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5.1 The environmental requirements of cassava
5.2
The cassava root
5.3 Economic aspects of cassava production
5.4 Causes of limitations to storage for fresh
cassava roots
5.5 Ways of and limits to. storing fresh cassava
roots
5.6 The processing of cassava roots
Cassava is a plant of the new world which originates in the northeast Brazil. Central America is assumed as another source (ONWUEME, 1978). Having begun with these two regions, cassava is now cultivated in all tropical regions of the world.
In contrast to yams, mere is only one species of cassava bearing the scientific name Manihot esculenta Crantz and belonging to the family of the Euphorbiaceae.
There is a wide range of cassava varieties. Individual varieties can be recognised by the leaf and root form, the duration of vegetation, the yield and the content of hydrogen cyanide. The latter constitutes the difference between the sweet and the bitter cassava.
The bitter varieties of cassava have a high hydrogen cyanide content which can amount to up to 250 mg per kg fresh root (GRACE, 1977). To avoid poisoning, the roots have to be detoxified before consumption. The vegetation period for bitter cassava varieties lies between 12 and 18 months. After ripening, the roots can be left unharvested in the soil for a long period and will not spoil (ONWUEME, 1978).
The sweet cassava varieties only contain low quantities of hydrogen cyanide so that detoxification prior to eating is normally not necessary. The vegetation period is relatively short at 6 - 9 months. The roots of this variety rot quickly if they are left in the soil after maturity.
The content of hydrogen cyanide is not constant according to the varieties, but is subject to fluctuation due to the environment. For this reason, the content of hydrogen cyanide is unsuitable as the only criterion in defining the varieties of cassava (ONWUEME, 1978).
Cassava is a perennial plant. Apart from for purposes of research and breeding, propagation is exclusively vegetative. In contrast to yams which are propagated via the tuber, the cassava can be reproduced by cuttings taken from the stalks of the plant. As the stalks, in contrast to the root, are used neither for consumption nor other economic purposes, the cost of propagating cassava where planting material is concerned, is practically zero.
5.1 The environmental requirements of cassava
Cassava is a plant of tropical lowlands. Its cultivation is restricted to regions between the latitudes of 30° north and 30° south It is most widespread near the equator between 15° north and south. Since cassava is a short-day plant, the highest yield of roots is in this region.
Cassava finds the most favourable growing conditions in humid-warm climates at temperatures of between 25 - 29°C and precipitations of between 1000 - 1500 mm which ideally should be evenly distributed (ONWUEME, 1978).
In view of the climate, cassava has an enormous ability to adapt. There are locations in the Andes where cassava is cultivated at an altitude of 2000 metres. Cassava can even survive slight frosts although the plant then loses its leaves which grow again when temperatures rise. Where mere are high temperature fluctuations, the annual average temperature must amount to 20°C. With low fluctuations in temperature, 17°C is also sufficient for successful cultivation (COCK, 1985).
Cassava is able to survive longer arid periods. During this period, the plant loses all its leaves and suspends growth even of the thick roots. When precipitation again begins, the plant regenerates without any great loss in yield occurring. This ability is why it is particularly suitable for locations marked by indefinite and irregular precipitation.
Cassava likes light, sandy loam soils with medium soil fertility and with good drainage. Saline, strongly alkaline and stony soils, and soils with stagnant water are unsuitable for the cultivation of cassava. Stony soils inhibit the formation of the root tuber. Where soil fertility is concerned, cassava is easily satisfied. Even on very poor and acidic soils which are totally unsuitable for the cultivation of other plants, the cassava will still provide a relatively good crop. For this reason, the cassava is frequently grown on edge locations which can otherwise not be used arably. The low demands of the cassava mean that it is often the last member in crop rotation.
The economically most important part of the cassava plant is the tuber-like thick root. This develops from thin roots which take the nutrients out of the soil. Only a few roots per plant develop into tuberous, thick roots.
The thick root is connected to the plant by a short, wooden neck. It has a longish round form and can grow to between 15 and 100 cm and reach a weight of 0.5 to 2.0 kg.
