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This chapter discusses, in particular, the drying of rice and maize. After describing the traditional methods of drying, the various improvements possible will be examined. Grain drying has been described in FAO Bulletins N° 40, 70, 109.
Most grain is physiologically ripe when its moisture content is around 30%. In this state it should be harvested rapidly to avoid attack in the field from predators and, in the case of rice, to diminish grain loss from shattering and cracking. The harvested grain is then dried sufficiently to minimise attack by insects and micro-organisms.
The key factor during drying is the humidity of the ambient air especially when drying is done without supplementary heat. Where there is only one rainy season, drying by natural air convection presents no major problems. For equatorial zones (two rainy seasons per year) drying conditions are not optimal.
Traditionally, drying rice in the sun poses no particular problems. In effect, in tropical regions the average level of insolation is greater than 0.5kw/m2 (that is, for 12 hours of sunshine a quantity of heat of 21.6MJ/m2) and this amount of heat is sufficient to evaporate 9kg of water.
Under conditions of maximum insolation the grain can attain a temperature of 60°C and the rate of drying is sufficiently high to provoke cracking and losses in milling. The drying rate should be reduced by covering the grain between mid-day and 3pm (in the experience of IRRI) when the weather is particularly sunny thereby reducing cracking by 25 % .
Maize drying takes place in the middle of the rainy season. Therefore, while the relative humidity remains around 80% for several weeks after the harvest, it is practically impossible, without additional heat, to reduce the moisture content of the maize to 13-15% at which it can be stored. Exceptionally, in the south of Nigeria, despite the humidity, maize cobs can be stored at about 20% moisture content in naturally ventilated structures in which it continues to dry for 1 to 3 months.
The humidity constraint is accentuated by increases in quantity harvested which are often beyond the drying capacity of the producers. A drying phase of varying length occurs in the field, on the stalk, although this practice leads to losses from predators. Also, after harvest, removal from field to homestead is delayed through waiting for transport. The result is considerable loss from fungal attack on the too moist grain (Table 4.1).
Table 4.1- Levels of loss during pre-drying of maize in the field
Source: FAO, AGS Bulletin N° 40
|End of August||End of September||End of October|
|Insect damage (%)||2.8||1 1.4||7 8||1 9||10.8||2 1|
to insects (%)
to Birds (%)
A maize drying practice current in the Gulf of Guinea consists of maintaining a wood fire alight in the storage structure to reduce the humidity relative to the ambient air. This is the only means of drying products under high rainfall conditions.
In the north west of Cameroon corn cobs, partially husked, are carried in baskets to the homestead and placed in a bed 30-40cm thick inside a structure called a "banda". The banda is the space above the ceiling of the kitchen (Figure 4.1). The base of the banda on which the cobs lie consists of an assembly of bamboo rails a few centimetres apart with an opening 60cmx60cm to provide access to the interior.
Drying is carried out by means of one or more wood fires which the women tend day and night for at least two weeks. The cobs are regularly re-arranged to place the drier ones on top. After this period drying proceeds more slowly for a further two months based only on fires lit to cook meals. The cobs remain stored in the banda and are removed as and when needed by the family.
In the context of a subsistence economy the banda is perfectly convenient for the storage of relatively small quantities of maize. At present a considerable increase in maize production is observable in the countries surrounding the Gulf of Guinea for various reasons (liberalization of the economies and a shift from cash crops to food crops). In order to accommodate all the crop in the banda it is thus necessary to increase the depth of the bed of cobs to the detriment of the quality of the drying operation.
Figure 4.1- Traditional Granary or "Banda" in North West Cameroon (Source: Cameroon, A.V. Anthony)
After the harvest, small quantities of maize are spread in the sun or the cobs are hung from trees, from poles or along verandas (Figure 4.2)
The "Tap" is a drying structure found in high altitude (1800-2000m) regions in North West Cameroon which is not subsequently used for storage (Figure 4. 3). It consists of a platform raised above the ground and covered with a roof of thatch or corrugated sheet (better drying according to the peasants). Taps are frequently erected in the field to reduce transport distances. During the harvest, the maize is carried to the tap where the outer husk is removed, leaving only the inner husk around the cob. A preliminary selection is made to eliminate mar-formed or mouldy cobs. Then the husked cobs are spread on the platform in a bed 40-60cm thick and dried for about 6 weeks.
The tap is used simultaneously for storage and as a shelter (rain, meals, grading). This structure permits field losses to be minimised, reduces transport and defers carriage of the cobs until such time as roads are in adequate condition.
