Craig Yencho likes to tell farmers that the new industrial sweet potatoes he's working on aren't your grandma's sweet potato. For one, they are being bred for high starch and high solids, which means they don't taste the same as the familiar sweet potato. Nor do they look the same; the industrial sweet potatoes have white flesh, and either white or purple skins. Finally, they wouldn't be grown for food, but rather for ethanol.

Yencho recently visited southwest China, where sweet potatoes are joining cassava and sweet sorghum as ethanol feedstocks. Yencho reports there are two ethanol plants there using those feedstocks and one under construction that will use those feedstocks, as well. He says the biggest difference between China's agricultural system and the U.S. ag system is that China has a multitude of farmers with small farms under a quarter-acre in size, where hand labor is the norm. In the United States, sweet potatoes require hand transplanting and hand harvesting, which boosts production costs between $1,500 and $1,700 per acre, compared with $175 to $225 per acre for corn in North Carolina. Thus, even though sweet potatoes can outyield corn in biomass per acre and equal corn's ethanol yield per acre, they aren't considered an ethanol feedstock because of the high production costs.

Yencho leads research at North Carolina State University that aims to create an industrial sweet potato for the emerging ethanol industry in the South. The harvesting limitation should be relatively easy to overcome, using modified potato diggers, Yencho says. The primary reason for hand digging is to protect the easily skinned sweet potato in order to maximize its visual appeal for supermarket customers. An industrial market wouldn't worry about cosmetic appearance and could tolerate machine digging. Overcoming the transplanting obstacle will take more time. Unlike Irish potatoes, which are planted mechanically using cut seed tubers, the sweet potato isn't a tuber, but a swollen part of the plant's root, Yencho explains. He says the trait to sprout from cut sweet potatoes is heritable, and new varieties should be forthcoming that can be mechanically planted. "After two years of study, I think the planting modifications are solvable," he says. Additionally, he is working to boost yields between 30,000 and 60,000 pounds per acre, which will yield an estimated 400 to 600 gallons of ethanol per acre. Because uniform size, shape and nice appearance aren't as important, the industrial sweet potato could be grown on more marginal land than the table varieties, Yencho adds. Other advantages to an industrial sweet potato crop in the South would be the plant's drought tolerance and low nitrogen requirement.

In parallel work, molecular biologist Byron Sosinski, Yencho's colleague, is experimenting with genetic modifications that would make industrial sweet potatoes self-processing. Sosinski is using genes identified in bacteria found in extreme deep-sea environments that create enzymes capable of breaking down starch at high temperatures. The theory is that the genes would be inserted into sweet potatoes to be expressed only in the sweet potato itself and not other plant tissue. The enzymes would lie dormant in the sweet potato until it was taken from storage, ground into slurry and heated. Under the higher temperatures, the enzymes would become active and turn the starch into fermentable sugars. "We have proof of this concept in tobacco cell cultures," Yencho reports. "The next step is to place the genes in sweet potatoes, which we've done in Jewel." Jewel is a prime tabletop sweet potato variety that is amenable to genetic manipulation. Moving the genetic trait into the new industrial varieties is the next challenge for the researchers. Definitely, these potatoes won't be your grandma's sweet potato.