Learn more about Yam (vegetable)
Yam is the common name for members of the genus Dioscorea (family Dioscoreaceae). There are more than 600 species of yam. Some species are cultivated for the consumption of their starchy tubers in Africa, Asia, Latin America and Oceania. They are used in a similar fashion to potatoes and sweet potatoes. There are hundreds of cultivars (varieties) among the cultivated species.
Yam tubers can grow up to seven feet (approx. two meters) in length and weigh up to 150 pounds (68 kg). The yam has a rough skin which is difficult to peel, but which softens after heating. Yam skins vary in color from dark brown to light pink. The majority of the yam is composed of a much softer substance known as the "meat". This substance ranges in color from white to bright orange in ripe yams.
Yams are a primary agricultural commodity in West Africa and New Guinea. They were first cultivated in Africa and Asia about 8000 B.C.. To this day, the yams are important for survival in these regions. Yam tubers can be stored for four 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.
Yams of African species must be cooked to be safely eaten because various natural substances in raw yams can cause illness if consumed; the most common cooking method in Western and Central Africa is fufu. Preparing some species of yam is a time-consuming process, involving days of pounding, leaching, and boiling to remove the toxins. Yams may be served fried, boiled or pounded into a paste. In the Philippines, the purple ube variety of yam (Dioscorea alata, also known in India as ratalu or violet yam) is eaten as a sweetened dessert called halaya, and is also an ingredient in another Filipino dessert, halo halo.
An exception to the cooking rule is the Japanese mountain yam (Dioscorea opposita), known as nagaimo or yamaimo depending on the root shape. It is eaten raw and grated, after only a relatively minimal preparation: the whole tubers are briefly soaked in a vinegar-water solution, to neutralize irritant oxalate crystals found in their skin. The raw vegetable is starchy and bland, mucilaginous when grated, and may be eaten plain as a side dish, or added to noodles.
 Yams and sweet potatoes
In the United States, sweet potatoes (Ipomoea batatas) are often referred to as "yams". Sweet potatoes labeled (incorrectly, according to some) as "yams" are widely available in U.S. grocery stores. True yams are mostly only found in specialty markets such as those that serve Asian or Caribbean communities.
|Top Yam Producers - 2005|
(million metric ton)
|Image:Flag of Nigeria.svg Nigeria||26.6|
|Image:Flag of Ghana.svg Ghana||3.9|
|Image:Flag of Cote d'Ivoire.svg Côte d'Ivoire||3.0|
|Image:Flag of Benin.svg Benin||2.3|
|Image:Flag of Togo.svg Togo||0.6|
|Image:Flag of Colombia.svg Colombia||0.3|
UN Food & Agriculture Organisation (FAO)
 Yam culture
In many societies yams are so important that one can speak of a yam culture. Growing the tuber is associated with magic; the best ones must be given to the chief or king; there is a series of myths connected to a divine origin; a farmer may gain a lot of prestige by growing the largest or longest yam; etc.
- Micronesia, for example Pohnpei.
- Melanesia, for example Vanuatu, Papua New Guinea.
- Polynesia, west Polynesia only, like Samoa, Tonga. In Tonga the ancient names of the months of the year and the names of the days of the moon-month were all geared towards the yam culture.
 Major cultivated yam species
 Dioscorea rotundata and D. cayenensis
Dioscorea rotunda, the white yam, and Dioscorea cayenensis, the yellow yam, are native to Africa. They are the most important cultivated yams. In the past they were considered two species but most taxonomists now regard them as the same species. There are over 200 cultivated varieties between them. They are large plants; the vines can be as long as 10 to 12 meters (35 to 40 feet). The tubers most often weigh about 2.5 to 5 kg (6 to 12 lbs) each but can weigh as much as 25 kg (60 lbs). After 7 to 12 months growth the tubers are harvested. In Africa most are pounded into a paste to make the traditional dish "fufu" (Kay 1987).
 D. alata
Dioscorea alata, called water yam, winged yam, and purple yam, was first cultivated somewhere in Southeast Asia. Although it is not grown in the same quantities as the African yams it has the largest distribution world-wide of any cultivated yam, being grown in Asia, the Pacific islands, Africa, and the West Indies (Mignouna 2003). In the United States it has become an invasive species in some Southern states.
