Perspectives Online

The Sweet (Potato) Science - Sweet potatoes may be a fuel of the future, at least in North Carolina. By Dave Caldwell


Horticultural science researcher Ken Pecota inspects sweetpotatoes in the field at the Lower Coastal Plain Research Station near Kinston.
Photo by Marc Hall

Could the sweetpotato someday be to North Carolina and the Southeastern United States what sugar cane is to Brazil: an alternative and renewable source of fuel?


At last year’s biofuels field day in Duplin County, Dr. Craig Yencho (left) and Pecota described their efforts to make industrial sweet potatoes a viable alternative in large-scale ethanol production.
Photo by Dave Caldwell
It’s too early just yet to divine the future of the sweetpotato (beyond the Thanksgiving table), but a team of College of Agriculture and Life Sciences researchers is working on several fronts to make what the scientists call industrial sweetpotatoes a viable crop for North Carolina growers.

Making a fuel like ethanol from sweetpotatoes is a relatively straight-forward process. To begin with, sweetpotatoes are a good source of starch. Just process sweetpotatoes to turn that starch into sugars, ferment the sugars, and you can make ethanol. The drawback is the cost.

At this point, sweetpotatoes are not an economically competitive fuel source. It costs more to grow and process sweetpotatoes than many other fuel sources. Indeed, Dr. Craig Yencho, associate professor of horticultural science and sweetpotato breeder, estimates it costs eight times as much to grow an acre of sweetpotatoes as an acre of corn. But Yencho and others are out to change that.

Yencho and a colleague, Ken Pecota, horticultural science researcher, are working both to breed industrial sweetpotatoes with high starch content and to lower growing costs.


Pecota (at right) and colleagues are working to lower production costs of industrial sweetpotatoes through more efficient planting methods.
Photo by Marc Hall
They have developed industrial sweetpotatoes with around 32 percent dry matter content. Dry matter refers to what is left after a sweet-potato is dried in an oven to remove the water, Pecota explains. Much of what is left is starch, so dry matter content is a good indication of starch content. So-called table stock sweetpotatoes, the kind that end up on your table, are typically around 20 percent dry matter. Industrial sweetpotatoes produce about 30 percent more starch per acre than corn.

But higher starch content doesn’t get at the high production cost for sweetpotatoes. That’s where what Yencho and Pecota call their “cut seed piece project” comes in.

Sweetpotatoes are typically planted as small plants by hand. They’re also harvested by hand. To reduce planting costs, Yencho, Pecota and Blake Bowen, a graduate student working on his master’s degree with the team, are conducting experiments to see if they can plant sweetpotatoes the same way white potatoes are planted, using cut seed pieces. They point out that seed pieces, pieces of the potato, can be planted mechanically, which has the potential to cut production costs by as much as 20 percent.


Dr. Mari Chinn (right) is developing efficient ways of processing sweetpotatoes to produce ethanol, and Dr. Byron Sosinski is working toward production of transgenic industrial sweetpotatoes.
Photo by Becky Kirkland
When a typical table stock sweetpotato is cut into pieces and the pieces planted, these “mother pieces” often “size up” as they grow, Pecota says. When the mother pieces become particularly large, they tend to crack and often the center rots away, leaving a hollow core, reducing biomass.

But some sweetpotato mother pieces don’t size up and instead produce daughter storage roots that look like normal sweetpotatoes and produce lots of biomass. Yencho and Pecota are breeding high-starch industrial sweetpotatoes for this trait. The result: several promising sweetpotato lines with high starch content that can be planted mechanically, producing quantities of biomass that can be turned into ethanol.

And turning sweetpotatoes into fuel is where Dr. Mari Chinn, assistant professor of Biological and Agricultural Engineering, comes into the picture. Chinn and her graduate students, William Duvernay and E. Nicole Hill, have been working with some success on developing more efficient ways to process sweetpotatoes to produce ethanol and, perhaps, other products.

Chinn is experimenting with various enzymes and combinations of enzymes to find the recipe that most efficiently converts the starch in sweetpotatoes to sugars. She has been able to convert up to 90 percent of the starch in the dry matter to fermentable sugars. She’s working on that last 10 percent.

But all Chinn’s work has been done in the lab, at a small scale. She anticipates challenges in scaling up from the lab to an industrial scale. She’s pretty sure that what works in the lab won’t work exactly the same way in large-scale production. Yet she also estimates that what she and her students have done in the lab equates to producing 700 gallons of ethanol per acre of sweet potatoes. By comparison, an acre of corn typically produces around 400 gallons of ethanol. A USDA Agricultural Research Service study showed that sweetpotatoes yielded two to three times as much carbohydrate for fuel ethanol production as field corn (see http://www.ars.usda.gov/is/pr/2008/080820.htm).


The purple-fleshed sweetpotato, rich in antioxidants and dry matter, has double product potential.
Photo by Becky Kirkland
Chinn emphasizes there are still many questions. For example, in an industrial setting, it is more efficient to process fresh sweetpotatoes or to dry sweet potatoes and make them into flour, which would then be processed? It takes more energy to dry sweetpotatoes and process them into flour, but the flour would be easier to store and require less processing volume. And the conditions needed for the starch conversion process would be different with flour.

Chinn is also working on sweetpotatoes with purple flesh. Purple sweetpotatoes tend to be high in anthocyanins, pigments that are also powerful antioxidants, and high in dry matter. It might be possible, Chinn thinks, to extract anthocyanins, which would have value as a natural food colorant, then process the remaining starch to produce sugar and ethanol, thus producing two valuable products from the same sweetpotato.

Why Sweetpotatoes?

Why are Sweetpotatoes considered a good biofuel for North Carolina? To begin with, North Carolina already grows a lot of sweetpotatoes. Close to 40 percent of the nation's sweetpotatoes come from North Carolina. Sweetpotatoes are grown in North Carolina by a range of farmers: small, medium, and large. There's plenty of sweetpotatoe-growing expertise in the state. Sweetpotatoes are well-adapted to North Carolina and the Southeastern U.S. Sweetpotatoes are drought-tolerant and can produce high yields with minimal ferticizer inputs on a variety of soils. Sweetpotatoes can be stored year round.

Then there’s the work of Dr. Bryon Sosinski, associate professor of Horticultural Science. Sosinski and Monica Santa-Maria, a Ph.D. candidate in horticulture, have inserted into sweetpotato plants genes from hypothermophilic bacteria.

The genes don’t become active unless they are heated. Heat up sweetpotatoes containing the genes, and the starch in the sweetpotatoes should hydrolyze into sugars, effectively self-processing.

The next step, Sosinski says, is a large-scale greenhouse test of the transgenic plants. He’ll grow transgenic and non-transgenic plants side-by-side in a greenhouse to see if insertion of the genes has altered yield or other characteristics.

If the greenhouse test is successful, Sosinski plans to begin working to produce transgenic industrial sweetpotatoes. The plants he’s working with now are not high dry matter sweetpotatoes.

It’s a step-by-step process, but then so, too, is the work of Yencho, Pecota and Chinn. At the end of all those steps may be an agricultural opportunity for North Carolina farmers and a big step toward energy independence for the nation.