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Will watermelon rejects be the next biofuel?

Twenty percent of watermelons never make it to the picnic table. Rather, one in every five is left to ripen and rot in the field, rejected for even the slightest of cosmetic imperfections. But U.S. Department of Agriculture (USDA) researchers may have found a way to elevate these outcasts to an even higher calling than the summer BBQ: biofuel production.

"As consumers, we would not choose that [misshapen or blemished] watermelon if we were in the supermarket," says Wayne Fish of the USDA's Agriculture Research Service's South Central Agricultural Research Laboratory in Lane, Okla., and lead author of a paper on the fruity biofuels idea published today in the journal Biotechnology for Biofuels. "So the growers won't even pick them."

With such a large percentage of their crop simply left to be plowed back into the field, farmers wondered what could be done. "Anything that they can do to add value to the crop is money in their pocket, and could be the difference between a loss for the year and a modest income," Fish says.

After hearing this enough times, the USDA team began to ask themselves: What could watermelon lacking a perfect complexion or physique have to offer? Given a growing biofuel industry, they wondered if the fruit might somehow be turned into fuel.

Ethanol biofuels are created through the breaking down of complex sugars, followed by fermentation. With corn, an increasingly popular biofuel source, the starch must first be treated with special enzymes to break it apart into basic sugar units. Watermelons, on the other hand, naturally harbor simple sugars, allowing more efficient conversion to ethanol. 

The resulting quantities of fuel are not insignificant. "If you took 10 tons of watermelon—about the amount of culled watermelons per acre—and fermented both the juice from the flesh and the rind, you would get about 115 gallons of ethanol," Fish says. He notes, however, that the fruit's relatively low sugar concentration, about 10 percent, may lend itself to more efficient use as a supplement or dilutant for other biofuel crops.

This juicy precursor to ethanol isn't the only treasure lying under the thick green skin. The watermelon is also rich in the nutrients lycopene—which makes its flesh red—and L-sitrulline. This inspired the researchers' idea of first harvesting these nutrients, and then fermenting the sugary fruit juice into a fuel for use on or off the farm. Between labor and transportation costs, such production may not be practical, but a company in Texas is working on a "mobilized processing system" that can be taken from field to field. Fish predicts a prototype in operation by next year's watermelon crop.

"Instead of taking Mohammad to the mountain," Fish says, "we can take the mountain to Mohammad." And with the world's heaviest watermelon on record weighing in at over 250 pounds, some harvesters may very well feel like they are moving mountains.


Picture by Yasonya via iStockphoto

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