Sweet potato

• 1. Introduction

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  The sweet potato, Ipomoea batatas, carries a name that is older than written history and a journey that crossed entire oceans before Europeans had any idea it existed. It is one of the few food crops that can honestly claim to have traveled the world twice over. The first time was carried by human hands across the Pacific. The second time came with the age of European exploration, which scattered it into kitchens and fields that would eventually make it the fifth most important food crop in developing countries today.

  Not a potato. Not a yam. Neither, technically, despite what labels in American grocery stores have insisted for generations. The sweet potato belongs to the morning glory family, Convolvulaceae, a vine that sprawls across the ground and produces trumpet-shaped lavender flowers that open before sunrise and close within a few hours. The part most people know is underground: a tapered, smooth-skinned root whose flesh can run from beige through white, red, pink, violet, yellow, and orange to deep purple.

  Radiocarbon dating puts the oldest known sweet potato remains in caves in Chilca Canyon in south-central Peru, to 8080 plus or minus 170 BC. That figure deserves a moment of stillness. Thousands of years before the pyramids, someone in coastal South America was eating this root. What brought it to a Polynesian island by the year 1210 CE? What compelled a Ming dynasty official to rush it into Fujian during a famine in 1594? And how did enslaved Africans in North America inadvertently give it a name that would confuse shoppers for centuries?

• 2. A Vine From the Morning Glories

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  Ipomoea batatas is the only crop plant of major importance within a family of roughly 50 genera and more than 1,000 species. The family includes several well-known garden flowers also called morning glories, though that name is not usually applied to the sweet potato itself. Some cultivars are grown purely for ornament under the name tuberous morning glory, prized for their green, yellow, and purple foliage rather than any edible root.

  The plant is a herbaceous perennial vine. Stems crawl along the ground and form adventitious roots at the nodes. Leaves vary dramatically in shape, ranging from heart-shaped to kidney-shaped to egg-shaped, with edges that can be entire, toothed, or cut into as many as seven lobes. Anthocyanins can accumulate along the leaf veins, turning otherwise green leaves distinctly purple. Some cultivars produce stems up to 16 meters in length.

  The flowers are five-petaled and funnel-shaped, often lavender to purple-lavender with a darker throat, though white flowers also occur. Seeds form only from cross-pollination, which means most propagation in practice happens through stem or root cuttings, or through adventitious shoots called slips that grow from the roots during storage. True seeds are reserved for breeding programs.

  I. trifida, a diploid species, is the closest wild relative. The sweet potato itself originated from an initial cross between a tetraploid and a diploid parent, followed by a second complete genome duplication. More unusually, its genome contains actively expressed sequences of DNA from Agrobacterium, a genus of soil bacteria. Those T-DNA transgenes were not found in closely related wild relatives, and studies suggest this natural genetic modification was part of what made domestication possible. This makes the sweet potato the first known example of a naturally transgenic food crop.

• 3. Pacific Crossing

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  Sweet potato has been radiocarbon-dated in the Cook Islands to between 1210 and 1400 CE, centuries before any European ship arrived in Polynesia. The most widely accepted explanation is that Polynesian voyagers traveled to South America and returned with vine cuttings. Spreading from central Polynesia, the plant reached Easter Island, Hawaii, and New Zealand.

  Genetic evidence adds texture to this picture. Researchers have identified similarities between Polynesian peoples and the Zenú, an indigenous group inhabiting the Pacific coast of what is now Colombia. Dutch linguists Willem Adelaar and Pieter Muysken pointed to something striking in the languages: the Proto-Polynesian word for the crop, reconstructed as kumala, appears in Rapa Nui as kumara, in Hawaiian as uala, and in Māori as kūmara. Quechua and Aymara, spoken in the Andes, have the forms k'umar and k'umara. Adelaar and Muysken described this resemblance as near proof of contact between the Central Andes and Polynesia.

  The Māori varieties that arrived in New Zealand bore elongated tubers with white skin and whitish flesh, distinct from the orange cultivars now most common there. Māori grew several small, yellow-skinned, finger-sized varieties with names including taputini, taroamahoe, pehu, hutihuti, and rekamaroa. Modern trials showed these varieties were capable of good yields, but American whalers, sealers, and trading vessels introduced larger cultivars in the early 19th century and those quickly came to dominate.

  A 2021 analysis of material excavated near Dunedin, roughly 250 kilometers south of Christchurch, found that sweet potatoes were grown and stored there during the 15th century. Previously, archaeologists had believed the crop could not survive that far south because of the colder climate. The site's abandonment may have been connected to the Little Ice Age.

