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The genus Dioscorea contains about 600 species, although most of the edible yams are derived from only about 10, and of these the Guinea yam, Dioscorea rotundata Poir. and D. cayenensis Lam. are generally the preferred carbohydrate staple in Africa while D. alata L. and D. esculenta (Lour.) Burk. are the more widely used species in the Caribbean and Pacific (Coursey, 1967). The first three named normally produce annually a single large tuber often weighing from 5 to 10 kilograms while D. esculenta produces a large number of small tubers. Also in widespread cultivation is D. bulbifera L. which forms small aerial tubers or bulbils in the leaf axils. Other edible species include D. hispida Dennst. grown in many parts of Asia, D. dumetorum (Knuth.) Pax. grown in Africa and D. trifida L.f. which is native to central America and the Caribbean. Also D. opposite Thunb. and D. japonica Thunb. are grown in temperate areas of China and Japan.
Yam tubers are organs of dormancy, having evolved to enable the plant to withstand the hot dry season of savanna areas in the dormant state and hence they are inherently well suited to storage in the fresh state when required by man as food. In practice most yams, unlike cassava, are normally stored in the fresh state while only a very small percentage are processed. The storage life varies greatly between species and even between cultivars, but the best keeping cultivars, principally forms of D. rotundata deriving from the drier areas of the savanna, can be stored for 3-4 months and sometimes for longer periods (Coursey, 1967).
The basic principles of successful yam storage are the provision of adequate ventilation, access for regular inspection and protection from direct sunlight (Wilson, n.d.) and many of the more sophisticated techniques incorporate these principles.
The simplest storage technique is to leave the tubers in the ground until needed, as is practiced with other root crops and this method is still used to a limited extent in some of the remoter parts of West Africa. Extending this technique, freshly harvested tubers may be packed in ashes and covered with soil or simply covered with a few inches of soil and grass mulch. Another simple method involves stacking the tubers into small heaps after harvest as is done in some parts of Africa and Asia. Some protection from sun and flooding is often afforded by careful selection of the sites in crevices of rock outcrops or in large trees and the small size of the heaps insures adequate ventilation throughout. However, the yams are very susceptible to termite and rodent attack and also to pilfering when stored in this way. Yams may also be stored in heaps on the floor or on shelves in sheds or huts which may or may not have been constucted for the purpose, or in the case of Southeast Asia where the houses are often built on stilts, underneath the house (Coursey, 1967; Irvine, 1969).
In the Ivory Coast, yams are often stored in small thatched shelters known as koukou, or in small earthen silos (Miège, 1957). The silos are simply hollowed out pits in the ground, the earth which is dug out being used to form a low wall around the edge: these are used mainly for early crop yams harvested before the end of the rainy season which will be kept only for limited periods. A similar technique is used in parts of northern Ghana (Coursey, 1967). The small thatched huts of one metre in height are generally constructed in the shade of large trees and the yams are stacked one on top of the other up to the top of the hut. Hence, only the harder varieties which do not bruise easily can be stored in this way and these stores are said to be conducive to attack by scale insects (Miège, 1957).
The commonest type of store used in West Africa is the yam barn which has been discussed in some detail by Coursey (1967). The barns vary considerably in design and construction between different areas but all consist in principle of a vertical or nearly vertical wooden framework to which the tubers are fastened individually with string or other local cordage material such as raffia. The frames are usually 1-2 metres in height but can be as much as 4 metres high and are from 2 metres upwards in length according the the amount of material to be stored. The vertical posts of the frame are often made from timbers which when left unbacked will take root when set in the ground, for example, species of Dracaena, Gliricidia, Ximenia and Gmelina. This reduces the risk of collapse as a result of termite attack or rotting and also helps to provide shade. Poles of 5-10 cm in diameter are placed in the ground about 1 metre apart and cross members of lighter wood, bamboo or palm-leaf midribs (Elaeis or Raphia species) attached, and finally to these are fastened lighter vertical sticks providing a fairly rigid structure.
