Traditional post-harvest system

This section describes the steps involved in the traditional processing of cereals. It is vital that project planners and managers consider the traditional technologies in their particular socioeconomic context before introducing any technical improvements or adaptations. The following main components of the post-harvest food system are discussed: harvesting, threshing and winnowing, drying, storage and primary processing methods.

Post-harvest grain losses are a major concern in the traditional system. This chapter deals with the traditional post-harvest system and local methods by which these losses are reduced. Chapter 2 describes some improved technologies which have been developed to further reduce losses and increase productivity in cereal processing, together with relevant technical background information. Most of the cereals discussed are processed in much the same way but, where relevant, differences between processing techniques are mentioned.

Harvesting

There is an optimum time for harvesting which depends upon the maturity of the crop and climatic conditions (FAO, 1970) and has a significant effect on the subsequent quality of the grain during storage. Harvesting often begins before the grain is fully ripe and extends until mould and insect damage are prevalent. Grain not fully ripened contains a higher proportion of moisture, and will deteriorate more quickly than mature grains because the enzyme systems are still active. If the grain remains in the field after maturity, repeated wetting from rain and dew at night, along with drying by the sun during the day, may cause grain to crack (particularly long-grain paddy) and increases the likelihood of insect damage (especially maize and paddy). Advice may therefore be necessary on the correct harvesting time.

Cereal crops are traditionally harvested manually, making high labour demands and therefore providing an important source of income for landless labourers.

Threshing and winnowing

Threshing is the removal of the grains from the rest of the plant. In the case of maize, the removal of the grain from the cob is referred to as shelling. Maize is shelled mainly with bare hands, by rubbing one cob against another. Most manual threshing methods use some implement; the simplest is a stick or hinged flail with which the crop, spread on the floor, is beaten. Such tools are simple and cheap but they are also laborious to use.

Threshing and shelling will contribute to losses if carried out in a manner that results in the cracking of grains. Other traditional methods of threshing, such as using animals to trample the sheaves on the threshing floor, or the modern equivalent using tractor wheels, may result in loss of unseparated grain. This method also allows impurities to become mixed with the grain, which may cause subsequent storage problems.

Winnowing involves separating the chaff from the grain. If there is plenty of wind, the threshed material is tossed in the air using forks, shovels or baskets. The lighter chaff and straw blow away while the heavy grains fall more or less vertically. Final cleaning may be done with a winnowing basket, which is shaken until any chaff and dust separate at the upper edge. An alternative method is to use winnowing sieves or open-weave baskets. Separating impurities from threshed grain can require almost as much labour as the original threshing.

Once threshed the grains must be dried and stored. In many cases, these two functions are performed together so that grain is dried during storage.

Drying

Drying the grain helps to prevent germination of seeds and the growth of bacteria and fungi, and considerably retards the development of mites and insects. With traditional methods, the rate and uniformity of drying is difficult to control, as it is dependent on environmental conditions. It is essential that food grains be dried quickly and effectively, but in most cases, regardless of the disadvantages, the small farmer still prefers sun-drying because it is cheap and simple.

Air is used as the drying medium, causing water to vaporize and conveying the moisture away from the grain. The moisture-carrying capacity of air is dependent upon its temperature, increasing with a rise in temperature (e.g. at 30°C the air is capable of holding twice as much moisture as at 16°C).

The simplest and most common method of drying is to lay the cut stalks on the ground in the fields, either in swathes of loose bundles or in stacks or heaps, until the crop is dry. When the plants are piled in large stacks they may suffer from a lack of circulating air, leading to sprouting, discoloration, and microbial damage. Sometimes racks are used for hanging un-threshed sorghum, millet, and paddy. Most racks are designed to permit air movement through the drying material.

At the homestead the crop is further dried by spreading on woven mats, hard surfaces (including roads), plastic sheets, or on the roof or ground. The drying time depends on the climatic conditions. Some farmers periodically turn or rake the grain during the drying period in order to obtain uniform drying. If it rains the crop must be protected. In other cases farmers dry their produce on raised platforms of various shapes. In Zambia, Malawi, and southern Tanzania, the platforms are shaped like a cone; in many other parts of Africa they are rectangular.

After drying many farmers store their produce in the home, where the smoke and heat produced during cooking helps complete the drying of the grain and reduces insect infestation. The smoke produced and heat lost in traditional cooking stoves thus serve a useful purpose which should not be ignored in the development of improved cooking stoves.

Storage

Traditional storage systems have evolved slowly within the limits of the local culture. Large amounts of grain for human consumption are stored in containers constructed of plant material, mud, or stones, often raised off the ground on platforms and protected from the weather by roofing material. The design and materials vary according to local resources and custom. In the humid areas of the Ivory Coast, Tanzania, and Kenya, maize is dried and stored by suspending it in bundles from a tree, by hanging it on tacks, or by suspending it from poles. Because of the problems of rain and rodents and other predators, these methods are becoming less popular. In parts of East Africa and Central America, wood ash or rice husk ash is mixed with grain being stored to control infestation.

Storage conditions influence the rate of deterioration of grains. High temperatures and humidities encourage mould growth and provide conditions for rapid growth of insect populations. Deterioration is minimal in cool, dry areas, more marked in hot, dry ones, high in cool and damp conditions, and very high in hot, damp climates.

Table 1 lists specific loss factors before milling for maize,...