with uniform weight and compact stacking.
- Risks: wind lifting the cover, rain leaks, rodent damage, condensation, higher losses than in godowns; to be avoided as a long-term solution — the policy emphasis is on moving to covered godowns and steel silos.