Yam (vegetable)

Yamis the common name for some plant species in the genusDioscorea(familyDioscoreaceae) that form edibletubers(some other species in the genus being toxic).
Yams areperennialherbaceousvinesnative to Africa, Asia, and the Americas and cultivated for the consumption of theirstarchytubers in manytemperateandtropicalregions. The tubers themselves, also called "yams", come in a variety of forms owing to numerouscultivarsand related species.
Amonocotrelated to lilies and grasses, yams are vigorousherbaceous,perenniallygrowingvinesfrom atuber.Some 870 species of yams are known,a few of which are widely grown for their edible tuber but others of which are toxic (such asD. communis).
Yam plants can grow up to 15 metres (49 feet) in length and 7.6 to 15.2 centimetres (3 to 6 inches) high.The tuber may grow into the soil up to 1.5 m (5 ft) deep.The plant disperses byseed.
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.
The name "yam" appears to derive from PortugueseinhameorCanarian Spanishñame, which derived fromFula, one of theWest African languagesduring trade.However, in Portuguese, this name commonly refers to thetaroplant (Colocasia esculenta) from the genusColocasia, as opposed toDioscorea.
The main derivations borrow from verbs meaning "to eat".True yams have variouscommon namesacross multiple world regions.
In some places, other (unrelated) root vegetables are sometimes referred to as "yams", including:
Yams are native to Africa, Asia, and the Americas.Three species were independently domesticated on those continents:D. rotundata(Africa),D. alata(Asia), andD. trifida(South America).
Some yams areinvasive plants, often considered anoxious weedoutside cultivated areas.
Yams are cultivated for the consumption of theirstarchytubers in manytemperateandtropicalregions, especially in West Africa, South America and theCaribbean, Asia, andOceania.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 oftenintercropyams 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 asnematodes. 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 1,500 millimetres (59 in) distributed uniformly throughout the growing season. White, yellow, and water yams typically produce a single large tuber per year, generally weighing 5 to 10 kilograms (11 to 22 pounds).
Yams suffer from relatively few pests and diseases.There is ananthracnosecaused byColletotrichum gloeosporioideswhich is widely distributed around the world's growing regions.Winchet al., 1984 findsC. gloeosporioidesafflicts a large number ofDioscoreaspp.
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.
Many cultivated species ofDioscoreayams are found throughout the humid tropics.The most economically important are discussed below.
Non-Dioscoreatubers that were historically important in Africa includePlectranthus rotundifolius(the Hausa potato) andP. esculentus(the Livingstone potato); these two tuber crops have now been largely displaced by the introduction ofcassava.
D. rotundata, the white yam, andD. 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.
TheKokorovariety is important in making dried yam chips.
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 asIyan.
D. alata, called purple yam (not to be confused with the Okinawan purple "yam", which is asweet potato), greater yam, winged yam, water yam, and (ambiguously) 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. alataandD. esculenta(lesser yam) were important staple crops to the seafaringAustronesiancultures. They were carried along with the Austronesian migrations ascanoe plants, fromIsland Southeast Asiato as far asMadagascarandPolynesia.
D. polystachya, Chinese yam, is native toChina. 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 inKoreaandJapan.
It was introduced toEuropein the 19th century, when thepotatocrop 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, 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 thebulbilswhich 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.
In 1905, the air potato was introduced to Florida and has since become aninvasive speciesin 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, 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.
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 saidthat they have also been used for criminal purposes.
D. trifida, the cush-cush yam, is native to theGuyanaregion of South America and is the most important cultivated New World yam. Since they originated in tropicalrainforestconditions, 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.
D. hirtiflorasubsp.pedicellata, lusala, busala or lwidi, is native toTropical 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.Research on propagation of this subspecies to alleviate the threat from wild harvest has been successful.
D. japonica– known as East Asian mountain yam, yamaimo, or Japanese mountain yam – is a type of yam (Dioscorea) native to Japan. Its other common names include cham ma, Chinese yam, dang ma, glutinous yam, jinenjo, pinyin, rìběn- shǔyù, shan yao, Taiwanese yam, and wild yam. Varieties includeD. japonicaThunb var.pseudojaponicaYamamoto,D. japonicaThunb. var.pseudojaponica(Hayata) Yamam,D. japonicavar.japonica,D. japonicavar.oldhamiiandD. japonicavar.pilifera. It is widely cultivated Japan, Korea, China, and neighbouring islands.
It is widely cultivated as a food crop in Japan, Korea, China and neighbouring islands.Jinenjo is a related variety of Japanese yam that is used as an ingredient in soba noodles.
Yams inWest Africaare typically harvested by hand, using sticks, spades, or diggers.Wood-based tools are preferred to metallic tools as they are less likely to damage the fragile tubers; however, wood tools need frequent replacement. Yam harvesting is labor-intensive and physically demanding. Tuber harvesting involves standing, bending,squatting, and sometimes sitting on the ground depending on the size of mound, size of tuber, or depth of tuber penetration. Care must be taken to avoid damage to the tuber, because damaged tubers do not store well and spoil rapidly. Some farmers use staking and mixed cropping, a practice that complicates harvesting in some cases.