There are two variations to this basic design which should be mentioned. One which is widely used in Ghana, involves the use of a larger number of vertical poles set closer together in the ground, only 10-30 cm apart with cross members as before, but avoiding the need for the secondary sticks. In the second, paired cross members are fixed parallel to one another either side of the vertical poles or on two poles set side by side. This means the yams can be tied perpendicularly to the frame instead of parallel to it, so increasing the storage capacity of the barn.
Two or more frames may be erected alongside each other and surrounded by a fence or hedge for security, or four frames may be constructed to form a rectangular enclosure with the yams tied only to the inner walls for security. A thatched roof may be added for shade or the branches sprouting from the "live" poles may provide the only shade. Barns are often built, however, under the shade of forest trees. Although a short period of exposure to the sun may facilitate the "curing" of yam tubers (Been et al., 1977), it is essential that they be protected from direct insolation for long term storage (Coursey and Nwankwo, 1968; Rickard and Coursey, 1979).
Essentially similar, but often elaborately ornamental structures are found in parts of Oceania such as Papua New Guinea, Trobriand Islands and New Caledonia (Barrau, 1956; Girard, 1967). Barrau (1956) describes a structure consisting of a platform of poles supported on vertical poles about half a metre above ground level and shaded by a thatched roof. The yams are stacked vertically on the platform held in place by a rim of horizontal poles around the edge of the platform. The same author also mentions a simpler method used in Vanuatu whereby individual tubers are suspended from a horizontal pole supported about 1 or 2 metres above the ground by 2 forked sticks set in the ground: a technique occasionally used in West Africa. In Papua New Guinea huts of two floors are sometimes used, closed in on 3 sides by walls of leaves and reeds and supported above the ground on poles. The yams for planting are placed on the lower platform while those for eating are placed on the upper floor. Yam tubers may also be suspended on hooks from the roof (Girard, 1967).
The storage life of yams is finally terminated by the breaking of dormancy and subsequent sprouting but storage of tubers for food use can be extended by as much as a month by breaking of the emergent sprouts when they are 20-30 mm long (Coursey, 1981a). Most farmers in traditional yam-growing societies are well aware that only sound, healthy tubers are suitable for storage and reject others for immediate consumption or processing. Further, they know what has only recently (Passam et al., 1976) been scientifically established, that a bruise or abrasion is far more likely to lead to decay in storage than a clean cut. It is normal traditional practice to cut away any bruised or decayed portions, and often rub the clean wound with alkaline material (lime, chalk or wood ashes) to discourage reinfection.
In both West Africa and Melanesia, yam is essentially a man's crop, and this extends to the post-harvest sector. Indeed, in most of West Africa, a well-built and well stocked yam barn is one of the major factors through which a man gains prestige in his community, although women, especially widows, also grow and store yams in their own right. The sexual tabu is much stronger in Melanesia, where women are usually totally excluded from all operations, pre- or post-harvest, connected with the ritually important D. alata, although they are allowed to grow and store D. esculenta, which although less highly regarded, probably makes a greater contribution to the total diet.
In most yam-growing areas the major protion of the yam crop is stored and transported in the fresh state and in West Africa long distance trade in fresh yams has existed for centuries, in contrast to the cassava trade which is generally in the processed product and is a recent development (Coursey, 1978b). A small proportion of the yams are, however, processed into a dried form and yam flour is particularly popular in Yoruba speaking districts of West Africa, although in other areas generally only those yams which are misshapen, damaged or partially decayed are processed. The whole topic of yam processing, both traditional and nontraditional, has recently been reviewed by Coursey and Ferber (1979).
