Yam (vegetable)

Top 10 Producers - 2008 (million metric ton)
Nigeria	35.0
Ivory Coast	6.9
Ghana	4.8
Benin	1.8
Togo	0.6
Cameroon	0.3
Colombia	0.2
Brazil	0.2
Haiti	0.2
World Total	50.0
Source: UN Food & Agriculture Organisation (FAO)

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 many cultivars of yam.

The sweet potato (Ipomoea batatas) has traditionally been referred to as a yam in parts of the United States and Canada, but it is not part of the Dioscoreaceae family.

Although it is unclear which came first, the word yam is related to 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 doya in Hausa or "to chew" in Dholuo language of the Luo of Kenya and Northern Tanzania.

There are over 100 ethnic groups and languages in Nigeria, and each has different language names for Yam, "Isu" is the Yoruba translation or "Iyan" when it has been prepared to be consumed as a main course for dinner. The yam is a versatile vegetable which has various derivative products after process, it can be barbecued; roasted; fried; grilled; boiled; 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.

Yam tubers can grow up to 2.5 m (8.2 ft) in length and weigh up to 70 kg (154 lb).

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 colour from white or yellow to purple or pink 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 used as a vegetable offered during blessings.

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

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 or application of red palm oil to the affected part of the body will stop the itching.) Yam is consumed in various ways, but is usually boiled and eaten. This involves cutting yam into pieces, then peeling the skin, and boiling the starchy "meat". This is usually consumed with palm oil (traditional way), or with other sauces. The boiled yam can also be pounded with a traditional mortar and pestle to create a thick starchy paste known as Pounded Yam Iyan, as well as Fufu. This is also eaten with traditional stews and sauces. Another method of consumption is to sun dry the raw yam pieces. When dry, the pieces turn a dark brown colour. This is then milled to create a powder known as "elubo" in Nigeria. The brown powder can be prepared with boiling water to create a thick brown starchy paste known as "amala". This is also consumed with the local stews and sauces. The most common cooking method in Western and Central Africa is cooked "boiled" yam.

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 "ube halaya", and is also used as an ingredient in another Filipino dessert, halo-halo.

Vietnam

In Vietnam, the same purple yam is used for preparing a special type of soup canh khoai mỡ or fatty yam soup. This involves mashing the yam and cooking it until very well done.

Indonesia

In Indonesia, the same purple yam is used for preparing desserts. This involves mashing the yam and mixing it with coconut milk and sugar.

Japan

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

In Japan, the purple yam is popular as lightly deep fried tempura as well as being grilled or boiled. Additionally, the purple yam is a common ingredient of yam ice cream with the signature purple colour.

India

In central parts of India, the yam (or Garadu) is prepared by being finely sliced, seasoned with spices and deep fried. In southern parts of India, the vegetable is a popular accompaniment to fish curry. In Assam, it is known as Kosu (কচু) and is normally boiled, mashed and lightly seasoned with salt.

The West

'Yam powder' is available in the West from grocers specializing 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 served with a heated sauce, such as tomato and chili, poured onto it. To avoid lumps forming, mix the powder and the water before putting on the heat, then stir continuously as the mixture is heating.

Cultural aspects

Nigeria and Ghana

A Yam Festival is usually held in the beginning of August at the end of the rainy season. A popular holiday in Ghana, 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 (Idoma in Benue state, Nigeria)

The New Yam Festival consists of prayers and thanks for the years past. Yam is the main agricultural crop of the Idomas, Tivs and Igbos. It is the "staple" food of the Idoma and Tiv people. The New Yam Festival, known as Orureshi in Owukpa in Idoma west and Ima-Ji, Iri-Ji or Iwa Ji in Igbo land is a celebration depicting the prominence of yam in the social and cultural life. The festival is very promiment among all the major tribes in Benue state, mainly around August.

The celebration is followed by various cultural dance with the display of masquerades from different clans or groups. This usually last to very late night. During the occasion is also a moment of reuniting old friends and family members.

Elsewhere

Historical records in West Africa and of African yams in Europe date back to the 16th century. Yams were taken to the Americas through precolonial Portuguese and Spanish on the borders of Brazil and Guyana, followed by a dispersion through the Caribbean.

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

In many societies yams are so important that one can speak of a 'yam civilization'. 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.

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. People of ancient times worshiped the yam. Olhuala is a type of local yam that is a staple food in the Maldives.

On the Japanese island of Rishiri, yams and yam products are regarded as a folk remedy for the treatment of impotence, possibly because of the vegetable's high vitamin E content, but likely because of its evocation of virile phallic imagery, according to the common folk medicine theory of sympathetic medicine.

Major cultivated species

Dioscorea rotundata and D. cayenensis

Dioscorea rotundata, 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 separate species but most taxonomists now regard them as the same species. There are over 200 cultivated varieties between them. The Kokoro variety is important in making dried yam chips.

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).

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 Vietnam, it is called khoai mỡ and is used mainly as an ingredient for soup. In India, it is known as ratalu or violet yam. In Hawaii it is known as uhi.

Uhi was brought to Hawaii by the early Polynesian settlers and became a major crop in the 19th century 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. 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 19th century 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).

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 four 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).

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

Yam provides around 110 calories per 100 grams of product. Yam is high in vitamins C and B₆, potassium, manganese and dietary fibre 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.

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.

It is also known to replenish fast-twitch fibers and West Indians use it as a way of recovering after sprinting.

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." In the United States, firm varieties of sweet potatoes were produced before soft varieties. When soft varieties were first grown commercially, there was a need to differentiate between the two. African slaves had already been calling the soft sweet potatoes "yams" because they resembled the yams in Africa. Thus, soft sweet potatoes were referred to as yams to distinguish them from the firm varieties. Sweet potatoes labeled as "yams" are widely available in markets that serve Asian or Caribbean communities.

Today the U.S. Department of Agriculture requires sweet potatoes labeled with the term "yam" to be accompanied by the term "sweet potato."

In New Zealand "yam" sometimes refers to the oca (Oxalis tuberosa). "Kumara" refers to the sweet potato.

The corm of the konjac is often colloquially referred to as a yam, although it bears no marked relation to tubers of the genus Dioscorea.

In Malaysia and Singapore, "yam" is used to refer to taro.