(Dioscorea alata – Family Dioscoreaceae)
Yam, white yam
ñame blanco (Spanish), igname (French)
Depending on the country, the term “yam” is used to embrace many tubers, including the sweet potato, which is no relation. The sweet potato is a member of the Morning Glory family (Convolvulaceae) and is not a true yam.
In the 1930s, promoters of southern US-grown sweet potatoes hit on the word “yam” for a campaign to set apart their Louisiana product from the drier, paler sweet potato. It may have helped consumers at the time, but it has since become a worldwide nightmare with no one completely sure of what it is they are buying.
To North Americans, yams are white fleshed, whereas sweet potatoes are orangey fleshed – but not always.
This can be just the opposite in other parts of the world, so travellers beware!
True yams generally grow only in the Caribbean, Africa, Asia, and Mexico, but not in the US. By 1999, the US was importing various types of yam from fourteen different countries.
An interesting story emerges as to how it got its name. While Portuguese slave traders were watching Africans dig up the roots, they asked what they were called. Failing to understand the question fully, they replied that it was “something to eat”, nyami in Guinea. This became inhame in Portuguese, then igname in French, and finally yam in English.
The botanical family name comes by way of Dioscorides, the 1st century Greek botanist and physician, who used them medicinally, as well as for food.
It is also the foundational name of its most important genus, Dioscorea, which comprises 600 or more species, and found widespread throughout the tropics.
The yam is a staple for the 700 or more different ethnolinguitic groups of southeast Asia and surrounding islands.
In Papua New Guinea, the people have managed their crops for thousands of years without disturbing the forest around them. The women may prepare them, but yams are strictly grown and harvested in secret by the men away from the women.
At special yam festivals, prize tubers can be as long as six feet and weigh as much as 200 pounds. Reflecting the status of the grower within the community, these ceremonial yams are the focus of exchanges and given as gifts to the women at the end of the festivities.
Yams were successfully grown in Europe to relieve the distress caused by the potato famine of the 1840s, but it never became very popular. Although cultivated in Asia, and America, 95% of the world’s yam crop is grown in Africa, where its high starch content makes it an important food.
It is a labour-intensive crop however, making it an expensive one to grow; so it is constantly being superseded by the cheaper crops of cassava and sweet potatoes.
Its one curious disadvantage is that it extends deep underground, by as much as six feet, and often vertical. Digging it up can be an exhausting business. Thus, it is understandable that, although known since ancient times in China, it was not seriously eaten, except in times of famine.
Most of the yam species are right- or left-twining climbers, producing tubers that greatly differ in colour, shape, and size.
Of the huge number of yam species in existence, many appear to have been domesticated independently in the Old and New Worlds. Some yam species have become very important to Western medicine.
Their reputation in making poultices is high.
Diosgenin, extracted from several species of Mexican yam, enabled the contraceptive pill to be produced, and is the starting material for many steroidal drugs today. Yams also contain phytoestrogens associated with relief from menopausal symptoms.
On the other hand, some species are so poisonous, that they were used by native hunters.
Yams are an excellent source of potassium, with twice the amount as found in a medium-sized banana. They are also a good source of vitamin C, B6, folate, iron, and magnesium.
Yams are high in starch and contain an enzyme, alpha amylase, which converts starches to sugars as the tuber matures, is stored, or when heated.
According to research at Brigham Young University, curing yams by storing them at 29°C (85°F) for four to six days immediately after harvest, increases the concentration of this enzyme. Curing also appears to heal small surface scratches, decreasing the risk of rotting.
Raw yams, like lima beans, contain cyanogenic glycosides, natural chemicals that break down into hydrogen cyanide in the stomach or when the potato is heated. If the potato is pierced while it is baking or when the lid is left off the pot as it is boiling, this gas escapes off into the air.
Moldy yams may be contaminated with a number of toxins, including the liver toxin ipomeamarone and a toxic derivative ipomeamaronol. These are generally destroyed through normal cooking.
Yams are usually planted on banks, mounds, or ridges at the end of the dry season while they are still dormant as they need the long rainy season to develop. Small tubers or sections containing two or three eyes are used for planting, with the latter being preferable as they sprout faster.
Yams do not grow well in temperatures below 20°C (68°F).
Depending on the variety, harvest takes place seven to twelve months after planting, when the leaves and stems start to die back. Tubers require air drying in the sun before they can be stored, but damaged tubers should not be stored.
Types of yams include the following:
African bitter yam (D. dumetorum) contains a bitter toxic alkaloid called dihydrodioscorine, which is removed with soaking and boiling in changes of water. Some cultivated forms have most of this dangerous substance bred out of them.
Asiatic bitter yam (D. hispida) also contains the deadly tropane alkaloid dioscorine, which was used as a source of poison by native hunters of Africa for eons, but was not isolated until 1951. Grated and sliced, the tubers are soaked and boiled after several changes of water to make them edible.
