A Sweet Idea

October 31, 2008 07:00 PM
It grows in nearly all temperate climates, needs little water and trumps corn's energy balance sheet. If researchers can find how to economically harvest it, sweet sorghum may be the next best ethanol feedstock.

"I think we have a real winner,” says Danielle Bellmer, a food processing engineer with the Food and Agricultural Products Center at Oklahoma State University (OSU). "You get a better carbohydrate yield from sweet sorghum than you do from other crops used for renewable energy.”

Sweet sorghum offers an estimated net energy ratio of 1 unit input for 5 units out, which is comparable to switchgrass values, Bellmer explains. In corn, the ratio is about 1 unit to 1.6 units. Depending on the sweet sorghum variety, its juice can yield more than 400 gal. of ethanol per acre in a single harvest. Corn produces about 420 gal. per acre.

And while crops like corn require complex heat processing to convert starch into simple sugars for ethanol, the simple sugars in sweet sorghum are directly "juiced” from the stalks, eliminating the starch conversion step.

"This opens up a whole new market for farmers who can't grow corn,” says Ray Huhnke, biosystems and agricultural engineer, who coordinates OSU's biofuels efforts. Currently, sweet sorghum is primarily grown in Tennessee and Kentucky for syrup markets.

Harvest hurdle. India and China already process ethanol from sweet sorghum. The stumbling block in the U.S. is the high cost of constructing a central processing plant that would only operate seasonally, Bellmer says.

While starch from corn can be stored for long periods, the simple sugars derived from sweet sorghum have to be fermented immediately, she says. Also, the harvest season for sweet sorghum is only a few months.

"Since the sorghum juice cannot be easily stored, the processing plant would only be in production for a few months out of the year, making it economically unfeasible,” she says. Bellmer and others are investigating in-field ethanol production, in which sweet sorghum juice will be collected, fermented and distilled on the farm.

Lee McClune, president of Sorganol Production Company Inc. in Knoxville, Iowa, has built and tested equipment to harvest and press sweet sorghum juice in a single pass through the field. The machine has been used at Iowa State University and OSU and has the potential to harvest 4,000 gal. to 6,000 gal. of juice per acre, McClune says.

This fall, the machine harvested a 20-acre plot at OSU's South Central Agricultural Research Station in Chickasha, Okla. Bellmer says other companies are showing interest in designing and building a sweet sorghum ethanol harvester.

All on-farm. Once the sorghum is harvested, the fermentation process must begin within 24 hours or the sugar will be lost. Researchers are working with new yeast strains that may allow fermentation to take place on-farm without the need for temperature control.

The next step is to concentrate the ethanol through distillation. Researchers are evaluating the economic feasibility of various distillation systems for small-scale ethanol production.

"Our goal is to make sweet sorghum harvesting low-cost and decentralized so farmers can process locally to minimize transportation costs,” Bellmer says. On-farm processing also reduces the need for large water supplies that are required for processing plants and that can burden local municipalities.

"I think we have a huge alternative fuel crop on our hands,” she adds.

You can e-mail Jeanne Bernick at jbernick@farmjournal.com.

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