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The major objectives of crop conditioning and storage . were discussed earlier. To be able to achieve these objectives, the store must satisfy the following parameters in so far as possible: a the grain must be kept dry; b the grain should be kept at a uniform temperature; c the grain should be protected from insect attack; d rodents and birds should be excluded.
It is evident from previous sections that drying and storage are in many cases provided for in mne structure. Combining these functions is economical and allows further conditioning at later stages if required. For example, if a hot spot develops in a storage bin, it can be easily ventilated again. It may also be possible to provide some low-volume ventilation in an otherwise pure storage system.
There are however, situations when the storage is considered quite separately from drying, ranging from the storage of naturally dried crops, to the storage of grain from a continuous-flow or batch dryer.
The size and type of storage facilities is likely to be dictated by: a Total volume of crop to be stored. b The storage requirements for the crop to be stored. c The unit cost of various types of storage. d The form in which the crop is stored, i.e. cob maize vs shelled maize or bagged wheat vs bulk wheat.
The volume of the store required can be estimated from the expected yield and the land area.
A comparison between different forms of storage is normally done through calculation of costs/tonne of capacity.
The form of storage depends not only on how the crop is harvested, the volume and the way it is delivered to the market, but also the overall costs.
Where drying is a problem bag storage has the advantage that it allows higher moisture content than bulk storage. For maize, the requirement for safe storage is maximum 15 and 12% moisture content respectively. in general terms the advantages and disadvantages of bag and bulk storage respectively, are:
|Flexibility of storage||Inflexible storage|
|Slow handling||Rapid handling|
|Considerable spillage||Little spillage|
|Low capital costs||High capital costs|
|High operating costs||Low operating costs|
|Easy inspection||Inspection more difficult|
Soild-wall bins and silos for bulk storage Soild wall bins may be anything from a small plastered basket to large steel or concrete silos holding several thousand tons. The traditional bins used by the African farmers are small with capacity of up to 2-3 tonnes and include gourds, clay pots, mud plastered baskets raised off the ground and mud walled silos ("rumbus'). Many of these have limitations, particularly in durability, protection against rodents and insects as well as moisture from ambient air. Solid wall bins or silos should only be used in areas where the produce can be dried sufficiently before storage. Several attempts have been made to improve on the traditional stores to make them more suitable for long term storage.
Figure 9.18 Clay silo for storing grain in 4 compartments. [he stone c hips forms the moisture barrier.
Improved traditional bins
Many traditional stores perform excellenctly in their appropriate climatic conditions and others can be made to do so with minor changes. Efforts should be made to prevent cracks in the surface of the walls and to seal the entrance to the bin. This can be done for instance by adding lime or cement to the mud (i.e. a stabilized soil technique) or by incorporating an airtight lining (e.g. plastic) in the wall.
Figure 9.19 Improved traditional hint
Figure 9.19 shows a woven basket made of sticks or split bamboo plastered with mud mixed with cement. The walls slope towards a covered manhole in the top An outlet is near the bottom. The bin, which is placed on a raised platform, is covered by a thatch roof or hat.
The major improvements are:
Floor raised from the ground to avoid moisture
Supporting legs of hardwood made 90cm long and equipped with baffles to keep rats away.
Instead of mud the walls may be plastered with cement or mud mixed with cement/lime.
Inlets and outlets should be made with airtight and lockable covers.
Thatched roof to protect the bin from rain and strong sun.
The area around the store is kept clean.
In a few countries, e.g. India, parts of Africa and Latin America, underground pits are claimed to keep grain without damage for many years.
The pits keep grain cool, and some of them are relatively airtight. Grain on top and around the sides can however often be mouldy.
There are several types of pits, most of them flask shaped covered with sticks, cowdung and mud, or a large stone embedded in soft mud. The area should be free from termites and relatively dry.
Improvements of the pit may include:
Figure 9.20 Underground pit.
Brick walled silo or bins are suitable for small and medium size stores. The strengthening requirements makes them uneconomical when the height exceeds about 7-8 m. The wall may be made of bricks or blocks of mud, stabilized soil, burnt clay, stones or cement. To withstand the pressure from the grain the wall will need reinforcement depending on the size and strength of the building materials. Reinforcement can be reduced and even omitted by building the walls thick and heavy (gravity walls). Figure 9.21 shows a silo with gravity-walls where the bricks are placed radially. No reinforcement is needed for this size but more building material is required.