Yam flour is prepared by cutting the tubers into slices of about 1 cm thickness, peeling the slices and sun-drying them. Slices may be boiled or parboiled before sun-drying to soften the tissues giving a more palatable product. After drying the pieces are ground in mortars to give a coarse flour or may be stored as pieces until needed. The main problems of storage of the pieces or flour are attacked by insects, most commonly Araecerus fasciculatus De G. and Sitophilus zeamays Mots. and rodent attack of unmilled pieces can be severe. The flour is prepared for consumption by reconstituting in boiling water to form a paste (Coursey, 1967). Preparation of dried yam slices and flour is also practiced in other parts of the world including Indonesia (Ochse, 1931), Madagascar (Decary, 1946) and parts of the Far East (Clemente, 1918).
Some edible yams such as D. dumetorum (Knuth.) Pax. and D. hispida Dennst., are highly toxic due to presence of alkaloids of the dioscorine group and consumption of these without careful preparation can be fatal. Traditionally, the toxicity is overcome by soaking the sliced or grated tubers so that the alkaloid leaches out into the water. Sulit (1932) describes a technique used in the Philippines for D. hispida. Thin slices of peeled tuber are placed in a basket and submerged in the sea or a solution of salt water for 2 to 3 hours. They are then removed and squeezed under weights for a few hours and then replaced in the baskets and left in a running stream for 36 to 48 hours with occasional stirring. The slices are tested for toxicity by squeezing a drop of liquid into the eye and if the eye smarts, soaking is continued for a further period. Similar techniques are used for
D. dumetorum in West Africa. The soaked yams are prepared for consumption by flavouring with coconut and sugar. Other techniques reported from India, Solomon Islands and Madagascar, for example, involve boiling the yams before immersing in running water either whole, mashed or sliced (Barrau, 1956; Decary, 1946; Karnik, 1969). Wood ashes or tamarind are also sometimes added to the water for boiling in parts of India as an aid to the removal of the acridity of the toxic yams (Karnik, 1969).
Non-toxic yams are prepared for consumption by a variety of ways, often simply by boiling, roasting or frying and may be eaten raw (Decary, 1946; Coursey, 1967; Ochse, 1931). For boiling, the yams are usually peeled and cut into pieces and boiled until soft, although smaller tubers may be left whole and unpeeled (Coursey, 1967). Yams are generally roasted by simply placing the unpeeled tubers in the ashes of a fire, although in the Pacific Islands the tubers are usually cut into pieces or, less commonly, grated and wrapped in green leaves often with other ingredients such as coconut cream and chicken or fish and roasted in stone ovens (Barrau, 1956; Malcolm and Barrau, 1954). Frying pieces of yam in vegetable oil is common practice; in West Africa, palm oil is frequently used, the pieces usually having been boiled or parboiled first, although the most important edible product prepared in this area from yams is fufu which is eaten as an accompaniment to stew (see also sections on cassava, plantains, cocoyams). The tubers are peeled, cut into pieces and boiled until soft. The water is then drained off and the pieces pounded in a wooden mortar and pestle until a stiff glutinous dough is formed, usually taking 15-30 minutes (Coursey, 1967).
The sweet potato (Ipomoea batatas (L.) Lam. is grown throughout the tropics and to a greater extent in some warmer temperate areas such as southern United States, southern China, Japan and New Zealand. In tropical regions the fresh tuber is generally considered to be difficult to store, especially in areas where pre-harvest attack by Cylas weevils is common and so storage is usually avoided by "mumuting", or progressive harvesting of the crop only when it is needed (Siki, 1979). There are few examples of storage techniques in these areas (Keleny, 1965). It is in the temperate areas where the tuber is grown as a summer crop that the more elaborate storage techniques have been developed in order to protect the sweet potato from cold during the winter (Cooley, 1951), the tubers being susceptible to chilling injury at about 12°C. A period of curing at high temperature and humidity before storing is also necessary to encourage the development of a suberized layer in any wounds present and so inhibit invasion by pathogens. A technique which meets the requirements of curing and protection from the cold has been used by the Maoris in New Zealand for centuries (Cooley, 1951).
