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 contextwhen 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 methodsPost harvest grain losses are a major concern in the traditional system. This section describes someimproved technologies, which have been developed to further reduce losses and increase productivity incereal 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 climaticconditions 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 prevented. Grain notfully ripened contains a higher proportion of moisture, and will deteriorate more quickly than maturegrains because the enzyme systems are still active. If the grain remains in the field after maturity, repeatedwetting from rain and dew at night, along with drying by the hot sun by day, may cause grain to crack .Advice is therefore frequently necessary on the correct harvesting time.Cereal crops are traditionally harvested manually, requiring high labor demand and therefore inmany 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 removalof 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 aresimple and cheap but they are also laborious to use. Maize is shelled mainly with the bare hands, byrubbing one cob against another. Threshing and shelling will contribute to losses if carried out in amanner that results in cracking of grains. Other traditional methods of threshing, such as use of animals totrample the sheaves on the threshing floor or the modern equivalent using tractor wheels may result inloss of unseparated grain. This method also allows impurities to become mixed with the grain, which maycause subsequent storage problems.Winnowing involves separating the chaff from the grain, if there is plenty of wind, the threshedmaterial is tossed in the air using forks, shovels, baskets, etc. The lighter chaff and straw blow away whilethe heavy grains fall more or less vertically. Final cleaning may be done with a winnowing basket, whichis shaken until any chaff and dust separate at the upper edge. An alternative method is to use winnowingsieves 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 twofunctions 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 and considerably retards the development of mites and insects. Intraditional method the rate and uniformity of drying is difficult to control, as it depends on the prevailingenvironmental 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 becauseit is cheap and simple.Air is one environmental factor used as the drying medium, causing water to vaporize andconveying the moisture vapour away from the grain. The moisture carrying capacity of air is dependentupon its temperature and increases with the rise in temperature (e.g. at 30 ° C the air is capable of holdingtwice as much moisture as 16 ° C). 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 canoccur.The simplest and most common method is to lay the cut stalks on the ground in the fields, either inswaths of loose bundles or stacks or heaps, until the crop is dry. When the plants are piled in large stacksthey may suffer from lack of circulation leading to sprouting, discoloration, and microbial damage
Posted on: Tue, 24 Sep 2013 14:40:09 +0000
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