Starting May 1, Minnesota will become the first state in the nation to require buses, trucks and other diesel vehicles to fill up on a 5 percent biodiesel blend. But with even higher biodiesel requirements on the horizon, some are calling for the state to do more testing.

Since 2005, the state has required a mix of regular diesel and 2 percent biodiesel -- nonpetroleum-based fuel usually made from soybean oil. Advocates say biodiesel burns cleaner than regular diesel and is a renewable energy.

Under a measure passed last year, the required biodiesel blends will bump up to 10 percent in 2012 and 20 percent in 2015, but those will be in effect for only seven months out of the year.

That's because biodiesel has a higher freezing point, causing it to turn to gel more easily on extremely cold days. Some critics argue that as the biodiesel concentrations increase, the fuel will cause engines to stall during Minnesota's cold winters, shutting down diesel engine operators across the state.

A state task force, organized by Gov. Tim Pawlenty in 2003 to help carry out the biodiesel mandate, reported to legislators in February that the increase to 5 percent biodiesel shouldn't cause engine problems but that "there are significant issues to be addressed" concerning the increases in 2012 and 2015.

Last winter, Bloomington school officials blamed biodiesel when buses stalled in subzero weather, but later state officials said fuel filters, not biodiesel, were at fault.

John Hausladen, president of Minnesota Trucking Association and a member of the state task force, said he's not too worried about the 5 percent increase, but he is worried about the fuel price fluctuations and future increases in biodiesel concentration.

And even though the mandates will be eased during the coldest winter months, Hausladen said he still thinks there needs to be more performance testing on biodiesel.

"It's one thing to test in a lab, but it's another thing to test biodiesel in a truck on the road in below-freezing temperatures," he said.

Before the 2005 mandate, the Minnesota Trucking Association was one of the biggest opponents of biodiesel, arguing that the fuel was unproven and would cause serious problems. But since then, the group's position has softened.

"We're not against biodiesel, we just want to make sure it's used properly," Hausladen said. "Not to be smug about it, but some of the questions and concerns we raised back then ended up coming true."

Chuck Neece, a member of Pawlenty's task force and one of the biggest players in Minnesota's biodiesel movement, said 5 percent biodiesel won't cause any extra engine problems that aren't already caused by regular diesel.

"We're all comfortable with 5 percent," said Neece, who is a member of a number of organizations that deal with biodiesel and is a director of manufacturing at one of the state's three biodiesel plants. "We're proud Minnesota is leading the nation."

Neece also said he's confident that the state will be able to work out any kinks before the next increase.

The University of Minnesota has been testing biodiesel for years, and university research engineer Kelly Strebig said biodiesel can be used in cold weather and problems are rare unless the fuel isn't stored or mixed properly.

Rep. Al Juhnke, DFL-Willmar, chief author of the bill mandating the biodiesel increases, said he isn't concerned about the new fuel blend not meeting quality standards. There are exceptions built into the law, so if the fuel begins to cause problems or if the price spikes too high, the state will be able to temporarily back off the mandate, Juhnke said.

Alex Robinson is a University of Minnesota student on assignment for the Star Tribune.