Purple Sweet Potatoes: The Ancient Superfood
“It’s definitely not your typical sweet potato,” says Mary Landis, recounting an episode in which her microwave nearly erupted in flames. She’s a former “forager” for Frieda’s, a specialty produce company based in California that introduces unusual fruits and vegetables to American tables, including kiwi and dragon fruit. Companies like Frieda’s that specialize in exotic produce make it their priority to supply community supermarkets that would otherwise go without access to unique produce items.
These businesses have helped spark the appearance of more recent and now-familiar items, such as persimmons, rambutan, and cucamelon. Fruits aren’t the only products that have gained in popularity, though — specialty vegetables have also been attracting attention, particularly an unusual variety of sweet potato. Unbeknownst to some consumers, sweet potatoes come in a variety of colors besides the traditional orange, including red, yellow, and white. Perhaps most striking is the purple sweet potato, offering unrivaled health benefits and rich color.
In 2010, a sweet potato supplier of Frieda’s, A.V. Thomas Produce, told Landis they’d planted a trial of a North Carolina sweet potato that had rich, nearly black-purple skin and flesh that turned brilliant violet in the oven. “It’s a gorgeous sweet potato,” says Landis. “And the antioxidants don’t cook out. Usually when you cook a vegetable with dark coloring, the color is lost. A purple bell pepper turns brown when you cook it, and a purple cauliflower doesn’t keep its color in the pot.” Jeremy Fookes, retail sales specialist for A.V. Thomas, says this sweet potato’s color retention post-cooking was “what we thought was quite special. It’s not lavender-y, but purple-purple.”
Purple-fleshed sweet potatoes are rich in anthocyanins, the same antioxidant compound that turns blueberries blue and gives them a superfood reputation. The same year A.V. Thomas trialed the mysterious purple-purple sweet potato, TV personality Dr. Mehmet Oz listed another cultivar — pale-skinned, mauve-fleshed beni-imo from Okinawa, Japan — as No. 1 of his top five superfoods. A year earlier, in 2009, The Oprah Winfrey Show had featured beni-imo when she interviewed researchers about Okinawa Island’s many centenarians and their diet consisting of about 60 percent sweet potatoes. Americans started looking to put the ‘Okinawan’ purples on the table, but these tubers were hard to come by. “They’re only grown in Hawaii and on Japan’s Okinawa Island, and they have to be irradiated when they come into the U.S.,” says Landis. “A lot of people don’t like that.”
A.V. Thomas tried growing the Japanese sweet potatoes on its plot in Merced County, but the crop failed dismally. “The ‘Okinawan’ sweet potato can have a difficult time growing in California because it’s very susceptible to disease,” says Fookes.
In contrast to the dismal planting success the ‘Okinawan’ has had on the U.S. mainland, the unknown purple sweet potato has thrived. To grow it commercially, A.V. Thomas was legally required to discover its origins. The produce supplier traced them as far as Stokes County, North Carolina, where, at the 2003 state fair, a woman gave fairgoer Mike Sizemore a slip (a young plant grown from the potato eye) of an unknown purple sweet potato. When it turned out to be good, Sizemore patented it as ‘Stokes Purple’, after his home county in North Carolina. Sizemore has no idea who this woman was or how she came to have the purple sweet potato. “It could have been a discarded cultivar from North Carolina State University’s Sweetpotato and Potato Breeding and Genetics Program,” suggests Fookes. “But quite frankly, I don’t know.”
Sweet Beginnings In History
The origins of domesticated sweet potatoes (Ipomoea batatas) have puzzled botanists and breeders until just recently. Sweet potatoes are grown all over the world, to the tune of 9 million tons annually, and are the seventh most important food crop because of their high global yield. Most researchers agree they originated in Central America, and carbon dating proves they were domesticated in Peru 10,000 years ago. But they’re also a staple in Africa, a fixture of Polynesian culture, and so popular in China that 60 percent of the world’s total crop is now grown there. It’d be easy to blame European explorers for distributing sweet potatoes, but the historical records just don’t add up.
Christopher Columbus was the first European to record tasting the sweet potato, on his infamous 1492 trip to the Caribbean. In his journal, he writes of two white-fleshed varieties, a sweet one the Taino people called batata, and a starchy version called aje that “has the smell of a chestnut. And anyone would think he was eating chestnuts.”
Columbus was far more taken with the sweeter batata, as was Gonzalo Fernández de Oviedo a few years later. “A batata well cured and well prepared is just like fine marzipan,” Oviedo wrote in delight, describing the way locals left the harvested roots in the shade for 10 or so days to cure before roasting. Bartolomé de Las Casas echoed that they were “as sweet as if they had been dipped in a jar of jam.” Both Columbus and Oviedo brought samples home to Spain, and from there the batata traveled south as part of the African slave trade route. Eventually, the vegetable ended up in Asia.
