Watermelon Juice May Be Next "Green" Fuel

Watermelon, the quintessential summer fruit, may soon be helping to fuel your car as well as your picnic guests.

According to a new U.S. government study, juice from unwanted watermelons could be a promising new source for making the biofuel ethanol.

Up to a fifth of all watermelons grown each year have odd shapes or scarred rinds that turn off consumers, said study co-author Wayne Fish, a chemist with the Agricultural Research Service in Lane, Oklahoma.

Instead of picking the fruit, farmers leave these reject melons on the vine.

"If you figure a field of watermelon may yield somewhere between 60 and 100 tons per acre of watermelon, a fifth of that can be substantial," Fish said.

When he and colleagues were experimenting with extracting antioxidant compounds from watermelon juice, they realized the waste stream of sugary fluids could be a source of ethanol.

(Compare the costs and benefits of different biofuels.)

Mobile Homebrew

The researchers brewed several experimental batches of the fruity fuel in the lab and optimized the process to produce about 23 gallons (87 liters) of ethanol from an acre's worth of the unused fruit.

"For average-size growers that have 300 to 1,000 acres [121 to 405 hectares], they may just keep the ethanol themselves and use it in their own production," Fish said.

Larger farms could even produce enough fuel to sell.

(Related: "Alcohol, Feces, Carcasses Fuel 'Green' Vehicles in Sweden.")

However, it doesn't make economic sense to haul the unwanted watermelons to a processing facility. Rather, Fish envisions mobile breweries that go from farm to farm.

"In terms of the actual process that goes on, it is no different than making homebrew," he said, except on a larger scale and with a few special laboratory tweaks.

Watermelon Beer?

When brewing the watermelon biofuel, the researchers were focused on its energy potential, not flavor. But they couldn't resist sampling the brew.

"It's not going to kill you, for goodness sake," Fish said.

Their process tended to produce fusel oils, which give alcohols an "off" flavor.

"Don't expect to see any watermelon beer at your local tavern anytime soon," Fish said, "at least as produced at Lane, Oklahoma."

Findings published August 26 in the journal Biotechnology for Biofuels.