21 Apr. 1985|
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The more important traditionally processed products include dried chips and flours, and in the particular case of cassava, non-gelatinized flours, both fermented and unfermented, fermented pre-gelatinized grits and pre-gelatinized flours. A pre-gelatinized cassava paste usually in the form of balls wrapped in leaves.In Congo and Zaire chicouangue is steamed before being sold. The objective of chipping is to expose the maximum surface of starchy flesh and encourage a rapid drying. The tool used by villagers is often only a heavy knife with which it is difficult to make slices sufficiently thin for rapid drying. Soaking in water is often done for one to three days, either before or after the chipping operation during which some fermentation takes place that gives the chips the sour flavour favoured by some consumers. Sun-drying of cassava chips is carried out on any convenient flat surface; on roofs, concrete surfaces, mats or along road-sides. With traditional sun-drying little can be done to change or control the conditions of wind, which affects air movement over the chips, the air temperature or relative humidity. This explanation underscores the difficulties of drying chips during the rainy season when the relative humidity of the air is frequently above 80% and when drying the chips may take several days. A combined research programme by The Asian Institute of Technology, the NRI and CIAT has shown that drying rates can be greatly improved by placing the cassava chips in suspended trays with a bottom of wooden slats or chicken wire. The use of solar dryers could constitute a major improvement on the drying of cassava chips (Figure 5.2). Wood fired dryers also represent a simple drying technique which can alleviate the constraints of drying chips during the rainy season (Figure 5.3).
Once the chips are dried to a moisture content of 13% or lower they can be stored for several months, provided they are kept dry, and particularly are not exposed to high ambient relative humidities.
Gari is a fermented and gelatinized dry coarse flour, very popular in West Africa and a staple food in Nigeria, Ghana, Benin and Togo. During the fermentation period the container is put under pressure by piling heavy stones on it, by strongly twisting the neck of the sacks and pressing the bag or sack between wooden poles tightened by ropes (Figure 5.6). After pressing the de-watered cassava mash is a solid cake which has to be broken up and sieved to remove the large lumps and fibre (from the central vascular strands) and to obtain an homogenous product. The FAO post-harvest project in Cameroon introduced improved fireplaces for the frying operation (Figure 5.9) consisting of an elongated fireplace built of burnt-clay bricks and with a three metres high chimney at one end to remove smoke and fumes from the fire. A mechanical "garifier" or a mechanised system of frying and drying usually takes the form of a mild-steel drum or trough with rotary rakes and paddles fixed to a steel shaft slowly rotating on the axis of the drum. The operations involved in its production are essentially the same as for gari; peeling and washing, chopping (into 3-4cm cubes), mashing or grating, fermenting, de-watering, sieving, granulating, drying and steaming. After this short drying period, which may in some cases be omitted, the grains are steamed for about 30 minutes by putting the batch of granules inside a steamer. CIP has developed simple processing machinery for the production of dehydrated potatoes, including a washer, peeler, slicer and dryer. The drier is simply a range of two-tier wooden racks inclined at an angle of 30 degrees to the prevailing wind. The production method is simply slicing or cutting the roots into pieces and drying them in the sun. Villagers have learned that the best drying, in terms of quickness and quality of the end product, is achieved when the peeled cassava is thinly sliced. The objective is to produce dry cassava chips which are clean, have a white colour, are free from extraneous matter and can be safely stored for long periods. During the drying process there are two discrete phases; in the first phase the chips lose moisture very quickly down to about 20% mc, wind speed passing over the chips is more important than air temperature and relative humidity.
The trays allow more air to flow through the mass of the chips and eliminate the need to turn the chips periodically. Solar dryers of the cabinet type and of the hot-house types are reported to be in use in Nigeria.
One of the first to be developed was the Brook drum dryer which is basically a large tray supported on a block or mudbrick base which has air inlets in its sides.
At village level gari is fried in shallow cast-iron pans, or in the more traditional areas in earthenware pans, over an open wood fire (Figure 5.8).
At family and village levels, the fresh or precooked roots or tubers are simply peeled, cut into chips and sun-dried by being spread out in the open, on a mat or any clean surface. The improved methods for drying cassava chips have been applied to the drying of yams, potatoes, sweet potatoes and cocoyams. Roots and tubers chips which are still warm after being boiled, parboiled or blanched should not be left exposed to air but should be cooled as quickly as possible, for example by immersion cold water. It was intended for use at village level and involved washing, peeling, slicing, blanching, cooling and drying.
After cooling in cold water baths, the slices are spread to dry on elevated wooden platforms. For these reasons cassava is usually sold as a processed product whilst other roots and tubers are most frequently sold as fresh produce.
