The objective of this synthesis is to provide an updated analysis of the evolution of postharvest technologies used for the processing and conservation of grains and tubers by African farmers. These technologies permit to ensure food security to rural families, and avoid losses during harvesting, threshing or damage by pests or inadequate processing methods. Farmers will only increase their production if they are provided with adequate post-harvest techniques.
The majority of post-harvest techniques used in Africa are still based on traditional practices, however, there has been a large effort to improve existing practices. There have been various post-harvest projects executed by several organizations and a large number of projects operated by the Agricultural Engineering Service (AGSE) and the Prevention of Food Losses Programme (PFL) of FAO.
This synthesis has been based on a collection of data from FAO projects as well as on contributions of the participants in the Workshop on African Experience in the Improvement of Post-Harvest Technology, organized in Accra in July 1994 by the Ministry of Agriculture in collaboration with the Agricultural Engineering Service of FAO (AGSE). The Government of France and the Prevention of Food Losses Prograrnme of FAO have provided a financial contribution to the Workshop.
Without pretending to be exhaustive, this synthesis will present, for each stage of the post-harvest process, both traditional and improved techniques of interest to African producers.
This publication has been written by Mr. Aliou Diop and revised by Mr. El Houssine Bartali amd Ms. Cécile Guérin (FAO Consultants). Its production has been supervised by Mr. Francis Troude and Ms. Annemieke Schoemaker. Our special thanks go to Mrs. Beatrice GraniPolidori of the Agricultural Engineering Service (AGSE) for the presentation of the synthesis in its final form.
A.G. Rijk
Chief
Agricultural Engineering Service
Agricultural Support Systems Division
This report describes the evolution of post harvest technologies employed in the conservation of grains and tubers in Sub-Saharan Africa. The triple objectives of this study are: to review existing technology, to permit the wider dissemination of successful developments, and to reflect on the means to adapt certain technologies to the actual environment. This synthesis has been carried out following the "Workshop on African Experience in the Improvement of Post-Harvest Technology" organized in Accra in July 1994 by the Government of Ghana, and from a review of published data resulting from FAO field projects.
These traditional technologies are described in detail in this report. In addition, developments in new technology will be revealed together with their impact and their dissemination among interested populations. Causes of the limited success enjoyed by certain innovations will be discussed as well as the insufficient consideration of the needs of the actors involved in the development, poorly evaluated economic and social criteria, and the lack of global vision of the post-harvest system.
Account will be taken of the evolution of a socio-economic environment in the postharvest system in order to better understand and anticipate future technological changes. Numerous factors impinge directly on post-harvest techniques, in particular the liberalization of commerce, the decrease in local production due to desertification and the relative reduction in the farm work force.
Numerous development projects and agricultural research bodies focus on post-harvest problems relating to food products. A great deal of effort is thus invested in the quest for appropriate solutions to these problems. This report is a synthesis of different technologies employed in the conservation of grains and tubers which does not pretend to be exhaustive. The structure of chapters follows the post-harvest process: harvest, threshing or shelling, drying, storage, control of predators, quality control, and processing. A sole chapter concerns the conservation of roots and tubers.
This chapter describes the various ecological, technological and economic changes which have profoundly modified the environment post-harvest system. It must be noted that experience of the improvement of traditional technology has enjoyed varied success, often neglecting the "human" side of development. This aspect, appearing as a thread running through the report, is dealt with in more detail at the end of this chapter.
From time immemorial, rural agricultural producers have always sought to improve their techniques and methods of production, of handling and of conservation of the crops on which they depend for the survival of their families. Traditional post-harvest techniques for food crops are thus the result of a long process of experimentation and adaptation which have been largely empirical. After many centuries and generations they have perhaps approached a certain degree of perfection. This is explained by the constant necessity to find appropriate solutions based only on resources available in the local environment.
These techniques have been applied in an economy essentially oriented towards subsistence and self-sufficiency. The patriarchal family played therein a preponderant role with the functions and tasks defined in terms of gender, age and social rank. The exchange of goods and services was hardly monetarized and urbanization poorly developed. However, the phenomena of desertification and massive emigration as a result of drought did not have the amplitude which they demonstrate today. Thus, since one generation at least, consecutive changes are observable in response to new constraints in the operation of traditional postharvest systems.