The Maoris traditionally stored sweet potatoes in specially constructed underground storage houses, dug into the side of a hill. The floor is covered with a layer of gravel and rotten wood or dunnage of dried manukau (Leptospermum species or fern brush) and the tubers placed on top. The seed stock is placed in first at the back of the store with the food tubers in front, the two types being separated by fern leaves. Any cut or bruised tubers are placed nearest the entrance so that they are used first and the whole store is then sealed and left for some time, presumably to allow curing to occur under the influence of respiratory heating, before any tubers are removed for use (Best, 1925; Cooley, 1951; Keleny, 1965 ) .
Pit storage of sweet potatoes is also practiced in Simbabwe (Blyth, 1943) and Malawi (Anonymous, 1949) where the tubers are placed in pits with alternate layers of wood ash and in Papua New Guinea where the tubers are alternated with layers of grass in grass-lined pits (Siki, 1979) . The Kakoli people of the Kaugel Valley in Papua New Guinea store the tubers in groups of 3 or 4 tunnels dug into natural banks, each tunnel capable of holding about 11 kg of tubers and sealed with turf (Yen, 1974). The Bantoc of Luzon in the Philippines often adapt their wooden rice storehouses for the storage of sweet potato tubers, with additional space provided in the form of plank bins (Yen, 1974).
Sweet potato tubers are often stored for short periods of two to three weeks before use (Hrishi and Balagopal, 1979; Kimber, 1972; Siki, 1979; Villanueva, 1979). For example, in Papua New Guinea tubers are placed on platforms in the house in a dark, well-ventilated area where heat or smoke from the cooking fires appears to aid curing of the tubers (Siki, 1979) . Storage for a few days is said to improve the eating quality due to loss of moisture giving a greater energy value per unit weight and also to allow hydrolysis of starch to sugars (Kimber, 1972).
Owing to the perishability of the crop, in parts of East Africa where there is a pronounced dry season a proportion of the sweet potato crop is peeled, sliced and sun-dried for storage (Acland, 1971; Aldrich, 1963; Allnutt, 1942; MacDonald, 1970). In Tanzania, the tubers are boiled for an hour before drying and the dried product is said to be capable of being stored for two years without deteriorating (Allnutt, 1942). When needed for consumption the dried pieces are washed and boiled (Allnutt, 1942) or ground into a flour for making local dishes (Acland, 1971) .
The Maoris of New Zealand also stored some of the sweet potato crop in the dry state, two methods of preparation being known (Best, 1925). In the first, the tubers are simply scraped and sun-dried and either stored or prepared for consumption immediately, after first soaking and mashing with warm water to form a gruel. In the second method, roots from storage pits are oven-cooked and then dried (Best, 1925). The production of dried sweet potato as a snack is still practiced in parts of New Zealand (Yen, 1974). In the Philippines, dried flakes of sweet potato are prepared for storage and pounded into a flour as required for use in the preparation of a gruel. A popular dish is prepared by mixing the flour with water, and sometimes with sugar, and making small balls of dough which are wrapped in sugar cane leaves and boiled (Yen, 1974). The production of sweet potato flour is also commonly practiced in China (Yen, 1974).
There are no particularly elaborate processing techniques traditionally associated with the sweet potato, the tubers generally being prepared for consumption by boiling, steaming, baking or frying. In the islands of the South Pacific unbolted tubers are usually baked or braised in an oven or peeled tubers are boiled to be eaten as they are or mashed with coconut milk. Sliced tubers are also sometimes mixed with other tubers and green leaves, moistened with coconut milk, wrapped in a banana leaf and braised in an oven (Massal and Barrau, 1955c).
Roasting peeled or unpeeled tubers in the ashes of a fire is popular in parts of East Africa where they are usually eaten alone or perhaps with milk; however, boiling or steaming tubers is more common (Acland, 1971; Goode, 1974). Occasionally, slices of sweet potatoes are fried with other root crops, cereals or vegetables (Acland, 1971) . Ochse ( 1931) describes several dishes made in Indonesia, for example, in Java getook is made by pounding boiled roots with grated coconut and is eaten with sugar and salt; while groobi consists of peeled tubers cut into fine pieces, fried until hard and dry, dipped in a solution of sugar, fried again until dry and finally scooped into a banana leaf and pressed.