In the Pacific, Spanish galleons plying the Acapulco-to-Manila trade route encountered kamote, the mostly cream, yellow, or occasionally orange-fleshed roots cultivated by the Aztecs. In the Philippines, kamotes met batatas being introduced from the other direction, interbred, and meandered their intermingled way on Chinese junks north to Taiwan, China, and Korea. By 1615, these tubers had arrived on the only Japanese island not enshrouded by nationalist isolation policies — Okinawa.
‘Okinawan,’ ‘Stokes Purple,’ And ‘Beauregard’
“The ‘Stokes Purple’ is not like an ‘Okinawan,’” says Landis. Craig Yencho, a sweet potato breeder at North Carolina State University (NCSU), agrees. “It’s got a dry flavor, a little bit mealy, semisweet — It’s just kind of unique.”
North Carolina grows about 60 percent of the sweet potatoes consumed in the U.S., primarily the melting, deep-orange-fleshed cultivar called ‘Beauregard’ that we throw marshmallows onto and mistakenly call “yam” every Thanksgiving and Christmas. Purples are rare.
“In the U.S., ‘Stokes’ is the only really popular or widely grown purple cultivar,” says Yencho. “I think it’s more of a recent arrival to this country, coming here in the pockets of any number of immigrants from Southeast Asia.”
So much for Fooke’s hypothesis. NCSU has bred purple sweet potatoes for years, but until recently, focused on using them for alternative functions, such as dye. “Anywhere you use reds and blues, you can use sweet potato pigment,” Yencho explains. “The average purple sweet potatoes are very, very high in anthocyanins — much darker than the ‘Okinawan,’ and they typically taste terrible. They’re very bitter.”
Which is why A.V. Thomas Produce was so excited to find a palatable purple. But Yencho believes they can do better than the ‘Stokes Purple.’ Soon, Yencho’s team will release four new purple cultivars that are sweeter, smoother, and moister, more like a purple-fleshed ‘Beauregard.’ The main difference is moisture content. While ‘Beauregard’ has about 30 percent moisture content, ‘Stokes’ is as low as 10 percent. It tastes dry because it is dry, and that makes it next to impossible to microwave — because microwaves function by heating the water molecules inside the food. “It almost caught on fire,” says Landis, describing a microwaving incident with a ‘Stokes’ sweet potato. “It was like a charcoal briquette. There was just no way to get it fully cooked without obliterating it. So don’t try it.”
The trick to cooking the ‘Stokes,’ she says, is roasting it, and for a longer time than you would a typical sweet potato. If you’d wrap a ‘Beauregard’ in aluminum foil and pop it in the oven for an hour, you’d cook a ‘Stokes Purple’ the same way, but for 1-1/2 hours.
“The first few years it was really difficult to get people to understand that this is something different,” Landis says. But unlike Yencho, she doesn’t see the dryness as a bad thing. It’s part of what makes ‘Stokes’ unique among American sweet potatoes.
“It’s quite simple,” explains Vincent Lebot, a roots crop researcher based in Vanuatu. “In the North American market, sweet potato is often used for dessert-type dishes. In Polynesia and some parts of Southeast Asia, they cook and eat them like cassava or taro — something you can eat every day in significant quantities, like rice.”
Sweet potatoes were historically the staple carbohydrate for many Polynesian cultures. When Captain James Cook explored uncharted Pacific islands in the 1770s, he found that the Samoans already had ‘umala, the Hawaiians ‘uala, and the Maori kumara. Each culture had hundreds of varieties in all colors ingrained into their foodways. Most of these tubers were dry, starchy, and flavorful, but not very sweet — more like the chestnutty aje that Columbus neglected to send back to Spain. Polynesian sweet potatoes were, and still are, no marzipan-flavored batatas.
For around a century, the question of how sweet potatoes beat Captain Cook to the South Seas troubled botanists. The linguistics clearly linked them to the Peruvian Quechua word kumal. Researchers wrote papers like Hornell’s 1946 work “How Did the Sweet Potato Reach Oceania?” speculating that perhaps, just perhaps, Polynesian sailors might’ve had the technology to beat Europeans to the Americas.
Then, between 2009 and 2013, Lebot oversaw a series of studies that compared modern samples with the DNA of 200-year-old sweet potato plants collected during Cook’s voyages. The resulting genetic family tree proved that the Polynesian sweet potato really did depart Peru more than 400 years before Columbus tasted his first batata, and evolved in isolation into something just a bit different.
As far as ‘Stokes Purple,’ we may never know where it came from, but at least it’s easy to find. In 2015, Trader Joe’s began carrying the starchy vegetable in all continental states.
“I’m really happy to see sweet potato growers succeed in any form, and I like to see the appearance of new varieties,” says Yencho.” This one has a unique history, though — there’s no doubt about it.”
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