Unit 17: Post – harvest system

Unit 17: POST – HARVEST SYSTEM
This section outlines the steps involved in traditional processing of cereals. It is vital that project planners and managers consider the traditional technologies in their particular socio – economic context when introducing any technical improvements or adaptations. The following main components of the post – harvest food system are discussed.
Harvesting threshing, winnowing, drying and storage primary processing methods.
Post harvest grain losses are a major concern in the traditional system. This section describes some improved technologies, which have been developed to further reduce losses and increase productivity in cereal processing together with essential relevant technical background. Most of the cereals discussed are processed in much the same way, but where relevant, differences in 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 grain during storage. Harvesting often begins before the grain is fully ripe and extends until mould and insect damages 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 is the field after maturity, repeated wetting from rain and dew at night, along with drying by the hot sun by day, may cause grain to crack (particularly long – grain paddy). Advice is therefore frequently necessary on the correct harvesting time.
Cereal crops are traditionally harvested manually, requiring high labor demand and therefore in many situations providing an important means of work to landless laborers.
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. Most manual threshing methods use some implement, the simplest is 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. Maize is shelled mainly with the bare hands, by rubbing one cob against another. Threshing and shelling will contribute to losses if carried out in a manner those results in cracking of grains. Other traditional methods of threshing, such as use of 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, baskets, etc. 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 labor as the original threshing. Once threshed the grains much be dried and stored. In many cases these two functions are performed together so that grain is drying during storage.
Drying
During drying the moisture content of the grain is reduced. This helps prevent germination of seeds, the growth of bacteria and fungi ang considerably retards the development of mites and insects. In traditional method the rate and uniformity of drying is difficult to control, as it depends on the prevailing environmental conditions. Moreover, it is essential that food grains be dried quickly and effectively. However, in most cases, regardless of the disadvantages, the small farmer still prefers sun drying because it is cheap and simple.
Air is one environmental factor used as the drying medium, causing water to vaporize and conveying the moisture vapor away from the grain. The moisture carrying capacity of air is dependent upon its temperature and increases with the rise in temperature (e.g. at 300C the air is capable of holding twice as much moisture as 160C). Reducing post harvest grain losses during drying is a major objective of an improved technology. Some of the following traditional drying methods highlight where losses can occur.
The simplest and most common method is to lay the cut stalks on the ground in the fields, either in swaths of loose bundles or stacks they may suffer from lack of circulation leading to sprouting, discoloration, and microbial damage. Sometimes racks are used for hanging unthreshed 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 prevailing climatic conditions. Some farmers periodically turn or rake the grain during the drying period in order to obtain uniform drying. During rainy period the crop must be protected until the weather is again favorable. In other cases some farmers dry their produce on raised platforms of various shapes.
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 serves a useful purpose, which should not be ignored in the development of improved stoves.
Storage 
Traditional storage systems have evolved over long periods within the limits of the local culture. Large amount of grain for human consumption is stored 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 lvory Coast, Tanzania, and Kenya, maize is dried from a tree, by hanging it on tacks, or by suspending it from poles. Because of the fear of theft, and because of the problem of rain, rodent, and other predator, these methods are becoming less popular. In the parts of East Africa and Central America wood ashes 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 humidifies encourage mould growth and provide condition for rapid growth of insect in cool, dry areas, more marked in hot, dry ones, high in cool and damp conditions, and very high in hot, damp climates.