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Sweet Potato

SWEET POTATO

SWEET POTATO. Sweet potato (Ipomoea batatas) is in the botanical family Convolvulaceae along with common plants, such as bindweed and morning glory. The generic name Ipomoea comes from the Greek words "ips," meaning bindweed, and "homoios," meaning similar. Sweet potatoes should not be confused with ordinary potatoes (Solanum tuberosum) as they are entirely unrelated, although their uses can be similar. Orange-fleshed sweet potatoes are often known as yams, especially in the southern United States, but they are quite different from true yams (Dioscorea sp.) in growth habit and use. Furthermore, unlike true yams, the greens of sweet potatoes are edible and provide an important source of food in Africa and Asia.

Identification

Sweet potato is a perennial that is usually grown as an annual. It grows from underground tuberous roots with trailing, twisting stems that can be as long as twenty feet (six meters). Leaves are variable in shape, size, and color but are generally more or less heart-shaped and green with purple markings. The single flowers are funnelshaped and white or pale purple but are rarely seen in temperate regions. Roots grow where stem nodes touch the ground, and most develop into the edible storage roots, usually four to ten storage roots per plant.

Diversity

The International Potato Center (CIP) in Peru holds the largest sweet potato gene bank in the world with more than 6,500 wild, traditional, and improved varieties. Many of these are unique to a particular country or region. For example, an anthropologist in Irian Jaya found forty different cultivars of sweet potato growing in just one community garden. In contrast, Stephen Facciola's Cornucopia II (1998) lists only twenty-five different varieties available for the whole United States. Sweet potato flesh can be white, yellow, purple, red, pink, violet, and orange, while skin color varies among yellow, red, orange, and brown. Varieties with pale yellow or white flesh are less sweet and moist than those with red, pink, or orange flesh. They also have little or no beta-carotene and higher levels of dry matter, which means their textures are drier and more mealy and they stay firmer when cooked. Sweet potatoes also vary enormously in size, shape, taste, and texture, although all are smooth-skinned with roots always tapered at both ends.

Nutrients

All varieties of sweet potato are good sources of vitamins C and E as well as dietary fiber, potassium, and iron, and they are low in fat and cholesterol (see the Table for more detail). The orange-and red-fleshed forms of sweet potato are particularly high in beta-carotene, the vitamin A precursor.

Origins

Scientists debate the exact place of origin of I. batatas, although the evidence points toward Central America rather than South America. They are no longer found growing in the wild, but it is possible that the wild Mexican sweet potato I. trifida is an ancestor. Sweet potatoes have been cultivated for more than five thousand years, and fossilized remains found in the Andes have been dated at about 8,000 years old. Genetic studies suggest the likelihood that in early times sweet potatoes were carried by the local people from island to island, spreading gradually across the Pacific from Central and South America to eastern Indonesia, New Guinea, Polynesia, and New Zealand. Christopher Columbus is credited with taking sweet potatoes from the New World back to Spain, from where they spread through the warmer regions of Europe and were transported to other parts of

Constituents of the sweet potato: values per 100g (3.5 oz.) edible portion
  Units Raw sweet potato Cooked, baked in skin Cooked, boiled without skin
Water g 72.84 72.84 72.84
Energy kcal 105 103 105
  kj 439 431 439
Protein g 1.65 1.72 1.65
Total lipid (fat) g 0.30 0.11 0.30
Carbohydrate by difference g 24.28 24.27 24.28
Fiber, total dietary g 3.0 3.0 1.8
Ash g 0.95 1.06 0.95
Calcium Ca mg 22 28 21
Iron Fe mg 0.59 0.45 0.56
Magnesium Mg mg 10 20 10
Phosphorous P mg 28 55 27
Potassium K mg 204 348 184
Sodium Na mg 13 10 13
Zinc Zn mg 0.28 0.29 0.27
Copper Cu mg 0.169 0.208 0.161
Manganese Mn mg 0.355 0.560 0.337
Selenium Se mcg 0.6 0.7 0.7
Vitamin C mg 22.7 24.6 17.1
Thiamin B1 mg 0.066 0.073 0.053
Riboflavin B2 mg 0.147 0.127 0.14
Niacin B3 mg 0.674 0.604 0.64
Pantothenic acid B5 mg 0.591 0.646 0.532
Vitamin B6 mg 0.257 0.241 0.244
Folate, total mcg 14 23 11
Vitamin B12 mcg 0 0 0
Vitamin A, IU IU 20,063 21,822 17,054
Vitamin A, RE mcg-RE 2,006 2,182 1,705
Vitamin E mg-ATE 0.280 0.280 0.280
source: U.S. Department of Agriculture. Agriculture Research
Service Nutrient Database for Standard Reference, Release 14, 2001.

