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Modern-Day hermetic Storage
Above-ground rigid structures
Documented data on successful application of hermetic storage to above-ground constructions are largely lacking. Many existing silos and warehouses have been modified to provide a high degree of hermetic seal especially in Australia (Delmenico 1993). However, the objectives have been to convert these storages for modified atmosphere (MA) treatments or improved fumigations, and not for hermetic storage as such.
In contrast, the sealing of both bagged stacks and bulk grain in warehouses in China using plastic liners is part of a grain preservation regime termed "Triple-Low". This is an integrated approach to insect control consisting of obtaining reduced O₂ concentrations by metabolic activity within the grain bulk in combination with phosphine and low temperature treatments (Wang et al., 1993; Xu and Wang, 1993). This procedure is claimed to provide effective protection.
Above-ground flexible structures
In the early 70s, above-ground structures were designed in England for emergency storage using flexible plastic liners supported by a weldmesh frame. These liners were made of butyl rubber, sometimes laminated with white EPDM, and consisted either of a wall-floor section, plus a roof section attached after loading, or both sections welded into a single unit. These silos were recommended for both conventional storage and hermetic storage of dry grain (Kenneford and O'Dowd, 1981). However, under tropical and sub-tropical climates the liners were found to deteriorate, and gas permeability increased to a level where the liners could no longer be used for hermetic storage (Navarro and Donahaye, 1976; O'Dowd and Kenneford, 1982). In Israel, the manufacture of PVC liners that conform to pre-requisite specifications of durability to climate, gas-permeabilities, and physical properties, enabled the development of three other storage systems based on the hermetic principle.
Navarro and Donahaye have developed three storage systems based on the hermetic storage technology. These are:
- Bunker storage for conservation of large bulks of 10,000 to 15,000 tonnes capacity (Navarro et al., 1984; Navarro, et al., 1993).
- GrainPro cocoons. These are flexible liners for enclosing stacks of 10 - 150 tonnes capacity and designed for storage at the farmer-cooperative, small trader, and even national grain-reserve levels (Donahaye et al., 1991).
- Grainsafe. The problem of applying present-day technology to provide hermetic storage for subsistence farmers lies in the need to provide an easily sealable low-cost container of about a ton or even smaller capacity. The high surface area to volume ratio necessitates a liner with a very low permeability to gases. Our most recent R&D project to address this challenge has been through the "GrainSafe" granary, which is specifically tailored to the farmer's needs.
- The most recent attempt to address this problem has been through the "Joseph bag," which is made of a plastic-metal foil laminate, sealable by means of a hot iron (Murray ,1990). Also recently a transparent multilayer with gas barrier for storage of cereals up to 50 kg capacity under the name of SuperBag has been proposed for sealed storage (Tom deBruin, personal communication,2004).