The cassava root consists of three layers. The cork periderm and the cortex below this form the exterior protection for the root. Both cell layers are only a few millimetres thick. The central part of the root is a storage tissue where starch is kept. In the centre of the root there is a small vascular bundle running lengthwise. There are cells which can secrete latex in the storage tissue as well as in the cortex.
The thickening growth of the roots does not begin until the roots absorbing the nutrients have penetrated the soil to prepare the way. The arrangement of thick roots is influenced by how the cuttings are planted. If these are planted vertically, thick roots develop and lie close to each other like in a bundle. If the cuttings are planted horizontally, roots will form at each node. The thick roots then develop at some distance from each other at the nodes of the cuttings (ONWUEME, 1977).
The thick roots have no function in vegetative propagation which occurs through cuttings from the stalk. The reason why reserve substances accumulate in the thick roots has not been completely clarified. It can however be assumed that these reserves serve to help the plant survive unfavourable situations e.g., longer arid periods. This ultimately also defines the good resistance of cassava to dryness.
The thick root in a fresh condition contains approx. 62% water, 35% carbohydrates (mainly in the form of starch), 1 - 2% proteins, 0.3% fats, 1 - 2 % fibres and 1 % minerals (ONWUEME, 1977). In comparison to the yam tuber, the cassava root contains more energy but far less protein. An unbalanced diet containing only cassava can lead to deficiency. Deficiency and poisoning can also be caused by the high concentration of hydrogen cyanide especially when cassava is not processed or insufficiently processed before eating (cf. Chapter 6.2).
5.3 Economic aspects of cassava production
Cassava was introduced to Africa in the 16th century and became established at various locations on the continent in the subsequent centuries. However, not until the beginning of the 20th century did cassava become extensively widespread and find a permanent home in numerous small farm systems. In some cases, cassava clearly took over from other staple foods e.g. bananas in East Africa and maize and sorghum in southern parts (LYNAM, 1991).
In Africa, but also on other tropical continents, cassava is mostly grown by small farmers. In Africa, only 10% of production reaches the market; 90% is cultivated as food for the producers themselves. Cassava provides central benefits particularly for subsistence-oriented farms from an economic aspect.
Cassava has a potential tuber yield of 70 tons per hectare and with this, has the highest output per unit area among all staple foods providing starch (COCK, 1985). Decisive for subsistence-oriented small farmers who avoid risks is the ability of cassava to provide secure yields of 7 - 9 tons of roots per hectare even on marginal and acid soils and under unreliable precipitation conditions (ONAYEMI, 1982). In addition, the annual fluctuations in the yield of cassava are among the lowest for all food crops (HAHN, 1987).
In comparison to other roots and tubers, the labour productivity of cassava is very high. For a yield of 10 tons per hectare, a labour input of approx. 120 days (manual phase) can be estimated (COCK, 1985). This corresponds to about one quarter of the work input required to produce the same quantity of yams.
After the plants have closed their leaves, cassava can be left to itself. On the one hand, a contribution is made here to evening out peak seasonal work (HAHN, 1987). On the other hand, this encourages the seasonal migration of male labourers in search of an income, without endangering the production of cassava.
Production input, e.g. fertilisers, plant protection and propagation, is very low. Fertilisers can be completely dispensed with without fear of losing any part of the yield (COCK, 1985).
The economic features and modest requirements of the plant are the reason for it being called a "starving plant". Cassava is able to provide secure yields on marginal sites and under unfavourable weather conditions which cause crop failure for other plants.
5.4 Causes of limitations to storage for fresh cassava roots
The starch-storing root of cassava is of no importance for vegetative propagation. This means that the cassava, in contrast to the yam tuber, has no period of dormancy which naturally favours storage after the harvest.
When the cassava root has been harvested, a rapid process of deterioration sets in after 2 - 3 days at the latest. This can be differentiated in two phases.
Primary deterioration comes from the central vascular bundle in the root. This begins to take on a dark-blue to black colouring starting from broken and cut surfaces. The adjacent storage tissue is also affected and the starch stored undergoes structural changes (PLUMBLEY and RICKARD, 1991).
Experiments have shown that no microorganisms are involved in the change of colour. This is based on an endogenous oxidative process. The colouring can be delayed by cutting off oxygen, e.g. by storing the roots in a water bath (PLUMBLEY and RICKARD, 1991).