Asia and to Africa by Spanish and Portuguese explorers and traders. Sweet potatoes were grown in gardens by North American Indians and were an important staple food during the American Revolutionary War and the American Civil War. They were also an essential part of the diet of the slave population in southern states. Most large plantations had a sweet potato plot and root cellars beneath cabins for potato storage.

Popularity

At the beginning of the twentieth century sweet potatoes were the second most important root crop in the United States. In 1920 the per capita consumption of sweet potatoes was thirty-one pounds (fourteen kilograms), but consumption steadily declined. In 1999 consumption was only 4 pounds (1.8 kilograms) per person (U.S. Department of Agriculture, 2000).

Sweet potatoes grow in all warm, humid areas of the world and at the beginning of the twenty-first century were the seventh largest world food crop, 95 percent of which is produced in developing countries. They are typically grown by small farmers, often on marginal ground. This crop plant has a long history of saving lives. It matures fast, is rich in nutrients, and is often the first crop planted after a natural disaster, providing abundant food for otherwise starving populations. In eastern Africa the sweet potato is known as "the protector of children" or cilera abana because it is often the only food that stands between a child's survival and starvation.

Growing Sweet Potatoes

Sweet potatoes are tropical plants that can also be grown in the summer in temperate regions as long as they have at least five frost-free months combined with fairly warm days and nights. They can be grown from vine cuttings or by planting pieces of the roots. To grow new plants, place one or more sweet potatoes in a bed of sand and cover with a couple of inches of moist, sandy soil. When the sprouts reach about 10 inches (25.4 centimeters), detach by twisting and transplant to the place they are to grow. Push sprouts about 3 inches (7.6 centimeters) into the ground and water well. Leave a distance of about 1 foot (30 centimeters) between plants and 3 feet, 3 inches (1 meter) between rows. Sweet potatoes do best in full sun with fertile, open sandy-loam soils. They also like some added manure (well rotted) and compost, although they should not be given too much nitrogen as this encourages leaf growth at the expense of root growth. They benefit from regular additions of potash. Roots will be bigger and easier to harvest if sprouts are planted into raised mounds about 1 foot (30 centimeters) high. This is particularly important in heavy or wet soil. Once established, apart from occasional weeding, sweet potatoes need little care.

Harvesting

Roots are harvested as the leaves begin to yellow in the fall. They are then brushed clean and left to cure. Traditionally curing involved stacking the potatoes in the field or garden, covering them with sand, and leaving them for several weeks. Sweet potatoes in commercial production are cured in rooms with humidity between 75 percent and 80 percent and temperatures between 80°F and 86°F (27°C and 30°C). Curing heals cuts and reduces decay and shrinkage during storage, and it converts some starches to sugars, improving the flavor. Once cured, sweet potatoes can be stored for several months, and white-fleshed varieties last as long as ten months.

In the United States most of the sweet potato crop is canned. These are usually the smaller roots. Roots of good size are sold fresh, and any that are too large are generally processed into baby food.

Buying

When buying sweet potatoes, always choose ones that are firm with even skin coloration and no signs of decay. They should never be stored in the refrigerator. Keep them in a cool, dry, well-ventilated container (a basket is ideal) at about 55°F to 60°F (13°C16°C). Generally they should be used within two to three weeks of purchase because it is not possible to determine how long they have already been stored before purchase.

Who Grows Sweet Potatoes?

