The Associated Press

A few years ago, drums of used french fry grease were only of interest to a small network of underground biofuel brewers who would use the slimy oil to power their souped-up antique Mercedes.

Now, restaurants from Berkeley, Calif., to Sedgwick, Kan., are reporting thefts of old cooking oil worth thousands of dollars by rustlers who are refining it into barrels of biofuel in backyard stills.

"It's like a war zone going on right now over grease," said David Levenson, who owns a grease hauling business in San Francisco's Mission District. "We're seeing more and more people stealing grease because it lets them stay away from the pump, but it's hurting our bottom line."

Levenson, who converted the engine in his 1983 Mercedes to run on straight canola oil, has built up contracts to collect the liquid leftovers from 400 restaurants in the last two years.

Last week when his pump truck arrived at Thee Parkside, a dive bar known for its chili-cheese fries, his driver found someone had already helped himself to the barrel of yellow oil.

Grease is transformed into fuel through a chemical process called transesterification, which removes glycerine and adds methanol to the oil, leaving a thinner product that can power a diesel engine.

Biodiesel can also be blended with petroleum diesel, and blends of the alternative fuel are now sold at 1,400 gas stations across the United States.

But as the price of diesel shoots up, so, too, does the value of grease.

In the last three years, the price of soybean oil -- the main feedstock for biodiesel made in the United States -- has tripled.

Those kinds of numbers have encouraged biofuel enthusiasts to plunder restaurants' greasy waste, and have even spurred the City of San Francisco to get into the grease-trap cleaning business.

"Restaurants and staff are no longer looking at this material as trash. They're looking at is as something that's about to go into city vehicles," said Karri Ving, who runs the city's new waste
cooking oil collection program. "Unless you lock down every trash can, thefts are going to happen."