Secondary deterioration mainly results from microbial activities but can also be due to fermentation and softening of the root tissue (PLUMBLEY and RICKARD, 1991). Secondary deterioration is caused by rot viruses which can occur in very complex compositions and vary from location to location (ibid.).
Considered economically, primary deterioration is more significant than secondary deterioration. Discolouring parallel to primary deterioration causes a distinct decline in the value of the roots and makes them impossible to sell. For this reason, it is initially essential to develop processes which allow primary deterioration to be controlled.
5.5 Ways of and limits to. storing fresh cassava roots
5.5.1 Storing cassava roots in the soil after
maturity
5.5.2 Traditional methods of storing fresh
cassava roots
5.5.3 Storage of fresh cassava roots in clamp
silos
5.5.4 Storing fresh cassava roots in crates
5.5.5 Storing fresh cassava roots in a dip
5.5.6 Storing fresh cassava roots in plastic
bags
5.5.7 Use of modern methods to store fresh
cassava roots
5.5.8 Measures to prepare fresh cassava roots
for storage
5.5.9 Suitability of storage systems for fresh
cassava roots on a small farmholder level
The cassava roots deteriorate within 2 - 3 days of harvesting. This means a high selling risk for the seller as the produce becomes unsaleable after a short time. The seller tries to compensate for his sales risk by asking the appropriate prices. This means that urban consumers have to pay relatively high prices for fresh cassava roots (FAO, 1988).
The problem of the storage ability of fresh cassava roots is also known to the traditional producers in tropical America. These societies already developed processes during historical times to allow extension of storage (RICKARD and COURSEY, 1981).
Also various research establishments have concerned themselves with the specific problems around cassava and have searched for a solution on how to lengthen storage of fresh cassava roots. The most significant results of these efforts and traditional methods are described below.
5.5.1 Storing cassava roots in the soil after maturity
The method of leaving cassava roots in the soil after maturity is still widespread today. The roots can be kept in this way for several months without deteriorating.
With this method of storage, the rhythm of the harvest can be adapted to that of consumption. If the optimum harvest age has been missed, the root loses more and more substance and particularly starch, the constituent which defines its value (cf. Fig. 14), the longer storage is. At the same time, the root begins to become woody and impairments to the flavour occur (LANCASTER and COURSEY, 1984).
During storage in the soil there is also the danger of roots being infested by pathogens. Another disadvantage of this method of storage is that area which could be planted with other crops is occupied by storage (CHINSMAN and FIAGAN, 1987). Particularly in densely populated areas, this leads to shortage of land and increases production costs for cassava as the opportunity costs incurred have to be allocated to this method of production.
5.5.2 Traditional methods of storing fresh cassava roots
Freshly harvested roots can be buried in the soil to preserve them. This method is evidently oriented to the process of leaving ripe cassava roots unharvested in the earth (INGRAM and HUMPHRIES, 1972). It is said that by using this method in South America, cassava roots have been stored from one season to the next (RICKARD and COURSEY, 1981).
Storage methods oriented to this process are widely distributed. In West Africa and India, roots which cannot be directly consumed or processed after the harvest are piled into heaps and watered daily. The roots can also be coated with a loam paste to attain a storage ability of 4 - 6 days (RICKARD and COURSEY, 1981).
In older reports on traditional storage methods processes are described which allow a storage of up to 12 months (RICKARD and COURSEY, 1981). However, there is justified doubt here as recent practical experiments have not been able to confirm these results. BAYBAY (1922) tested various traditional methods of storage on the Philippines. He came to the conclusion that all the traditional processes he had tested could only prolong storage by a few days. Only storage in trench silos showed a somewhat more favourable picture.
5.5.3 Storage of fresh cassava roots in clamp silos
The storage of fresh cassava roots in clamp silos was tested by the Tropical Products Institute (TPI) and the Centro Internacional de Agricultura Tropical (CIAT) in Columbia. Setting up the clamp silos was oriented to traditional silos of the Indians and to experience gained in northern Europe with the storage of potatoes.
