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, sorghum, millet, and paddy (NAS, 1978).

Primary processing methods

Whole grains store better than those ground into flour, so in many rural families women grind and pound only small amounts of grain for immediate consumption. Traditional grinding methods for various cereals are discussed below.

Maize

Maize may be either dry or wet milled. In dry milling, maize is usually ground between stones or by using a small hand-powered plate mill; otherwise custom or co-operatively owned power-driven hammer or plate mills are used. At other times, the grain is milled wet after it has been soaked and allowed to ferment slightly to improve its flavour. In Latin America maize is partially cooked in alkaline conditions to facilitate the removal of bran before it is milled. Where very small quantities are needed, the wet maize may be ground at home using a saddle stone or similar device. More commonly, hand- or power-driven plate mills are used to either roughly break the grain or mill it further to a smoother paste.

Maize grain can be pounded using a wooden pestle and mortar, or ground by hand using a quern (a rotating, hand-driven stone mill). The amount of maize required for several meals is taken off the cob and transferred to a wooden mortar. One or two cups of water are added and the whole is pounded.

If the maize meal is not used whole, it is transferred into a flat basket and shaken, so that the bran is separated from the floury endosperm. The flour is again collected into the mortar and pounded in the same way for three or four shorter periods, followed by the traditional separation of the bran. The resulting product is called 'pure' and the bran is often used to feed chickens.

Some of this 'pure' meal may be cooked as it is or with beans, but most undergoes further processing, such as soaking in water for one or two days until an odour develops owing to fermentation. Fermentation produces acidic conditions which inhibit the growth of undesirable bacteria. The water is then poured off, the soaked 'pure' meal is washed up to three times with fresh water, decanted, and again pounded in the mortar. It is then graded as before, into fine and coarse particles. The bigger particles are kept for further pounding until everything is reduced to a semolina-like flour. This product is ready for preparing into foods such as uji and ugali; it cannot be kept for more than a day. If dried it may be stored for a few weeks (Stewart, 1978). The shelf-life of maize meal is very short because maize has a relatively high fat content, which tends to go rancid quickly.

Paddy

In some countries paddy is parboiled before the husk is removed. Parboiling is partial cooking which causes the starch of the kernel to gelatinize, making it tougher. There is also a slight change in flavour which is preferred by some. The toughening process makes the seed more resistant to insect attack, to shattering during husking and to the absorption of moisture from the air. Paddy that has been parboiled has better nutritional quality owing to the migration of nutrients towards the centre of the grain during the process.

The parboiling process involves three stages:

* soaking or steeping of the paddy in cold or hot water to increase its moisture content;

* steaming to gelatinize the starch in the kernel;

* drying.

Traditional parboiling techniques differ greatly from country to country. In West Africa paddy is frequently parboiled in small quantities in earthenware pots or oil drums after soaking in cold water overnight. Sometimes the soaking water is brought to the boil, the fire extinguished, and the pot left overnight. The next day the water is drained off, a little fresh water added and the pot put over a fire until all the water evaporates. The paddy is then sun-dried. Traditional paddy parboiling techniques are slow and can only handle small quantities at a time.

The 'off' odours produced during prolonged steeping of the paddy in the first stage of parboiling has been recognized as a problem. Two methods of eliminating the offensive smell, which is caused mainly by microbial fermentation, are to reduce the steeping time by hot soaking (60–80°C) and to pre-steam the paddy before soaking, which reduces both the steeping time and the number of microorganisms in the paddy.

Careful drying after parboiling is essential to minimize post-harvest losses.

Husking paddy, which is sometimes referred to as de-husking or milling, is the process of removing the outer husk. Husked paddy is referred to as brown rice. Further milling of brown rice produces white rice. The most widespread traditional method uses a mortar and pestle. This may be carried out by one woman working by herself or by a number of women working in rhythm together. The method is very slow and laborious and output rarely exceeds 5kg per worker, per hour.