Sweet Potato tubers are also sometimes used as a substitute in the more complex processing techniques normally applied to other starchy root crops. For example, the tubers are used by Amerindians for making an alcoholic beverage normally prepared from macerated cassava (Yen, 1974), while in Hawaii sweet potato tubers are sometimes used instead of taro to make poi (Handy, 1940). (see Aroid Root Crops).
The most important food crops within the family Araceae belong to the two species Colocasia esculenta (L.) Schott (taro, "old" cocoyam, dsaheen eddoe) and Xanthosoma sagittiforium (L.)
Schott ("new" cocoyam, tannia). The edible portion of the plant is a corm or a group of small corms or cormels found largely or entirely underground, although the leaves of several species are also eaten as green vegetables.
The cultivars of C. esculenta can be grouped roughly into two types: one with a relatively small corm surrounded by large, well-developed cormels, known in the West Indies as the eddoe and the other, known as dasheen in the West Indies which has a large central corm and few side cormels. However, the names dasheen and eddoe are not always strictly applied to these groups in this way, while further there are intermediate forms, and in the South Pacific islands the term taro is used for both types. To add to the confusion the name cocoyam in West Africa is used to describe both Colocasia and Xanthosoma although "old" cocoyam is sometimes used for the former and "new" cocoyam for the latter. Purseglove (1968) considers it most reasonable to differentiate two varieties of C. esculenta, var. esculenta for the dasheen type and var. antiquorum for the eddoe type.
Colocasia and Xanthosoma are particularly important food crops in the islands of the South Pacific, the Carribbean and West Africa (Alexander, 1969; Coursey, 1968; Lambert, 1979; Massal and Barrau, 1955b) and Colocasia is also a staple food in parts of the Philippines where it is known as gabi (Pardales, 1980). Other genera, Alocasia, Amorphophallus and Cyrtosoerma also contain edible species but these are of localized importance only.
There are few descriptions of traditional storage systems for aroids. Normally the corms are consumed shortly after harvest so avoiding the necessity for storage and similarly to other root crops some aroids such as Colocasia and Amorphophallus can be left unharvested in the ground until needed (Lewis, 1976; Plucknett, 1970; Plucknett and White, 1979). Cyrtosperma, in particular, is usually left in the ground for several years, for its gigantic cormous system to attain its full size of 50 to 100 kg (Plucknett, 1977). Reports of postharvest storage life vary considerably from several months to less than a week, experimental work having demonstrated severe losses due to sprouting and diseases, but reported storage losses vary considerably from country to country and also appear to vary with cultivar (Baybay, 1922; Gollifer and Booth, 1973; Praquin and Miche, 1971). The variation may be due, at least in part, to variations in the degree of dormancy of the corms at the time of harvest.
For storage for short periods of 2 to 4 weeks in the Philippines, Colocasia corms are tied in bundles and hung in the shade or left in the baskets or jute sacks in which they were transported from the field, or simply left in a pile in the shade (Villanueva, 1979). For longer term storage of up to 6 months corms may be placed with alternate layers of grass, straw or leaves and covered with a final layer of leaves and soil either in piles above ground or leaflined pits below. These techniques are used in Papua New Guinea, Nigeria and China (Nwana and Onochie, 1979; Plucknett and White, 1979; Siki, 1979). In India Amorphonhallus corms are said to keep for months after dipping in a cow dung slurry and ash (Hrishi and Balagopal, 1979). Sometimes in China a special enclosure is built within the house into which alternate layers of taro corms and soil are placed (Plucknett and White, 1979), while in Nigeria, Colocasia and Xanthosoma corms are often stored in specially constructed barns made from palm (Nwana and Onochie, 1979).
