Ipomoea batatas, the most important of the tropical root crops, is the starchy tuber of a vine of the convolvulus and morning glory family. It is not related to the ordinary potato, although both plants are of American origin.
The sweet potato is the cultivated descendant of a wild plant, remains of whose tubers have been found in a cave in Peru inhabited before 8000 bc. It was taken into cultivation during the last centuries bc, well before the time of the Incas, and became a staple food all over tropical America as far north as Mexico and on the Caribbean islands.
It is likely that it was during the 13th century ad that the sweet potato was taken westward to Easter Island and Hawaii, and in the next century to New Zealand. These dates have been calculated from Hawaiian and Maori records. It could be that Peruvians aboard a raft were blown out into the ocean and made a lucky landfall on the sparse islands to the west. However, the voyage may have been deliberate; Thor Heyerdahl repeated it in his Kon-Tiki expedition of 1947, using a similar raft, to prove that it was possible. It had previously been supposed that the sweet potato must have been carried to the Pacific islands by Europeans, at a later time. However, the validity of Heyerdahl's demonstration is supported by the fact that the ancient Peruvian name for the sweet potato, kumar, is found with only minor variations (kumala, gumala, umala) in several Pacific island languages.
The first Europeans to taste sweet potatoes were members of Columbus' expedition to Haiti, in 1492. Later explorers found many varieties as they extended the range of their expeditions. Early accounts give various local names, aji, camote, apichu, and others; but the name which stuck was the first known Haitian one, batata. Later this name was accidentally transferred to the ordinary potato after its discovery by Europeans in 1537, causing a confusion which still persists.
Native American sweet potatoes in use at the time were not all sweet. Some were plainly starchy and others markedly fibrous, as were those which had been taken to the Pacific islands. But the European explorers were interested only in the sweet kinds, and it is these which have been spread by European influence while the others have largely died out.
The sweet potato was cultivated in the south of Spain from the early 16th century, and proved a popular novelty. Attempts to grow it further to the north were only partly successful. The plant is easily grown from roots or cuttings in hot countries; but in cooler regions it has to be started in a hothouse. Although for this reason it could never become a major food crop in Europe, it enjoyed two brief vogues in France during the 18th and early 19th centuries, since both Louis XV and the Empress Josephine, who was a Creole, were partial to it.
Sixteenth-century Spanish explorers took the sweet potato to the Philippines; and from there Portuguese traders spread it to the E. Indies and India. It is generally accepted that the sweet potato reached China at the end of the 16th century. There was a famine in Fujian province in 1593 and the governor sent an expedition to the Philippines to search for food plants. Next year the ships returned with sweet potatoes, which soon became a staple of that part of China. Early in the 18th century the sweet potato passed into Japan. The fact that it is called karaimo (i.e. Chinese potato) in the Ryukyu Islands (Okinawa), ryukyu-imo in Satsuma, and satsumaimo in the rest of the country is said to indicate the route by which it arrived.
It was probably slave traders who introduced the sweet potato to Africa, where it was called igname or nyam, which simply means ‘yam’. Since that time the sweet potato has been steadily displacing the true yam as a major carbohydrate food in tropical Africa.
On the N. American mainland sweet potatoes had long been grown by the Indians in Louisiana, where de Soto found them in 1540, and as far north as Georgia. By 1648 the colonists in Virginia were cultivating them. The sweet potato was especially valued during the war against the British and the Civil War, for it grows quickly and its underground habit makes it less vulnerable than surface crops to deliberate destruction.
There are numerous varieties. Most have tubers which are about the size of medium ordinary potatoes, and generally of an elongated, slightly pointed shape, though there are also round kinds. The skin may be white, yellow, red, purple, or brown, and the flesh white, yellow, orange, or even orange-red. The two main categories of sweet potato now grown are best distinguished as ‘soft’ and ‘firm’; one becomes soft and moist when cooked, while the other remains mealy and relatively firm. The flesh of the soft ones is apt to be orange, and that of the firm ones white or yellow. In the USA the soft kind is sometimes called ‘yam’; a misnomer, as the true yam is a different plant.
The boniato, often regarded as a vegetable in its own right although correctly known as a cultivar of the sweet potato, is of the firm kind, with a brown or red skin and white flesh. It is outstandingly mealy when cooked, and has found its way from Cuba and the Antilles, where it has long been popular, to the USA.
Yellow flesh indicates the presence of carotene, a source of vitamin A. Sweet potatoes provide only half as much protein as ordinary potatoes, but contain much starch and a little sugar. The tubers do not store well. Traditional methods of preserving them have included partial drying of whole roots by the Maori, slicing and sun-drying in China, and candying in many countries. They can also be made into a starchy meal. Around 1766 such a product was being exported from Savannah, Georgia, to England. It never became popular, but sweet potato meal or starch is used by the food-processing industry. The tubers are sometimes canned.
Most sweet potatoes are used fresh: baked or boiled in their skins (after which they can be slipped out of their skins and mashed), or fried. They can also be used in baking breads or cakes; and chips (US French fries) can be made of them. In China and Japan, roasted sweet potatoes are sold by street vendors. Many find the flavour of sweet potato more attractive if it is further sweetened and spiced with cinnamon or nutmeg. It is often made into desserts in the USA, on the lines of pumpkin pie.
The most remarkable root in the genus Ipomoea is the manroot or man of the earth, I. leptophylla, which grows in the western USA. This huge tuber, the size and shape of a fully grown man, is not very good to eat, and remarkably difficult to dig up; but Indians have reluctantly turned to it in hungry times. Other species with edible roots are of minor importance. See also jicama, a plant which was formerly classed in the same genus.
Alan Davidson was a distinguished author and publisher, and one of the world's best-known writers on fish and fish cookery. In 1975 he retired early from the diplomatic service—after serving in, among other places, Washington, Egypt, Tunisia, and Laos, where he was British Ambassador—to pursue a fruitful second career as a food historian and food writer extraordinaire. Among his popular books are Seafood of South-East Asia, North Atlantic Seafood, and Mediterranean Seafood. In 2003, shortly before his death, he was awarded the Erasmus Prize for his contribution to European culture.
