Information about Yam (vegetable)

The term "yam" may be used in the United States in place of sweet potato.

Yam is the common name for some species in the genus Dioscorea (family Dioscoreaceae). These are perennial herbaceous vines cultivated for the consumption of their starchy tubers in Africa, Asia, Latin America and Oceania. They are used in a fashion similar to potatoes and sweet potatoes. There are hundreds of cultivars among the cultivated species.

The word yam comes from Portuguese inhame or Spanish ñame, which both ultimately derive from the Wolof word nyam, meaning "to sample" or "taste", in other African languages it can also mean "to eat" e.g. yamyam and nyama in Hausa.^[1]

Yam tubers can grow up to 2.5 meters in length^[2] and weigh up to 70 kg (150 pounds).

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.

Top Producers - 2005 (million metric ton)
Nigeria	26.6
Ghana	3.9
Cte d'Ivoire	3.0
Benin	2.3
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Colombia	0.3
World Total	39.9
Source: UN Food & Agriculture Organisation (FAO)[1]

The majority of the vegetable 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.

Yams are important to this day for survival in these regions. The 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.

Yams are used as food plants by the larvae of some Lepidoptera species including Palpifer sordida.

Preparation

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.

'Yam powder' is available in the West from grocers specialising in African products, and may be used in a similar manner to instant mashed potato powder, although preparation is a little more difficult because of the tendency to form lumps. (The best way to eliminate the lumps is to make a very stiff paste with as little hot water as possible, and then press out the lumps, before diluting with more hot water to achieve normal consistency.) The 'yam powder' is sprinkled onto a pan containing a small amount of boiling water, and stirred vigorously.

The resulting mixture is more filling than mashed potatoes, and can make an agreeable meal if warmed cooking sauce, preferably tomato and chilli, is poured onto it.

Cultural aspects

The coming of the yams (one of the numerous versions from Maré) is described in Pene Nengone (Loyalty Islands - New Caledonia) [2]

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.

In many cultures the yam is considered the most sensual of the tubers.. Here are some examples of where this applies:

• In Micronesia, see for example Pohnpei.
• In Melanesia, see for example Vanuatu, Papua New Guinea.
• In Polynesia (west Polynesia only), see 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 growing of yam.

Major cultivated 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 1800s 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).

Nutritional value

Yams are high in Vitamin C, dietary fiber, Vitamin B6, potassium, and manganese; while being low in saturated fat, sodium, and cholesterol. Vitamin C, dietary fiber and Vitamin B6 may all promote good health^[3]. Furthermore, 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 protect against osteoporosis and heart disease^[4]. Having a low level of saturated fat is also helpful for protection against heart disease^[5].

Yam products generally have a lower glycemic index than potato products^[6], which means that they will provide a more sustained form of energy, and give better protection against obesity and diabetes^[7].

Other uses of the term yam

In the United States, sweet potatoes (Ipomoea batatas) are often referred to as "yams". Sweet potatoes labeled as "yams" are widely available in markets such as those that serve Asian or Caribbean communities.

In contrast, people in New Zealand sometimes use the word "yam" to refer to the oca (Oxalis tuberosa), and the word "kumara" to refer to the sweet potato.

References

• Brand-Miller, J., Burani, J., Foster-Powell, K. (2003). The New Glucose Revolution - Pocket Guide to The Top 100 Low GI Foods. ISBN 1-56924-500-2.
• Consultative Group on International Agricultural Research (CGIAR) (1994). A Breakthrough in Yam Breeding.
• Consultative Group on International Agricultural Research (CGIAR) (2006). Yam.
• Holford, P. (1998). The Optimum Nutrition Bible. ISBN 0-7499-1855-1.
• Huxley, A., ed. (1992). New RHS Dictionary of Gardening. Macmillan.
• Kay, D.E. (1987). . 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. Available online.
• Schultz, G.E. (1993). Element Stewardship Abstract for Dioscorea bulbifera, Air potato. Nature Conservancy
• Walsh, S. (2003). Plant Based Nutrition and Health. ISBN 0-907337-26-0.
• White, L.D. (2003). Canoe Plants of Ancient Hawai'i: Uhi

External links

• Yam Research at the International Institute of Tropical Agriculture (IITA)
• Purdue University's Famine Foods Database includes a page about how various Dioscoreaceae species are used
• African Pygmies - Wild yams gathering
• The African Table by Jessica Harris. Simon & Schuster: 1996. ISBN 0-684-81837-X.
• Sweet Potato Awareness, education about the differences between yams and sweet potatoes, with a downloadable flyer to distribute.
• The Straight Dope answers "What's the difference between yams and sweet potatoes?"