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Hermetic grain bag

PICS bag · Purdue Improved Crop Storage bag

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A hermetic grain bag stores suitably dried grain in an airtight, multilayer package. The PICS design pairs two inner liners with a woven outer sack; it is a storage tool, not a fermenter or dryer.

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Generated photorealistic image of a filled white woven hermetic grain bag with a gathered tie.

A layered bag for dry grain

This entry focuses on the PICS (Purdue Improved Crop Storage) design as a documented example of a hermetic grain bag. Purdue describes a composite package: two high-density polyethylene (HDPE) liners inside a woven polypropylene (PP) outer sack. The plastic liners form the gas barrier; the tougher outer sack makes the filled bag easier to handle. These materials describe PICS bags, not every bag sold as hermetic.

The closure matters because the storage effect depends on limiting gas exchange. Purdue explains that insects use oxygen while feeding and growing; in a well-sealed package, oxygen can fall and carbon dioxide rise until insect activity and reproduction are suppressed. The bag does not need a powered pump or heater. It creates a storage environment around grain already harvested and dried.

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Dryness and seal integrity come first

A hermetic bag is not a dryer. Purdue's guide says to store clean, dry grain, check both inner liners and their seals for holes, and close the package correctly. Dirt, sharp debris and careless handling can damage a liner. If the barrier leaks, the storage conditions described for an intact bag cannot be assumed. Purdue's care guidance also recommends keeping bags away from direct sunlight and extreme heat, and patching small holes when appropriate for the product.

The same limit appears in FAO's account of the Zimbabwe Idai Recovery Project: grain had to be adequately dried before it was stored. The FAO story reports 25,600 hermetic bags distributed to 6,400 households across four districts. It says the bags were reusable and that the grain could last up to two years in that programme. Those details describe the local project; they are not a universal shelf-life guarantee for every crop, bag or storage room.

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Sizes and crop choices vary

PICS bags are sold in different capacities. Purdue's extension bulletin compares 50 kg and 100 kg models, while a current Purdue Edustore listing offers 25 kg and 50 kg options. The numbers are grain-mass capacities, not litres, and the exact range depends on the product and market. The sources do not give one standard height or diameter for this equipment class.

Purdue's programme names maize, sorghum, wheat, rice, peanuts, common beans and other crops as uses for PICS storage. One field study compared hermetic bags and other storage approaches for sorghum in Zimbabwe; its result belongs to the tested crop and storage conditions, rather than proving that every bag produces the same outcome for every grain.

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Modern distribution and geographic context

The PICS system began as Purdue research with African collaborators and was first developed for cowpea storage. Purdue's Extension materials describe its later use across cereal and legume crops and dissemination in multiple parts of Africa and Asia. Purdue's current programme page describes a wider vendor network and use beyond its focus countries. This page therefore records the contemporary manufactured equipment as modern and global rather than assigning the bag a single place of origin.

Zimbabwe is one documented setting, not a claim of origin. The 2021 sorghum field study and FAO's post-Cyclone Idai report describe hermetic storage in Zimbabwe. FAO names districts in Manicaland and Masvingo affected by the project, while the separate research study evaluates sorghum storage. These examples show use and context, not one universal local design.

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Equipment details

CapacityCapacity is reported by grain mass, not litres. Purdue sources document PICS sizes of 25 or 50 kg in one current listing and 50 or 100 kg in extension materials; the size depends on model and market.
SizeThe sources report nominal grain-mass sizes rather than a standard height or diameter.
PowerNo power Hermetic storage is passive; the documented bag has no powered components.
How you get oneBought, manufactured PICS bags are manufactured and sold through vendors and distribution networks; other hermetic bag designs may differ.
CareUse clean, adequately dried grain; remove debris that can puncture the liners, inspect both inner liners and their seals, tie them correctly, and protect the bag from punctures, rodents, direct sunlight and extreme heat. Purdue Extension describes repair of small liner holes with tape; follow the instructions for the specific bag.

Made of

  • High-density polyethylene (HDPE) — Two inner liners in the documented PICS design
    Purdue describes two 80-micron HDPE liners; this specification is PICS-specific and does not describe every hermetic bag.
  • Woven polypropylene (PP) — Outer handling sack in the documented PICS design
    Purdue describes a woven PP outer sack around the two inner liners; not a universal material specification for every hermetic bag.

Limits

The bag does not dry grain. Purdue and FAO guidance says to store only adequately dried grain; wet or damp material is not covered by the PICS storage recommendation.

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A punctured or poorly closed liner compromises the hermetic barrier and can reduce effectiveness.

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Places

Zimbabwe

Documented use · Country evidence

The cited field study evaluates hermetic bags in smallholder sorghum storage in Zimbabwe; this is one documented-use context and not an origin or exclusive-use claim.

More equipment from ZimbabweLocation source 1 →

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