According to figures released by CIP, more than 148.77 million short tons (135 million metric tons) of sweet potatoes are grown worldwide. China is by far the largest producer with about 87 percent of the crop, nearly half of which is fed to animals. The rest of Asia accounts for 6 percent, Africa 5 percent, Latin America 1.5 percent, and the United States 0.45 percent. In most developing countries, where the sweet potato is part of the staple diet, the white-or cream-fleshed forms with a bland taste are usually grown. These potatoes have a high dry matter content, which means they are a good energy source, which is vital for a staple food. In developed countries, where the sweet potato is used more as a vegetable or for sweet dishes, the red-or orange-fleshed types are preferred for their moist flesh and sweet flavor. The U.S. sweet potato crop was worth $214,980,000 in 1999, and just under a third of the crop was grown in North Carolina. Louisiana, Mississippi, and California also grow significant quantities. The largest European producer is Portugal with only .02 percent of world production. These figures clearly illustrate that sweet potatoes are an important crop in third world countries but are a secondary foodstuff in first world countries.

Preparing and Eating

Sweet potato roots can be boiled, steamed, baked, and fried. They are also canned or dried and made into flour, cereal, and noodles. Like pumpkins, sweet potato roots are often used in sweet dishes, such as pies, puddings, biscuits, cakes, and desserts. In some countries roots are processed to produce starch and fermented to make alcohol. Cooked red-or orange-fleshed sweet potato roots are sweet, soft, and starchy with a flavor that resembles roasted chestnuts and baked squash. Sweet potatoes are prepared by scrubbing and cutting into appropriately sized pieces. Leave the skin on if they are to be baked, boiled, or steamed; peel before frying. Cooking in the skin preserves more of the nutrients. Once the roots are cooked (when a knife can be easily inserted), they can be served whole or peeled and mashed, pureed, or sieved and served as a sweet or savory vegetable, depending on what is added.

In the United States sweet potatoes are probably best known for their use in pies and as a candied vegetable. They are a traditional accompaniment to Thanksgiving dinner and often appear on the menu at other festival times, such as Christmas and Easter. Sweet potatoes can be substituted for potatoes, apples, or squash in almost any recipe. Cooked, mashed sweet potatoes are also used to replace some of the wheat flour in breads, cakes, muffins, and cookie recipes, as is sweet potato flour. Sweet potatoes cooked in their skins can be frozen. Wrap each piece in aluminum foil or freezer wrap, place into a freezer bag, and freeze.

In third world countries sweet potatoes are processed into starch, noodles, candy, desserts, and flour. This allows the farm household to extend the availability of the crop. In China, for example, sweet potato starch production has become an important cottage industry, while in Uganda sweet potatoes are sliced and dried, which allows them to be kept for about five months. The dried pieces are also ground into flour, which is then rehydrated and eaten as a thick porridge known as atapa.

Although usually the roots are eaten, young leaves and the tips of vines can be harvested, washed, and boiled as a green vegetable or added to stir-fries. All parts of the sweet potato are used as stock feed, although the roots are often cooked first.

As a Medicine

Sweet potato roots and leaves are used in folk remedies to treat illnesses as diverse as asthma, night blindness, and diarrhea. Easily digestible, they are good for the eliminative system. It is believed they bind heavy metals, so they have been used to detoxify the system.

Sweet Potatoes in Africa

In eastern and southern Africa some 3 million children under the age of five suffer from xerophthalmia or dry eye, which causes blindness. Dry eye is caused by a lack of vitamin A in the diet, and many of the affected children die within a few months of becoming blind. The yellow-and orange-fleshed varieties of sweet potatoes are high in beta-carotene, which can be converted into vitamin A in the intestines and liver. It has been shown that even small amounts of these sweet potatoes as a regular part of the diet will eliminate vitamin A deficiency in adults and children. African countries have traditionally grown white-fleshed sweet potatoes, which are low in vitamin A. A ten-year research project concluded that varieties high in beta-carotene could compete with production levels of the white-fleshed varieties and would be acceptable to local tastes. Consequently CIP and related organizations launched a regional effort to encourage African women to also grow orange-fleshed varieties.

At the same time researchers at CIP have combined parental clones of sweet potatoes to yield a group of yellow and orange potatoes with high dry matter (a characteristic of the white-fleshed forms of sweet potato) that they believe will be more acceptable to African consumers. In Kenya sweet potatoes are mostly grown in the densely populated Western Province, where often more than half the crop is destroyed by a virus. In 2000 the Kenyan Agricultural Research Institute released genetically modified sweet potatoes with increased disease resistance and assured the public that these potatoes would be largely resistant to the virus.

