Cassava is a staple crop and is particularly important in Africa and South America. It is a perennial shrub that grows to approximately 2 metres tall and has the ability to grow on marginal lands in low-nutrient soils where other crops do not grow well. It is also fairly drought tolerant. It is grown for its enlarged starch-rich tuberous roots. The amount of carbohydrates contained in dry cassava root is higher than other staple crops, such as maize or cereals but, by contrast, the protein content is very low.
Figure 1: The cassava root
Illustration: Neil Noble/Practical Action
Although cassava is a staple it is poisonous in its raw state as the plant contains cyanogenic glucosides. These glucosides are converted to hydrogen cyanide (HCN) by an enzyme called linamarase, which is also present in cassava and acts on the glucosides when the plant cells are ruptured either when it is eaten or during processing. The amount of cyanide present depends on the variety. There are two main types of cassava, bitter and sweet. While, in general, bitter varieties have higher levels of cyanide it must not be assumed that all sweet varieties have low cyanide levels. The cyanide levels range from 10 to 450 mg/kg of fresh root. The poison tends to be more concentrated in the skin of the root. The cyanide is readily removed during processing, resulting in a safe and versatile product that can be made into many different foods and non-food products. After proper processing a final residue of hydrogen cyanide will remain at very low concentrations but it does not cause any problem regarding the consumption of cassava products. Versatility of cassava Traditionally cassava has been regarded as a subsistence crop for low-income families - providing high levels of carbohydrates during shortages of other crops because of its tolerance to drought and ability to grow in poor soils. Recently the view of cassava as simply a subsistence crop has begun to change and there is growing interest in developing its commercial potential through improved varieties, increased productivity, harvesting and processing technologies. Along with the increased production of cassava, new markets and uses for the crop are being developed. Cassava roots are processed in many different ways to make it edible, to change its properties, remove cyanide and improve its storage capacity. Cassava is made into flour, commonly known as gari in West Africa and Farinha in Latin America, which is an ingredient in many recipes and used to make cassava bread or to replace up to 10% wheat flour in conventional bread. In many cassava-producing countries there is interest in reducing the wheat flour importations. Brazil incorporates 2% of cassava flour in wheat flour bread and Nigeria has recently made it a requirement to have 10% cassava flour in its bread.
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Cassava dough known as fufu in Ghana or amala lafun in Nigeria is made from cassava flour and plantain to produce rounded balls that can be eaten with fish, meat and soups. In Southern Africa cassava fritters are produced from grated or pulped fresh cassava combined with egg, onions, spices, breadcrumbs or cassava flour. Similar products are produced in South America. In the Amazonian region cassava juice, with the starch removed, is boiled down and skimmed repeatedly until it ceases to froth, seasoned with pepper, garlic and other flavours and is used as a source for flavouring meat and fish. The product is known as Tucupy or Casareep. It can be boiled down further to make syrup. Tucupy de sol is made by putting the seasoned cassava juice in a lightly corked bottle and exposing it to the sun for several days. During the slow evaporation the cyanide is eliminated. Other uses for cassava include animal feed made from the leaves and the dried root chips, and starch that is used as a food ingredient and in industrial processes.
Cassava can grow in poor conditions but yields increase when soil fertility is maintained and a good supply of water is used. Under very good conditions yields of fresh roots can reach 90 tonnes per hectare while yields from subsistence agricultural systems average 9.8 tonnes per hectare. Traditionally, cassava is grown on small farms at a subsistence level. The application of fertiliser is usually limited among small-scale farmers due to the high cost and lack of availability. Soil depletion can be reduced by intercropping with crops such as vegetables, coconuts, yams, sweet potatoes, melons, maize, peanuts and other crops. In this respect legumes, which fix nitrogen are very important. Commercially produced fungicides and pesticides are not commonly used. Cassava is planted by hand, burying the lower half of the cutting, taken form the mature stem, upright in moist soil. The cuttings produce new roots and shoots within a few days but early growth is relatively slow. In the early stages of growth it is important to keep weeds under control. If fertilizer is being used it should be applied during the first few months of growth. Organisations that have carried out research into developing cassava cultivation include The Natural Resources Institute (NRI), which aims to develop environmentally-friendly technologies to reduce crop losses from pests and diseases in developing countries, and the International Institute of Tropical Agriculture (IITA), that has developed improved varieties of cassava. Production has been increasing over recent years, partly in response to the new varieties introduced by IITA between 1988 and 1992 and because of government initiatives in the promotion of production and market development. A number of new initiatives are currently being implemented to improve yields and expand the growing area to increase cassava production. For example the Cassava Growers Association in Nigeria has set out to purchase parcels of land suitable for commercial growing of cassava in a cluster farming system. This allows for improved farming methods including the use of high yielding plant varieties and shared use of mechanised equipment such as a hired tractor for ploughing. The production of cassava is dependent on a supply of good quality disease-free and pest-free stem cuttings. The stem cuttings are sometimes referred to as 'stakes'. Dr. Robertson of the Faculty of Agriculture in the University of Zimbabwe has developed a low cost approach to propagating disease-free cuttings by growing the clean cuttings in cabinets before they are passed onto farmers. The yields from the disease free plants are very much higher than standard plants.
