Yam (vegetable)
Yam is the common name for some plant species in the genus Dioscorea that form edible tubers.
Yams are perennial herbaceous vines native to Africa, Asia, and the Americas and cultivated for the consumption of their starchy tubers in many temperate and tropical regions. The tubers themselves, also called "yams", come in a variety of forms owing to numerous cultivars and related species.
Description
A monocot related to lilies and grasses, yams are vigorous herbaceous, perennially growing vines from a tuber. Some 870 species of yams are known, a few of which are widely grown for their edible tuber but others of which are toxic.Yam plants can grow up to in length and high. The tuber may grow into the soil up to deep. The plant disperses by seed.
The edible tuber has a rough skin that is difficult to peel but readily softened by cooking. The skins vary in color from dark brown to light pink. The majority, or meat, of the vegetable is composed of a much softer substance ranging in color from white or yellow to purple or pink in mature yams.
Etymology
The name "yam" appears to derive from Portuguese inhame or Canarian Spanish ñame, which derived from Fula, one of the West African languages during trade. However, in Portuguese, this name commonly refers to the taro plant from the genus Colocasia, as opposed to Dioscorea.The main derivations borrow from verbs meaning "to eat". True yams have various common names across multiple world regions.
In some places, other root vegetables are sometimes referred to as "yams", including:
- In the United States, sweet potatoes, especially those with orange flesh, are often referred to as "yams"
- In Australia, the tubers of the Microseris walteri, or yam daisy, were a staple food of Aboriginal Australians in some regions.
- In New Zealand, oca is typically referred to as "yam".
- In Malaysia and Singapore, taro is referred to as "yam".
- In Africa, South and Southeast Asia as well as the tropical Pacific islands Amorphophallus paeoniifolius is grown and known as "elephant foot yam".
Distribution and habitat
Ecology
Some yams are invasive plants, often considered a noxious weed outside cultivated areas.Cultivation
Yams are cultivated for the consumption of their starchy tubers in many temperate and tropical regions, especially in West Africa, South America and the Caribbean, Asia, and Oceania. About 95% of yam crops are grown in Africa.A yam crop begins when whole seed tubers or tuber portions are planted into mounds or ridges, at the beginning of the rainy season. The crop yield depends on how and where the sets are planted, sizes of mounds, interplant spacing, provision of stakes for the resultant plants, yam species, and tuber sizes desired at harvest. Small-scale farmers in West and Central Africa often intercrop yams with cereals and vegetables. The seed yams are perishable and bulky to transport. Farmers who do not buy new seed yams usually set aside up to 30% of their harvest for planting the next year. Yam crops face pressure from a range of insect pests and fungal and viral diseases, as well as nematodes. Their growth and dormant phases correspond respectively to the wet season and the dry season. For maximum yield, the yams require a humid tropical environment, with an annual rainfall over distributed uniformly throughout the growing season. White, yellow, and water yams typically produce a single large tuber per year, generally weighing.
Yams suffer from relatively few pests and diseases. There is an caused by Colletotrichum gloeosporioides which is widely distributed around the world's growing regions. Winch et al., 1984 finds C. gloeosporioides afflicts a large number of Dioscorea spp.
Despite the high labor requirements and production costs, consumer demand for yam is high in certain subregions of Africa, making yam cultivation quite profitable to certain farmers.
Major cultivated species
Many cultivated species of Dioscorea yams are found throughout the humid tropics. The most economically important are discussed below.Non-Dioscorea tubers that were historically important in Africa include Plectranthus rotundifolius and P. esculentus ; these two tuber crops have now been largely displaced by the introduction of cassava.
''D. rotundata'' and ''D. cayennensis''
D. rotundata, the white yam, and D. cayenensis, 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.
They are large plants; the vines can be as long as. The tubers most often weigh about each, but can weigh as much as. 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.
''D. alata''
D. alata, called purple yam, greater yam, winged yam, water yam, and white yam, was first cultivated in Southeast Asia. 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. 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.D. alata and D. esculenta were important staple crops to the seafaring Austronesian cultures. They were carried along with the Austronesian migrations as canoe plants, from Island Southeast Asia to as far as Madagascar and Polynesia.
''D. polystachya''
D. polystachya, Chinese yam, is native to China. The Chinese yam plant is somewhat smaller than the African, with the vines about 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.
''D. bulbifera''
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, 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, weighing from.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.
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.
''D. esculenta''
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 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.