In the Philippines it is known as ube (or ubi) and is used as an ingredient in many sweet desserts. In India, it is known as ratalu or violet yam or the Moraga Surprise. In Hawaii it is known as uhi. Uhi was brought to Hawaii by the early Polynesian settlers and became a major crop in the 1800s when the tubers were sold to visiting ships as an easily stored food supply for their voyages (White 2003).
 D. opposita
Dioscorea opposita, Chinese yam, is native to China. It is tolerant to frost and can be grown in much cooler conditions than other yams. It is now grown in China, Korea, and Japan. It was introduced to Europe in the 1800's when the potato crop there was falling victim to disease. It is still grown in France for the Asian food market. The Chinese yam plant is somewhat smaller than the African yam, with the vines about 3 meters (10 feet) long. The tubers are harvested after about 6 months of growth. Some are eaten right after harvesting and some are used as ingredients for other dishes, including noodles, and for traditional medicines (Kay 1987).
 D. bulbifera
Dioscorea bulbifera, the air potato, is found in both Africa and Asia with slight differences between those found in the two places. It is a large vine 6 meters (20 ft) or more in length. It produces tubers; however the bulbils which grow at the base of its leaves are the more important food product. They are about the size of potatoes (hence the name air potato) weighing from 0.5 to 2 kg (1 to 5 lbs). Some varieties can be eaten raw while some require soaking or boiling for detoxification before eating. It is not grown much commercially since the flavor of other yams is preferred by most people. However it is popular in home vegetable gardens because it produces a crop after only 4 months of growth and continues for the life of the vine, as long as two years. Also the bulbils are easy to harvest and cook (Kay 1987).
In 1905 the air potato was introduced to Florida and has since become an invasive species in much of the state. Its rapid growth crowds out native vegetation and is very difficult to remove since it can grow back from the tubers and new vines can grow from the bulbils even after being cut down or burned (Schultz 1993).
 D. esculenta
Dioscorea esculenta, the lesser yam, was one of the first yam species cultivated. It is native to Southeast Asia and is the third most commonly cultivated species there, although it is cultivated very little in other parts of the world. Its vines seldom reach more than 3 meters (10 feet) in length and the tubers are fairly small in most varieties. The tubers are eaten baked, boiled, or fried much like potatoes. Because of the small size of the tubers mechanical cultivation is possible; which, along with its easy preparation and good flavor, could help the lesser yam to become more popular in the future (Kay 1987).
 D. trifida
Dioscorea trifida, the cush-cush yam, is native to the Guyana region of South America and is the most important cultivated New World yam. Since they originated in tropical rain forest conditions their growth cycle is less related to seasonal changes than other yams. Because of their relative ease of cultivation and their good flavor they are considered to have a great potential for increased production (Kay 1987).
 D. dumetorum
Dioscorea dumetorum, the bitter yam, is popular as a vegetable in parts of West Africa; one reason being that their cultivation requires less labor than other yams. The wild forms are very toxic and are sometimes used to poison animals when mixed with bait. It is said that they have also been used for criminal purposes (Kay 1987).
 Some other species
- Dioscorea altissima - dunguey
- Dioscorea elephantipes - Hottentot yam
- Dioscorea floridana - Florida yam
- Dioscorea hispida - intoxicating yam
- Dioscorea polygonoides - mata gallina
- Dioscorea preussii - Preuss' dioscorea
- Dioscorea sansibarensis - Zanzibar yam
- Consultative Group on International Agricultural Research (CGIAR), 1994, "A Breakthrough in Yam Breeding"
- Consultative Group on International Agricultural Research (CGIAR), 2006, "Yam"
- Kay, D.E., 1987, Root Crops, Tropical Development and Research Institute : London
- Mignouna, H.D., Abang, M.M., & Asiedu, R., 2003, "Harnessing modern biotechnology for tropical tuber crop improvement: Yam (Dioscorea spp.) molecular breeding"
- Schultz, G.E., 1993, "Element Stewardship Abstract for Dioscorea bulbifera, Air potato", The Nature Conservancy
- White, L.D., 2003, Canoe Plants of Ancient Hawai'i, "Uhi"
 External links and resources
- See also the United States Department of Agriculture's Plant Profile for a list of 28 yam species.
- Yam Research at the International Institute of Tropical Agriculture (IITA)
- Purdue University's Famine Foods Database includes a page about how various Dioscoreacea species are used
- African Pygmies - Wild yams gathering
- The African Table by Jessica Harris. Simon & Schuster: 1996. ISBN 068481837.bm:Ku