3. Roots and tuber crops
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3.1 The root/tuber crop resource
Useful insights into the post-harvest loss complex of root crops are provided by a study of their characteristics. The principal roots and tubers are derived from nine species. They are all of high Moisture content, and their parenchyma cells are packed with starch grains. These materials show variable degrees of inherent keeping life, from some species of yam, like Dioscorea alata which remains dormant for 3-4 months to cassava in which there is no natural dormancy. When such material is stored in the fresh state, respiration and transpiration continue with inevitable losses of water and dry matter. Prolonged high levels of transpiration result in a change of texture affecting quality and weight loss. The dry matter loss, caused by superation constitutes a real food value as distinct from the moisture loss which only has an economic value. Sprouting at the end of dormancy can result in dramatic loss as the physiological state is altered. At this stage the stored starch is transformed to sugars and utilized by the elongating shoots with appreciable loss of both food value and moisture.
The main root/tuber crops are cassava (Manihot esculenta Crantz), the yam (Dioscorea spp.), the potato (Solanum spp.), the sweet potato (Ipomoea batatas L. (Lam) and the edible aroids (Colocasia spp. and Xanthosama sagittifolium). With the yam, the potato, and the aroids many different species are used in the cultivated complex. Differences between these are usually sufficient to merit separate treatment. A list of the more frequently used root/tuber species is given in Appendix 2.
Tare (Colocasia esculenta var esculenta) illustrates the system of maintenance of continuous food supply well. This species which has no seasonal growth constraint is planted in moist situations conducive to year-round growth. By systematic replanting whenever material is harvested, continuous supplies are assured. A similar absence of seasonality in cassava (Manihot esculenta) together with its ability to produce mature roots at various time intervals after planting permits year-round harvesting in most ecological systems. By opting for continuous food supply the opportunity for post-production losses is restricted to the food preparation phase. The same benefits accrue from the use of crop scheduling
Maintenance of food supplies ever restricted periods through storage is an essential feature of yam-based food supply systems. The success with yam storage lies in recognition of the limits to the potential for storage and in producing an environment conducive to keeping the material in sound condition. Perceptive farmers like those in Nigeria and the Ivory Coast often construct different stores to meet the separate requirements of D. alata and the different culivars in the D. rotundata/cavenesie complex. These systems work well within the normal period of yam tuber dormancy. They are so successful in West Africa that improvements in the beat indigenous practice could hardly be expected without recourse to the technologies of reduced temperature storage, the use of controlled gas environments or ionising radiation. These latter technologies are however not commonly applicable in developing countries because of cost and lack of know-how.
Considerable quantities of roots and tubers are transformed into more durable products by drying, fermentation and commination in different combinations and sequences producing a variety of materials each with distinctive characteristics. Other benefits that often accrue from transformation include the removal of toxic substances naturally present and the convenient-to-use nature of the transformed product. Although the ultimate keeping potential is constrained by environmental conditions, transformations have good prospects for long-form maintenance of root/tuber crop supply because they convert a perishable commodity into a form with stability similar to that of the cereal grains.
A feature of much of the indigenous processing technology is its high labour requirement. Because of this, food supply strategies based on transformation tend to be restricted to economic environments where labour is available. As a result root crop transformation is widely practiced among subsistence economies whore labour inputs have zero or low cash value. Attempts are being made to establish root/tuber crop transformation commercially, but success has been variable. Radical changes in the entire system of production may be required, as has been suggested with yam (D. rotundata/cavonenais) in Nigeria for manufacture of instant pounded yam.
3.2 The major root/tuber crops
3.2.1 Cassava (Manihot esculenta Crantz)
Known variously as manioc, yuca, tapioca and mandioca, the edible portion of cassava is a starchy root which matures in harvestable state in 8-24 months according to cultivar and climate. In addition to the roots, the leaves also have potential for use as food as for instance in pert e of Central and West Africa. Cassava cultivars are often grouped into "sweet" with relatively low contorts of cyanogenic glucosides and "bitter" with high cyanogenic glucoside content although many intermediate forms exist. The "sweet" types may be eaten raw or lightly boiled without harm while the "bitter" forma require processing to remove cyanogenic glucosides.
