• A
• B
• C
• D
• E
• F
• G
• H
• I
• J
• K
• L
• M
• N
• O
• P
• Q
• R
• S
• T
• U
• V
• W
• X
• Y
• Z

Yam

Properly the name of an edible tuber of plants of the genus Dioscorea, ‘yam’ is often used in a general sense to embrace other tropical root crops such as sweet potato, taro, oca, etc. The wider usage is an inconvenience; all the more so since the genus Dioscorea itself comprises scores of species which are often difficult to distinguish from each other. However, one must not complain. The origin of the word ‘yam’ was such that its meaning had to be elastic. The story goes that Portuguese slave traders, watching Africans digging up some roots, asked what they were called. Failing to understand the question aright, the Africans replied that it was ‘something to eat’, nyami in Guinea. This became inhame in Portuguese and then igname in French, and yam in English.

The scientific name Dioscorea refers to the classical Greek writer on medicine, Dioscorides, and serves as a reminder that many yams have medicinal as well as culinary uses. Their reputation in making poultices is high. Some also serve as a source of steroid drugs, including a main ingredient of the contraceptive pill. An even more negative function is fulfilled by the few which produce toxins for arrow poisons, in Malaysia, and for fish poisons.

All edible yams have to be cooked before consumption, to destroy the bitter, toxic substance dioscorine which they contain in the raw state. When cooked they are starchy and bland, sometimes slightly sweet. No yams constitute a gastronomic excitement; they are just plain, filling food, providing plenty of carbohydrate and about as much protein as the potato.

In some regions yams are an important staple. This is true of Fiji and other Pacific islands; and also of Nigeria and other W. African countries, although the position of the yam there as the most popular staple food has been weakened by increased cultivation of the sweet potato.

The yam has one serious disadvantage; it extends deep underground, as far as 2 m (6′), and digging it up can be an exhausting business. Thus, although yams have been known in most parts of China since ancient times they have not been much eaten there except during famines. And, when a hardy variety of yam was introduced experimentally to Europe, to relieve the distress caused by the potato blights of the 1840s, it was grown successfully enough but failed to become popular.

Principal Species

Yams are so numerous that only the main cultivated species can be mentioned. Most of them are natives of the Old World, from SE Asia, nearby Pacific islands, and Africa; but there is also a small group native to S. America. Yams existed at least as far back as the beginning of the Jurassic era, when dinosaurs had not yet been succeeded by mammals and S. America and Asia were still joined. After the continents separated at the end of the Cretaceous era, the evolution of American yams proceeded separately, but they are still not much different from their Old World relatives. It is true that, as a group, they tend to produce clumps of small tubers rather than single huge ones; but they share this convenient habit with certain Old World yams. The differences between Asian and African yams, which were separated only in historic times by the drying up of the intervening land, Arabia, are also slight. The transfer of useful species by human agency has anyway confused the picture.

Even within the main cultivated species, yams vary to a remarkable extent in size, shape, and colour. The typical yam, if there be such a thing, is a large, oblong root which looks something like a mortadella sausage encased in a barklike skin. It is this sort of yam which is seen in markets in industrial countries where there are immigrants from yam-eating countries. But where yams are indigenous and widely eaten the diversity is great. Some yams produce many small tubers, no larger than potatoes. Others produce single giants. The greatest recorded weight is 60 kg (130 lb), for a greater Asiatic yam dug up in Malaysia (source cited by Burkill, 1965–6). There are intermediate sizes and shapes, including some with branching ‘fingers’ and others which first develop downwards and then curve back up towards the surface of the soil. (The diversity of shapes can be illustrated by just a few of the many Malay common names for particular kinds: peaked-cap yam, elephant's ear yam, snake yam, buffalo-thigh yam.) The rind may be rough or smooth, pale in colour, or brown, or purple. The flesh is often white or yellow, but sometimes pink or purple, either just under the skin or all through.

