Watermelons Tapped for Ethanol

With their sweet, refreshing juices and succulent interior, watermelons are a favorite summertime treat, especially around July 4th. But now this Independence Day favorite could become even more of a patriotic commodity.

Agricultural Research Service (ARS) studies in Lane, Okla., have shown that simple sugars in watermelon juice can be made into ethanol. In 2007, growers harvested four billion pounds of watermelon for fresh and cut-fruit markets. Around 800 million pounds--or 20 percent of the total--were left in fields because of external blemishes or deformities.

Now, instead of being plowed under, such melons could get an economic "new lease on life" as ethanol. Normally, this biofuel is produced from cane crops like corn, sorghum or sugarcane as a cleaner-burning alternative to gasoline. The watermelon work reflects a national push by ARS to diversify America's "portfolio" of biofuel crops that can diminish the reliance on petroleum, especially from foreign suppliers.

Chemist Wayne Fish's ethanol studies at the ARS South Central Agricultural Research Laboratory in Lane complement ongoing research there to commercially extract lycopene and citrulline from the crop. Both are valued nutraceutical compounds thought to promote cardiovascular and other health benefits.

In publication-pending studies, Fish showed ethanol can be fermented from the glucose, fructose and sucrose in waste-stream juices--what's left after lycopene and citrulline are extracted. Making ethanol offers the potential benefits of helping to defray sewage treatment costs associated with nutraceutical extraction, and providing watermelon growers with a new market for their crop.

On average, a 20-pound watermelon will yield about 1.4 pounds of sugar from the flesh and rind, from which about seven-tenths of a pound of ethanol can be derived. To extract all the possible sugars, Fish is seeking to degrade the rind with chemical and enzyme treatments. He's also evaluating different combination of temperatures, yeasts, antifoaming agents and pH levels to optimize the system.

Lane scientists also are examining annual ryegrass, sorghum and other crops that could be rotated with watermelons to furnish processing plants with a year-round supply of nutraceuticals or ethanol.

ARS is the principal intramural scientific research agency of the U.S. Department of Agriculture.