Figure 4.3 - The "Tap", North West Cameroon (Source: FAO, A.V. Anthony)
Traditional structures with walls in cylindrical form made of woven vegetable material are widely used, not only for storage but also for complementary drying of cobs when maize is harvested in the rainy season. The traditional structure used for drying and storage of maize in southern Togo, called a "bliva" is a special case where the careful arrangement of the cobs forms the walls of the granary (Figure 4.4). The platform which supports such a structure is raised 03100cm above the ground: this is called a "low granary". When the platform is raised higher (200cm) to permit a fire to be lit underneath, it is called a "high granary". The maize is harvested and stored at a moisture content between 22-25%. The husks are not removed and constitute a barrier against insect attack.
The maize crib
Experience at the African Rural Storage Centre (Ibadan, Nigeria) have led to the design of a narrow structure called a "crib" (Figure 4.5) where maize with a high moisture content (30%) harvested when ripe, may be stored conveniently. The width of the crib should not be more than 60cm in a humid climate to avoid fungal attack. The walls are made from grilles or vertical rails of wood, bamboo or palm frond stems (3-5cm thick). A roof ensures supplementary protection from the rain.
The principle of the crib is based on the drying capacity of the air which blows through the mass of husked cobs. It is best to treat the cobs with an appropriate insecticide before loading into the crib in order to protect them from insect attack.
Figure 4.5 - The maize crib (Source: FAO Bulletin No 66)
The hot air dryer
A hot air dryer has been designed by IRRI (Figure 4.7). With a drying capacity of 5001000kg of grain per day, it is made from local materials capable of being assembled on the spot. Batches of freshly harvested cobs are dried to 20% moisture content, then shelled. Then the grain is dried to 13% moisture content.
This model, if made mobile, can be used to provide a rental service in order to optimise its use in the same area or village.
Figure 4.7 - IRRI-type hot air dryer (Source: CEEMAT, 1988)
The growth in production of maize in recent years has encountered a lack of drying capacity. Traditional drying structures which, as has been shown, perform relatively well, can accommodate the quantity to be processed. Grain which remains too long at a high moisture content risks attack from micro-organisms and consequent mycotoxin infection. (cf Chapter 6). Extension of new types of drying structure of higher capacity should be encouraged together with improvements in shelling and storage.
The mechanism of drying in the "bliva" has been studied by the University of Benin in the context of a project financed by the CRDI (International Development Research Centre). As well as the two types of bliva, two other structures (granaries of woven vegetable fibres and the crib) have been studied to provide a comparison (Figure 4.6).
Observation of traditional granaries confirms their role as dryers. In effect, maize harvested at 18-25% moisture content is dried to 13% (or less) while this level is not achieved by cribs. The explanation commonly advanced of possible drying by movement of ambient air appears inadequate as the granaries are poorly ventilated. Another hypothesis has been put forward: the heat of the sun, stored within the mass of cobs contributes to maintaining a relatively stable temperature despite temperature changes in the ambient air. In parallel, differences in temperature are observable, firstly between different points in the interior of the granary and secondly between the inside of the granary and the ambient air. These temperature gradients cause air movement by convection in the interior of the mass of maize and thus correspond to a secondary drying mechanism (Smith et al., 1994).
Drying follows a sequence of 4 phases:
In addition, there exist horizontal and vertical air movements as a function of climatic conditions. When there is little wind, the mechanism for an exchange of air between the granary and the environment is due to convection of air in the heart of the mass of cobs. Air exchange happens essentially during day time when humidity is at its lowest. This mechanism is facilitated by passing smoke through the structure.
Contrary to the bliva, the crib has been conceived in order to permit the maximum exchange of air with the exterior environment. The wind assists drying during the day but the crib permits a greater uptake of humidity during the night. Because of this fact, the crib cannot dry maize to as low a level as the traditional granary in the tropical conditions of southern Togo.
Figure 4.6 - The four different types of granary in the drying/storage studies (Source: Smith and Kpakote, 1994)
Between 60% and 70% of grain production in Africa is stored at farm level, generally for family consumption but also for sale and for seed. Storage methods which have evolved over many generations are often well adapted to local conditions.
The objective of all grain storage structures is to furnish a protection against deterioration due to rain and soil moisture and to provide a barrier against insects and vertebrate predators. In Africa, a considerable variety of post-harvest systems is found, depending on ethnic and agro-climatic zones. Therefore, the first part of this chapter will be limited to the presentation of harvest-drying-storage practices in the Sudan-Sahel regions of West Africa (Grolleau and Diop, 1987). Subsequently, techniques for improvement to traditional structures will be discussed. Finally, the specific cases of silos and cereal banks will be described.
A large number of publications have been produced on storage. This chapter is based on summaries from FAO Bulletins N° 40, 53, 69 and 109 as well as on the paper concerning the Togo Experience from Smith and Kpakoté presented at the Accra Workshop.