• 4. Famine, Trade, and Deliberate Spread

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  The plant reached the Philippines during the Spanish colonial period of 1521 to 1898, carried aboard the Manila galleons alongside other New World crops. From Luzon, it crossed to the Chinese mainland in about 1594, when the Governor Chin Hsüeh-tseng introduced it to Fujian in response to a major crop failure.

  Japan received the sweet potato from Portugal in the early 1600s, arriving via the Ryukyu Kingdom, present-day Okinawa. It became a buffer against famine, valued precisely because it could feed people when rice harvests failed. Aoki Konyō promoted its cultivation, and the Tokugawa bakufu sponsored, published, and distributed a Japanese-language translation of his research monograph to encourage wider growing. The shogun Tokugawa Yoshimune planted sweet potatoes in his private garden.

  Korea received the crop in 1764. Two men, Kang P'il-ri and Yi Kwang-ryŏ, traveled to Tongnae starting in 1764 to learn cultivation techniques from Japanese growers, then returned to Seoul in 1766 to put that knowledge into practice. The project succeeded for a year but ultimately collapsed in the winter of 1767 after Kang's unexpected death.

  Meanwhile, the crop arrived in Europe through the Columbian exchange. An English reference appears in Elinor Fettiplace's Receipt Book, compiled in 1604, making it one of the earliest documented appearances in European cookery. In Spain, sweet potato is called boniato, and in the Veneto region of northeastern Italy it is still known in the Venetian dialect as patata mericana, the American potato. As of 2023, Spain is the largest sweet potato producer in Europe.

• 5. The Yam Confusion

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  The first Europeans to taste sweet potatoes were members of Christopher Columbus's expedition in 1492. Later explorers encountered the plant under many local names, but the one that persisted came from the Taíno people: batata. The Spanish then blended batata with the Quechua word for potato, papa, to create patata, the word for the common potato in Spanish. Two different roots. One borrowed name shared between them.

  The word traveled further. Spanish patata was borrowed into Arabic as batata because Arabic lacks a /p/ sound, while the sweet potato itself was called batata hilwa, meaning sweet potato. Arabic then passed batata into Hebrew, where it designated the sweet potato, because Hebrew already had its own word for the common potato: tapuakh adama, literally earth apple, the same logic as the French pomme de terre.

  In parts of North America, a different linguistic confusion took hold. When soft sweet potato varieties were first grown commercially, there was a need to distinguish them from firmer varieties. Enslaved Africans called the soft sweet potatoes yams because they resembled the unrelated yams they had known in Africa. The name stuck. Actual yams are monocots in the order Dioscoreales, botanically distant from sweet potatoes, which belong to Solanales. A third plant, the oca, Oxalis tuberosa, a wood sorrel, is called a yam in many other parts of the world.

  Some researchers and organizations now advocate for writing the name as a single word, sweetpotato, precisely to signal its genetic distinctiveness from both potatoes and yams. In current American English, the two-word version remains preferred.

• 6. Nutrition and Global Table

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  Baked sweet potato is 76% water and 21% carbohydrates. In a 100-gram serving baked in its skin, it provides 359 kilojoules of food energy and exceeds 20% of the daily value for vitamin A at 120%, vitamin C at 24%, manganese at 24%, and vitamin B6 at 20%. Cultivars with dark orange flesh carry more beta-carotene than light-fleshed varieties, and their increased cultivation is being actively encouraged in Africa, where vitamin A deficiency is a serious public health concern.

  According to the United Nations Food and Agriculture Organization, sweet potatoes are the most efficient staple food to grow in terms of farmland, yielding approximately 28,000 kilocalories per acre daily. Global production in 2020 reached 89 million tonnes, with China accounting for 55% of that total. Secondary producers were Malawi, Tanzania, and Nigeria. Per capita consumption in the United States, once about 13 kilograms annually around 1920, has fallen to roughly 1.5 to 2 kilograms today.

  The plant's culinary range is wider than most people realize. In northeastern Uganda, sun-dried root slices called amukeke are a breakfast staple served with peanut sauce, while emukaru, an earth-baked root, is eaten as a snack at any hour. In Egypt, vendors sell the tubers from carts fitted with ovens along the Nile or the sea, often drizzled with honey. In northeastern Chinese cuisine, chunks of sweet potato are fried and then submerged in boiling syrup. In Indonesia's West Papua highlands, sweet potatoes are cooked in a pit using the bakar batu method, heated by fire-warmed rocks layered beneath leaves, alongside vegetables and pork.

  In Japan, the sweet potato gave rise to imo-jōchū, a variety of shōchū distilled from the fermented root rather than rice. In Korea, sweet potato starch is used to make dangmyeon, the cellophane noodles used in japchae. And globally, chef Bun Lai of Miya's Sushi is credited with introducing sweet potato as a sushi ingredient in the 1990s, creating plant-based maki rolls that eventually spread into mainstream sushi cuisine worldwide.