Plants are ready for harvest as soon as the tubers are large enough to meet consumer requirements. Roots can be harvested from 6 months up to 3 years after planting depending on the variety, enabling harvests to be delayed until market, processing, or other conditions are most favourable. However, as the roots age, the central portion becomes woody and inedible. Most cassava is harvested by hand, lifting the lower part of the stem and pulling the 2
roots out of the ground, then removing them from the base of the plant by hand. The upper parts of the stems with the leaves are removed before harvesting the roots. Care must be taken during harvesting to minimise damage to the roots, as this greatly reduces shelf life. Mechanical harvesters have been developed that lift the roots from the ground. One such design comes from the National Centre for Agricultural Mechanization in Nigeria. During the harvesting process, the cuttings for the next crop are selected. These must be kept in a protected environment to prevent them from drying out.
Cassava can stay in the ground for a long time but once harvested can rot very quickly. Post harvest losses from rot, pests, rodents can be very high unless effective measures are taken. Traditionally cassava has been stored by piling the roots into heaps that are kept watered to prevent the tubers drying out. Sometimes cassava is stored in pits that are lined with straw or other vegetable material and kept watered. Cassava can also be stored in containers with moist sawdust that keep the tubers from touching. Other approaches to storing for short periods are to keep the tubers under water or to coat the tubers in mud or ash to prevent moisture loss. Modern storage methods have been adopted for commercial cassava production. One low-cost method developed by the Nigerian Stored Products Research Institute is to store the tubers in trenches. A layer of palm and raffia leaves is laid at the bottom of the trench and then a layer of roots is placed on top. A new layer of leaves is added, then more roots until the trench has been filled. The top of the trench is covered with earth and a roof is constructed above the trench. Harvested roots are now frequently packed in plastic bags. The bags are airtight so limiting the supply of oxygen, which helps prevent the crop from rotting, although fungicides are sometimes applied before bagging the crop. The universal fungicide "Benomyl" restricts mould growth. The crop can be stored in this fashion for more than 14 days reducing losses during transportation. This has worked successfully in Columbia although it is not as well developed in African countries. Another element to consider is temperature - the optimum storage temperature for fresh root is 3°C. Once cassava has been packed into plastic bags it can also be frozen and this approach is currently used in South America on a commercial basis. While the texture can be affected by freezing, the flavour is not. Other preservation methods such as coating the roots with a wax containing fungicide, and dipping the cassava into cooled paraffin have been tried to a lesser extent.