A more or less thick layer of straw is laid out on a dry area and the roots are piled on this in conical heaps. The heaps, weighing between 300 and 500 kg are covered by straw and soil and - as with potatoes - openings are left for ventilation (RICKARD and COURSEY, 1981). Storage periods of up to 4 weeks were reached with this method in experiments. Losses in weight and the formation of rot were low (BOOTH, 1976).
Controlling temperature for this method which should be below 40°C for successful curing of wounds and for storage, was difficult. Several structural changes towards improving temperature control were tested. These led to very varied and unforeseen results (BOOTH, 1976).
Although storage in clamp silos allowed a substantial lengthening of storage duration of up to 4 weeks, the system hardly experienced any practical dissemination. On the one hand, building the silos requires a relatively high labour input. On the other hand, management of such storage demands a great deal of experience (LOZANO et al., 1978). What remains completely open is whether the storage duration of 4 weeks reached corresponds to the requirements of the farmers.
5.5.4 Storing fresh cassava roots in crates
Freshly harvested cassava roots can be stored in wooden crates. The crates are lined with a layer of sawdust. The spaces between the roots are also filled with sawdust. Finally, the roots are then covered with sawdust..
The sawdust, which can be replaced by any other resorbent material e.g. dust from coconut fibres, has to be damp but must not be wet. If the sawdust is too dry the roots will deteriorate quickly. Sawdust which is too moist promotes the formation of mould and rot. To prevent the roots drying out too early, the crate should be lined with plastic foil (RICKARD and COURSEY, 1981). A storage period of 4 - 8 weeks was attained with crates in experiments.
In Ghana this method of storage was modified and the crates were replaced by large baskets. The baskets were lined with fresh banana leaves which also served as a cover for the stored produce. Before storing the roots these were subjected to three days of curing. Storage periods in Ghana using this method reached 2 months (injured and cured roots) and up to 6 months (uninjured roots) (OSEI-OPARE, 1990).
The limited availability of crates and lack of suitable baskets which can only take up a small amount of roots in comparison to the value of products, have prevented this storage method from spreading. Both types of container are relatively expensive and the labour input involved in preparing the store and the produce is quite high.
However, this storage method could be interesting where fresh (sweet) cassava roots are sold over long distances. On the one hand, this method allows sufficient storage ability and distinctly reduces the risk of early deterioration. Secondly, the crates or baskets can simultaneously be used as containers during transport (also several times) which saves on handling costs and also reduces injury to the roots during transport.
5.5.5 Storing fresh cassava roots in a dip
Storing fresh cassava roots in water is a widespread method on a household level and with traders in Ghana. For this, various sized containers are filled with water and the roots are completely submerged (OSEI-OPARE, 1990).
Storage duration can only be extended minimally by this method. The roots stored in this way normally begin to ferment or spoil after 3 days. The effectiveness of this method depends greatly on the degree of freshness of the roots when they are stored (OSEI-OPARE, 1990). As the roots passed on to the dealers are mostly already 1 - 2 days old, the storage ability of the roots is hardly improved by this method.
The limited extension of storage is not the sole criterion for the selection of this method of storage. This process is far more a method of simultaneously detoxifying the roots which contain hydrogen cyanide (cf. Chapter 5.6.2).
5.5.6 Storing fresh cassava roots in plastic bags
The use of plastic bags to preserve cassava roots can be seen as a consistent extension of traditional storage methods which serve the purpose of avoiding the loss of moisture and water stress (RICKARD and COURSEY, 1981).
Freshly harvested roots are put into bags. Fungicides should be applied before the bags are closed to avoid the formation of mould and rot (BEST, 1990). When the roots which are packed airtight, breathe the oxygen content in the bags is reduced creating a preserving effect (RICKARD and COURSEY, 1981). High temperatures (above 40°C) as well as low temperatures (below 10°C) both have a positive effect on the duration of storage.
A storage duration of more than 14 days was reached in Columbia using this method (BEST, 1990). This method is particularly interesting for dealers and consumers. As with storing in crates, the risks involved in transport and sales is reduced for the trader. Consumers profit as the roots can be kept for a certain time after purchase. with the relevant infrastructure, this method of storage can provide new sales potential for production locations which are distant from the market.
