Dioscorea rotundata
Encyclopedia
Dioscorea rotundata, the "white yam", is a species of yam
native to Africa
. It is one of the most important cultivated yams, along with the Dioscorea cayenensis. It is sometimes known as the "Guinea Yam" D. rotundata can be defined as a group of cultivated Dioscoreaceae yams of African origin belonging to the botanical section Enantiophyllum Uline, with a short annual vegetative cycle (6–8 months), a tuber with a long dormancy period (3–5 months) and a slightly or non-pigmented (creamy or white) flesh. We thus make a distinction between D. rotundata and D. cayenensis. These two yams probably have a common ancestor at a yet undetermined phylogenetic level, but only the former is involved in domestication in West Africa
. Dioscorea rotundata is grown in West Africa especially Nigeria. It is about 1.6m in height and weighs about 2–5 kg depending on size. The tuber has a rough skin usually dark to light brown in color. This rough skin can be peeled with minimal degree of difficulty. The yam become edible only if it is well washed and properly cooked. These steps are necessary in order to reduce the anti-nutritional componenets of yam before consumption. Planting of yam tuber starts from march and harvesting is between September and October and late harvesting is between November and December for those not living in coastal region. White yam consists of starch, which is the only quantitatively important digestible polysaccharides being regarded nutritionally superior to low molecular weight carbohydrate or sugar. Studies have shown that D. rotundata yams have distinctive genetic traits, thus separating them from the other yams, while still having some diversity. This finding suggests that this type of yam is of hybrid origin, although other hypotheses are not excluded. At present, D. rotundata yams are widely regarded as being tetraploid, but this preconception could be challenged in the future.(Dainou et al.)
The West African area in which D. rotunda is grown has changed over the last few decades. In southeastern Nigeria, commercial agriculture has pushed the yam cropping area northwards, while production has developed in Burkina Faso along the Ghanaian border in the vicinity of Ouagadougou, the country's capital and largest city. Urban market demand has also prompted D. rotunda production to southern Benin. The same phenomenon is apparent in the extreme southern part of Mali, in the Segou region (near Bamako), and in the area bordering on Senegal, which supplies the Dakar market.
Yams are also reported to be commercially cropped in savannah areas of the Central African Republic and southern Chad (Mbailao Kemdigao, 1998). Few inventories have been made of the plant material used, so the proportion of D. rotunda yams cropped is unknown.
Fresh tubers are used to prepare traditional food dishes. From Cote d' Ivoire to Nigeria, yams are most appreciated eaten in pounded form called futu or fufu, a dough obtained by mortar pounding tubers that have been boiled until soft. These countries account for over 90% of D. rotundata production in Africa (according to FAOSTAT, 2003). North of this area and throughout Central Africa, yam preparation is simpler, the tubers are just boiled and served with various stews. Recent studies have shown that most inhabitants of urban areas of West Africa eat boiled or fried yams, often as a snack away from home (Bricas et al., 2003). The use of yam flour (produced by milling dried chips) is another emerging habit. Yam flour is very well adapted to urban cooking requirements and is used to prepare a dough called Amala, a staple or occasional food for about 50% of the population of Cotonou (Benin) and towns of southwestern Nigeria. Amala is not perceived as a substitute for fufu but rather as a food in its own right (Bricas et al., 1997).
Polyphenol oxidation is responsible for browning of the flesh when the tuber is cut. In traditional cuisine, this deterioration in quality seems to be specific to cultivars with a high mucilage content (Dumont, personal observation). The same alteration occurs in food prepared from dried yam chips. What starts as a pinkish-white flour ends up as a light or dark brown dough called Amala whose tast is modified to various extents by lactic fermentation. The intensity of these phenomena depends on the cultivar used, the length of soaking after parboiling and drying. Many D. rotundata yams are more or less bitter. The bitterness is usually greater at the tip of the tuber, which is why this part is often removed before pounding. Physiological immaturity of the tuber and unfavorable environmental conditions (drought, waterlogging) are factors that increase the bitterness. The latter is due to the presence of saponins and tannins, which both occur in the tuber flesh in the form of several chemical compounds (Osagie, 1992).
Yam (vegetable)
Yam is the common name for some species in the genus Dioscorea . These are perennial herbaceous vines cultivated for the consumption of their starchy tubers in Africa, Asia, Latin America and Oceania...
native to Africa
Africa
Africa is the world's second largest and second most populous continent, after Asia. At about 30.2 million km² including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area...