Good quality raw materials are required for processing if the final product is to be of a good quality. Cassava should be free from disease, infestations and damage. The crop should be processed within two days of harvest to maintain product quality. Attention should be paid to the proper application of hygienic practices to prevent contamination. The cassava should be thoroughly washed, in potable water, to avoid contamination and waste material from the process should be removed to prevent cross-contamination. Peeling Ideally the cassava roots are peeled immediately after harvesting either with a traditional cutting tools such as a machete or with a mechanical peeling device. Typically one woman can peel 3
Figure 2: A cassava peeler
Illustration: Neil Noble/Practical Action
about 20 to 25 kg of roots in an hour. Around 30% of the fresh weight is lost during the manual peeling, when woody tips are also removed. Various peeling machines that have been developed but they have not been universally accepted because the cost is too high for many small-scale producers and the machines result in too much wastage in the peeling process. The peeled roots then need to be washed to remove dirt and pieces of peel. The peeled tubers are next chipped, grated, sliced, cubed or pelleted before further processing can take place. Chipping If the chips come from bitter cassava varieties, the roots are often kept in water for 2 to 4 days. This allows hydrogen cyanide to be released, reducing any public health hazard. The water must be carefully disposed of afterwards. Chipping can be done manually or with chipping equipment. Grating and Rasping Cassava is frequently grated or rasped as part of its processing. Traditionally it was a laborious activity carried out by hand. Now there is a large range of graters that can be used form the most basic manual and pedal operated graters through to fully motorised machines. Drying Cassava chips have to be dried to a moisture content of around 12 to 15%; from the original moisture content of 65 to 75%. Once dried the chips should break easily without crumbling. If no other storage method is used drying should take place within 2 days of harvesting to ensure Figure 3: A cassava chipper that the quality of the product is maintained. Illustration: Neil Noble/Practical Action Cassava chips are often infested by insects during the drying process so the time taken to dry the chips is important. The drying process can be shortened by increasing the surface area of the chips in relation to their volume - so the smaller the chips the faster the drying time. The larger chips therefore have to be reduced in size to improve their drying properties. Traditionally cassava has been sun dried in the open air, either on the ground or on a raised platform. Solar drying techniques improve upon this traditional approach by protecting the product from the elements and by improving the airflow which reduces the time for drying. Artificial drying can be used when the climate is not reliable enough or where the humidity is too high to use solar drying equipment. Artificial dryers are common as they are more predictable and controllable. Practical Action has developed various designs of tray dryer that can be used to dry fruit and vegetables and would be suitable for drying chopped cassava. Here economic factors must be considered as artificial dryers use fuel.
Making cassava flour
As flour, cassava can be stored up to a year if properly packaged. Traditionally cassava flour is made by pounding chips with a pestle and mortar. The process involves softening the tubers, cutting, drying and then pounding again or milling. The moisture content is high so the roots need to be dewatered, which can remove up to 50% of the water. This can be done by various types of press. The more simple types consist of parallel press boards that can be screwed together. Hydraulic jack presses can also be used. The 4
The basic steps for making flour Sort Peel Wash Grate Dewater Pound or mill Dry Fine mill Sieve Pack
fermented paste is put into hessian or polypropylene sacks and placed in to the press. An alternative traditional approach is to use rocks to compress the sacks, which allows the fermentation and pressing to take place at the same time. Sieving will produce a high quality product with similar sized granules. Larger particles that are separated during this process are sold as a cheaper grade. The product should be packed into polythene bags to prevent any moisture being absorbed from the air. This is especially important in areas of high humidity. The bags should then be stored in a cool dry location. Another approach to making cassava flour is to incorporate a fermentation stage which produces a slightly sour flavour product known as Gari. This can be done either during the dewatering stage or through retting which involves soaking the whole or cut roots in water for 3 to 5 days until root has fermented. The duration of fermentation should be carefully monitored to ensure that detoxification is complete but the product must also have an acceptable flavour and texture. The basic steps for making Gari Sort Peel Wash Cut/grate Pound or mill Ferment Dewater/dry Sieve Roast Cool Sieve Pack Store
Roasting the flour improves the storage capacity. It is done in a large, shallow pan over a fire, with constant stirring with wooden paddles for 20 to 30 minutes. It can be done with palm oil on its own. Cylindrical rotating roasters are used for in larger scale production setups. Fufu is made from cassava flour by pounding the material until a gelatinous sticky product results. Traditionally this was done using a pestle and mortar and could take up to an hour. Motorised pounding machines reduce the amount of work required. One such design was developed by the Department of Agricultural Engineering at the University of Ife in Nigeria.
Figure 4: Cassava flour roaster
Illustration: Neil Noble/Practical Action
Starch is traditionally used for foods such as biscuits, bread, and as a base for puddings. It is also used for non-food applications such as in papermaking and in the textile industry. There are two approaches to producing starch - non-fermented starch or soft starch, and fermented starch or sour starch. Constraints to cassava fermented starch production are the variable quality of the end product. Colour, expansion during cooking and water absorption are important qualities of starch. The basic steps for making starch Wash, Chop or grate Mill with water Sieve Mill again Allow to settle Separate - Drain the water Remove the top layers of fine fibre Ferment (optional) Remove the starch, break and dry.
By Neil Noble, Published by Practical Action on 06/20/06
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