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|Yams at Brixton market in London|
|Yams at Brixton market in London|
Yam is the common name for some plant species in the genus Dioscorea (family Dioscoreaceae) that form edible tubers. These are perennial herbaceous vines cultivated for the consumption of their starchy tubers in Africa, Asia, Latin America, the Caribbean and Oceania. There are many cultivars of yam. Although some varieties of sweet potato (Ipomoea batatas) are also called yam in parts of the United States and Canada, it is not part of the family Dioscoreaceae but belongs in the unrelated morning glory family Convolvulaceae.
Yams are monocots, related to lilies and grasses. Native to Africa and Asia, yam tubers vary in size from that of a small potato to over 60 kilograms. There are over 600 varieties of yams and 95 percent of these crops are grown in Africa.
Yams are a monocot (a plant having one embryonic seed leaf) and from the Dioscoreaceae family. Sweet Potatoes are a dicot (a plant having two embryonic seed leaves) and are from the Convolvulacea family. They are therefore about as distant as two flowering plants can be. Culinarily, yams are starchier and drier than sweet potatoes. The table below lists some differences between yam and sweet potato.
|Plant Family||Morning glory||Yam|
|Origin||Tropical America (Peru, Ecuador)||West Africa, Asia|
|Edible part||Storage root||Tuber|
|Appearance||Smooth, with thin skin||Rough, scaly|
|Shape||Short, blocky, tapered ends||Long, cylindrical, some with "toes"|
|Beta carotene||Usually high||Usually very low|
|Propagation||Transplants/vine cuttings||Tuber pieces|
Yam tubers can grow up to 1.5 meters (4.9 feet) in length and weigh up to 70 kilograms (154 lb) and 3 to 6 inches high. 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 color from white or yellow to purple or pink in mature yams.
There are many cultivars of yam throughout the humid tropics. The most economically important are discussed below.
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.
White yam's tuber is roughly cylindrical in shape, the skin is smooth and brown and the flesh usually white and firm. Yellow yam is named after its yellow flesh, a color 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.
They are large plants; the vines can be as long as 10 to 12 meters (33 to 39 ft). The tubers most often weigh about 2.5 to 5 kilograms (5.5 to 11.0 lb) each but can weigh as much as 25 kilograms (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" (Kay 1987).
Dioscorea alata, called "water yam", "winged yam" and "purple yam" (not to be confused with the Okinawan purple "yam", which is a sweet potato), 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). 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.
In the Philippines it is known as "ube" and is used as an ingredient in many sweet desserts. In Indonesia it is known as "ubi". 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. In Japan it is called daijo or beniimo.
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).
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).
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 kilograms (1.1 to 4.4 lb).
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).
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).
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).
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).
|7||Central African Republic||460,000|
|9||Papua New Guinea||345,000|
UN Food & Agriculture Organization 
Yams are farmed on about 5 million hectares in about 47 countries in tropical and subtropical regions of the world.
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 setts 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 percent of their harvest for planting the next year. Yam crops face pressure from a range of insect pests, fungal and viral diseases, as well as nematodes.
Yam typically grows for six to ten months and is dormant for two to four months, depending on the species. The growth and dormant phases correspond respectively to the wet season and the dry season. For maximum yield the yam requires a humid tropical environment, with an annual rainfall of over 1500 millimeters 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.
Yam production requires high labor. The crop has low yield per hectare compared to crops such as cassava (manioc) or sweet potato. It is not an efficient food staple given the relatively large amount of planting material that is required and its long growing season. Yam's labour requirement exceeds that of other comparable crops. Yam is also difficult to preserve and store over extended periods of time. The cost per 1,000 calories of yam is four times greater than those of other root and tuber crops.
For these reasons and problems of storing harvested yam, the costs of yam production are high and yam crops are slowly losing ground to cassava and other food staples.
Despite these high costs and low nutrient density, when compared to other tubers and roots, low technology yam farming produces the highest amount of food calorie and protein annually per hectare per season, on average. Given this nutritional value of yam and its high cultural acceptance in certain parts of Africa, there is an interest in developing knowledge that can improve yam agriculture.
In 2010, the world harvested 48.7 million tonnes of yam, 95 percent of which was produced in Africa. The biggest yam harvest, globally, was in 2008, when the world produced 54 million metric tonnes of yam.
The world average annual yield of yams was 10.2 tonnes per hectare in 2010. The most productive yam farms in the world were in Colombia, where nationwide average annual yield was 28.3 tonnes per hectare. These are average national yields; certain farms report yields significantly above 30 tonnes per hectare, for example for yellow yam; other farms report yields less than 1 tonnes per hectare.