Chinese yam/Cinnamon yam (D. opposita, D. batatas) is cultivated in China, Japan, Korea, and Taiwan. It is a cultivated form of another species which grows wild in China and Japan, and is more resistant to cold than other yams. The tubers can be over five feet long, and is the variety most often eaten in Southeast Asia.It is difficult to harvest, however, as the tubers descend vertically. A cooking starch called Guiana arrowroot is made from the tubers. Used as a medicinal plant in China, it was successfully grown in Europe, for a time, as a potato substitute after the potato famine in the 1840s. The Chinese yam is also called the lesser yam, which is a different species and, typically, also called by numerous other local names. It originated in Indochina and is fairly uncommon in the West. It is a good all-purpose yam that blends in well with almost anything, much like the potato.
Cush-cush yam (D. trifida) is the only yam indigenous to northern South America, but now grown mainly in the Caribbean. It is one of the four most important food yams of the world. It is also called mapuey and many other local names, depending on the country. This type bears clusters of up to a dozen tubers of good quality. It is small and elongated with charcoal to purplish skin that is lightly striated but fairly smooth. Its slippery, compact ivory flesh turns to a rosey underlayer just beneath the skin. There are also dark-fleshed types. When boiled or steamed, they emit an unexpected odour of bacon and eggs. Turning dry and fluffy, the starchy texture melts in the mouth and is much softer and lighter than other yams. The flavour suggests a slightly sweet, smokey baking potato, but a firmer and drier texture. A cushcush yam and a white yam are as different as delicate fingerling potatoes are to the sturdy russets.
Globe yam (D. globosa) is the species preferred for cultivation in India. All varieties are white.
Greater yam/Asiatic yam (D. alata) is believed to have originated and first cultivated in Southeast Asia, but now widely grown throughout the humid tropics. It requires at least sixty inches of rain per year, and is the highest yielding of the yam crops. Tubers can weigh up to 130 pounds and take ten months to a year to mature, and keeps well for five or six months after harvest. The fingered yam is a cultivar which has finger-like protuberances, dark brown skin and white flesh that is sometimes tinged with pink. The Greater yam is thought to have reached Madagascar by 1000 CE and by the 16th century, Portuguese and Spanish traders had taken it to West Africa and the New World. Christopher Columbus knew it as a staple food regularly used for ships’ supplies because it was easy to handle and could be stored for months without deteriorating. There are literally hundreds of different forms producing tubers, with an average weight of eight to twenty-two pounds, although massive specimens are not uncommon. One is recorded as having a weight of 136 pounds.
Japanese yam (D. japonica), called yamatoimo (mountain potato), is cultivated in several varieties and has been prized as a food since antiquity. It can sometimes reach lengths of six feet. The skin is brown and the flesh is white. Because it is slim and brittle, digging it up requires a high degree of extertise; and, whenaccomplished, it is transported with great care coddled in straw. It has always been considered a particularly nourishing food whose energy giving qualities rival that of the eel, and so, sometimes is referred to as the “eel of the mountains”. It does contain a large amount of the enzyme diatase, which helps the digestion. It is almost always eaten raw in the form of ‘tororo’.
Lesser yam (D. esculenta) originated in Thailand and has been grown in China since about 300 CE. Production is limited in Southeast Asia and the islands of the western Pacific as it does not keep well. All yams, except this species, contain the toxin, dioscorine, which is destroyed when cooked. It is especially small and aromatic and cooked like potatoes. It is cultivated in India, all of Southeast Asia, and northern Australia and is called ‘lesser’ because it does not keep well and generally eaten as a cooked vegetable. There are single and bunched varieties. The flesh is softer than most kinds with a slightly sweet taste.
Potato yam/Aerial yam/Air potato (D. bulbifera) grows wild in both Africa and Asia. It is also cultivated in tropical South America, the Caribbean, and parts of the southern US, but not a major crop. There are a few other species that also have aerial tubers. The Asiatic tubers are less toxic than the African varieties. It is the oddest yam as it has poor underground tubers; but bears small tubers above ground on its long, climbing vines. The aerial tubers usually weigh between one and four pounds and may contain a toxic substance. The underground varieties are hard and bitter.
White Guinea yam/White yam (D. rotunda) originated in West Africa. It is considered to be one of the four most important food yam varieties in West Africa, where it is thought to be the best for making the national dish of “fufu”. The White stores better than the Yellow and is more tolerant of the long dry season typical of West Africa. It is also known by a host of local names, and is quite similar to water yam but more solid. Many yams have been introduced into Africa where yam is synonymous with D. rotundata and the closely related D. cayensis. The two species have been the base for hybrids, creating a botanical challenge when it comes to official classification. The varieties are sometimes referred to as eight-months (white) and twelve-months (yellow) yams, referring to the time it takes for full maturity. The white variety has moister, softer flesh than the yellow.
Yellow Guinea yam/Yellow yam (D. cayensis) is indigenous to the forest zone of West Africa which has a short dry season. It requires a full year for maturity and is considered one of the most important yam types in the world. Grown mainly in Africa and the West Indies, it is not as popular as the white nor does it store well.