Hand-pounding produces an under-milled rice which is of greater nutritional value since it retains part of the bran which has high thiamine content and contains protein. Hand-pounding also results in a high proportion of broken kernels. Winnowing is carried out at intervals during this process (Mphuru, 1982).

Sorghum/millet

The outer layers of certain varieties of sorghum seed contain tannins which are slightly toxic, have a bitter taste and inhibit protein digestion when consumed. For these reasons sorghum is generally hulled (i.e. the outer layers are removed) and then pounded into flour.

Red or brown sorghum varieties continue to be grown in many parts of Africa, owing to their resistance to bird attack, in spite of the availability of white non-tannin varieties. Traditionally, the processing of sorghum and millet has been carried out by grinding the whole grain in querns, between stones, or by pounding the grain using a pestle and mortar. The last of these is the commonest method for sorghum. Once the seed has been winnowed to remove foreign matter, it is put in a large mortar and wetted. It is then pounded to strip the bran or 'shell' from the grain, followed by winnowing to remove the bran entirely. Pounding and winnowing are repeated several times before a good quality milled seed is obtained.

The milled seed is then washed with water to remove any small pieces of bran and soaked in water for 24 hours to 'condition' or 'temper' it. This is followed by drying to the correct moisture content and then re-grinding in a pestle and mortar. In order to obtain a good flour, sifting and pounding has to be repeated several times.

The flour obtained contains a large proportion of the oil-rich germ and the nutrients of the grain (Mphuru, 1982). Grains are moistened to facilitate the removal of the bran, but this can result in a slightly fermented flour. Although the keeping quality of this type of flour may be diminished because of fermentation, it has a modified flavour which is often considered desirable. The objective of hand-pounding is thus two-fold. In the first stage, the bran and pigmented layers are removed and in the second stage, the grain is pounded progressively, with intermittent sieving, into flour suitable for various end-products. These methods of hand-pounding are time-consuming and laborious and an output of only 1–3kg of flour per person per hour is possible.

Improved processes and technologies

In most developing countries, it is women who are largely responsible for the threshing, winnowing, drying, husking, shelling, and milling necessary to prepare grain for human consumption. These tasks are often arduous, monotonous, and time-consuming.

Care needs to be taken, however, regarding the introduction of new techniques into a rural community, in case they unintentionally worsen the women's situation instead of improving it. Although a technology may in theory lighten the women's workload it may lead to men taking over a task traditionally performed by women, so depriving them of income.

Harvesting

Crop harvesting equipment available to small-scale fanners in developing countries has changed little over the years. Knives, sickles and scythes continue to be the traditional tools used. Some low horsepower reapers are being developed, but because of their low capacity, high cost, and other problems, they are often not considered a suitable alternative to manual methods. The time at which a crop is harvested can have a significant effect on both the quantity of grain obtained and its quality.

Threshing

Although time-consuming and laborious, traditional manual and animal-powered threshing should not be dismissed out of hand. The problems of impurities and grain damage during manual threshing can be partially overcome with minor improvements; for example, carrying out the task in clean areas.

A range of small hand- and engine-driven mechanical threshers is available to improve not only the quality, in terms of damage and yield, but also the efficiency of human effort required. These fill the gap between traditional methods and highly sophisticated machines, which are mostly inappropriate for use by villagers owing to their high cost and maintenance problems.

Most mechanical threshers operate on the same basic principle, consisting of a cylinder or drum with teeth which strip the grain from the stalks as it passes between the revolving drum and a metal grate (known as the concave). Many of these machines have been developed for rice but can be used for other crops. However, it is reported that treadle operated machines are not suitable for threshing wheat as the power requirement is too great.

It is important to note that most mechanical threshers work best with grain dried to the correct moisture content. Under-drying leaves grain still attached to the stems; over-drying leads to excessive grain damage. Threshers are often classified according to the method of feeding, the design of the threshing drum/ concave and the power source.