Yam (vegetable)

In some places, other (unrelated) root vegetables are sometimes referred to as "yams", including:[1]

• In the United States, sweet potatoes (Ipomoea batatas), especially those with orange flesh, are often referred to as "yams"[5][6]
• In Australia, the tubers of the Microseris lanceolata, or yam daisy, were a staple food of Aboriginal Australians in some regions.[7]
• In New Zealand, oca (Oxalis tuberosa) is typically referred to as "yam".[8][9]
• In Malaysia, Singapore and Portuguese speaking countries, taro (Colocasia esculenta) is referred to as "yam".[10][3]
• In Africa, South and Southeast Asia as well as the tropical Pacific islands Amorphophallus paeoniifolius is grown and known as "elephant foot yam".[11]

Many cultivated species of Dioscorea yams are found throughout the humid tropics.[1] The most economically important are discussed below.[13]

Non-Dioscorea tubers that were historically important in Africa include Plectranthus rotundifolius (the Hausa potato) and Plectranthus esculentus (the Livingstone potato); these two tuber crops have now been largely displaced by the introduction of cassava.[14]

Dioscorea rotundata, the white yam, and D. cayennensis, the yellow yam, are native to Africa. They are the most important cultivated yams. In the past, they were considered as two separate species, but most taxonomists now regard them as the same species. Over 200 varieties between them are cultivated.

White yam tuber is roughly cylindrical in shape, the skin is smooth and brown, and the flesh is usually white and firm. Yellow yam has yellow flesh, caused by the presence of carotenoids. It looks similar to the white yam in outer appearance; its tuber skin is usually a bit firmer and less extensively grooved. The yellow yam has a longer period of vegetation and a shorter dormancy than white yam.

The Kokoro variety is important in making dried yam chips.[15]

They are large plants; the vines can be as long as 10 to 12 m (33 to 39 ft). The tubers most often weigh about 2.5 to 5 kg (6 to 11 lb) each, but can weigh as much as 25 kg (55 lb). 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", known as Iyan.[16]

Water yam (D. alata)

D. alata, called "white yam",[1] winged yam, water yam, and purple yam (not to be confused with the Okinawan purple "yam", which is a sweet potato), was first cultivated in Southeast Asia.[1] Although not grown in the same quantities as the African yams, it has the largest distribution worldwide of any cultivated yam, being grown in Asia, the Pacific islands, Africa, and the West Indies.[1] Even in Africa, the popularity of water yam is second only to white yam. The tuber shape is generally cylindrical, but can vary. Tuber flesh is white and watery in texture.

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.[17]

Segment of a Chinese yam (D. polystachya)

D. polystachya, Chinese yam, is native to China. The Chinese yam plant is somewhat smaller than the African, with the vines about 3 m (10 ft) long. It is tolerant to frost and can be grown in much cooler conditions than other yams. It is also grown in 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.[16]

Air potato (D. bulbifera)

Wild yams (D. sp.)

D. 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 m (20 ft) or more in length. It produces tubers, but 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 2.0 kg (1 lb 2 oz to 4 lb 7 oz).

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.[16]

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 it 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.[18]

Wild bitter yam vines (D. dumetorum)

D. 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 m (10 ft) 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.[16]

D. dumetorum, the bitter yam, is popular as a vegetable in parts of West Africa, in part because 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.[16]

D. 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 rainforest 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.[16]

Dioscorea hirtiflora subsp. pedicellata, lusala, busala or lwidi, is native to Tropical Africa. It is widely harvested and eaten in Southern Zambia where it grows in open forest areas. In Southern Zambia, it is an important addition to the March–September diets of almost all, and income of over half, of rural households.[19] Research on propagation of this subspecies to alleviate the threat from wild harvest has been successful.[20]

Dioscorea japonica known as East Asian mountain yam, yamaimo, or Japanese mountain yam, is a type of yam (Dioscorea) native to Japan (including Ryukyu and Bonin Islands), Korea, China, Taiwan, and Assam.

Dioscorea japonica is used for food. Jinenjo, also called the wild yam, is a related variety of Japanese yam that is used as an ingredient in soba noodles.

Tongan farmer showing off his prize yams.

Young women preparing pounded yam (which includes mashed and pounded yam flour) in the Democratic Republic of Congo.

