Manual of the prevention of post-harvest grain losses

As a result of extensive social and economic changes (e.g. expansion of trade with basic foodstuffs, supplies of food aid, increase in urbanisation), central stores have grown both in number and importance in all countries. These stores have usually been run under state control by cereal offices. Structural adjustments in many countries led to substantial retrenchments of food quantities stored for emergencies, but still storage in medium- and large-scale facilities plays a vital part in the provision of the population with staple foods.

5.1 Storage facilities

It is quite apparent that a number of the storage facilities set up in tropical and sub-tropical countries are unsuitable for storing goods without a reduction in quality and considerable losses.

Although appropriate warehouse designs have been provided as far as suitable construction material, ventilation facilities and favourable constructional properties are concerned, little use is made of these. This can be observed even in recently-built stores.

The storage structures described in the following paragraphs can frequently be found in hot climates. The potential for the use of each type and their main shortcomings are highlighted in the text.

- Corrugated iron halls

Easy-to-erect corrugated iron halls made of prefabricated parts may be justifiable for short-term storage in case of emergencies. Buildings of this kind are unsuitable for long-term storage due to the poor control of climatic conditions and condensation problems.

- Flexible plastic silos

Under arid climate conditions, instant erection flexible plastic silos can be used for bag storage of locally produced grain as a pan of maintaining security reserves. To avoid condensation problems it is a precondition that the produce is absolutely dry (about 10% moisture content) at the moment of storage.

In the case of supplies of foodstuffs coming from temperate areas or produce with higher moisture content, flexible plastic silos can only be used as emergency stores for a short period as the same problems may be anticipated as for corrugated iron halls. Fumigation can easily be performed in silos of this kind. Flexible plastic silos are not suitable as transit stores due to the longer filling procedure and their fragility.

- Bag storage in warehouses

Assuming that the basic storage requirements are adhered to, the system of bag storage in well-designed warehouses is most suitable in tropical or sub tropical areas. This system is easy to manage, cheap, and efficient and well adapted to the existing infrastructure in most countries. It involves little risk in particular as far as long-term storage of security reserves is concerned.

- Bulk storage

Bulk storage in warehouses and silos is a system whose strengths lie particularly in rapid and labour-saving turnover of produce. The system enables relatively simple and efficient pest control measures and may considerably simplify transport tasks. The transit silos found in ports are typical of this usage. The relatively sophisticated bulk storage system demands, however, a high degree of management qualities, in particular with respect to loss prevention as a result of moist grain and condensation problems.

Successful bulk storage in silos requires adequate funds being made available for the relatively high overheads (maintenance, service, energy). in general operating costs are higher for bulk storage than for bag storage. Setting up silo units demands also considerably higher investments and capital costs than for the comparable warehouse capacity for bag storage. A minimum annual turnover is necessary to justify these investments, thus making silos uneconomical for long-term storage. In the Federal Republic of Germany, for example, the rough figure for the economic operation of a silo plant is an annual turnover of 13 times.

In developing countries the bulk storage system may gain importance in the long term if the conditions in terms of infrastructure and management permit the introduction of such a system.

- Open-air storage

Open-air storage is a short-term emergency measure. The produce must always be stored on pallets in order to avoid any ground moisture being absorbed. Tarpaulins spread on the ground, or concrete platforms, afford also protection, and must be present in any case for fumigation. The stacks also have to be covered with tarpaulins against adverse weather conditions.

A decision in favour of one system or the other must take into account all the pros and cons of the systems and the framework conditions in the country in question. Moreover, storage must be seen only as one element in the entire chain of food supply from the producer to the consumer, and any particular storage system must become an integrated, well adapted part of the existing structures.

5.1.1 Instructions for the Construction of Warehouses

As many serious mistakes are made in the construction of medium-sized and large warehouses, instructions are given here for the basic design of stores which provide optimum conditions for the storage of grains and other foodstuff

5.1.1.1 Siting and Orientation

A raised site and good drainage ensure that there is no stagnant water in the vicinity of stores.
Setting up the store with its longitudinal side on an East-West axis (less sun radiation on the building) or exposed to the main wind direction creates balanced temperature conditions, thus reducing the danger of condensation.
Firm soil and good road connections enable easy delivery and transportation of produce.

5.1.1 2 General Constructional Features

Not more than four corners and a simple and effective spatial design without any more angles, pillars, beams, windows or doors than necessary make work and especially cleaning easier and make it more difficult for pests to enter or find a hiding place.
Offices and sanitary facilities which are separate from the warehouse enable fumigation and pest control measures to take place without any danger to staff.
Pesticides, fertilizers and other material which are stored separately prevent any damaging effect on the stored produce and improve storage hygiene.

Figure 39

5.1.1.3 Flooring

A floor at a height of I m above the ground with a ramp prevents ground moisture to penetrate the store, protects the walls and doors from being damaged by vehicles and simplifies loading and unloading of trucks,
A vapour barrier stops ground moisture from rising. A suitable method is to insert polyethylene foil of at least 0.2 mm thickness, or a 5 cm layer of bitumen in the floor and in the first 25 cm of the walls.
A concrete floor hard enough to bear the load expected prevents cracks.
A smooth surface without any cracks or holes is easy to clean and does not afford insects any place to hide.

5.1.1.4 Walls

Roof - bearing pillars integrated into the walls facilitate storage hygiene.
A smooth surface without any cracks ether inside or out affords no hiding-places for pests. Even the smallest of holes must be filled in.
A white, water-resistant and, if possible, plastifying outer coat of paint reflects the sun radiation and prevents penetration by moisture.
Corrugated iron walls are unsuitable due to temperature variations inside the store.

5.1.1.5 Roofing

Eaves overhanging the walls by at least 1 metre ensure that the store is kept in the shade and its walls are protected against rain.
Eaves overhanging by at least 2-3 metres at the doors enable vehicles to be loaded and unloaded when it is raining.
Properly sealed connections from roof to walls prevent any insects or birds from entering.
Insulation under the roof in case of corrugated iron sheets reduces the effect of the sun radiation and creates better storage temperatures. Insulating material does, however, have the danger of being an ideal hiding place for pests and also makes it more difficult to maintain good storage hygiene.
Aluminium sheets or fibre concrete roofing do not become as hot as corrugated iron and create better storage temperatures.
Gutters linked to a drainage system prevent the outer walls and the foundation becoming wet in case of rain.

5.1.1.6 Doors

One door at each gable end is normally sufficient.
Tight-sealing hinged doors prevent rodents from entering. Sliding doors always leave a gap between door and wall, A gap of 6 mm is sufficient to enable a mouse to enter. Roll-up doors rust and often become defect when older.
Metal doors are most resistant against any damage by rodents. Wooden doors should be fitted at the bottom with a panel of sheet steel of half a metre in height.

5.1.1.7 Ventilation Openings

Ventilation openings with flaps which can be regulated enable controlled ventilation and the evacuation of heat from the store.

Ventilation openings should have a size of:

0.5 m²/100 m² storage area for incoming air (lower ventilation openings) and 1.5 m²/100 m² storage area for outgoing air (upper ventilation openings).

The lower ventilation openings should be situated approx. 1/2 metre above the floor, the upper ones approx. 1/2 metre below the roof on both sides of the store.

Tightly-sealing ventilation openings permit fogging with insecticides.
Wire gauze and grilles in the ventilation openings prevent insects, rodents and birds from entering.

Roofing over the ventilation openings prevents any penetration by rainwater.

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