One problem however, is that the consumer has to be convinced of the quality and the benefits (e.g. less frequent buying, storing to some extent in the home) of this "product innovation". The experience gained here in Columbia is quite positive (BEST, 1990). Direct transfer of this experience to conditions in Africa is however a problem as there are considerable differences between the living and eating habits. In addition, it must be determined whether the consumer is willing to bear the extra costs involved in storage.
5.5.7 Use of modern methods to store fresh cassava roots
The modern methods of storage involved here comprise refrigeration and freezing, waxing of the roots and chemical storage protection.
Reduced temperatures extend the storage ability of cassava roots by delaying the rot processes which occur rapidly at normal storage temperatures. Experiments have shown that the most favourable temperature for the storage of fresh cassava roots is 3°C. Stored at this temperature, the total loss after 14 days amounted to 14% and after 4 weeks, 23% (RICKARD and COURSEY, 1981). A bluish mould occurs on the surface of the roots at higher storage temperatures and the flesh of the roots turns brownish. Both cause quality and storage losses (ibid.).
Cassava roots, or pieces of these, can be packed into plastic bags and frozen. Although the texture of the tissue becomes somewhat spongy the flavour is preserved (RICHARD and COURSEY, 1981). After defrosting, the roots remain edible for about 4 days. In some Latin American countries this method of preservation is used commercially. There are various preparations of freshly frozen cassava roots in shop refrigerators. These products are also entering supermarkets in European and American cities where a large number of African or Latin American inhabitants are potential customers.
Preliminary experiments towards preserving fresh cassava roots by coating them in wax were carried out in India. The wax contained a fungicide and the roots were dipped in it to coat them Storage duration could be extended to about 10 days with weight losses amounting to 10% (RICKARD and COURSEY, 1981). In Columbia fresh cassava roots were simply dipped in paraffin at a temperature of 90° - 95°C. Without any fungicide being used, the storage duration could be extended to 1 - 2 months (ibid.). Whether the storage ability is improved by the effect of the fungicide or whether this is due to the wax coating reducing respiration and the supply of oxygen has not finally been investigated.
The use of chemical agents to avoid mould and rot on foodstuffs is restricted for reasons of hygiene. The universal fungicide "Benomyl" was the only agent with which the formation of rot could be satisfactorily controlled for more man 10 days (RICKARD and COURSEY, 1981). This substance also had a reliable effect on treating the mould on roots stored in plastic bags.
Various commercial products tested had no effect on the discolouring of the vascular bundle. Only when this initial stage of deterioration can be controlled, will the control of the second phase of microbial root deterioration become interesting (RICKARD and COURSEY, 1981).
5.5.8 Measures to prepare fresh cassava roots for storage
For physiological reasons cassava roots are far less suitable for fresh storage than yam tubers. Despite this, the cassava roots have to be treated with just as much care as the yam tubers so that the maximum period of storage may be attained (cf. Chapter 3.7. 1).
It must be made sure that the cassava roots are not injured or squashed during harvesting, transport and storage as injuries accelerate the physiological destruction of the tissue (blue coloration of the vascular bundle).
The most serious injuries occur at the shoulder of the root where it is connected to the plant by the root collar. This kind of injury can be avoided by harvesting the whole plant or by leaving a short piece of stalk on the root (INGRAM and HUMPHRIES, 1972). The roots harvested in this way discolour far more slowly than those harvested in a conventional fashion.
The deterioration of the roots can be delayed by cutting off the parts of the plant above the ground except for a short stalk stump. This should be done about 3 weeks prior to harvesting. The positive effect of cutting the above-ground parts of the plant off on storage ability is only retained when the roots are stored without any injuries (RICKARD and COURSEY, 1981).
5.5.9 Suitability of storage systems for fresh cassava roots on a small farmholder level
There are differences among farmers cultivating cassava, e.g. regarding the economical status of the crop, the resources for production input (work, capital and soil) and the market orientation and proximity. This makes the requirements of small farmholders regarding the storage of fresh cassava roots, very varied and not at all homogeneous.
