Stokes Purple sweet potato grown in Livingston, Calif. (David Karp )
When the Stokes Purple sweet potato shows up in markets next week, it's hard to say what will be more intriguing: its look, with dramatically deep purple skin and flesh, its flavor or the mystery of its origins.
It was discovered in the United States by Mike Sizemore, 61, who grew up on a farm in North Carolina, the nation's largest sweet potato-producing state. He said in a phone interview, speaking in a delicious Southern drawl, that he worked for 30 years catching car thieves for the state government before retiring in 2003. He always wanted to farm and saw sweet potatoes as a replacement for tobacco, cultivation of which was declining in his area, Stokes County. One day in 2003 he won a prize for his sweet potatoes at a state fair, and an unidentified woman gave him some deep purple sweet potatoes of unknown origin.
Sizemore propagated them and found that he loved the variety, which keeps its deep color remarkably well when cooked. Stokes Purple has a rich, almost winy flavor but is denser and drier than regular sweet potatoes. The key is to bake it for longer than regular sweet potatoes, and at moderate heat, about 90 to 120 minutes at 350 degrees, at which point it becomes pleasingly moist.
Sizemore obtained a patent on his discovery and started marketing it commercially in 2006. He and contract growers for his Stokes Foods raised up to 80 acres of the variety and now have 30 acres. This year he licensed it to California's largest sweet potato grower and distributor, A.V. Thomas Produce of Livingston. (Ninety percent of the state's crop is grown nearby in the sandy soil of Merced County.) They planted 40 acres, harvested a good crop in September and are selling it exclusively through specialty produce distributor Frieda's Inc. — appropriately, since that company's signature color is purple.
The deep color of Stokes Purple derives from multiple anthocyanins, the pigments responsible for the red and purple hues of cherries, strawberries, purple carrots and many other fruits, vegetables and flowers. Medical researchers have found evidence that anthocyanins and related phenolic compounds may have beneficial health effects, scavenging free radicals that can cause cancer, protecting the liver and lowering blood pressure. Scientists debate just how beneficial such compounds are, but the growers and marketers of Stokes Purple are counting on the possible health benefits to boost demand.
Native to Colombia and southern Central America, sweet potatoes typically have brown, red-orange or white skin and orange or white flesh. Soon after Columbus, they were brought to Asia, and varieties with white skin and speckled pale purple, dry flesh developed on the Japanese island of Okinawa. Such kinds are widely grown in Hawaii; 12 million pounds a year, irradiated to kill insect pests, are exported to the United States mainland, where they are popular with Asians and Latinos.
Okinawan types are considerably drier and lighter in color than Stokes Purple and have a mealier texture when cooked. They don't yield particularly well on the mainland. Scott Stoddard, a vegetable crops farm advisor with UC Cooperative Extension in Merced County, estimates that Okinawan is grown on less than 40 acres in his area.
So where did the Stokes Purple really come from? A decade ago, when Sizemore was awarded his prize at the state fair, there were already a number of purple-fleshed sweet potato selections available to enthusiasts on the East Coast, said Craig Yencho, a sweet potato breeder and geneticist at North Carolina State University. In the last three or four decades Japanese sweet potato breeders have come up with several deep purple varieties, which are widely grown for use in cooking, in nutraceutical products and to replace synthetic red and blue food dyes. It is possible that one of these Japanese varieties found its way to North Carolina and into Sizemore's hands. Meanwhile, a company has asked Sizemore to grow large quantities of Stokes Purple for processing.
Deep purple sweet potatoes are tricky to breed, says Yencho, because the darker types tend to be bitter. He does have some purple-fleshed sweet potato test selections with improved disease resistance and yield in the pipeline, he says, so we may be seeing more deep purple sweet potatoes before long.
California-grown Stokes Purple, which is not irradiated, will be available from next week at Sprouts and Bristol Farms stores in Southern California, at about $2 a pound, more than twice the price for regular sweet potatoes. Sizemore, who has spent a lot of money building his brand but has yet to see much profit and has battled pirates who illegally grew his variety, certainly hopes that Stokes Purple succeeds here.
It would be a shame, he says, if he had to go back to chasing car thieves.