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Coffee can do more than just fuel you through an afternoon slump. It might also power your car.

That's the idea behind a new study that turned used coffee grounds into biodiesel fuel. Coffee will probably never replace petroleum, but discarded cappuccino scraps might someday help reduce our impact on the environment, say the study's authors. They imagine a day when the byproducts of your latte end up in the gas tank of your car -- with hardly any waste left behind.

"It's a very simple two-step process," said Susanta Mohapatra, a chemical engineer at the University of Nevada, Reno. "We can definitely make a big impact on our environment with fuel made out of nature."

Scientists have known for decades that coffee beans contain oil. Mohapatra and colleagues, however, were the first to analyze coffee grounds.

Used grounds usually end up in landfills, though gardeners sometimes use them as compost material. The scientists collected used grounds from Starbucks, which gives bags of grounds away as part of the company's "Grounds for your Garden" program.

To prepare the grounds for analysis, the team first dried them in an oven. They mixed the resulting powder with a combination of solvents that caused the oil to separate from the solution. They extracted the oil, saving the solvents for the next round of processing. The remains could still be used as compost, ethanol feedstock, and fuel pellets.

"We're not wasting anything," Mohaptra told Discovery News. "It's a recycling process."

The study showed that used grounds contain about 15 percent oil by weight, depending on the type of coffee. That's not too far off the proportions in soybean, rapeseed, and palm oils, which are also used as sources for biodiesel. And coffee oil is more stable than these other sources because of its high antioxidant content, found the study, which appeared in December in the American Chemical Society's Journal of Agricultural and Food Chemistry.

Around the world, growers produce more than 16 billion pounds of coffee each year, according to the U.S. Department of Agriculture. The scientists estimate that spent grounds could add 340 million gallons of biodiesel to the global fuel supply.

Mohapatra envisions a streamlined coffee recycling system, in which the same trucks that deliver beans to Starbucks could pick up the brewed waste and head to a biodiesel plant. The plant would be close by, to save on transportation costs and emissions.

Coffee grounds appear to produce high-quality oil, granted Robert McCormick, an engineer at The National Renewable Energy Laboratory in Golden, Colorado. But, he said, coffee probably won't be a practical solution to the world's energy needs.

For one thing, the country's main sources of biodiesel -- cooking oil and animal fat -- are 100 percent oil, compared to coffee's 15 percent. And even when a cafe brews a large amount of coffee, relatively few grounds are left behind. It takes 50 gallons of spent grounds to produce just 1 gallon of oil, Mohapatra said.

Still, McCormick commends the researchers for thinking outside the box about the world's energy issues.

"Anything that takes a waste product and makes a fuel out of it is really a positive," he said. "This is pretty cool."