Depending on the type of material used, granaries of plant material, usually well aerated, are distinguished from those made from clay with rigid walls impermeable to the external air. Construction or re-construction of granaries is done at harvest time.
These "straw granaries" are typical of the Guinea or Sudan-Guinea zones. They are also found in dry zones such as the Sudan-Sahel zone. They are used for storing millet, sorghum and maize by a wide range of ethnic communities living in these zones (Figure 5.1).
The granaries are raised above ground level (from 25-30cm to 1m or more for smoke drying in the more humid regions of the south). The base of the platform, made from three or four beams braced at right angles, is supported on forked posts. The wood used should be resistant to termites, for example Prosopis africana, Burkea africana, Anogeissus leiocarpus, and Khaya senegalensis.
The granary walls
The cylindrical walls are made from plaited reeds, Andropogon guyanus or Guiera senegalensis, of bamboo cut into strips or of palm leaf veins. Among the Djerma of Niger and the Bariba of Benin the body and roof of the granary constitute the same conical assembly (Figure 5.2).
Figure 5.1- Traditional granaries of woven plant fibres (Source: FAO Bulletin N° 69) (a) Traditional granary of the Mossi and Gourmantché (Burkina Faso), for storage of cereals in ear. Platform on rocks. Granary of reed mats (plaited stems of Andropogon)
Figure 5.1- Traditional granaries of woven plant fibres (Source: FAO Bulletin N° 69) (b) Mandingo granary entirely constructed from plant material, called a "Bountoung" Used for storing cereals in ear (south Senegal; north Guinea-Bissau).
The roof, typically conical, is of straw. It is composed of several layers of grass thatch (Imperata cylindrica = Akwa or Andongo) covering a frame of branches or bamboo fastened to the body of the granary with lianas. The fixed or semi-permanent roof may last up to ten years. Its life is reduced by frequent removal for extraction of grain through the roof. Some granaries are provided with lateral openings, inserted in the body of the granary.
Maintenance of a straw granary consists of re-tightening the reinforcing framework and of replacing the roof thatch every two to five years. The increasing scarcity of traditional material (wood, reeds, straw) leads to use of substitute material of lower quality (less durable and more permeable) and changes the life span of the granaries. Frequently, plant material is replaced with clay (for example in the Djerma villages near Niamey).
Figure 5.2 - Djerma Granaries in Niger (roof and walls a single assembly composed of superimposed layers of millet stalks, reed mats and grass straw) (Source: FAO Bulletin N° 69)
These granaries, known often as "mud granaries" are characteristic of the driest zones (sahel and sudan-sahel). Located inside or outside dwellings, depending on local custom, they take varied forms: cylindrical, trapezoidal, oval or spherical.
For a square-section granary, the platform consists of a base of six large stones supporting, normally, three main beams across which are laid the wooden floor beams (figure 5.3). When the section is circular, the platform is based on either a circular bed of stones or a circle of stones around a central stone. Very short beams placed between the stones carry floor beams laid out as spokes of a wheel. These different types of platform are used for large granaries (8 - 12m3 up to 60m3 among the Hausa of Niger). For small granaries (0.5 to 2m3) notably among the Gourmantché of Burkina, the platform is made on a base of 3, 5 or 9 stones in a circle. The height from the ground never exceeds 30cm.
The chosen clay comes from the banks of rivers or swamps or from termite mounds. Generally, it is mixed with chaff (fonio chaff) to reduce the effects of shrinkage. The Senoufo of the north of the Côte d'Ivoire use a mixture of clay and threshed rice panicles. A vegetable oil, Parkia filicodae or even karité, Butyrospermum parkii, are sometimes used as stabilisers. The mud floor (5 to 7cm thick) and the first layer of the walls are placed in one operation without mixing straw with the mud. While this base layer dries (3 days) kneading of the mortar of clay and straw may commence (the wet mixture ferments for two to three days). Then the walls are raised in courses 7 to 10cm thick. The need to let successive courses dry can make the operation last several weeks. This explains the tendency to replace continuous mud walls with mud brick walls, built in a day but less resistant (Figure 5.4).
For square-section granaries which are relatively high (2.5 to 3m) ties placed across the walls rigidity the structure and serve as support or attachment points. Where the roof is fixed, an opening averaging 50 x 50cm is placed towards the top of the wall on the side most protected from the rain.
Among the Lobi and the Gourmantché of Burkina, the Dogon of Mali and the Somba of Benin, the store is divided into two, three or four compartments, permitting separation of different products.
