N.C. State scientists brewing energy from sweet potatoes

Sweet potatoes, a staple on holiday dinner tables, are being re-engineered by North Carolina State University scientists as source of ethanol to help the U.S. reduce its dependence on imported oil and ease the biofuel industry's troublesome reliance on corn.

This industrial sweet potato doesn't look, or taste, much like the Southern classic, but can produce twice the starch content of corn, the leading source of ethanol. More starch means more sugars that can be fermented into biofuel.

"These are not your grandmother's sweet potatoes," says Craig Yencho, an N.C. State associate professor of Horticultural Science, who is leading a project to develop alternative uses for the vegetable. "The industrial sweet potato is edible, but not palatable."

While the table version is orange inside and becomes sweet during baking as enzymes break down starch into sugar, the industrial sweet potato typically has a purple or white skin and white inside with a much higher starch content that limits its sweet taste.

North Carolina produces about 40 percent of the U.S. sweet potato crop. The industrial sweet potato could help diversify the state's farm income.

The biggest challenge is lowering production costs to take advantage of that higher starch content. Sweet potatoes traditionally are planted by hand using transplants.

"But if we could plant them the same way you plant an Irish potato – by planting cut 'seed' pieces and mechanically planting them into the ground, we could cut planting costs in half," Yencho said.

"The ethanol production from sweet potatoes then becomes much more cost effective and feasible. Not only would these sweet potatoes be a much more viable ethanol source than corn, but because they are industrial sweet potatoes, we wouldn't be taking away from a food source," said Yencho, who is currently in China helping the world's No. 1 producer of sweet potatoes tap the crop's biofuel potential.

While the best of conventional breeding techniques have been used to develop N.C. State's industrial sweet potato, Yencho is also teaming with colleague Bryon Sosinski on an unconventional approach to further boost sugar – and thus ethanol – yield. By using bacteria from deep-sea thermal vents they are creating an industrial sweet potato that practically processes itself into ethanol.

"Our ultimate goal is to develop a self-processing sweet potato," says Dr. Yencho, noting that the special genes could reduce the cost of enzymes that are used by biofuel processors to break down the starch in corn to sugars which are then converted into alcohol by fermentation. Sosinski hopes to move into greenhouse trials next year.

Ultimately, N.C. State scientists believe the industrial sweet potato can compete with corn – now much cheaper to produce – as a viable alternative source of ethanol. Corn is by far the leading source of ethanol, but corn- based biofuel has come under increasing attack by poverty-fighting and other groups who argue, among other things, that diversion of corn crops for biofuels aggravates world-hunger problems. At the same time, Congress and state legislative leaders concerned about dependence on imported oil are pushing for increased use of biofuels.

"There isn't one magical crop that will solve our energy problems, but the industrial sweet potato can play an important role, especially in the southeastern U.S. where the crop is grown," says Dr. Yencho.

Moreover, he adds, research into the sweet potato can further enhance its value as a nutritional food staple while simultaneously finding new ways the crop can help replace petroleum as source for industrial products ranging from plastics to natural colorants and high-value specialty chemicals. And in their zeal to mine the tuber's variability, Yencho and his team of N.C. State researchers have created a hybrid intended for neither food nor fuel – the non-bearing "Sweet Caroline" variety developed strictly for ornamental use.