Small and medium producers of biodiesel had "significant failure rates" in the U.S. Department of Energy's 2007 biodiesel quality survey, while about 90 percent of all the biodiesel tested met specifications.

To evaluate the quality of biodiesel produced in the U.S., the DOE's National Renewable Energy Laboratory conduct periodic quality surveys to see if the fuel meetings ASTM International D6751 specifications. Biodiesel must meet these specs to be considered a legal fuel and qualify for tax credits.

The samples were tested for properties deemed critical for engine operation: oxidation stability, flash point and alcohol content, cloud point, water and sediment, acid value, and free and total glycerin. They also were analyzed for phosphorus, sodium, potassium, magnesium and calcium, which are poisons to advanced emissions control equipment.

For the 2007 survey, the agency collected samples representing 70 percent of the U.S. market. Producers meeting the National Biodiesel Board's voluntary BQ-9000 specifications accounted for 74 percent of the volume in the survey. Large producers and BQ-9000 producers hardly ever failed to meet the specifications.

However, small and medium biodiesel producers "had significant failure rates," according to the NREL's report. The small and medium producers failed to meet oxidation stability specifications most often - 30 percent of their samples failed.

Samples collected from BQ-9000 producers were "overwhelmingly on specification," with the exception of one. That one failed to meet water and sediment specifications, indicating that contamination of the sample was likely.