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E11 Staple Foods - Root and Tuber Crops
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Sweet potatoes |
In many tropical areas, people rely on root and tuber crops, such as yams, taro, cassava and sweet potato, to provide the bulk of the carbohydrate, and hence the energy, in their diets. The temperate equivalent of such crops is the potato (Solanum tuberosum).
Plants convert the sugars they produce in the course of photosynthesis into starch as a storage product. Some plants store starch in underground organs such as tubers, rhizomes or corms. They use this starch to fuel growth of new shoots in spring.
Yams
Yams (Dioscorea spp. in the family Dioscoreaceae) are cultivated in tropical America, Asia and Africa, with different species domesticated in each area. The plants are sprawling vines with edible underground stem tubers. The tubers have higher levels of protein than many of the other major root crops (4-8 per cent of fresh weight) and are rich in vitamin C. Just beneath the skin, however, they contain poisonous oxalic acid which is destroyed by peeling and boiling.
The major species cultivated is the white guinea yam (Dioscorea rotundata) from Africa, which was taken to the West Indies as food on slave ships during the sixteenth century. The tubers are long, smooth and cylindrical with a white mealy flesh.
Yams are particularly important in the rainforest zones of West Africa, where local peoples eat between 0.5 and 1 kg each per day. They can be boiled, fried, baked or roasted, or more commonly, boiled peeled yams are pounded to a pulp in a traditional wooden pestle and mortar to make the thick dough known as "fufu". This is eaten with a thick soup made from palm oil, meat or fish together with spices and vegetables.
In West Africa and New Guinea yams are considered so important that they have become the subject of religious cults with their own priesthoods. At the yam harvest, major festivals take place.
Wild yams are used as foods in times of famine. They also contain steroidal compounds which are used as precursors in the manufacture of hormones for oral contraceptives and cortisone creams. In some areas yams are being replaced by other tuber crops which are easier to cultivate or store.
Sweet potatoes
Sweet potatoes are the root tubers of the trailing vine Ipomoea batatas (family Convolvulaceae), which is related to the morning glories and bindweeds. The plant probably originated in Brazil, but had reached the islands of Polynesia before European explorers visited them. The occurrence of the plant on both sides of the Pacific was just one of the pieces of evidence used to support the theory that early peoples had travelled across the ocean and had taken sweet potato with them. In the 1940s Thor Heyerdahl re-enacted the voyage that these people would have made, sailing from Peru on the balsa wood raft, Kon Tiki. There is, however, still much discussion about exactly how the plant reached Polynesia. Sweet potatoes were brought to Europe by Christopher Columbus in 1492, some fifty years before the plant that we now call the potato (Solanum tuberosum).
Sweet potatoes vary considerably in size, shape and colour; in Britain the most readily available generally has pink skin and white flesh. The tubers contain almost as much vitamin A as carrots. In Asia and Africa, dry-fleshed mealy tubers are the most popular, whilst in the USA the soft moist-fleshed varieties form part of the traditional Thanksgiving Dinner, glazed with butter, brown sugar and orange juice. The world’s major producer of sweet potatoes is China and it is the second most important crop in Japan where it is used for the production of starch, wine and alcohol.
Cassava
Cassava, or manioc as it is often known, is made from the root tubers of Manihot esculenta in the family Euphorbiaceae - the same family as rubber (Hevea brasiliensis) and the spurges (Euphorbia species). It is the staple food for over 500 million people, providing 37 per cent of the total calories consumed in Africa and 11 per cent in Latin America. It is, however, low in protein and contains almost no vitamins.
Many different cultivated varieties are grown around the world – for example in Brazil the Yanomami know of 140 cultivars. The cultivars can be divided into two groups - sweet and bitter - according to the amount of toxic compounds that they contain. When cassava skin is damaged prussic acid (hydrocyanic acid) is produced. The toxic compounds occur only in the skin and outer layers of sweet cultivars but are distributed throughout the tubers of bitter cultivars. The bitter cultivars are generally considered to be more nutritious but require careful preparation.
In Brazil, the corky brown skin and the white rind around the starchy centre of the tubers is peeled off. The cassava is then grated and pounded to release all the toxins. The pulp is drained in a long sleeve-like basket called a tipiti and allowed to stand overnight to destroy the toxins. It is then used to make griddle bread which can be stored for long periods or dried, powdered and toasted to produce farinha or farofa. Any residual starch in the juice can be heated on a hot metal plate to produce tapioca, once so popular for making milk puddings. Considerable amounts of cassava starch are broken down into sugars for fermentation to produce fuel alcohol. The watery juice is fermented to produce beer.
Cassava originated in tropical America but was introduced to West Africa in 1558. It is a useful crop plant because it grows well in wet or dry climates, will produce tubers even in poor soils, can be stored in the ground, is resistant to pests and diseases and requires minimum labour for cultivation.
Other tuber crops
Taro and eddoes are crops produced by different varieties of Colocasia esculenta. This species is in the family Araceae, which also contains familiar species such as lords and ladies (Arum maculatum) and the swiss cheese plant (Monstera deliciosa).
Taro, sometimes known as dasheen or cocoyam, is probably native to Polynesia and South-east Asia. It is used as a basic subsistence crop on some of the Pacific Islands (particularly Hawaii, Fiji and New Caledonia) as well as in West Africa. The crop consists of the large starchy corms (short swollen underground stems). These contain calcium oxalate crystals and have to be prepared carefully before they can be used. Poi, the Hawaiian national dish, is a thick glutinous paste made from peeled boiled taro that has been mashed and fermented. In Cypriot cookery, taro is popularly known as kolokassi. Eddoes are smaller corms produced by a different variety of C. esculenta (var. antiquorum) and are popular in Asia and the West Indies.
Xanthosoma sagittifolium, which is also in the Araceae, produces the crop known as new cocoyam, tannia or yautia. Its corms are more nutritious than those of taro or eddoe.
Arrowroot is the fine easily digested starch made from the roots of Maranta arundinacea (family Marantaceae). It was eaten as an invalid food in Europe, and is still used to thicken sauces and to make arrowroot biscuits.
Further information
Wayne's Word. Vegetables from underground