Human World

Sweet potato tops corn in ethanol fuel study

Yes, the tasty, tuberous root is superior to corn and almost as good as sugar cane for producing ethanol for fuel, according to a new USDA study.

Scientists with the agency’s Agricultural Research Service (ARS) compared sweet potato and cassava grown in Maryland and Alabama to corn grown in those states. They found that sweet potatoes yield two to three times as many carbohydrates for fuel ethanol production, per acre, as corn does. That puts sweet potatoes at the low end of yields for sugar cane — the top ethanol crop. Cassava performed similarly in Alabama.

“Sweet potato looks really good and could serve as one potential source of bioethanol for the future,” said Lew Ziska, an ARS plant physiologist at the Crop Systems and Global Change Laboratory in Beltsville, Md.

Here are the numbers from the study: For the sweet potatoes, carbohydrate production was 4.2 tons an acre in Alabama and 5.7 tons an acre in Maryland. For corn, carb production was 1.5 tons an acre in Alabama and 2.5 tons an acre in Maryland.

Planting more sweet potatoes for fuel production would allow farmers to move corn crops out of the ethanol market and back into production for food or animal feed. This could help lower food prices related to corn worldwide (tortillas, high fructose corn syrup).

Ziska said his team were looking for crops that could be grown on marginal land with few synthetic inputs, such as fertilizers and pesticides, and could compete with corn in ethanol production.
Sweet potato and cassava fit the bill.

Right now we get most of our ethanol from the starch, or carbohydrates, in corn kernels. It’s a two-step process: First the starch gets converted into sugar, then the sugar into ethanol.

“There are a couple of advantages with sweet potato over corn,” Ziska said in an interview. “The first one is that you have a lot more carbohydrates overall. Corn typically has about 60 to 65 percent starch in the kernels; and that’s the principal source for ethanol. Sweet potato has about 80 to 90 percent total carbohydrates in the tuber. The other advantage is that, of those carbohydrates, in sweet potato, about 20 to 30 percent of them are sucrose, are sugar, and can be more easily converted into ethanol than starch can.”

Sweet potato has another advantage: It does not require as much fertilizer or pesticide as corn does. Corn uses large amounts of nitrogen fertilizer to grow and Ziska estimates that at least half of the nitrogen in use in the United States is converted from natural gas — a fossil fuel for which prices have shot up recently.

Ziska said sweet potatoes can be grown along the Eastern seaboard from New Jersey to Florida, and possibly in the Northwest as well.

We’re not talking small potatoes, either.

“These are not the sweet potatoes you would buy at Thanksgiving. These things are 20-, 30-pound tubers. So they’re huge,” said Ziska. These roots are bred to be very large and are normally chopped up and used as animal feed, he explained.

(I spoke today to John Kimber of the North Carolina Sweet Potato Commission Foundation, who said that not many sweet potatoes are grown for animal feed in the U.S., but in China — the world’s largest producer of sweet potatoes — about 80 percent of them are grown for animal feed. And those potatoes are higher in dry matter or carbs.)

Sweet potatoes sound like a sweet solution to the biofuel vs. food problem, but there are some hurdles to overcome. Start-up costs for sweet potato are high, due to the labor needed. “A lot of the sweet potato acreage is not entirely mechanized either for planting or for harvesting,” said Ziska. Sweet potato harvester machines are coming on line now and could make it more practical for more farmers to grow the crop commercially.

Ziska has submitted the paper describing the study to the journal Biomass and Bioenergy. The ARS published a press release August 20.

I think this is a great development. Now we have a crop that can efficiently produce ethanol — and we don’t have to saturate the environment with chemicals to do it. Remember, all that fertilizer leads to dead zones in the oceans.

We should ramp up sweet potato production for ethanol, so that we can put more corn back into the food supply. Balance is good, diversifying ethanol sources is good. What do you think? Post your comments below!

September 5, 2008
Human World

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