Sweet biofuel

Sweet sorghum might have the right juice for the job

By James D. Ritchie

A few years from now, you may be hauling your crop out of the field with a tanker truck rather than a grain cart.

Sweet sorghum has the potential to produce more ethanol per acre than corn, and do it with less nitrogen fertilizer and less water. Farm families used to grow a version of this plant to make “long sweetening” or sorghum syrup. The tall-growing grass, a native of central Africa, is more closely related to milo than to sugar cane and is best adapted to warmer climates.

For the past 2 years, Gene Stevens, University of Missouri crops scientist, and Roland Holou, UMC graduate student, have planted several varieties of sweet sorghum at the Delta Center near Portageville, and compared the ethanol yield with that of corn. Using 2.8 gallons per bushel conversion factor, the highest-yielding corn produced 574 gallons of ethanol per acre, compared with sweet sorghum, which produced 587 gallons of ethanol, with less nitrogen fertilizer. When the cellulose in the stalks was also broken down to sugars, the sweet sorghum produced a total of 724 gallons of ethanol.

“Our results showed that corn yield peaked at 160 lbs. of N per acre, compared with sweet sorghum with top yields at 60 lbs. of N per acre,” said Stevens, who also studied the cold tolerance of the sugary sorghum.

“In the Bootheel, we usually plant corn in late March or early April,” he added. “With sweet sorghum, we plant in May, after the soil has warmed above 60 degrees.”

“In Africa, we can grow two crops of sweet sorghum per year,” said Holou, who is a native of the central African nation of Belen. “That’s a big benefit, to get two yields each season.”

He and Stevens are planting sweet sorghum varieties at different times to test which is most cold hardy. The best-performing varieties will be candidates to breed (perhaps with genetic engineering) a crop better suited to Missouri conditions.

“If we can take genes from a cold-tolerant variety and put them into a variety that produces more sugar, we’ll get the best of both worlds,” said Stevens. “An early-maturing hybrid could increase ethanol yields. If we can harvest earlier, the sorghum will grow back again and we could get two crops instead of one.”

In his research, Stevens has noticed few insect and disease pests attacking sweet sorghum.

And, because sweet sorghum demands less nitrogen than corn and tolerates drier conditions, the crop could be grown in areas where corn doesn’t do well.

Sweet sorghum should be cheaper than corn to make into ethanol, too. With corn, an initial fermentation is needed to convert starches to sugars. With sorghum, this step is eliminated; sweet sorghum juice is already sugar.

But, there are some drawbacks, at least right now. One is the shortage of seed of sweet sorghum varieties adapted to the Midwest. That hurdle could be quickly overcome, if—or when—demand increases.

A more stubborn challenge is the bulk of the plant, which contains about 80 percent water. Handling this mass of material almost mandates in-field processing, with some combination of cutting stalks and pressing out the juice.

“Also, sugars in the juice begin to sour right away,” said Stevens. “This hurdle can be overcome by adding enzymes to begin the fermentation process.”

Which still leaves the challenge of transporting the watery juice. Growers with a conventional ethanol plant nearby would face less of a problem, but those producers who grow sweet sorghum outside major corn-growing areas might be in more of a bind to get their juicy crop to market.

Perhaps a viable solution might be portable stills that could be towed from farm to farm to “cook off” relatively small batches of ethanol. Unfortunately, federal laws dating back to the Prohibition Era outlaw such equipment.

If sweet sorghum catches on as a biofuels quencher for America’s petro-thirst, these infrastructure problems will need to be solved in relatively short order.

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