Deforestation
In many African countries, the natural materials used for construction of grain stores have become rare or have even disappeared through the effects of deforestation (increase in land clearing and urban expansion), of desertification (climate) and of the use of improved, short-strawed varieties of grain.
This relatively new situation has brought about different construction methods for storage structures with the use of alternative materials (Chapter 5) and the quality of storage has sometimes diminished (figure 1.1). Also, the problems of desertification and prolonged drought have resulted in reduction of long term stocks which assured food security for the rural population. This almost total disappearance of traditional reserve stocks has stimulated village initiatives to develop communal storage structures, notably cereal banks (end of Chapter 5).
New Predators
In certain regions new predators have appeared. Notably, a storage pest, Prostephanus truncatus (The Larger Grain Borer) discovered in Tanzania. This insect is now widely spread in East and West Africa. It remains difficult to control by traditional methods and causes heavy losses. This point is further discussed in Chapter 6.
New equipment
The introduction of new post-harvest equipment generally permits an increase in working capacity and productivity among farmers. Technological innovations may always be found to be ill-adapted if they are set in motion without taking into account the whole postharvest system. This is composed of a suite of interdependent operations. Any modification to one operation produces more or less serious effects on other operations and may destroy the equilibrium of the system. For example, the introduction of mechanical threshing leads farmers to change towards storage of grain rather than unthreshed ears (Chapter 5).
Improved varieties
The introduction of new, high yielding varieties tends to increase production and begins to solve the problem of food deficits. Nevertheless, new problems arise: the farmer must address problems in handling and storage of larger volumes of grain and, in addition, the new, more productive varieties are more susceptible to insect attack. They often exhibit greater tenderness or a sheath which does not ensure complete protection for the ear.
Chemical pesticides
Traditionally, farmers use various types of natural insecticides of either vegetable or mineral origin to preserve their grain from insect attack. These methods are described more precisely in Chapter 6. For some decades the employment of visibly effective chemical products has been very successful among farmers. Actually, certain products, on free sale in local markets can, if badly used, have serious consequences both on the effectiveness of the treatment and the health of consumers. This situation, discussed further in Chapter 6 of this report, is due to the absence or non-application of appropriate legislation on the subject.
In the majority of African countries the production of food crops has changed during the past thirty years from a subsistence economy to a market economy. On one hand the monetarization of the economy obliges the farmer to sell part of the crop to satisfy new needs. On the other hand rural areas, affected by the phenomenon of exodus, must produce more to respond to the growing demand of urban areas. The increase in production per farmer implies needs at different levels of the post-harvest system. For example, at the level of storage, the initiation of grain storage in bulk stores is a consequence of several socioeconomic changes such as: overlapping several post-harvest operations and the lack of time for construction of new granaries, increase in demand for storage capacity, fear of theft and fire, and the evaluation of social status as a function of the storage utilised. This point will be discussed in depth in Chapters 4 and 5.
Before discussing the new techniques now established, we will rapidly examine the experience gained in the 1970s and 1980s. The first steps to improvement of post-harvest techniques during the 1970s were focused on the storage of millet and sorghum in the Sahel zone following periods of drought and famine. In these ancient post-harvest systems based on self sufficiency the losses during storage were somewhat neglected by the farmers as long as people did not perceive losses to affect household food security severely. The quality of storage assured food security, and there were ancient traditions in the production, handling and conservation of millet and sorghum.
Since the end of the 1970s numerous research and development programmes prioritized action on maize and rice which constituted the basic food for people in many countries.
Maize:
The main focus has been on drying and storage. Since the end of the 1980s priority was given to the improvement of traditional systems (Chapters 4 and 5).
Operations of shelling and milling were little studied. Equipment for improvement of these operations was introduced (Chapters 3 and 7).
Rice:
The policies of liberalization of the economies of Senegal, Mali, Cameroun and Burkina Faso in the 1980s established a process of disengagement of parastatal bodies from the post-harvest processes in rice. The need for intermediate technologies adapted to the threshing and processing of rice at village level greatly increased in the last few years. Interesting trials (adaptive research and distribution of appropriate equipment) were conducted on the operations of husking, threshing and drying (Chapters 3, 4 and 7).