Walls made of bricks, mud or cement will absorb moisture from the ambient air. In areas with high relative humidity it is therefore necessary to protect the grain by adding a moisture barrier to the silo walls. It will help considerably to bagwash or plaster the walls on the outside with a mortar of cement-lime-sand ( 1 :1:5) for burnt bricks or cement, and cement-sand-mud (1:2:6) for mud walls. Then the walls can be painted with plastic paint or coaltar if better protection is needed.
An alternative to plastering and painting the silo is to incorporate a lining of plastic sheeting in the middle of the wall, floor and roof in such a way as to make the container airtight. The Pusa Bin is such a structure and has been developed by the Agricultural Research Institute in New Delhi. Originally the bin was rectangular with walls of two layers of brick; the floor and the roof made of two layers of mud. The system can be used for silos of any shape and will, if properly done, give a good protection against air and moisture.
Figure 9.21 Silo built of bricks laid radially (gravity wall).
Reinforced Concrete Silos
Concrete can take very little tension and needs to be reinforced when used for silos. Small silos suitable for farm level may be reinforced with chicken-wire
The ferrocement store or ferrumbu is a typical example, figure 9.22. One or two layers of 12mm chickenwire are tied to vertical sticks or rods placed in a circle. The chickenwire is then plastered from both out and inside. The verticals are removed after the outside isfinished. Taller silos 3-4m or more may be framed by welded mesh wire and with 12mm chicken-wire tied to the outside. With bags or plastic tied to the outside it is possible to plaster the silo from the inside first and then from the outside a few days later after having removed the bags. These techniques make it possible to construct walls with thickness of 3 to 6 cm.
Figure 9.22 Cross-section of a ferro-cement store (ferrumbu).
Larger concrete silos are built by using a sliding mould which is moved upwards continuously or step by step. Reinforcement and concrete are supplied from the top Concrete silos can be made airtight if openings are properly sealed.
Steel bins ranging from properly cleaned-out steel oildrums to commercial stores with capacity of several thousand tonnes figure 9.23. In most cases steel silos are more expensive than concrete silos but have the advantage of being easy to erect, and for the small sizes, also possible to move. The welded steel silo is normally airtight if the openings are properly sealed, but even a silo assembled of corrugated iron sheets can be made air tight if all joints are sealed with rubber gaskets or bitumen.
Figure 9.23 Commercial storage silos.
The most common method for grain storage in many countries is bag storage in a variety of buildings, e.g. stone, local brick, corrugated iron, and mud and wattle, with or without plastered walls and with an earth, stone, or cement floor and corrugated iron or thatched roof.
As mentioned before, the form in which the produce should be stored will depend on the quantity, harvest method, handling method, moisture content and costs.
The advantage of bag storage is listed earlier. The disadvantage is that jutebags do not give any protection against insects which means that an insecticide has to be used. In some countries with a dry climate it is common practice to stack the bags on plinths and cover them with a tarpaulin for temporary storage. Examples are the hard stands used in Zambia and the groundnut pyramids near Kano in Northern Nigeria. However, if the grain is going to be kept for some time it is recommended to store the bags in a building. A simple store would be to use the ventilated maize crib that was used for drying, with the only difference being that the walls should be covered as protection against rain see figure 9.24.
If the bags are stored in a multi-purpose farm shed or even in the farmers dwelling they should be kept out range from rats and mice. A raised free-standing platform equipped with ratguards will serve the purpose.
For larger quantities a special building is recommended Figure 9.25 shows a small block-built bag store (20mē) with the capacity of about 15 tonnes of cereals.
Figure 9.24 Ventilated maize crib used for storage of shelled maize in bags.
Figure 9.25 Small block-built bag store.
Whatever the size, the floor should be of good quality concrete, the door should fit tightly to prevent entry of rodents, and ventilation openings should be screened to keep out birds. The gaps between the wall and the roofing sheets must be closed for instance with cement.
If fine mesh is used to prevent insects from coming in through the ventilation openings it must be maintained regularly; dust should be brushed away and holes repaired immediately. Figure 9.26 shows a multi-purpose store with 90mē (extendable) storage space suitable for cooperatives and villages.
Figure 9.26 Multipurpose store (196mē).
Storage management is important for all types of storage. For bag-storage the four important points are:
1 Prevent damp from the floor and walls to reach the produce by stacking the bags on pallets off the ground and away from the walls.
2 Stack the bags properly to allow:
3 Control insects and rodents
Figure 9.27 Proper stacking for easy management. The bags are placed on dunnage.
Losses caused by insects: a Weight loss. Insects as they develop will feed on the produce. Losses vary with the commodity, for grain and legumes
Figure 9.28 Bags stacked in five respective eight bags layers.