In forested areas, tubers grow in areas where other tree roots are present. Harvesting the tuber then involves the additional step of freeing them from other roots. This often causes tuber damage.
Aerial tubers or bulbils are harvested by manual plucking from the vine.
Yields may improve and cost of yam production be lower if mechanization were to be developed and adopted. However, current crop production practices and species used pose considerable hurdles to successful mechanization of yam production, particularly for small-scale rural farmers. Extensive changes in traditional cultivation practices, such as mixed cropping, may be required. Modification of current tuber harvesting equipment is necessary given yam tuber architecture and its different physical properties.
In 2020, world production of yams was 75 million metric tons (74,000,000 long tons; 83,000,000 short tons), led byNigeriawith 67% of the total (table).
Unlikecassava, most varieties of edible, mature, cultivated yam do not contain toxic compounds. However, there are exceptions. Bitter compounds tend to accumulate in immature tuber tissues of white and yellow yams. These may be polyphenols ortannin-like compounds.
Wild forms of bitter yams (D. dumetorum) do contain some toxins, such asdihydrodioscorine, that taste bitter, hence are referred to as bitter yam.Bitter yams are not normally eaten except at times of desperation in poor countries and in times of local food scarcity. They are usually detoxified by soaking in a vessel of salt water, in cold or hot fresh water or in a stream. The bitter compounds in these yams are water-soluble alkaloids which, on ingestion, produce severe and distressing symptoms. Severe cases of alkaloid intoxication may prove fatal.
Aerial or potato yams (D. bulbifera) have antinutritional factors. In Asia, detoxification methods, involving water extraction, fermentation, and roasting of the grated tuber, are used for bitter cultivars of this yam. The bitter compounds in yams also known locally as air potato include diosbulbin and possiblysaponins, such asdiosgenin.In Indonesia, an extract of air potato is used in the preparation of arrow poison.
Raw yam has only moderatenutrient density, withappreciable content(10% or more of theDaily Value, DV) limited topotassium,vitamin B6,manganese,thiamin,dietary fiber, andvitamin C(table).But raw yam has the highest potassium levels amongst the 10 major staple foods of the world (see nutritional chart). Yam supplies 118caloriesper 100 grams. Yam generally has a lowerglycemic index, about 54% ofglucoseper 150 gram serving, compared to potato products.
The protein content and quality of roots and tubers is lower than other food staples, with the content of yam and potato  being around 2% on a fresh-weight basis. Yams, withcassava, provide a much greater proportion of the protein intake in Africa, ranging from 5.9% in East and South Africa to about 15.9% in humid West Africa.
As a relatively low-protein food, yam is not a good source ofessential amino acids. Experts emphasize the need to supplement a yam-dominant diet with more protein-rich foods to support healthy growth in children.
Yam is an important dietary element for Nigerian and West African people. It contributes more than 200 calories per person per day for more than 150 million people in West Africa, and is an important source of income. Yam is an attractive crop in poor farms with limited resources. It is rich in starch, and can be prepared in many ways. It is available all year round, unlike other, unreliable, seasonal crops. These characteristics make yam a preferred food and a culturally important food security crop in some sub-Saharan African countries.
The following table shows the nutrient content of yam and major staple foods in a raw harvested form on adry weight basisto account for their different water contents. Raw forms, however, are not edible and cannot be digested. These must be sprouted, or prepared and cooked for human consumption. In sprouted or cooked form, the relative nutritional and antinutritional contents of each of these staples is remarkably different from that of raw form of these staples.
Roots and tubers such as yam are living organisms. When stored, they continue to respire, which results in the oxidation of thestarch(a polymer of glucose) contained in the cells of the tuber, which converts it into water,carbon dioxide, and heat energy. During this transformation of the starch, the dry matter of the tuber is reduced.
Amongst the major roots and tubers, properly stored yam is considered to be the least perishable. Successful storage of yams requires:
Storing yam at low temperature reduces the respiration rates. However, temperatures below 12 °C (54 °F) cause damage through chilling, causing a breakdown of internal tissues, increasing water loss and yam's susceptibility to decay. The symptoms of chilling injury are not always obvious when the tubers are still in cold storage. The injury becomes noticeable as soon as the tubers are restored to ambient temperatures.
The best temperature to store yams is between 14 and 16 °C (57 and 61 °F), with high-technology-controlled humidity and climatic conditions, after a process of curing. Most countries that grow yams as a staple food are too poor to afford high-technology storage systems.
Sprouting rapidly increases a tuber's respiration rates, and accelerates the rate at which its food value decreases.