Yams of African species must be cooked to be safely eaten, because various natural substances in yams can cause illness if consumed raw. The most common cooking methods in Western and Central Africa are by boiling, frying or roasting.[38]

Among the Akan of Ghana, boiled yam can be mashed with palm oil into eto in a similar manner to the plantain dish matoke, and is served with eggs. The boiled yam can also be pounded with a traditional mortar and pestle to create a thick, starchy paste known as iyan (pounded yam) or fufu which is eaten with traditional sauces such as egusi and palm nut soup.

Another method of consumption is to leave the raw yam pieces to dry in the sun. When dry, the pieces turn a dark brown color. These are then milled to create a brown powder known in Nigeria as elubo. The powder can be mixed with boiling water to create a thick starchy paste, a kind of pudding known as amala, which is then eaten with local soups and sauces.

Yams are a primary agricultural and culturally important commodity in West Africa,[38] where over 95% 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 cultivated in other humid tropical countries.[1]

Yam is the main staple crop of the Igbos in south eastern Nigeria where for centuries it played a dominant role in both their agricultural and cultural life. It is celebrated with annual yam festivals.

Yams are particularly consumed in the coastal area of the Northeast region, although they can be found in other parts of the country. In Pernambuco state, it is usually boiled and served cut in slices at breakfast, along with cheese spread or molasses.

In Colombia yam production has been specifically located in the Caribbean region, where it has become a key product in the diet of the population of this area. In 2010, Colombia was among the 12 countries with the highest yam production worldwide, and ranked first in yield of tons per hectare planted. Although its main use is for food, several studies have shown its usefulness in the pharmaceutical industry and the manufacture of bioplastics. However, in Colombia, there is no evidence of the use of this product, other than food.[39]

Yams at Port-Vila market (Vanuatu)

A piece of cake made with ube (purple yam; Philippines)

In the Philippines, the purple ube species of yam (Dioscorea alata), is eaten as a sweetened dessert called ube halaya, and is also used as an ingredient in another Filipino dessert, halo-halo. It is also used as a popular ingredient for ice cream.

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. The yam root was traditionally used by peasants in Vietnam to dye cotton clothes throughout the Red River and Mekong delta regions as late as the mid-20th century, and is still used by others in the Sapa region of northern Vietnam.[40]

In Indonesia, the same purple yam is used for preparing desserts. This involves mashing the yam and mixing it with coconut milk and sugar. White- and off-white-fleshed yams are cut in cubes, cooked, lightly fermented, and eaten as afternoon snacks.

Yamakake is a yam-based Japanese dish prepared from tororo (D. polystachya) and maguro (tuna).

An exception to the cooking rule is the mountain yam (Dioscorea polystachya), known as nagaimo and can be further classified into ichōimo (lit. 'ginkgo-leaf yam'; kanji: 銀杏芋), or yamatoimo (lit. Yamato yam; kanji: 大和芋), depending on the root shape.

Mountain yam 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.

Another variety of yam, jinenjo, is used in Japan as an ingredient in soba noodles. In Okinawa, purple yams (Dioscorea alata) are grown. This 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 color. Purple yam is also used in other types of traditional wagashi sweets, cakes, and candy.

In central parts of India, the yam is prepared by being finely sliced, seasoned with spices, and deep fried. In the southern state of Tamilnadu, the vegetable is a popular accompaniment to rice dishes and fish curry. The purple yam, D. alata, is also eaten in India, where it is also called the violet yam.

Dioscorea root is traditionally eaten on Māgh Sankrānti (a midwinter festival) in Nepal. It is usually steamed and then cooked with spices.

Yam[41] is, along with cassava and taro, a staple food, and is consumed boiled, roasted in a lovo, or steamed with fish or meat in curry sauce or coconut milk and served with rice. The cost of yam is higher due to the difficulty in farming and relatively low volume of production.[41]

Because of their abundance and importance to survival, yams were highly regarded in Jamaican ceremonies and constitute part of many traditional West African ceremonies.[42]

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 of the yam powder to form lumps. The 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.

Skinned and cut frozen yams may also be available from specialty grocers.

A yam festival is usually held in the beginning of August at the end of the rainy season. 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.

The New Yam Festival celebrates the main agricultural crop of the Igbos, Idomas, and Tivs. 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 social and cultural life. The festival is prominent among southeastern states and major tribes in Benue State, mainly around August.

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.[44]

Yams are used in Papua New Guinea, where they are called kaukau. Their cultivation and harvesting is accompanied by complex rituals and taboos. The coming of the yams (one of the numerous versions from Maré) is described in Pene Nengone (Loyalty Islands of New Caledonia).