Despite the high labor requirements and production costs, consumer demand for yam is very high in certain subregions of Africa, making yam cultivation quite profitable to certain farmers.
Yams in west Africa are 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. For each 2-10 kilogram tuber harvested, it involves standing, bending, squatting, and sometimes sitting on the ground depending 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.
Roots and tubers such as yam are living organisms. When stored, they continue to respire. The respiration process results in the oxidation of the starch (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.
Storing yam at low temperature reduces the respiration rates. However, temperatures below 12 °C (53.6 °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 14-16 °C (57.2-60.8 °F), with high technology controlled humidity and climatic conditions, after a process of curing. Most countries that grow yam as food staple 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 yam store better than others. The easier to store yams are those that are adapted to arid climate, where they tend to stay in dormant low-respiration stage for much longer than yam breeds that are 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 losses for yams are very high in Africa, with insects alone causing over 25 percent harvest loss within 4 months.
Unlike cassava, 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 or tannin-like compounds.
Wild forms of bitter yams do contain some toxins that taste bitter, and 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 too have anti-nutritional 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 possibly saponins. These substances are toxic, causing paralysis. Extracts are sometimes used in fishing to immobilize the fish and thus facilitate capture. The community of Zulus use this yam as bait for monkeys, while hunters in Malaysia use it to poison wildlife such as tigers. In Indonesia, an extract of air potato is used in the preparation of arrow poison.
|Nutritional value per 100 g (3.5 oz)|
|Energy||494 kJ (118 kcal)|
|Dietary fiber||4.1 g|
|Vitamin A equiv.|
|Percentages are roughly approximated using US recommendations for adults.|
Source: USDA Nutrient Database
The protein content and quality of roots and tubers is lower than other food staples. Of all roots and tubers, the protein content of yam and potato is the highest, being approximately 2 percent on a fresh weight basis. Yams, with cassava, provide a much greater proportion of the protein intake in Africa, ranging from 6 percent in East and southern Africa to about 16 percent in humid West Africa.
Yam, like other root crops, is not a good source of essential amino acids. It is rich in phenylalanine and threonine but limiting in the sulphur amino-acids, cystine and methionine and in tryptophan. Yam consuming areas of Africa have a high incidence of kwashiorkor, a serious medical condition in children caused by protein deficiency. Experts emphasize the need to supplement a yam-driven diet with more protein-rich foods in order to support active and healthy growth in infants.
Except for potassium, vitamin B6 and vitamin C, yam is a food with moderate nutrient density. Yam provides around 110 calories per 100 grams. It has good levels of potassium, manganese, thiamin and dietary fiber, while being low in saturated fat and sodium. Yam generally has a lower glycemic index, about 54% of glucose per 150 gram serving, compared to potato products.
The African yam (Dioscorea sp) contains thiocyanate; it was suggested in one 1986 paper that potentially it is protective against sickle cell anemia (SCA). The paper points out that SCA is known to be relatively rarer in Africans than in the African-American population of the United States, and suggests that the higher thiocyanate content of the African diet explains the high incidence of SCA among African-Americans and its rarity in Africans.
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 tubers of certain wild yam, a variant of kokoro yam and other species of Dioscorea, such as Dioscorea nipponica, are a source for the extraction of diosgenin, a steroid sapogenin. The extracted diosgenin is used for the commercial synthesis of cortisone, pregnenolone, progesterone, and other steroid products. Such preparations were used in early combined oral contraceptive pills. The unmodified steroid has estrogenic activity.
The following table shows the nutrient content of yam and major staple foods in a raw harvested form. Raw forms, however, aren't edible and can not be digested. These must be sprouted, or prepared and cooked for human consumption. In sprouted or cooked form, the relative nutritional and anti-nutritional contents of each of these staples is remarkably different from that of raw form of these staples reported in the table below.