Certain cultivars of yams store better than others. The easier to store yams are those adapted to arid climate, where they tend to stay in a dormant low-respiration stage much longer than yam breeds adapted to humid tropical lands, where they do not need dormancy. Yellow yam and cush-cush yam, by nature, have much shorter dormancy periods than water yam, white yam, or lesser yam.
Storage lossesfor yams are very high in Africa, with bacteria, insects, nematodes, and mammals being the most commonstorage pests.
Yams are consumed in a variety of preparations, such as flour or whole vegetable pieces across their range of distribution in Asia, Africa, North America,Central America, theCaribbean, South America, andOceania.
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 inWesternandCentral Africaare by boiling, frying or roasting.
Among the Akan of Ghana, boiled yam can be mashed with palm oil intoetoin a similar manner to the plantain dishmatoke, 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 asiyan(pounded yam) which is eaten with traditional sauces such asegusiandpalm nutsoup.
Another method of consumption is to leave the raw yam pieces todry in the sun. When dry, the pieces turn a dark brown color. These are then milled to create a brown powder known in Nigeria aselubo. The powder can be mixed with boiling water to create a thick starchy paste, a kind of pudding known asamala, which is then eaten with local soups and sauces.
Yams are a staple agriculturalcommodityinWest Africawith cultural significance,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 thewet season. Yam cultivars are also cultivated in other humid tropical countries.
Yam is the main staple crop of theIgbosin south eastern Nigeria where for centuries it played a dominant role in both their agricultural and cultural life. It is celebrated with annualyam festivals.
Yams are particularly consumed in the coastal area of theNortheast region, although they can be found in other parts of the country. InPernambucostate, it is usually boiled and served cut in slices at breakfast, along withcheese spreadormolasses.
In Colombia yam production has been specifically located in theCaribbean 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.
In the Philippines, the purpleubespecies of yam (D. alata), is eaten as a sweetened dessert calledube 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, yams are used to prepare dishes such ascanh khoai mỡorcanh khoai từ. 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 theSapa regionof northern Vietnam.
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.
An exception to the cooking rule is the mountain yam (Dioscorea polystachya), known asnagaimoand can be further classified intoichōimo(lit. 'ginkgo-leaf yam'; kanji: 銀杏芋), oryamatoimo(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 irritantoxalatecrystals found in their skin. The raw vegetable is starchy and bland,mucilaginouswhen 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 traditionalwagashisweets, cakes, and candy.
In central parts of India, the yam is prepared by being finely sliced, seasoned with spices, and deep fried. In Southern India, the vegetable is a popular accompaniment to rice dishes and curry. The purple yam,D. alata, is also eaten in India, where it is also called the violet yam. Species may be called by the regional name "taradi", which can refer toD. belophylla,Dioscorea deltoidea,andD. bulbifera.Digging and selling taradi is a major source of income in the region ofPalampur.
Dioscorearoot is traditionally eaten onMāgh Sankrānti(a midwinter festival) in Nepal. It is usually steamed and then cooked with spices.
Yam is, along withcassavaandtaro, a staple food, and is consumed boiled, roasted in alovo, or steamed with fish or meat incurrysauce orcoconut milkand served with rice. The cost of yam is higher due to the difficulty in farming and relatively low volume of production.
Because of their abundance and importance to survival, yams were highly regarded inJamaicanceremonies and constitute part of many traditionalWest Africanceremonies.
Yam powder is available inthe Westfrom grocers specializing in African products, and may be used in a similar manner toinstant mashed potatopowder, 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 astomatoandchili, poured onto it.
Skinned and cut frozen yams may also be available from specialty grocers.
The tubers of certain wild yams, including a variant of 'Kokoro' yam and other species ofDioscorea, such asDioscorea nipponica, are a source for the extraction ofdiosgenin, a sapogenin steroid.The extracted diosgenin is used for the commercial synthesis ofcortisone,pregnenolone,progesterone, and other steroid products.Such preparations were used in earlycombined oral contraceptive pills.The unmodified steroid hasestrogenicactivity.
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.
Yams are used inPapua New Guinea, where they are calledkaukau. Their cultivation and harvesting is accompanied by complex rituals and taboos. The coming of the yams (one of the numerous versions fromMaré) is described in Pene Nengone (Loyalty IslandsofNew Caledonia).
A yam festival is usually held in the beginning ofAugustat the end of therainy season. People offer yams togods and ancestorsfirst, 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 theIgbos,Idomas, andTivs. The New Yam Festival, known asOrureshiin Owukpa in Idoma west andIma-Ji,Iri-JiorIwa JiinIgbo land, is a celebration depicting the prominence of yam in social and cultural life. The festival is prominent among southeastern states and major tribes inBenue State, mainly around August.
The Igbo people accord special respect to yam to the extent that no one eats the newly harvested yam until the New Yam celebrations / feast is marked. It is called Iri ji ọhụrụ. People return to their various communities for the celebrations.