• D. alata, the greater Asiatic yam or water yam, is the most important species in SE Asia and the nearby islands. A common name for it is ‘ten months yam’, since it takes that time for the tubers to develop. In Fiji, where this yam is the staple food, most of the names for the (11, not 12) months and seasons refer to the stages of its growth. Of all yams this species produces the largest tubers, though there are also smaller and branched ones. Some are white fleshed; others as purple as a beetroot. This species spread to Madagascar by ad 1000, the African mainland not long after, and was being grown in the W. Indies in the early 16th century, having been brought over by slave traders.
• D. cayenensis, the white or yellow Guinea yam, is the principal species of W. Africa, where its most usual African name is allato. The white and yellow varieties are sometimes called respectively ‘eight months’ and ‘twelve months yam’. The former, distinguished as var rotundata, has moister and softer flesh than the latter, and also keeps better in storage. It is normally prepared as a boiled vegetable but widely used in the preparation of foo-foo. Most W. African cultivars are considered to have a superior flavour and contain no toxic constituents.
• D. esculenta, the lesser Asiatic yam, has long been cultivated in India, the whole of SE Asia, and N. Australia, where one aboriginal name for it is ‘karro’. Despite its designation as ‘lesser’, it can reach a large size. There are single and bunched varieties. The flesh is soft and, in most kinds, has a slightly sweet taste. It does not store well, and is generally eaten as a cooked vegetable.
• D. batatus, the Chinese yam, is a cultivated form of another species which still grows wild in China and Japan. It is more resistant to cold than most other yams. The tubers are very long, as much as 1.5 m (5.5′). This yam, the one most eaten in China and Japan, is also cultivated in SE Asia, where the climate is not too hot, and in America. A starch used in cooking, called Guiana arrowroot, is made from the tubers.
• D. globosa, the globe yam, is the species preferred for cultivation in India. All its varieties are white.
• D. bulbifera, the oddest yam, is the ‘air potato’, which has poor underground tubers but also bears small tubers above ground on its long, climbing vine. This SE Asian yam is also cultivated in tropical S. America, the Caribbean, and parts of the southern USA, but is nowhere a major variety. A few other species also have aerial tubers.
• D. trifida, the only native American yam which is cultivated to any extent, is known as the cushcush yam. This bears clusters of up to a dozen small tubers whose texture and flavour, by the generally undemanding standards applied to yams, are good. It is grown in the Caribbean islands and tropical S. America. It is baked or cooked as a vegetable and some cultivars have an excellent flavour.
• D. opposita, Chinese yam, a native of China, is widely grown in China, S. Japan, Korea, and nearby islands. The tubers are eaten as a vegetable, usually after slicing or grating and boiling.
• D. japonica is cultivated in several varieties in Japan. Its name there, yamanoimo (or yamaimo) means mountain tuber. Since antiquity it has been highly prized as food. Its underground tuber has a long tapering shape, sometimes reaching 2 m (over 6′) in length. The skin is brown; the flesh white. Because it is slim and brittle, digging it up without breaking it requires a high degree of expertise and once it has been successfully dug up, it is transported with great care, coddled in straw. Yamanoimo has traditionally been considered a particularly nourishing food whose energy giving quality is compared to that of the eel, and so it is sometimes called ‘the eel of the mountains’. Some people even attribute to it an aphrodisiac property. This is doubtful, but it is true that it contains a large amount of diastase and thus helps digestion. Yamanoimo is nearly always eaten raw, in the form of tororo.

Uses and Customs

Most yams keep well and were often used as provisions for lengthy sea voyages. Steaming, or boiling and mashing (or boiling and then grilling), or roasting, or frying (including fried croquettes of mashed yam) are recommended ways of cooking them. See also foo-foo.

Yams can be ground to make a kind of meal, which also keeps well.

Lovelock (1972) describes some unusual practices. In Fiji, the yam is ‘grown in earth that is hard and unprepared in the belief that it is a sporting sort of vegetable that likes to feel resistance before it will show its strength (and therefore grow large)’. He continues:

In the D'Entrecasteaux Islands, off the east coast of New Guinea, there lives a gloomy and suspicious people who believe that yams travel underground from garden to garden. They therefore spend a good deal of time trying to entice their neighbours' yams into their own plots by magic; and yet are righteously indignant if someone else's superior magic (or husbandry) produces a crop better than their own. In the neighbouring Trobriand Islands there is a much happier and more open race. These make a parade of their wealth by constructing fairly open yam-houses in order that all can see the quality of their produce, putting roots of best quality well to the fore, of course. Particularly fine yams, however, are displayed outside the stores, often framed and decorated with paint.

Contributors

Reading