Yam (vegetable)
From Wikipedia, the free encyclopedia
For the vegetable sometimes called yam in the United States, see sweet potato. For the vegetable called yam in New Zealand, see Oxalis tuberosa.
| Yam | |
|---|---|
| Yams at Brixton market | |
| Scientific classification | |
| Kingdom: | Plantae |
| (unranked): | Angiosperms |
| (unranked): | Monocots |
| Order: | Dioscoreales |
| Family: | Dioscoreaceae |
| Genus: | Dioscorea |
| Top 20 Producers of Yams - 2011[1] (million metric ton) | |
 Nigeria | 37.1 |
 Ghana | 6.2 |
 Ivory Coast | 5.5 |
 Benin | 2.3 |
 Togo | 0.7 |
 Cameroon | 0.5 |
 Central African Republic | 0.45 |
 Chad | 0.4 |
 Papua New Guinea | 0.39 |
 Cuba | 0.349 |
 Colombia | 0.346 |
 Ethiopia | 0.33 |
 Haiti | 0.32 |
 Brazil | 0.24 |
 Gabon | 0.20 |
 Japan | 0.17 |
 Jamaica | 0.13 |
 Venezuela | 0.10 |
 Burkina Faso | 0.099 |
 Democratic Republic of the Congo | 0.093 |
| World Total | 50.0 |
| Source: UN Food & Agriculture Organisation (FAO) | |
Yam is the common name for some plant species in the genus Dioscorea (family Dioscoreaceae) that form edible tubers. These are perennialherbaceous vines cultivated for the consumption of their starchy tubers in Africa, Asia, Latin America, the Caribbean and Oceania. There are manycultivars of yam. Although the sweet potato (Ipomoea batatas) has also been referred to as a yam in parts of the United States and Canada, it is not part of the family Dioscoreaceae, rather it is in the Morning glory family Convolvulaceae.
The true yam is a versatile vegetable. It can be barbecued; roasted; fried; grilled; boiled; baked; smoked and when grated it is processed into a dessert recipe. Yams are the staple crop of the Igbo people of Nigeria, in their language it is known as ji, and they commemorate it by having yam festivals known as Iri-ji or Iwa-Ji depending on the dialect.
Yams are a primary agricultural and culturally important commodity in West Africa, where over 95 percent of the world's yam crop is harvested. Yams are still important for survival in these regions. Some varieties of these tubers can be stored up to six months without refrigeration, which makes them a valuable resource for the yearly period of food scarcity at the beginning of the wet season. Yam cultivars are also cropped in other humid tropical countries.
Yam tubers can grow up to 1.5 meters (4.9 feet) in length[2] and weigh up to 70 kilograms (154 lb) and 3 to 6 inches high. The vegetable has a rough skin which is difficult to peel, but which softens after heating. The skins vary in color from dark brown to light pink. The majority of the vegetable is composed of a much softer substance known as the "meat". This substance ranges in color from white or yellow to purple or pink in mature yams.
Because of their abundance and importance to survival, yams were highly regarded in Jamaican ceremonies and constitute part of many West African ceremonies.[3] Certain species of yams are a competing phytochemical source. Yams are used as food plants by the larvae of some Lepidopteraspecies including Palpifer sordida.
Nutritional value
| Nutritional value per 100 g (3.5 oz) | |
|---|---|
| Energy | 494 kJ (118 kcal) |
| Carbohydrates | 27.9 g |
| - Sugars | 0.5 g |
| - Dietary fiber | 4.1 g |
| Fat | 0.17 g |
| Protein | 1.5 g |
| Vitamin A equiv. | 7 μg (1%) |
| Thiamine (vit. B1) | 0.112 mg (10%) |
| Riboflavin (vit. B2) | 0.032 mg (3%) |
| Niacin (vit. B3) | 0.552 mg (4%) |
| Pantothenic acid (B5) | 0.314 mg (6%) |
| Vitamin B6 | 0.293 mg (23%) |
| Folate (vit. B9) | 23 μg (6%) |
| Vitamin C | 17.1 mg (21%) |
| Vitamin E | 0.35 mg (2%) |
| Vitamin K | 2.3 μg (2%) |
| Calcium | 17 mg (2%) |
| Iron | 0.54 mg (4%) |
| Magnesium | 21 mg (6%) |
| Manganese | 0.397 mg (19%) |
| Phosphorus | 55 mg (8%) |
| Potassium | 816 mg (17%) |
| Zinc | 0.24 mg (3%) |
| Link to USDA Database entry Percentages are relative to US recommendations for adults. Source: USDA Nutrient Database | |
The protein content and quality of roots and tubers is lower than other food staples. Of all roots and tubers, the protein content of yam and potato is the highest, being approximately 2 percent on a fresh weight basis. Yams, with cassava, provide a much greater proportion of the protein intake in Africa, ranging from 6 percent in East and southern Africa to about 16 percent in humid West Africa.
Yam, like other root crops, is not a good source of essential amino acids. It is rich in phenylalanine and threonine but limiting in the sulphur amino-acids, cystine and methionine and in tryptophan. Yam consuming areas of Africa have a high incidence of kwashiorkor, a serious medical condition in children caused by protein deficiency. Experts emphasize the need to supplement a yam-driven diet with more protein-rich foods in order to support active and healthy growth in infants.[13][14]
Except for potassium, vitamin B6 and vitamin C, yam is a food with low nutrient density.[15]
Yam provides around 110 calories per 100 grams. Yam is high in vitamins C and B6, potassium, manganese and dietary fiber while being low in saturated fat and sodium. A product that is high in potassium and low in sodium is likely to produce a good potassium-sodium balance in the human body, and so protects against osteoporosis and heart disease.[16]
Yam products generally have a lower glycemic index than potato products.[17]
The African yam (Dioscorea sp) contains thiocyanate; it was suggested in one 1986 paper that potentially it is protective against sickle cell anemia (SCA).[18] The paper points out that SCA is known to be relatively rarer in Africans than in the African-American population of the United States, and suggests that the higher thiocyanate content of the African diet explains the high incidence of SCA among African-Americans and its rarity in Africans.
- Nutritional significance of yam
Yam is an extremely important element of the diet of the people of Nigeria and West African countries. It contributes more than 200 calories per person per day for more than 150 million people in West Africa, and is an important source of income. Yam is an attractive crop in poor farms with limited resources. It is rich in starch, and can be prepared in many ways. It is available all year round making, unlike other, unreliable, seasonal crops. These characteristics make yam a preferred food and a culturally important food security crop in some sub-Saharan African countries.[19]
[edit]Phytochemical value
The tubers of certain wild yam, a variant of kokoro yam and other species of Dioscorea, such as Dioscorea nipponica, are a source for the extraction of diosgenin, a steroid sapogenin. The extracted diosgenin is used for the commercial synthesis of cortisone, pregnenolone, progesterone, and other steroid products.[20] Such preparations were used in early combined oral contraceptive pills.[21] The unmodified steroid has estrogenic activity.[22]
[edit]Comparison to other staple foods
The following table shows the nutrient content of yam and major staple foods in a raw harvested form. Raw forms, however, aren't edible and can not be digested. These must be sprouted, or prepared and cooked for human consumption. In sprouted or cooked form, the relative nutritional and anti-nutritional contents of each of these staples is remarkably different from that of raw form of these staples reported in the table below.
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