Not only are thick pieces more difficult to dry, the surface will dry whilst the inside is still wet and because of the dried outer cells have shrunk the rate of moisture diffusion from the inside is greatly reduced and the time of complete drying considerably extended. Hence in sun-drying systems the chips are dried more by the passage of air over them than by the direct effects of the sun's rays. Notwithstanding, hand chipping is still adequate where only up to 50kg of roots are dealt with in one day. The National Root Crop Research Institute in Nigeria suggests that optimal hydrocyanic acid reduction can be achieved through a combination of 15 minutes soaking and 2 minutes blanching of the cassava chips. Under cloudy weather or even at night, the first drying phase can be completed so long as there is sufficient air movement through the chips. Discolouration of the chips detracts from their appearance and noxious odours and off-flavours are unacceptable to the consumer. The cabinet type, often referred to as a Brace dryer, is usually a wooden structure covered with plastic sheets which form the solar collector. A flue made of empty 200 litre metal drums passes under the tray through which hot air flows from a firebox to a chimney.
In West Africa the chips are sometimes parboiled before drying them which is considered to enhance the storability of the chips. This grating surface is fixed on a wooden frame (Figure 5.4) and the cassava pieces are pressed against the jagged side of the metal and rubbed vigorously with strong downward movements.
The abrasive surface can be either cylindrical or a flat disc (figure 5.5) and is frequently a galvanized metal sheet with nail-punched holes, as in the hand grater, and attached to a wooden frame. Where the hopper or the grating cylinders are made of wood, they should be lined with aluminium or galvanized sheeting to make thorough cleaning easier. The improved fireplace are said to reduce wood consumption by 55% and the frying time by about half. In a design of the Federal Institute of Industrial Research, Oshodi (FIIRO) in Nigeria a gas or wood fire below the hopper produces a wall temperature of about 250°C with the hot gasses contained within a housing of mudblocks or burnt-clay bricks. After baking the fresh cassava bread is sun-dried on raised platforms for few hour to improve its keeping quality, before being packed in paper or polyethylene bags for marketing, which is frequently sold in the retail market directly from the bakers and traders vehicles.
But dried chips of these roots and tubers can be affected by discolouring compounds which do not occur in cassava. They are intended to be built inside a structure, covered with clear plastic, effectively a simple solar drier, and surrounded by screens in the hope of keeping out insects and birds, but sufficiently open to allow free passage of air. The initial drying of the chips occurs as the water on the chip surface evaporates and is then replaced by water vapour diffusing from the inner layers of the chips. In the second phase drying is much slower and needs a relative humidity of not higher than 65% to dry the chips to a moisture content of 13% which is considered safe for long term storage. Both are prepared in the household by mixing dry gari with hot or cold water and cooking and are served with soup or stew.
A chimney helps draw air over the fire which reduces the effects of smoke and heat on the operator. Whilst the technical aspects of the production have reportedly been solved and it is now considered possible to produce attieke with a moisture content of 8% to 10%, with a shelf life of several months (Muchnik and Vinck, 1984), there remains the problems of the product's acceptability by traders and consumers and its economic competitiveness against traditional attieke and imported cereals.
The dried chips are ground into flour before being incorporated into the traditional foods. Hot solutions of sodium hydroxide (Lye) can be used to loosen the skin to facilitate later peeling, such as removal by water spray or scrubbing with brushes. Although diffusion and the rate of drying are fastest in small chips, when they are in a thick layer they can easily become compacted, which prevents free air movement through the mass. Except during periods of actual rain, in most places the temperature will increase sufficiently during the day to reduce ambient humidity to the required level to complete this second stage of drying. While this type of dryer is cheap to construct and have been tried in many situations around the world they so not seem to have achieved wide acceptability.
Natural convection creates enough pressure differential to move the heated air through the drying crop without any mechanical device. This traditional technology can be improved by mounting the grating surface on a wooden table at a convenient height so the rubbing action is horizontal rather than in a downward slant when the grating surface is supported against the operators legs It is not possible to completely grate a whole cassava piece, 3% to 5% of the cassava has to be left ungrated (Flach 1990, Bencini 1991). The drying of the slices down to 10% moisture content is done in 14-18 hours depending on weather conditions. As a result, for effective drying the chip's shape should permit air to readily pass through a large mass of chips. On the following day the difficult final drying to about 13% mc occurs during the early afternoon when the relative humidity is lowest (Cock 1985). The "Wadwha" disc grater was developed in Ghana and consisted of a disc shaped wooden block to which a perforated metal sheet was nailed. Although this type of dryers may be beyond the financial reach of most farmers, they may be suitable for cooperatives or groups of women processors. In 1990 manual graters were sold for US$2 to $3 each in village markets of the north-west province of Cameroon (Flach, 1990). The outer surface of the drum was covered with a sheet of perforated metal and as it rotated the cassava was pressed against the grating surface by a wooden block. During dewatering some soluble cyanide and organic acid is removed with the press liquor It also contains some starch and may be used as a base for stews and soups or the starch can be recovered by allowing the liquor to settle and decanting off the liquid.