For immediate consumption aroid corms are normally boiled, baked, roasted or fried, a thorough cooking for several hours often being needed to remove the irritating effect of calcium oxalate raphides and other acrid principles present in many species (Sakai, 1979). In Southeast Asia, tamarind or lime are often added when cooking Colocasia or Alocasia corms to counteract the acridity (Allen, 1940; Ghani, 1982). The aroids are so ubiquitous in the humid tropics that there are numerous local variations to the dishes prepared from them. For example in Fiji and other Pacific Islands, Colocasia is often grated and mixed with coconut milk and then wrapped in leaves before boiling or baking, or peeled corms may be boiled or steamed and then pounded and made into balls to be eaten with coconut milk and sugar (Anonymous, 1951; Greenwell, 1947; Harwood, 1938; Massal and Barrau, 1955b; Parham and Raiqiso, 1939). Similar dishes are commonly prepared in Java where fried corms are also prepared (Ochse, 1931). In West Africa Xanthosoma cormels are used to prepare fufu by pounding boiled pieces into a doughy mass often eaten with soup (see also cassava and yam) (Karikari, 1971; Pele and Berre, 1967). In Egypt, cubes of Colocasia corms are commonly given an initial washing and soaking in warm water for 15 minutes or are washed and lightly fried in order to remove the mucilaginous material and are then prepared for consumption by cooking with meat to which either onion and tomato or garlic and chard are added (Warid, 1970).
A fermented product known as poi is prepared in Hawaii from Colocasia. Traditionally, it was prepared by the men as it was said to be too important to be left to women. Traditionally, corms are first baked or steamed, then peeled and pounded with a stone pestle in a long, shallowly hollowed-out board or stone. Water is added gradually until the correct consistency is obtained after which time the dough is placed in a calabash and left to ferment for several days, the extent of the fermentation being varied according to taste (Greenwell, 1947; Stewart, 1928 (in Allen and Allen, 1933)). Poi prepared in this manner will keep for only a few days but is said to keep for months if prepared without adding water and pounding, so obtaining a harder, drier product, which is diluted with water only when needed (Stewart, 1928 (in Allen and Allen, 1933)). A similar preservation technique is used on Rapa in the Anuta Islands with Colocasia and with Cyrtosperma on Kiribati (formerly Gilbert Islands) (Massal and Barrau, 1955b). Again in the Anuta Islands, Colocasia is made into the fermented ma by placing grated corms in leaf lined pits which are sealed over and the material left to ferment (Yen, 1973b) (see also sections on cassava, breadfruit, banana).
In some regions a storable product is obtained from aroids by sun-drying the corms as, for example, in Papua New Guinea where the Colocasia corms are processed in this way when there has been a particularly good harvest (Ochse, 1931). In Kiribati, Cyrtosperma tubers are sometimes scalded, chopped and sun-dried and are said to keep for several months (Massal and Barrau, 1955b). In Japan Amorphophallus rivieri Durieu is stored in the form of a flour konnyaku or konjac (Chevalier, 1931; Motte, 1932). The tubers (which consist of mannans, not starch) are first peeled, washed and cut into pieces which are then skewered on lengths of bamboo to dry in the sun for about a week. The dried pieces are broken into fragments known as arako and further pulverized into flour. The flour is prepared for eating by mixing with water to form a paste which is mixed with slaked lime and water and boiled until it forms a gelatinous mass. This can be eaten in this form or processed further to produce another storable dry product, by cutting the dough into pieces and placing these in hot ashes for 5 to 6 days. The pieces are then left to dry for 2 weeks and are said to keep indefinitely in this form (Motte, 1932). Both Colocasia and Xanthosoma are used in West Africa to prepare sun-dried chips, known in Nigeria as achicha (Nwana and Onochie, 1979), essentially similar to cassava kokonte: like the cultivation of these crops, this is essentially a woman's function in this part of the world.
* Mannana are starch-like compounds which are formed, however, by the condensation of mannose instead of glucose and similarly under hydrolysis split into mannose rather than glucose.
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