The cassava roots have no natural dormancy and are highly susceptible to deterioration. The nature of this deterioration is as yet not fully understood, although two factors, one an indigenous physiological one and the other associated with microbiological infection, have been identified. The physiological deterioration' of which the symptoms are tissue discolouration, can commence as soon as 24 hours after harvest. Acting independently of, but usually following, the physiological changes, microorganisms, both fungi and bacteria develop in the flesh of the roots causing additional damage and loss: this microbial deterioration usually develops only 5-7 days after harvest. (Booth, 1974; Lozano, Cook and Castano, 1978).
This rapid post-harvest deterioration of cassava roots places serious constraints on their use with fresh produce trade and on the holding of buffer stocks for large-scale processing.
The short storage life of fresh cassava imposes constraints on its distribution and use. As a result, the choice of market for which production is intended becomes influenced by location. The range over which production for the fresh market extends is therefore a function of the distance from their markets and the efficiency of transport. The time of delivery rather than distance per se is the important limiting factor. In the vicinity of Bogota, Colombia, up to an 8-hour interval from harvest to delivery is acceptable. With the current status of road a and vehicles this permits production within a 300 km radius. With more effective preservation or a faster delivery system, this range could be extended. It is claimed that the market for fresh cassava in Colombia is so structured that as quality deteriorates, the price is lowered, thereby enabling roots of decreasing quality to be bought by successively poorer income groups.
Techniques to extend the fresh life of cassava roots have been independently developed in several countries. At the Instituto de Investigaciones Tecnologicas in Colombia deterioration has been delayed by coating tubers with a film of paraffin wax (IIT 1973). At CIAT fresh tubers were shown to have improved keeping characteristics when plant stoma are out and removed but tubers are loft unharvested for up to 14 days. Tubers so treated remained sound for as long as 30 days. This technique has much to commend it, but has not yet been tested on a large scale. Other practices developed in joint CIAT/TPI projects include storage in earth silos' and storage of roots in plastic bags and in absorbent packs. These methods appear to have some utility in small and medium scale production but have not yet been proved in large-scale commercial operations. None of those techniques have as yet attained general application.
The transformation of cassava to more durable forms, frequently in forms more convenient to use than fresh roots, is partly associated with the detoxification process for the removal of hydrocyanic acid. From the crop loss point of view these products are important since they are frequently more able to avoid the rapid deterioration of the fresh tubers. Cassava products may be classified in a variety of ways, but for simplicity these may be regarded as food starch ant whole root products. These latter may be fermented or unfermented and may either be "flour" or "gari" types.
Cassava starch is extracted from roots after pealing, washing, reaping, squeezing settling and drying. Two types of starch are produced, sour or fermented and sweet or unfermented. Sweet starch is dried immediately following extraction while sour stanch is stored for 3-4 weeks when some fermentation involving lactobacilli and yeasts occurs. Sour starch is favoured for baking and commands a better price than sweet starch at least in some countries. This latter has similar conformation to corn starch with which it competes disadvantageously.
Gari, the most commonly used form of cassava in West Africa, accounts for some 70% of the entire cassava production in Nigeria. It is estimated that between 4 and 5 million tone of roots are used each year for this purpose. Although there are close parallels between the production of gari and of Brazilian "farinha de mandioca", the West African variant has some distinctive features.
The effect of the longer fermentation affects the taste by the production of lactic acid, as wall as well as reducing the content of free hydrocyanic acid. Production at levels of indigenous technology is highly labour-intensive, but this constraint is being progressively removed through a variety of innovations aimed at increasing labour productivity. The hygroscopic nature of gari is a major constraint to its keeping quality. 1a a humid atmosphere it can absorb sufficient moisture to make it vulnerable to the growth of fungal organisms. The recent practice of packaging gari dried to a safe moisture contort of around 12% in sealed polyethylene bags enables the product to be kept in good condition for extended periods.
Farinha de mandioca is the Brazilian product from which gari derives. The stages of preparation are similar to those for gari, the roots being washed, peeled, grated, pressed, toasted and classified by size. Unlike gari, the mashed or grated mass is not permitted to ferment. The heating dries the product and allows some gelatinization and dextrinization of the starch as also occurs in gari making.
Other durable cassava products include cassava flour, dried chips and "cossettes' from which flour can subsequently be prepared by milling or grinding.
3.2.2 Tam (Dioscorea Spp.)
With yams, storage life is determined by dormancy since attempts to store yams after sprouting has commenced are impractical. During the period of dormancy the metabolic processes continue at reduced rate thereby keeping dry netter losses at relatively low levels. The various yam species have different periods of dormancy with 3-6 months for D. rotundata and D. alata and 1-2 months with D. cavenensis or D. trifida. Pathological factors are also of great importance in causing postharvest losses in yams.
Traditionally, in West Africa, yams are stored in "yam barns" where good but uncontrolled conditions for storage exist by building the barns in the deepest available shade. This prevents heating by the nun and also helps to maintain high humidity. Several other indigenous yam storage systems are used but moat of these are less satisfactory than the "barn" or "claie verticale".
It is generally believed that these systems are well adept d to the yam varieties and to the climatic conditions prevailing in West Africa and offer little scope for improvement at current levels of technology.
Of the methods used to extend storage only the use of ionising radiation has been an unreserved technical success. The practical application of this technique is not yet generally available. Reduced temperature storage is limited by possibility of chilling damage around 10-12 C and by the growth of fungal organisms at temperatures around 14° C. Treatment with chemicals used to suppress sprouting in other crops has had little success but active research is being pursued by TPI. Controlled gas storage has not thoroughly been investigated.
Transformation of yams to more durable forma is possible but these methods have not yet met with much commercial success. Traditional products such as "amala" are variable. This may be related to the intrusion of undesirable dark colours into the final product from the use of a mixture of varieties and the failure to deactivate the polyphenol oxidases during processing rather than to any inherent basic defect of the process. The attempts of commercial organizations to produce an instant pounded yam failed more as a result of nontechnical problem such as the logistics of raw material supply than to technical defects in the process. New processes recently developed at the University of Ife, Nigeria, may provide the required answer. A yam flake product developed to pilot project level in the West Indies has not continued long in production at a commercial level.
3.2.3 The Potato (Solanum Spp.)
The potato, Solanum tuberosum, originally of highland tropical origin, but developed mainly as a temperate crop, is now being developed as a lowland tropical crop. Selections tolerant to high temperature and short days are being made in several parts of the tropics. At higher elevations in the tropics it is already established as a crop of some importance.
She extension of potato growing to these areas is a major concern of International Potato Centre (Centro Internacional de Papa - CIP). This Centre works together with existing bodies selecting areas for activity on the basis of comparative advantages. This approach forms a rational basis for co-operation and avoids the sometimes unintended, but nonetheless real, tendancy towards replacement of national effort with that of an international organization.
The work on post-harvest lose in this Centre is integrated with that of the social science unit permitting a broader view of the problem. This approach, taking account of social, economic and ecological factors, could provide useful guidelines in the development of other work on post-production losses. An account of the studies in the Mantaro Valley is given by way of an illustration of the CIP approach.
The Mantaro Valley lying some 300 km to the South-East of Lima has been described by Mayer (1979). It is a riverine valley acme 60 km in length and from 2 to 22 ho in breath. Three broad ecological zones are recognized. The valley floor is some 3,200 - 3,450 m above sea level. The eastern slopes between 3,400 and 3,950 m and the western slopes of similar elevation.
Finally a high zone above 3,900 m. The flat riverine soils of the valley floor have a high productivity with a capacity to support a diversified agriculture and a high population density. In this zone a contrast between large and small farm a is noticed. In the zone of sloping land above 3,450 m soil productivity declines as dose the number of crop species. The practices on the East and West slopes differ. On eastern slopes potatoes maintain their place in cropping systems but play a diminished role on western slopes. Under the more severe conditions of the High Zone potatoes become the most important component of the cropping system occupying some 57% of the arable land. These zones are constrained by three main environmental factors: precipitation, temperature and water balance. These are all subject to changes relative to elevation. Generally precipitation and exposure to frost increases with increasing elevation.
