PreviousThe technical quality of a grain is the link between the characteristics of the grain (texture of the endosperm, size and shape, thickness of the pericarp) and its behaviour in processing (hulling or milling). These differences in technical quality are often raised as issues when new varieties selected by agronomists for high agronomic potential give poorer results on processing than traditional local varieties.
The quality of cereal products has different implications depending on the person's role in the production chain, from the producer right through to the consumer (Faure, 1994):
These quality criteria, reflected in the price, are always present even if the current quality standards are not always respected.
Varieties with a floury texture are easier to mill than varieties with a vitreous texture which give more semolina and grits. To classify varieties according to their degree of vitreosity (from 1 to 5), the transversal cut surface of the endosperm is visually examined. A vitreous (or "glassy") grain will be Class 1, while a farinaceous (or "toothy") grain will be Class 5.
There is also a simple grinding test for grain which determines an index of hardness based on the size of ground particles. The lower the index the more difficult the grain is to grind. It has thus been possible to identify the yield of broken maize among varieties grown in Senegal.
In Togo, varieties of soft maize are more appreciated since they give a fine flour with minimum grinding. Improved varieties such as NH1F, Mexico 8049 or TZSR having similar processing qualities have been well accepted. On the other hand, a variety like La Posta, needing 2 or 3 passes though the mill are more costly to process and therefore attract less interest.
Contrary to maize, sorghum is usually hulled before grinding into flour. The yield from hulled Bin is higher for sorghum types with a hard endosperm compared to those types having a farinaceous endosperm (same classification principle as for maize).
Sorghum varieties with round grain and/or large grain have a higher hulling yield. To obtain maximum yield, the grain should be divided into three groups - small, medium and large for separate hulling. If this is not done, small grain should be screened out before hulling. This is technically possible in the industrial installations which exist in Africa (Sudan, Senegal, Tanzania) but is rarely utilized.
Mechanical hulling by the dry process gives better results from sorghum with a thin pericarp. On the other hand, manual pounding is easier with sorghum varieties with a thick pericarp.
The presence of tannins in certain varieties of sorghum is generally linked to the presence of a farinaceous endosperm and gives lower yields on hulling. Mechanical processing should be adapted to give a lighter abrasion of the pericarp.
This criterion of quality has a direct influence on the acceptability of the finished product. Before processing, the grain (maize, millet or sorghum) should be free of foreign matter (small stones, sand, soil, straw, string, metallic debris). Screening, winnowing, and, finally, washing are obligatory. The operators who sell the grain know this and always do it before pounding the grain or taking it to the local mill.
When selling grain to travelling traders, cereal banks or State authorities, the farmer does not worry about cleaning the grain after threshing or shelling because payment does not depend on quality (cleanliness or homogeneity). Thus there is often 5% to 10% of impurities in the batch. The cost of cleaning (loss of weight and supplementary expenses) falls on the industrial buyer-operator and on the artisanal processors and users. When the industrial operator works with imported grain (maize or wheat) only a less costly complementary cleaning operation is needed. To decrease or increase the price according to the degree of cleanliness of the product would undoubtedly improve the respect given to cleanliness in commercial transactions.
Traditionally in African couontries where maize, millet and sorghum are consumed, the flour has to have precise characteristics adapted to each dish. In Benin there are about forty different foods prepared from maize (Nago, 1989). The culinary quality depends on the skill and experience of the cook, and on the quality of the raw materials used. There are several predominant criteria of quality, mentioned below.
The colour of the milled product will be the same in the cooked product. In general, light coloured products (white, yellow, pink) are preferred to dark colours (grey or red).
This depends on the size of the grains in the final product: coarseness of semolina or fineness of flour; hulled grain more or less broken. The measure of granularity is easily achieved with the aid of standard mesh sieves available on the market.
The flour obtained from pounding in a traditional mortar has 22% to 46% moisture content. Its rapid fermentation, appreciated in culinary preparations, does not permit conservation beyond one or two days. In town, where the grain is milled dry or is processed in mini-mills, it is difficult to produce a flour or semolina correctly de-germed and ground for good conservation for 1 to 2 months which gives, after storage, the same cooking quality as a fresh product.
At present there is no small scale equipment capable of de-germing grain effectively. Hulling by external abrasion does not eliminate the portion of germ embedded in the endosperm. To remove this, it is first necessary to break the grain as is done in industrial installations. But since the final de-germed product achieves a yield of no more than 70%, it is more expensive.
There is currently a need to market a partially processed product having an intermediate granularity which can be used in certain dishes or re-ground by the cook for other uses (as in the case of "Sanka!" or crushed millet and "soungouf" or milled flour in Senegal).
The texture of a product is linked to its capacity to absorb water and its behaviour on cooking. The measure of the texture of the "to" or "porridge" made from millet, maize or sorghum has been the object of laboratory study at the IER in Mali and CIRAD in France. The same procedure is in progress to characterize other dough-type products such as "ogi" and "akassa" in Benin or granulated or rolled products such as couscous, "arrow" and "aklui" in other parts of West Africa. When planning to industrialize the manufacture of ground products for used in traditional dishes, it is possible to determine the influence of the varieties and the milling technique employed on the quality of the final cooked product.
A review of the difficulties failures encountered in various technology extension projects in Africa has enabled us to draw up the following list of bottlenecks to be avoided:
After having reviewed a large number of post-harvest technologies, we have made an assessment of the experience acquired in sub-Saharan Africa. On one side, it seems that traditional technologies carried out by farmers over the years have proven useful. They are still are still predominantly in use today because they are well anchored in the culture of the various ethnic groups. Nevertheless, the socio-economic evolution and the ecological changes have considerably altered the production and marketing conditions for staple foods (grain and tubers). The post-harvest system must therefore be adapted to the new context which is continuously evolving.
The first technological adaptation is already taking place more or less successfully. The constant improvements in existing technologies respond more positively to new marketing requirements following the course of technical progress. In fact, technical solutions do exist and can be disseminated through shared experiences such as is given in this report, by developing field research and training farmers to adapt to the new technologies.
The second adaptation is another matter. It means modifying people's mentality with regard to socio-economic changes in order that they can adapt to the market, and improve the quality of production, taking into account environmental constraints. This new type of "human" experience is, undoubtedly, the most difficult to surmount as it comprises technical, collective and individual factors. This document is reporting the African experience in postharvest technology. Let us hope it will allow each person involved in development to draw some valuable information to help him achieve his own experience.
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