|STAPLE:||Maize / Corn[A]||Rice[B]||Wheat[C]||Potato[D]||Cassava[E]||Soybean (Green)[F]||Sweet potato[G]||Sorghum[H]||Yam[Y]||Plantain[Z]|
|Component (per 100g portion)||Amount||Amount||Amount||Amount||Amount||Amount||Amount||Amount||Amount||Amount|
|Vitamin C (mg)||0||0||0||19.7||20.6||29||2.4||0||17.1||18.4|
|Pantothenic acid (mg)||0.42||1.01||0.95||0.30||0.11||0.15||0.80||-||0.31||0.26|
|Vitamin B6 (mg)||0.62||0.16||0.3||0.30||0.09||0.07||0.21||-||0.29||0.30|
|Folate Total (μg)||19||8||38||16||27||165||11||0||23||22|
|Vitamin A (IU)||214||0||9||2||13||180||14187||0||138||1127|
|Vitamin E, alpha-tocopherol (mg)||0.49||0.11||1.01||0.01||0.19||0||0.26||0||0.39||0.14|
|Vitamin K1 (μg)||0.3||0.1||1.9||1.9||1.9||0||1.8||0||2.6||0.7|
|Saturated fatty acids (g)||0.67||0.18||0.26||0.03||0.07||0.79||0.02||0.46||0.04||0.14|
|Monounsaturated fatty acids (g)||1.25||0.21||0.2||0.00||0.08||1.28||0.00||0.99||0.01||0.03|
|Polyunsaturated fatty acids (g)||2.16||0.18||0.63||0.04||0.05||3.20||0.01||1.37||0.08||0.07|
|A corn, yellow||B rice, white, long-grain, regular, raw, unenriched|
|C wheat, hard red winter||D potato, flesh and skin, raw|
|E cassava, raw||F soybeans, green, raw|
|G sweet potato, raw, unprepared||H sorghum, raw|
|Y yam, raw||Z plantains, raw|
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). The most common cooking method in Western and Central Africa is by boiling, frying and roasting the yam.
Preparing boiled yam involves cutting the tuber into five or so thick slices, then peeling the skin from each cut piece and boiling the whitish starchy flesh. The older the yam, the smaller the chunks must be cut and the more water it will need to boil. Boiled yam is traditionally consumed with palm oil, but is also commonly served with a sauce such as pepper sauce or palaver sauce. In big cities and border towns in West Africa, fried yam and pepper sauce is a popular street food and has a similar position as french fries and ketchup in Western cuisine. Yam balls (styled after meat balls) have also gained some popularity in contemporary West African cuisine.
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 palmnut 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, where over 95 percent 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 cropped in other humid tropical countries.
Yams are the staple crop in Nigeria where it is celebrated with yam festivals known as Iri-ji or Iwa-Ji depending on the dialect.
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.
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.
Nagaimo 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 is known locally as daijo (大薯) or beniimo (紅芋). 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.
The Sweet potato (Ipomoea batatas) is also grown in Japan, where it is known as "satsuma imo" (サツマイモ). In Winter, street vendors with carts often sell hot sweet potatoes called "yakiimo" (焼き芋) which are baked over wood-fired hot stones. Sweet potatoes are also used to make a traditional distilled liquor called "imo shochu" (芋焼酎).
In central parts of India, the yam (Khamalu or Suran or Chupri alu) is prepared by being finely sliced, seasoned with spices and deep fried. In southern parts of India, known as Karunai Kizhangu (கருணைக்கிழங்கு) in Tamil, the vegetable is a popular accompaniment to rice dishes and fish curry. Also eaten in India, Dioscorea alata, a purple-pigmented species, is known as ratalu or violet yam.
In the southern part, especially in Kerala, you can see both purple and white colored yams, locally known as "Kaachil or Kavuttu".
In southern part of India, Karnataka, it is known as Suvarna gadde.
Dioscorea root is traditionally eaten on Māgh Sankrānti (a midwinter festival) in Nepal. In Nepali it is called Tarul and in Newari Hī (ही). It is usually steamed and then cooked with spices.
'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, the powder and the water are mixed before putting on the heat, then stirred continuously as the mixture is heating.
Because of their abundance and importance to survival, yams were highly regarded in Jamaican ceremonies and constitute part of many traditional West African ceremonies.
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.
The New Yam Festival consists of prayers and thanks for the years past. Yam is the main agricultural crop of the Igbos, Idomas, and Tivs. It is the "staple" food of the Igbo 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 prominent among south-eastern states and 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.
Traditionally, there was "The Feast of the New Yam," which was held every year before the harvest began in many Igbo villages prior to colonialism. This feast was held to honor the earth goddess and the ancestral spirits of the clan. New yams could not be eaten until some had first been offered to these powers.
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 in Papua New Guinea. 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 - 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.
From Portuguese inhame or Canarian (Spain) ñame, which are probably derived from some West African language, such as Fulani, Serer, Wolof, etc., many of which have verbs meaning "to eat", which could have been the source of the plant name that would have been borrowed.
Several other unrelated root vegetables are sometimes referred to as "yams", including: