Watermelon Juice: The New Fuel?

Aug. 26, 2009 -- A staple of backyard barbecues and summer time snacks, watermelon is also a promising new source of renewable energy.

According to a new study, leftover watermelons from farms' harvests could be converted into up to 9.4 million liters (2.5 million gallons) of clean, renewable ethanol fuel every year destined for your car, truck, or airplane's gas tank.

Agriculturally, watermelon is a peculiar fruit -- each year farmers across the country leave between 20 and 40 percent of their crop to rot on the ground. These are the ugly ducklings of the lot; though perfectly fine on the inside, the misshapen or blemished melons simply won't sell at the grocery store.

"If a crow lands on a melon, takes two pecks at the rind, and then flies away, it's no good," Wayne Fish of the United States Department of Agriculture in Lane, Oklahoma said. "I had farmers telling me, 'I'm leaving one-fifth of my melons on the land. Is there anything I can do with them?'"

Across the United States, he estimated that 360,000 tons of watermelons spoil in fields every year.

Some local growers wondered whether the waste melons could be turned into ethanol, the clean-burning fuel derived from plant sugars. In a series of new experiments published yesterday in the journal Biotechnology for Biofuels, Fish and a team of researchers showed that they can.

What's more, watermelon juice may turn out to be the perfect way to optimize industrial-scale production of ethanol from corn, molasses and sugar cane.

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Watermelon juice is about 10 percent sugar by volume, about half the concentration that manufacturers consider right for producing ethanol. But it's chock full of amino acids that provide a crucial source of nitrogen for yeast to feed on during fermentation.

On its own, the team calculated they could make about 2.5 million gallons of ethanol each year from waste melons, a drop in the bucket of an industry that last year produced 9 billion gallons from corn and other feedstock in the United States alone.

But both corn and molasses require lots of water, and sometimes nitrogen supplements to prepare for fermentation. The team suggests that watermelon juice from reject melons could drastically cut down on water usage, supply needed nitrogen, and even add some sugar to the mix, cutting the amount of corn or molasses by up to 15 percent.

"This is not going to replace corn. In that sense it will remain a niche source of biofuel," said Jim Rausch, president of the College Station, Texas-based company Common Sense Agriculture, LLC, which is developing a prototype watermelon juice-to-ethanol processing plant.

"But unlike algae biodiesel or cellulosic ethanol, it's a right now thing. There's no new technology that needs to be developed to make it economical."

Related Links:

HowStuffWorks.com: Watermelon

TreeHugger.com: Square Watermelons

Discovery Earth blog: Earth Pub