Figure 5.3 - The different types of platform (Source: FAO Bulletin N° 69)
The straw roof overhanging the walls is constructed of the same materials as the granaries of vegetable material. It protects both the stored grain and the walls from rain and sunlight. Adequate materials are becoming increasingly rare so warehouses (flat roofed in corrugated sheeting) tend to replace traditional granaries in certain regions (around Tillabéry in Niger and Sélibaby in Mauretania). The use of the corrugated sheeting is explained also by its greater durability and its status as "an external sign of wealth" among some villagers. However, the sheeting, because of its high thermal conductivity compared with straw thatch, functions as a solar collector and does not ensure as good conservation of the stored products (Figure 5.5).
Maintenance of a mud-walled granary consists of rehabilitating the roughcast exterior coating, especially on walls exposed to rainfall, every four or five years. The roof thatch is replaced every two to five years. The mud brick granary needs a new exterior coating almost every year.
Before storing the new harvest, certain precautions are important:
The transport of the ears in sheaves or bundles facilitates handling and examination of the crop before storage. If the residue of old stock can be consumed in two or three months it is removed from the granary, threshed and kept in sacks in order to prevent mixing. Otherwise, the farmer constructs another granary for the new harvest.
Storage in ears
This practice is the most widespread whatever the type of granary since, according to family heads, "it is more economical and limits risk of wastage, given that the women sell off the grain in small lots". Another argument is that it permits better conservation, according to farmers.
Storage of chopped millet sorghum ears
The Dagari (Burkina Faso) practice this particular method of conservation for millet and sorghum. Millet ears are chopped in small pieces of 2 to 3cm before storing in bulk. Sorghum is threshed then stored in bulk without winnowing out the husks and panicles. This method seems to ensure better protection against insects in physically limiting their activity.
Storage as grain
Storage as grain is a recent practice in some zones of the sub-region and in some villages of Burkina Faso, Mali and Senegal (Moss), Marka, Bambara and Wolof people). This method of storage has assumed greater importance following environmental changes such as monetarization of exchange, decrease in vegetation and mechanization of threshing (Chapter 1). Among the Bambara, this practice leads to plastering the interior and exterior of the straw granaries to the detriment of the natural aeration of the granary. Similarly, the Wolof are progressively abandoning their straw granaries in Nguer and storing grain in sacks in houses or the new warehouses. This could lead to insect infestation and serious moulding (see Chapter 6).
Traditional structures in local materials have the characteristics necessary to good conservation of harvested crops at relatively low cost. Nevertheless there are always opportunities for improvement and above all where traditional materials are scarce or where the technical knowledge for construction is tending to disappear.
In order to improve waterproofing or combat rodents and insects, either existing granaries must be modified or new types of store must be introduced, based on industrial materials. These two approaches are difficult to implement due to the diffuse nature of the storage environment. A valuable technical improvement, even a small one, may seem unacceptable or too costly for the people involved. The farmer may be reluctant to adopt a new storage structure while the granary needs neither repair nor replacement. In addition, the lack of, or poor estimation, of costs and real benefits for proposed storage structures has led to promotion of technologies which were often inapt. These errors, over-estimated the level of losses to be reduced, the rate of refilling of stores (variable during the course of the same season), the price and demand of the stored product. On the other hand, aspects such as production, extension, and credit availability have been under-estimated. The following experiences in Togo and Benin are minmr modifications to the traditional structure which brought significant improvements.
Maize occupies a prime position among the food crops of Togo. As the cereal of greatest use in the Togolese diet, maize is used in some twenty local dishes. In the southern half of the country, maize has been harvested twice a year for a long time. In the northern regions maize is produced, to the detriment of sorghum, in one annual crop. In the cotton zones, maize has been grown in rotation with cotton for the last twenty years while in other areas it is usually associated with cassava or "niébé".
A well maintained maize granary permits the farmer to profit from the rise in price from one crop to the next (175% to 325% between July-August and the next crop in May-June). Ninety to ninetyfive percent of the maize stored and consumed is produced by small farmers. This was taken into account in the application of low cost technical improvements based on existing structures. Intensification of production and the introduction of more modern techniques is envisaged for the future.
As a result of research on the drying effected in the traditional "Bliva" granary (see Chapter 4), Smith et al. (1994) arrived at the following conclusions. To achieve optimum drying of the grain (low moisture content) it is necessary to
The smoke drying achieved during the first two weeks of storage and after each rainfall permits the redistribution of moisture in a homogeneous fashion. In order to allow the smoke to penetrate well throughout the mass of cobs after passing through the granary floor, a conical device was placed on the floor (figure 5.6). The ideal configuration would be to construct the floor with several adjustable vents. The vents would be opened during the day while the air is dry and smoke drying is possible, and closed at night when the air is humid. This configuration is difficult to achieve where the floor is made of tree branches. Studies should be carried out to devise an appropriate floor design.
Figure 5.6 - Granary fitted with a cone on the floor to improve smoke drying (Source: Kpakote et al, 1994)
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