. It is one of the most important cultivated yams, along with the Dioscorea cayenensis. It is sometimes known as the "Guinea Yam" D. rotundata can be defined as a group of cultivated Dioscoreaceae yams of African origin belonging to the botanical section Enantiophyllum Uline, with a short annual vegetative cycle (6–8 months), a tuber with a long dormancy period (3–5 months) and a slightly or non-pigmented (creamy or white) flesh. We thus make a distinction between D. rotundata and D. cayenensis. These two yams probably have a common ancestor at a yet undetermined phylogenetic level, but only the former is involved in domestication in West Africa
Africa
Africa is the world's second largest and second most populous continent, after Asia. At about 30.2 million km² including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area...
. Dioscorea rotundata is grown in West Africa especially Nigeria. It is about 1.6m in height and weighs about 2–5 kg depending on size. The tuber has a rough skin usually dark to light brown in color. This rough skin can be peeled with minimal degree of difficulty. The yam become edible only if it is well washed and properly cooked. These steps are necessary in order to reduce the anti-nutritional componenets of yam before consumption. Planting of yam tuber starts from march and harvesting is between September and October and late harvesting is between November and December for those not living in coastal region. White yam consists of starch, which is the only quantitatively important digestible polysaccharides being regarded nutritionally superior to low molecular weight carbohydrate or sugar. Studies have shown that D. rotundata yams have distinctive genetic traits, thus separating them from the other yams, while still having some diversity. This finding suggests that this type of yam is of hybrid origin, although other hypotheses are not excluded. At present, D. rotundata yams are widely regarded as being tetraploid, but this preconception could be challenged in the future.(Dainou et al.)
Names
The igbos of Eastern Nigeria call white yam Ji or Ji ukom, the Yorubas of Western Nigeria call it Isu ewura and Hausas of Northern Nigeria call it Doya. Yam in not only a preferred high energy food, but a king crop tied up with the socio-cultural life of the people in West Africa especially Nigeria.Geographical distribution
The African yam belt was described by Coursey as lying between western Cameroon and the Banfama River in Cote d' Ivoire, in the climatic area bounded by the 800mm isohyet in the north and extending southwards to the Atlantic Ocean. The area in which D. rotunda production is economically important does not fall exactly within these boundaries (figure 2). It starts in western Cameroon but extends as far as Upper Guinea, with a relatively empty space between the western border of Cameroon and Bandama River. It is highly concentrated in the savannah areas between latitudes 6 and 10 degrees North.The West African area in which D. rotunda is grown has changed over the last few decades. In southeastern Nigeria, commercial agriculture has pushed the yam cropping area northwards, while production has developed in Burkina Faso along the Ghanaian border in the vicinity of Ouagadougou, the country's capital and largest city. Urban market demand has also prompted D. rotunda production to southern Benin. The same phenomenon is apparent in the extreme southern part of Mali, in the Segou region (near Bamako), and in the area bordering on Senegal, which supplies the Dakar market.
Yams are also reported to be commercially cropped in savannah areas of the Central African Republic and southern Chad (Mbailao Kemdigao, 1998). Few inventories have been made of the plant material used, so the proportion of D. rotunda yams cropped is unknown.
Dioscorea rotunda vs. Dioscorea cayenensis
There has long been considerable confusion regarding the yams Dioscorea rotunda Poir. and D. cayenensis Lam. In English-speaking West Africa, particularly Nigeria, they are known as 'white yam' and 'yellow yam', respectively, and pooled under the term 'Guinea yam'. Farmers in French-speaking Africa, on the other hand, do not make a clear distinction between D. rotundata and D.cayenensis, whereas a generic name is used for all other cultivated yams (D. alata, D. bulbifera, D. dumetorum, D. esculenta) although the latter are not regarded as 'true yams' by many ethnic groups. D. rotundata and D.cayenensis are yams domesticated from wild Dioscoreaceae of the Enantiophyllum Uline section that have speciated in Africa. They differ with respect to various botanical and genetic traits but have never been definitively separated. It is thus essential to present D.cayenensis before investigating D. rotundata.Food uses
Polyphenol oxidation is responsible for browning of the flesh when the tuber is cut. In traditional cuisine, this deterioration in quality seems to be specific to cultivars with a high mucilage content (Dumont, personal observation). The same alteration occurs in food prepared from dried yam chips. What starts as a pinkish-white flour ends up as a light or dark brown dough called Amala whose tast is modified to various extents by lactic fermentation. The intensity of these phenomena depends on the cultivar used, the length of soaking after parboiling and drying. Many D. rotundata yams are more or less bitter. The bitterness is usually greater at the tip of the tuber, which is why this part is often removed before pounding. Physiological immaturity of the tuber and unfavorable environmental conditions (drought, waterlogging) are factors that increase the bitterness. The latter is due to the presence of saponins and tannins, which both occur in the tuber flesh in the form of several chemical compounds (Osagie, 1992).