The majority of West African small farmholders produce for the purpose of self-sufficiency with minimum resources. Cassava which is an undemanding plant in every respect, primarily serves the purpose of self-sufficiency and risk reduction. The proportion of production sold is generally very low.
The processes described above allow a very limited prolongation of storage. They mostly require an additional input of work and/or of capital which, in relation to the status of the cassava production, is relatively high. Some methods, i.e. cooling by means of external energy, constitute a technological leap and necessitate a functioning infrastructure.
For the majority of small farmholders, the methods described provide no solution to their specific storage problems (long-term, secure, low losses and low-cost).
For farmers who have attained a certain integration into the market (fresh selling), individual methods are definitely of some interest. These can serve to bridge time gaps by minimally prolonging storage ability and by solving logistic problems by providing transport containers. The use of the methods described however, will only be successful if production and sales up to the final consumer can be integrated into a system.
For the majority of farmers who produce cassava at some distance from the markets, other strategies become essential if their storage problems are to be solved. These strategies go in the direction of processing in order to produce products which can be stored. Some processes, e.g. the production of cassava chips as described below, can skill be included in the fields of storage and post-harvest technology. Other processes, e.g. the production of gari, are clearly a matter for foodstuff technology and are no longer a subject for this investigation.
5.6 The processing of cassava roots
5.6.1 The purpose of processing
5.6.2 Hydrogen cyanide and its release
5.6.3 The production of cassava chips
5.6.1 The purpose of processing
As stated in the preceding chapters, the storage ability of fresh cassava roots is very limited in time This can only be prolonged slightly by the use of technical processes which, in some cases, are very costly (e.g. refrigeration). In view of this it is not surprising that processes to conserve cassava roots have been developed.
There is a great variety of such processes ranging from simple drying through to processes which have to be considered as foodstuff technology (GRACE, 1977; COCK, 1985). Traditional methods of processing which are typical for some regions, e.g. the production of gari in West Africa, have been completely mechanised during the course of time. This has contributed to relieving particularly women of work (NZOLA-MESO and HAHN, 1982).
The main purpose of processing cassava roots is to get a product which will keep and which can be stored. Numerous production processes achieve this by drying the cassava roots. A welcome side-effect of drying is the concentration of the contents which determine its value. The ability of the product to be transported is considerably improved.
In addition to conserving, processing also detoxifies the cassava. This is necessary since the bitter varieties of cassava in particular have very high concentrations of hydrogen cyanide which can lead to serious health hazards.
5.6.2 Hydrogen cyanide and its release
Cassava roots contain hydrogen cyanide (HCN) which is a very strong poison. The lethal dose for an adult is approx. 60 mg per day (HAHN, 1989). Due to the high content of HCN, an unbalanced diet containing only fresh cassava products can lead to poisoning, deficiency and deformity. These occur especially if cassava roots have not been sufficiently detoxified and if there is a protein deficiency in particular of amino acids containing sulphur. The latter promote a very effective natural body detoxification process (HAHN, 1989).
The concentrations of hydrogen cyanide in the cassava roots depend on the variety. The content can amount to only a few milligrams but also to over 300 mg per kilogram of fresh root (HAHN, 1989). HCN is also unevenly distributed within the root. There are high concentrations in the outer cell layers and in the upper part of the root (HAHN, 1989). The bitter flavour cassava roots have does not indicate the content of HCN (LANCASTER and COURSEY, 1984).
Hydrogen cyanide does not occur freely in the cassava root but is combined with linamarin and lotaustralin, two cyanoglycosides. HCN is released by means of a hydrolytic process which is activated by the enzyme linamarase (COURSEY, 1982). Hydrolysis always takes place when the enzyme comes into contact with the cyanoglycoside. The natural release of hydrogen cyanide is encouraged by the mechanical destruction of the tissue or the disintegration of the cellular structures due to storage (LANCASTER and COURSEY, 1984).
Drying, boiling, immersing in water over a longer period and fermenting also encourage the release of HCN. What is promoted here is less hydrolysis and more the release of HCN which has already been detached due to the activity of the enzymes with the glycoside.
Even when the cassava roots are properly processed a residue of hydrogen cyanide remains. The concentrations however are mostly so minute that no hazard to health will occur from eating them (HAHN, 1989).
