Yam (vegetable)

Yams at Brixton market

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. There are hundreds of cultivars among the cultivated species.

A completely different plant, the sweet potato (Ipomoea batatas), is sometimes also called 'yam' in parts of the southern United States and Canada, though it is very distant from the Dioscoreaceae family.

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.

The Nigerian word for yam is adamwanga meaning "Adamo's food". Adamo was a chief notorious for his ability to consume incredible amounts of food, and was even banned from a neighboring village for his refusal to stop.

Yam tubers can grow up to 2.5 meters in length 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
Ivory Coast	3.0
Benin	2.3
Togo	0.6
Colombia	0.3
World Total	39.9
Source: UN Food & Agriculture Organisation (FAO)[1]

Yam output in 2005

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. Due to their abundance and consequently, their importance to survival, the yam was highly regarded in Nigerian ceremonial culture and even worshipped.

Yams are still important 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

Tongan farmer showing off his prize yams

Africa

Yams of African species must be cooked to be safely eaten, because various natural substances in raw yams can cause illness if consumed. (Excessive skin contact with uncooked yam fluids can cause the skin to itch. If this occurs, a quick cold bath will stop the itching.) Preparing these species is a time-consuming process, involving several minutes of pounding, leaching, and boiling to remove the toxins. The most common cooking method in Western and Central Africa is "pounded yam".

Yams may be served fried, boiled or pounded into a fufu dough form.

The Philippines

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.

Japan

An exception to the cooking rule is the Japanese mountain yam (Dioscorea opposita), known as nagaimo or yamaimo depending on the root shape.

Yams at Port-Vila market (Vanuatu)

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.

The West

'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 '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.

To improve taste and nutrition, water that has been used to cook vegetables (commonly referred to as "vegetable stock") (if available) may be used in place of plain boiling water.

Tips for lump avoidance / removal

There are various ways to deal with the tendency for lumps to form when yam powder is used.

• Use a large sugar sprinkler, of the type sometimes seen in 'greasy spoon' cafes, to sprinkle the powder onto the water. The sprinkler helps to disperse the powder. (If a sugar sprinkler is not available, it might be possible to improvise one by drilling / piercing / burning holes approx 2mm wide, at one end of a tin can or cardboard tube.)
• 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 easy way: put the yam powder in a bowl and add other, less sticky powders or granules to it. This seems to break up the yam powder and prevent lumps from forming. Stir the dry mixture thoroughly with a fork, so that the yam powder is evenly distributed, then pour on enough hot water to cover it. Allow the mix to stand for a minute, then stir with the fork once again, adding more hot water as necessary, until a smooth, powder-free paste is formed.

A mix of one part rice bran, two parts mashed potato granules and four parts yam powder can be used, or one part mashed potato granules to two parts yam powder.

If the 'easy way' is used, then of course, the Glycemic Index may not be so low as with pure yam powder. But it will be lower than for mashed potato granules on their own, and will therefore provide correspondingly better protection against diabetes (see under 'Nutritional value').

Cultural aspects

Nigeria & Ghana

A Yam Festival is usually held in the beginning of August at the end of the rainy season. A popular holiday in Ghana and Nigeria, the Yam Festival is so named because yam is the most common food in many African countries. Yams are the first crops to be harvested. People offer yams to gods and ancestors first, before distributing them to the villagers. This is their way of giving thanks to the spirits above them.

New Yam Festival (Igbo)

An Igbo writes: The New Yam Festival consists of prayers and thanks for the years past. Yam is the main agricultural crop of the Igbos and also the staple food of the people. The New Yam Festival, known as Iwa-Ji or Iri-Ji, is a celebration depicting the prominence of yam in our social and cultural life.

Men and women, young and old, look forward to this festival because it begins a new season…a New Year. On the last night before the festival, yams from the old year are disposed of. The New Year must begin with tasty, fresh yams. All cooking pots, calabashes and wooden bowls are thoroughly washed, especially the wooden mortar in which yam is pounded.

Pounded Yam and vegetable soup “edikayikor” is the main food in the celebration. So much of it is cooked that, no matter how heavily the family eats or how many friends and relatives they invite from the neighboring villages, there is always a large quantity of food left over at the end of the day.

Elsewhere

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 Bombei.
• 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.

On the Japanese island of Rishiri, yams and yam products are regarded as a folk remedy for the treatment of impotence, presumably because of the vegetable's high vitamin E content.

Major cultivated species

Young women preparing fufu in Democratic Republic of Congo

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 of "pounded yam" (Kay 1987).

A piece of cake made with Ube (water yam).

D. alata

Dioscorea alata, called "water yam", "winged yam" and "purple yam", was first cultivated in Southeast Asia. Although 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).

Segment of a Dioscorea opposita tuber

D. opposita

Dioscorea opposita, "Chinese yam", is native to China. The Chinese yam plant is somewhat smaller than the African, with the vines about 3 meters (10 feet) long. 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, and is still grown in France for the Asian food market.

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).

File:Dioscorea bulbifera.jpg

Air potato

D. bulbifera

Dioscorea bulbifera, the "air potato", is found in both Africa and Asia, with slight differences between those found in each place. 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 producing 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). Bock! Bock! *peckpeck* Bock!

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. These yams are often fertilized with human faeces. Because of their relative ease of cultivation and their good flavor they are considered to have a great potential for increased production (Kay 1987).

Wild bitter yam vines

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 and sodium. Vitamin C, dietary fiber and Vitamin B6 may all promote good health. 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. Having a low level of saturated fat is also helpful for protection against heart disease.

Yam products generally have a lower Glycemic Index than potato products, which means that they will provide a more sustained form of energy, and give better protection against obesity and diabetes.

Other uses of the term yam

In the United States, sweet potato (Ipomoea batatas), especially those with orange flesh, are often referred to as yams. Sweet potatoes labelled 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.

The word yam or yams is sometimes used as street slang for cocaine.

See also

• Shirataki noodles (yam noodles)

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.
• IITA has CGIAR global mandate for YAM. IITA's global research for development mandate.
• 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). 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. Available online.
• Schultz, G.E. (1993). Element Stewardship Abstract for Dioscorea bulbifera, Air potato. Nature Conservancy
• Sumiyoshi, S., ed. (1996). Nigerian culture and customs: A walk through time. Koerner.
• 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 Genebank - IITA has the world mandate to preserve the YAM genetic variety in its genebank, accessions can be searched here
• 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?"