Bag Stack Storage

A woven sack is a container for handling. It lets one person move a lot that would otherwise need a conveyor, lets a trader count and sell defined units, and stops grain spilling. It is not a barrier. Water vapour passes through the weave freely, which means grain in a sack sits at whatever moisture the surrounding air puts it at over time: bag dry grain in a humid store and it will take moisture back until it reaches equilibrium with that air. Storage insects move through and between sacks without obstruction, so an infestation that starts in one sack does not stay there. Rodents cut through jute and polypropylene without difficulty.

This matters because bagged storage is frequently discussed as though the sack were doing the protecting. It is not. The grain is protected by having been dried adequately before bagging, and by the store keeping rain, ground damp, and vermin away from the stack. Where a genuine barrier is wanted, it must be added: hermetic liner bags placed inside the woven sack seal the grain airtight and work by an entirely different principle, letting respiration deplete the oxygen inside. That is hermetic storage, and it is a different system that happens to use a sack-shaped container.

The conventions of bag stacking — pallets underneath, a gap to the walls, gangways between stacks, limited stack height — are each answers to a specific failure. Pallets lift the bottom course off a floor that conducts ground damp and pools any water that enters. The gap to the wall does the same for walls, which are cooler than the store air and condense moisture on their inner face, and it also denies rodents the sheltered run along the wall that they prefer. Gangways exist so that every face of every stack can be inspected: bag stacks fail from the outside inward and from the bottom up, and a stack pushed against its neighbours cannot be checked.

Height and tying patterns are about both stability and ventilation. Sacks slump as grain settles inside them, so a stack that was square when built leans as it ages, and a leaning stack of heavy sacks is a collapse hazard for the people dismantling it. Cross-tying the courses — laying alternate layers at right angles — binds the stack together. Ventilation is the other constraint: only the outer courses of a large stack see any air movement at all, so the interior of a big block stack is unventilated, and any heat generated there stays there. Stack dimensions, heights, and spacings are set by the applicable national storage guidance and the store’s own procedures.

Bagged storage is most common precisely where it works least well. In a warm, humid climate the air itself will hold grain at a moisture that supports mould growth, so grain dried properly at harvest does not stay dry in a permeable sack — it re-equilibrates with the store air. Warmth also means storage insects complete generations continuously, with no cold season to interrupt them, so an infestation that would take a season to develop in a temperate store develops in weeks. The two problems compound: insect activity generates heat and moisture, which favours mould.

The mould risk is not only a matter of visible spoilage and weight loss. Storage moulds on maize, groundnuts, and other susceptible commodities can produce mycotoxins, and mycotoxin contamination is not reliably visible — a sack can look sound and still carry it. That is a food-safety consequence of storage conditions, and it is why guidance for humid climates pushes toward hermetic liners, better drying before bagging, and shorter holding periods rather than toward better stacking of permeable sacks.

• Bag only grain dried adequately for the climate and the intended holding period — the stack cannot correct wet grain.
• Stack on pallets, clear of walls, with gangways that let every face be inspected.
• Clean the store and remove residual grain and old sacks before a new season; do not reuse sacks that carried an infested lot.
• Keep the roof sound and the floor sealed; the bottom and outer courses fail first.
• Proof the building against rodents and inspect for gnawing, droppings, and spillage.
• Cross-tie courses and limit stack height so stacks stay stable as sacks slump.
• Where the climate or the holding period demands a real barrier, use hermetic liners rather than expecting more from a woven sack.