Sources of infestation
A great variety of techniques are used for control of insect pests in stored produce, from sunning and smoking at the traditional farm level to irradiation in the largest scale bulk-handling. This paragraph is concerned with proven techniques, variously suitable for use in small to medium scale storage under tropical conditions. Specific recommendations are difficult to make, a technique must be tested for a particular situation depending on the value of the crop, occurrence and resistance of the pest, which farming system is used, and the availability of insecticides. When selecting a technique it is important to consider its effectiveness against the target pests; hazards to the farmer and the consumer, and will the result pay the cost of carrying it out?
Insect control techniques:
Chemical control involves, in most cases, the use of an insecticide which can be used on the produce as
Besides killing the insects, all insecticides are, more or less, toxic to mammals. The toxicity is usually expressed as a "LD50". Technically, this is the dose required, in mg active ingredient per kg of body weight of consumer, under specific conditions, method of application and time span, to kill 50% of the test population, usually rats.
Most insecticides do not kill all insects and mites; choose a chemical that is either "broad spectrum" or one that specifies toxicity to moths and beetles; mites may require special treatment.
With regard to persistence; insecticides will tend to lose their effectiveness with high humidity, high temperatures and sunlight.
It is important that the insecticide is applied, in the correct dosage. Excessive and/or inappropriate use of chemicals will lead to the insects becoming resistant and can be a hazard to human health.
Commercial insecticides usually consist of a small quantity of the toxic compound the "Active Ingredient" (A.I.) with other substances called the "filler". It is important to be able to convert from one basis to another; example: "Actellic should be applied at 15 ppm A.I." This means we should apply 15 grammes of active ingredient to every million grammes of produce i.e., to one tonne.
If we start with 5% dust, this means that 100 grammes of crude product (C.P.) contains only 5 gr of A.I. The dose of chemical to be applied will be
q = (15 ppm x 100) / 5% = 300gr/tonne of produce
Rodent and Bird Control in Stores
Besides consuming large quantities of stored grain and food, rodents contaminate stored produce through droppings, urine and hairs, and may spread human diseases. Control of rodents requires an integrated approach since no single method is completely effective. It should be focussed on creating an unfavourable environment and excluding rodents from stored grain. Methods used to minimize the damage caused by rodents include good housekeeping, proofing, repelling, trapping and poisoning. Keeping a cat around a grain store is another effective method of control. The requirement for good housekeeping is the same for rodent as for insect control; the store should be kept clean inside and outside and easy to inspect. In the following paragraph emphasis will be on how construction can be improved to keep rodents out.
Birds are likely to be a nuisance in ware houses if no precautions are taken.
With the protective measures described above it is possible to reduce and even eliminate the rodent problem if the measures are properly maintained.
Storage Management, Hygiene and Safety
Condensation and Moisture movement
If bins and in particular silos are exposed to direct sunlight or if the grain inside the silo is warmer than the external air, convection currents can be started. This results in the moist air being carried through the grain and where it meets a cooler surface i.e. the silo wall, the moisture will condense out and dampen the grain in the immediate vicinity. Clearly this can be a major problem with grain stored in steel silos in hot climates, particularly in areas where the sky is clear during both day and night. A clear sky results in high daytime temperatures and cool nights.
For small silos the problem can be reduced by covering the silo with a roof or hat that prevents the sun from heating up the surface. For larger silos other solutions have to be found, either by ventilating the grain in the store or moving the grain from one silo/cell to another. This will mix the grain enough to even out the moisture content. If the moisture content is too high it will be necessary to run the grain through a drier.
Reference is made to what has been said about insect and rodent control for bag storage. However, it is essential that all types and sizes of grain stores the cleaning will have to be done when it is empty. If the insect population is building up, the whole store may have to be fumigated or sprayed.
Dust is stirred up when grain is handled. Inhalation can cause respiratory problems, especially if exposed to slightly mouldy grain. Breathing filters should be used. Since grain dust is explosive it is important to enforce the "No Smoking" rule and ensure that all light bulbs and electric equipment are shielded. Good ventilation is recommended.
Falls: all catwalks where a person could fall more than 150cm should have guard rails 100cm high and a toes board of 15cm
Crusts: can be formed indamp grain beneath which the grain has run out. Walking on bridged grain can cause failure of the crust, resulting in being burried and suffocated.
Machinery: all moving parts should have guards fitted and all wiring should be maintained regularly.
Grain flowing out of a container tends to form a funnel at the centre. This highly unstable surface can stuck a man in, within seconds.
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