Plant breeder Ted Carey jokes that he´s spending so much time in developing a purple sweet potato because it could translate into a fan-favorite "niche" product. His vision is purple french fries, sold at Kansas State University ("purple pride") football games.
Carey´s No.1 reason for the research, however, is the cancer-reducing potential of the root vegetable´s anthocyanins - pigments that make fruits and vegetables purple, blue or red. Anthocyanins are a special class of phenolic compound. Foods with a high phenolic content can also provide both anti-aging and antioxidant factors.
No matter their color, however, all sweet potatoes are basically fat-free, he said. They´re a low-sodium, low-calorie powerhouse of vitamins and minerals. And, they´re "growable" in the central Great Plains.
Kansas farmers used to produce lots of them -- especially in the sandier soils of the Arkansas and Kansas river valleys. Carey´s based in the former production region at K-State´s Research and Extension Center near Olathe. He enjoys reminding people that Wamego, Kan., could once call itself the sweet potato capital of the world.
"Beyond that, though, Kansas is now 45 among the 50 states in vegetable production. So, Kansans´ providing more of all kinds of fresh-picked fruits and vegetables - including purple ones -- would be a health benefit for everyone in the state," he said.
K-State has been a member of the National Sweetpotato Collaborators Group since 1939. The group played a supportive role in K-State´s early development of such varieties as Kandee and Lakan (a joint project with Louisiana State University).
Even so, Carey got the start for his current "purple" search from scientists in Lima, Peru - home of the International Potato Center (Centro Internacional de la Papa or CIP).
The CIP is the germplasm bank for the world´s vast array of potatoes, sweet potatoes, and other root and tuber crops. It ensures that no variety´s genes disappear into history. It also provides documented source materials and improved germplasm for research worldwide.
"The CIP breeder sent me about 2000 seeds from crosses between purple parent plants that looked promising for regions like ours. In 2007, we planted those seeds at K-State´s John C. Pair Horticulture Center near Wichita. Each seed had the potential to be a unique new variety," Carey said.
In general, only plant breeders grow sweet potatoes from seed. If and when a promising plant emerges, they clone it -- reproduce it vegetatively -- from that point on.
Carey said the basic cloning process is simple: Root some vine cuttings from the seed-grown plant.
"But kids have been using another approach to since Grandma´s day. They stick four toothpicks into a sweet potato´s sides, so they can suspend it - pointed or root end down -- on the rim of a jar of water. Then they add water, as needed, and watch the sweet potato change," he said. "Within days, fibrous roots sprout into the water. In a few weeks, stems and leaves emerge on top, and the kids have a new plant."
In cold-winter regions, farmers and gardeners typically adapt the basics to make larger scale cloning easier, Carey said. They harvest and store selected plants´ storage roots. Then, in time for spring planting, they allow those vegetables to sprout (bud) - either above or below ground.
"You get more than one clone, because when the budded vines reach a foot long, they qualify as slips. And, every slip you twist off and plant can grow up to be just like its mother," the plant breeder said.
Except ...sweet potatoes do mutate occasionally while they´re sprouting slips, thus producing new types.
"In fact, that´s the traditional way of discovering new varieties," Carey said. "It was particularly important in our northern latitudes, where sweet potato plants don´t flower very often."
Today, CIP-type researchers carefully grow and cross-pollinate different varieties´ flowers. That way, they can experiment and have more control in how they get seeds for testing, he said. By nature, the plants have complex genetics. So, the crosses between varieties can vary greatly.
"Among the individuals in the families of seed I planted," Carey said, "many produced no sweet potatoes at all. None. For those that did, however, their storage roots´ flesh turned out to be white or yellow or anything on the range from lilac to deep purple. Their skin hues varied almost as widely, even when their parent seeds were as closely related as sisters or brothers."
From his first 2000 seedlings, Carey selected about 60 that had good yields of intensely pigmented, purple-fleshed storage roots. He saved that harvest to use in developing slips and growing plants to evaluate the following year.
The plant breeder now is down to on-going tests with less than a dozen possibilities. He was disappointed when last year´s best performer didn´t do well in 2009, but says his overall results still look promising.
Carey´s ultimate goal, of course, is foundation seed stock for a variety that grows vigorously and well in Kansas, plus produces sweet potatoes with purple skin and flesh, loaded with anthocyanins. The plant´s storage roots need to do well in storage, too, and be attractive enough to market.
"After each harvest, I´ve tasted the better performers, raw. So, I already can tell the winner isn´t going to be very sweet -- unless it turns out to be one of those vegetables that get sweeter with cooking. My best guess is the top variety will end up being useful in processed foods or as a health food supplement or even in potato chip production," Carey said. "But, discovering its best uses will require even more research, done by a different kind of scientist."
Carey isn´t quite as disinterested as he sounds, however. He really does like the idea of purple fries.
Purple sweet potatoes promising for promoting health
Scientists have already found evidence that anthocyanin - the pigment that makes blueberries blue and red cabbage red - is associated with a reduced risk of cancer. But, no one has known for sure whether the anthocyanin in purple sweet potatoes has the same anti-cancer abilities.
That is, no one has had much proof until two Kansas State University nutrition scientists started a new research program. They´re getting their materials from Ted Carey, K-State Research and Extension horticulturist, who sharing some preliminary selections from his purple sweet potato breeding program.
Human nutrition doctoral candidate Soyoung Lim and associate professor George Wang have already found that compared to an array of gene-related sweet potatoes, those with purple skin and meat can not only have a significantly higher anthocyanin content but also provide more anti-aging and antioxidant components.
In her initial studies, Lim also found that two anthocyanin derivatives she found in Carey´s purple potatoes -- cyanidin and peonidin -actually do inhibit human colon cancer cells´ growth. In summer 2009, Lim began a follow-up study that´s measuring the results of treating animal cancer cells with the pigments.