Preservation of Grain by Airtight Storage

Abstract

Preservation of grain quality in storage systems sufficiently airtight to control insects is reviewed. These systems are being used for long-term storage, as well as intermediate grain storage, especially for centralized storages, cooperatives, and the small scale farmer. The principle of the systems is based on the use of airtight synthetic liners. The structures investigated included bunkers for bulk storage of thousands of tonnes of grain, and silo of up to 1,000 tonne-capacity with wall support of weld-mesh fencing. For capacities of up to 50 tonnes of bagged grain, especially designed envelopes termed storage cubes have been developed. All these structures are recommended for dry grain storage only.

1. Introduction

Storage is an integral part of food security and its importance in many countries has been well documented (Anon., 1986; Bonner and Hirdy, 1987; O'Dowd et al., 1987). Food grains consisting of cereals and pulses stored at moisture contents permissible for safe storage are the principal commodities dealt with in this presentation.

Moisture content is the major factor in determining the storage behaviour of grain (Pixton, 1982). Initial grain deterioration due to molds can be prevented if the moisture content is sufficiently low. However, the amount of moisture in dry grain bulks is sufficient to permit development of most stored grain insects (Howe, 1965). Therefore, periodic insect control measures are usually required to prevent loss of quality and quantity of stored grain (Semple, 1985).

A method considered for the prevention of storage losses is airtight storage. The intrinsic advantage of the airtight storage of dry cereal grains lies in the generation - by the aerobic metabolism of insect pests and micro-organisms - of an oxygen-depleted and carbon-dioxide enriched intergranular atmosphere of the storage ecosystem. By doing so, their development is arrested and storage damage minimized. This principle has been used since prehistoric times, perhaps unwittingly, in traditional underground storage structures that are still used, particularly in semi-arid regions of the Mediterranean basin and Sahel (Curride and Navon, 1986, Gilman and Boxall, 1974).

A concept conceived to provide a low cost system for storing large quantities of grain for prolonged period was bunker storage. The first documented bunkers were those used to store grain surpluses in Argentina and Uruguay during the second world war. The initial bunkers were deep pits lined with bricks or straw and wood, or a soil cement mix, and roofed with bitumen-treated sheeting and sometimes covered with soil (Anon. 1949).

Although grain was stored successfully for prolonged periods, these pits suffered from shortcomings both in the quality of hermetic seal and in grain handling procedures. However, the more recent development of durable plastic sheeting has facilitated the design of above ground bunkers. In this case, the grain is loaded into the bunker to form an elongated peak according to the angle of repose of the grain. Above ground silos (concrete and metal) have also been constructed with specifications to provide a seal for hermetic storage. Earlier designs did not provide a sufficiently effective seal (Ctesiphon silos in Kenya) though sealing techniques have been developed that enable gas permeability to be reduced to a minimum (De Lima, 1980).

The present approach to sealing existing above-ground structures is more successful (Ripp et al., 1984). Plastic structures suitable for long-term storage systems, as well as intermediate grain storage for cooperatives and subsistence farmer for the storage of grain in bags or in bulk have been developed in Israel (Navarro et al., 1990). They differ from modern silo facilities constructed of metal or reinforced concrete, which are essentially more appropriate to permanent structures and where grain handling and storage is in bulk (Boumans, 1985).

Referring to the use of plastic structures for the storage of grain, we have considered that
  (a) loss prevention methods should not be very sophisticated; and
  (b) capital investment for the storage structure should be kept at a minimum.

In addition, on-farm storage or farm-level storage of small quantities of grain are considered as important supporting aspects in that they supply source material for food reserves that are stored in bags in warehouses. In areas of development or where bumper crops are expected, extra storage space should be provided near the production site. Therefore, rapid construction and possible translocation of the storage facilities from one site to another would be advantageous. In this paper plastic structures for storage of grain based on types of bunker storage, weld-mesh silos, and envelopes are presented. These are methods of storage based on different logistic principles but having a common structural component, namely, a flexible liner.

2. Materials and Method

Bunker-type storage is proposed for capacities larger than 1,000 tonnes. It consists of a bunker bordered on three sides by ramps of earth which should be excavated from both inside and outside the site to form the structural wall of the silo. The following methodology is merely provided for a description of the conditions under which the results were obtained. Detailed results of this storage trial have been published elsewhere (Navarro et al., 1984).

Dimensions of the Israeli bunker were 150 meters long and 50 meters wide. The earthen floor was bordered on three sides by earth ramps upturned from the floor during levelling and grading, and the fourth end was left open for grain loading. The ramps were 2 meters high, 8 meters wide at the base and levelled to permit drainage of rain water away from the outer sides (Fig. 1). Before loading, the floor and ramps were lined with overlapping strips of 0.25 mm polyethylene sheeting laid transversely to form a continuous under liner. The over liner was a 0.83 mm-thick PVC formulated sheeting. The two liners contacted at the top of the ramps where they were overlapped and folded, and buried in a 60 cm trench to form a hermetic seal (Fig. 2). The ramps are further protected by a 4 meter wide extension of the over liner against erosion in winter. Since the completion of the first bunker, this method of storing grain reserves has become routine practice in Israel.

Fig. 1 - Position of temperature and gas measurements, and grain sampling points, in the experimental structure containing 15,567 tonnes of wheat (Navarro et al., 1984).

Fig. 2 - Section of earth bank to show folded liners in trench and the protective apron.

3. Results

Bunker Storage
This type of storage has been in use in Israel since 1979. The typical oxygen and carbon dioxide concentrations that can be obtained with this type of storage are given in Fig. 6 for a bunker containing 15,567 tonnes of wheat averaging 11.4% moisture content (Navarro et al., 1984). The oxygen concentration fell to 6% and the carbon dioxide concentration increased to 9% within 3 months. At the end of 15 months' storage, the wheat was removed with minimum losses. In spite of some moisture migration in the bunker, mold-damaged grain unfit for human consumption was calculated at 0.06% and losses due to insect activity over the storage period amounted to 0.15% (total losses of 0.21%). Analysis of the PVC liner at the end of storage showed that its original elasticity and resistance to tear were preserved.

Fig. 6 - Average CO₂ and O₂ concentrations (%) in the bunker-type silo containing 15,567 tonnes of wheat bulk during the storage period (Navarro et al., 1984).

Bunker Cyprus	Bunker Israel
Walinga Agrivac 2614	Walinga Agrivac 510