Twenty-first Century Changes

In the last four decades of the twentieth century the uses of sweet potatoes diversified beyond their classification as subsistence, food security, and famine-relief crops. In particular the last decade of the century saw a concentrated, coordinated effort to fully realize the potential of this crop. The hoped for result is that millions of subsistence landholders in Africa, Asia, and Latin America will be able to use sweet potatoes for food, stock food, and processed products and to generate income.

The United States is also exploring the potential of sweet potato products. A patent was granted for the production of bread made from 100 percent sweet potato flour. It is hoped that these products will appeal to consumers who are allergic to grain breads and flours. Also scientists at two different institutes in the United States have developed genetically modified sweet potatoes containing edible vaccines. One of these vaccines works against hepatitis B and the other against the Norwalk virus found in food that has not been handled or stored correctly. Edible vaccines such as these may provide cheap protection for some of the poorest people in the world.

See also Columbian Exchange ; Potato ; Tubers ; Vitamins .

BIBLIOGRAPHY

Facciola, Stephen. Cornucopia II: A Sourcebook of Edible Plants. Vista, Calif.: Kampong Publications, 1998.

Herklots, G. A. C. Vegetables in South-East Asia. London: George Allen and Unwin, 1972.

International Potato Center. Available at http://www.cipotato.org.

Musau, Z. "Genetically Modified Sweet Potato Launched in Kenya." Nation, 19 August 2000.

North Carolina Sweet Potato Commission. Available at http://www.ncsweetpotatoes.com.

Onstad, Dianne. Whole Foods Companion: A Guide for Adventurous Cooks, Curious Shoppers, and Lovers of Natural Foods. White River Junction, Vt.: Chelsea Green Publishing, 1996.

U.S. Department of Agriculture, Agriculture Research Service. Nutrient Database for Standard Reference, Release 14. Washington, D.C.: Government Printing Office, 2001.

U.S. Department of Agriculture, Economic Research Service. Situation and Outlook, Vegetables and Specialties. Washington, D.C.: Economic Research Service, 2000.

Woodward, Penny. Asian Herbs and Vegetables. Flemington, Victoria: Hyland House Publishing, 2000.

Penny Woodward

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sweet potato

sweet potato, trailing perennial plant (Ipomoea batatas) of the family Convolvulaceae (morning glory family), native to the New World tropics. Cultivated from ancient times by the Aztecs for its edible tubers, it was introduced into Europe in the 16th cent. and later spread to Asia. It is now the most important of tropical root crops and is grown in many varieties (differentiated by their leaf shapes). In the United States it is cultivated chiefly in the South, though a few hardy varieties are grown as far north as Massachusetts. Sweet potatoes are used mostly for human consumption but are sometimes fed to swine. They yield starch, flour, glucose, and alcohol and are especially rich in vitamin A. The sweet potato is sometimes confused with the yam, which belongs to another family. Sweet potatoes are classified in the division Magnoliophyta, class Magnoliopsida, order Solanales, family Convolvulaceae.

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potato, sweet

potato, sweet Tubers of the herbaceous climbing plant Ipomoea batatas, known in Britain before the Irish potato. The flesh may be white, yellow, or pink (if carotene is present); the leaves are also edible. A 200‐g portion is a rich source of vitamins A (as carotene if pink) and C; a source of iron and vitamin B1; provides 4.5 g of dietary fibre; contains 0.4 g of fat, of which 16% is saturated; supplies 170 kcal (700 kJ).

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sweet potato

sweet po·ta·to • n. 1. an edible tropical tuber with pinkish orange, slightly sweet flesh. 2. the widely cultivated Central American climbing plant (Ipomoea batatas) of the morning glory family that yields this tuber. 3. inf. another term for ocarina.

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sweet potato

sweet potato Trailing plant native to South America and cultivated as a vegetable in Japan, Russia, USA, and the Pacific. Its flowers are pink or violet. The orange or yellow, tuber-like root is edible. Family Convolvulaceae; species Ipomoea batatas.

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sweet potato

sweet potato See potato, sweet.

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sweet potato

sweet potato See IPOMOEA.

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