The majority of development workers, executing agencies and extension specialists have long under-estimated the importance of the role of women in the economy of their countries and in post-harvest activities linked to the treatment, processing, conservation and marketing of food products. Because of their confinement to the home, women have little less mobility and little time; moreover they have limited access to education and financial or material resources. The lack of accessible and affordable technology, taking account of the needs and constraints of women is a constraint for development. This often leads to a loss of control for the women over current technology and transfer of employment opportunities and income to the men.
An in depth analysis of the distribution of tasks between women and men in the context of the agricultural enterprise as well as the identification of the needs and constraints of women (for the activities of storage, processing and marketing) is necessary if innovations in postharvest technology are to reach their potential beneficiaries.
Roots and tubers occupy an important place in the nutrition of the people and in the economy of some African countries, in particular those situated along the Gulf of Guinea. For some ten years, development assistance programmes have intervened at the post-harvest stage for roots and tubers under the same heading as for the long-privileged cereals. Yams and cassava are the two crops principally cultivated. This explains the volume of activity in research and development which have been devoted to them compared with other roots and tubers.
Roots and tubers constitute a good source of energy (see table 8.1), minerals and vitamins. The yields in calories per unit area are comparable to cereals. The quantity and quality of proteins are variable and relatively low in terms of fresh material. On a dry matter basis, their protein content is as good as cereals. Traditionally, dishes based on roots and tubers are accompanied by high protein foods (meat, legumes and pulses).
Roots and tubers have a high moisture content (up to 90%). This cumbersome produce, fragile and variable in shape, is damaged at all stages in the post-harvest handling chain, causing losses (for yams, 25% to 30% of losses registered in three months).
These constraints linked to the characteristics of roots and tubers make it difficult to process and market fresh products in urban centres. Despite several attempts at processing, often artisanal, the technology for adding value to these products remains under-exploited.
Table 2.1 - Level of nutritive elements in food products
| Maize | Sorghum | Paddy | Cassava | Yam | Sweet Potato |
|
| Average yield (kg/ha) | 336 | 746 | 1,756 | 6,182 | 9,973 | 5,893 |
| % Unusable | 60 | 10 | 35 | 26 | 16 | 21 |
| CALORIES | ||||||
| Per 100g useable | 359 | 347 | 364 | 149 | 119 | 121 |
| Per ha/season (´ 1000) | 4,137 | 2,330 | 4,155 | 6,816 | 9,969 | 5,633 |
| Rank | 5 | 6 | 4 | 2 | 1 | 3 |
| PROTEINS (g) | ||||||
| Per 100g useable | 9.3 | 11.1 | 7.0 | 1.2 | 1.9 | 1.6 |
| Per ha/season (kg) | 112 | 75 | 80 | 55 | 159 | 75 |
| Rank | 2 | 4 | 3 | 6 | 1 | 4 |
(Source: FAO, 1985 - Report of the Workshop on Production and Marketing Constraints on Roots, Tubers and Plantain in Africa, Vol. 1)
Cassava is a security crop for many farmers: it is well adapted to climatic conditions, even extreme drought and to different types of soil. It needs little inputs or labour. The roots can be left in the ground in order to delay harvest. The experience brought to bear on cassava concerns storage and processing, two operations which add value to the product.
Cassava roots deteriorate rapidly two or three days after harvest (physiological factors) or five to seven days after harvest (activity of micro-organisms) (Cooke et al. 1988). Several techniques for conservation of cassava roots in a fresh state have been tested: covering the root with a film of paraffin wax, or mud, or moist ash; storage underground; blanching with boiling water; immersion in cold water. These techniques have enabled conservation to be prolonged for two to ten days but they are laborious and sometimes costly for major quantities.
- Storage of the roots under plastic film
In Ghana, Gallat (1994) describes a technique developed by CIAT (International Centre for Tropical Agriculture) and the NRI (Natural Resources Institute). It consists of a "mobile" system of storage based on packing the roots in plastic film combined with chemical treatment to control microbial rot. The roots can be conserved for an extra two weeks in good condition to suit the requirements of merchants and consumers.
- Storage of the roots in moist sawdust
Agboola (1994) describes a technique for conservation of cassava roots in sawdust which is kept moist in baskets or wooden boxes. The cassava is conserved in good condition in a cool place for almost two weeks as long as the roots are of good quality and without bruises to begin with. This technique, introduced in Nigeria among peasant farmers and small traders, has not achieved an appreciable level of adoption.