Several products are made from potatoes including chuno, papa seca, tungush and potato starch. Ecological zone and economic circumstances result in different comparative advantages between choice of product. The most common product at low elevation is papa seca, a parboiled dried product which requires, fuel for boiling but which utilized what would otherwise be a waste product. This restricts production to the lower zones where fuel supplies are more assured. Hybrid or native varieties are used for papa seca since this process would not be sufficient to remove the glycoalkaloids present in the bitter varieties. Processing into chuno or tongosh which eliminates the bitter principle requires no cooking. The process however requires heavy night frosts and open areas on which to expose the potatoes to natural freeze drying. These conditions are all readily met in the high blue. Locally produced potato starch competes with commercially produced corn starch. It is clear from this example that changes due to the technological superiority of one process over another is unlikely since a different technique is adapted to meet different complexes of factors.
Potatoes are stored for use as seed or for food or sale. Different methods of storage have been described by CIP (Werge, 1977) from which the following summary has been taken. Ho recognizes three main systems: house' out-building and field. Peruvian rural houses according to Stein (1961) appear to have been cosigned with storage of agricultural produce in view. The convenience and security of storage in the house is evident. The actual technique of storage adopted appears related to the itended use for the potatoes. Potatoes to be boiled in their skins are kept is the attic where the conditions permit drying and some shrivelling, causing the potatoes to become sugary and thus more favoured. Potatoes which are to be pealed before cooking may be stored on the ground where they love leas moisture and HO remain firm.
Seed potatoes may be left on the ground on a thin layer of straw or eucalyptus leaves, in piles against the walls of houses or in shallow bins or "trojas". Where animals may have access to them if they are loft on the ground, the potatoes may be placed on a platform or chaclanka of eucalyptus branches in the rafters and covered lightly with et raw.
Field storage is more frequently adopted at higher evaluations. Here strawlined clamps, covered in turn with straw and soil, may be used. The cool moist conditions reduce dehydration but the system is vulnerable to flooding which may result in dramatic crop losses. At elevations of about 3,000 m ventilation with air at ambient temperature assists the storage process.
An interesting series of low-cost potato stores have been developed by the storage unit at CIP. These are developed and adapted to take advantage of prevailing conditions to facilitate storing. Control over the storage environment is attempted by using natural ventilation systems coupled with adequate insulation and shading. Where appropriate humidity modification using simple measures is also being researched. In the case of seed tubers, these are stored in simple low-cost stores exposed to natural diffused light which, to some extent, replaces the need for controlled low temperature storage.
3.2.4 Sweet Potato (Ipomoea batatas (L.)
Sweet potato is a crop of considerable unrealized potential. The roots are widely used as a carbohydrate food and recently attention has also been drawn to the nutritional value of its leaves as food. The ability of the roots to become suberized at high temperature and humidity in the classical curing process, gives the crop potential for extended storage. Storage is particularly successful when kept at relatively cool temperatures, although, it held below 12°C, chilling damage can occur. The most successful storage occurs in the southern states of America and in Japan, and successful storage for extended periods has still to be achieved in the Tropics.
There is interest in transforming the roots to more durable forms. In Trinidad, at the University of the West Indies, a sweet-potato-based flour was shown to have utility in wheat flour substitution in bakery products. There is interest there and elsewhere in the preparation of breakfast foods from sweet potato tubers. The manufacture of dried sweet potato flakes is already an established industry in the United States.
3.2.5 Taro (Colocasia esculenta) and Other Edible Avoids
The edible corms of these crops are normally eaten fresh after boiling or baking. Estimates in Fiji showed high peeling and trimming losses during preparation as food. The fresh corms to not normally remain sound for long periods, and signs of deterioration become apparent a few days after harvest. Observations in Fiji indicated that packaging in plastic bags enhances keeping quality. In the South Pacific it is traditional to keep taro by transforming it into a fermented product of which "Poi" is the Hawaiian variant. It is of interest to note that the presence of oxalic acid raphides in the corms acts as a deterrent to rats.
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