In Uganda, a similar technique has been tested with storage trials in moist sawdust or moist wood shavings (Nahdy, 1994).
Technical Description
Trials were held with cassava harvested less than 8 hours earlier (Tereka variety), on healthy batches and on damaged batches (2-3 cm removed from the tip of the root). Polythene sacks with perforated bottoms were placed in compartmented wooden boxes (170x65x80cm). Cassava roots were stored in three layers, alternating with sawdust or wood shavings. Storage of one batch in soil served as a control.
Results
Cassava is transformed into two principal products, flour and gari, in order to detoxify and better preserve the roots. Processing, organized by women in rural areas, causes the loss of some of the mineral and vitamin value.
Cassava Flour
Cassava flour, very common in the marketplace, is relatively easy to produce, store and market. It is used to make foufou, a basic African foodstuff. To produce the flour, the cassava roots are soaked in water for 3-4 days, then peeled, cut in chips and dried in the sun. Finally, the chips are ground into flour. The quality of the drying operation is primordial if dangerous microbiological infections are to be avoided. This operation, which is carried out sometimes in the wet season (in high rainfall areas) is a serious constraint to production. The introduction of equipment to cut the cassava into thin chips would lighten the work of the women and facilitate drying.
Gari
Gari is a product based on cassava found frequently in African markets. In Nigeria, for example, it represents 70% of the cassava-based products (Sadik, 1987). In North West Cameroon, the women make gari manually: 3 bowls of 14kg per week in the wet season and 6 to 8 bowls in the dry season. The production of gari requires a whole series of operations: peeling, washing, grating, pressing, fermenting, sieving, cooking (or gasification) packing and storing. Some operations are problematic for the women occupied in this artisanal production (Flach, 1990).
The artisanal method consists of cooking each batch (2 to 3kg), stirring continuously, for 30 minutes in pots heated over wood fires. The final product represents 20% to 25% of the fresh cassava.
The principal improvement brought to this operation concerns the reduction of firewood consumption (Flach, 1990). In Cameroon, improved hearths of baked mud bricks, managed by groups of women on a rental basis have achieved a reduction of 50% in firewood consumption as well as reducing cooking time. However, the introduction of mechanized ovens has been a failure. For semi-industrial gari production, an appropriate cooking apparatus capable of drying the product down to 12% moisture content has still to be developed. This would permit packaging the gari in plastic bags and conserving and marketing it over several months.
The types of yam most cultivated in Africa are Dioscorea rotundata, or white yam; Dioscorea cayenensis, or yellow yam and Dioscorea alata. Yams reach maturity after 6 to 9 months and have a dormant period of 3 to 6 months, depending on variety.
Traditionally, the yam store is an enclosure, partially shaded and constructed of vertical supports (wooden posts or live trees) about a metre apart and linked with bamboo slats. A straw roof and walls of woven plant material complete the structure. The yam tubers are hung (a laborious task) up to a height of two to three metres (Ezeike, 1994). This structure is popular for its simplicity and modest cost. However, the conditions of temperature and humidity are practically the same as the ambient air outside and thus not really favourable for storage.
The Department of Rural Engineering at the University of Nigeria at Nsukka has designed a ventilated cellar for yam storage (Figure 2.1). The cellar measures 2.9m long by 1.3m wide and 1.5m deep. The roof, in the shape of a cupola, protects the cellar against penetration of rain water. Grilled openings (B in the figure) and a chimney (C) in the centre, improve the ventilation level. The chimney, painted black, accentuates air movement.
Figure 2.1- Ventilated cellar with central chimney; (a) Longitudinal Section; (b) Profile
A: Door;
B: Grille;
D: Stair;
E: Stairway Roof
This new storage structure, conceived for the conservation about 200 tubers, has been evaluated over 6 years and compared with traditional stores.
Table 2.2 - Weight loss in yams under various conditions
| Phase | Weight
Loss to Respiration (% per day) |
Total
Weight Loss (% per day) |
|
| Cellar | Store | ||
| Harvest | 0.076 | 0 25 | 0.25 |
| Dormant | 0.021 | 0.17 | 0.27 |
| Germination | 0.068 | 0.23 | 0.35 |
