A new study confirms what many experts already suspected: Most imported "extra-virgin" olive oils are not what they claim to be.

A new study confirms what many experts already suspected: Most imported "extra-virgin" olive oils are not what they claim to be.

In a first-of-its-kind study by a U.S. academic institution, a University of California, Davis, research team found that 69 percent of the imported oils sampled failed to meet internationally accepted standards for extra-virgin olive oil. By comparison, only 10 percent of the California-produced oils in the test failed to meet the standards.

"Before this study, we had anecdotal reports of poor-quality olive oil being sold as extra-virgin," said Dan Flynn, executive director of UC Davis' Olive Center, part of the Robert Mondavi Institute for Wine and Food Science. "Now there is empirical proof. ... The oils that failed in our tests had defects such as rancidity. Many of these oils just did not taste good."

All 19 brands tested — 14 imported and five California-made — were purchased in March at supermarkets or big-box stores in Sacramento, San Francisco and Los Angeles counties. Three bottles of each brand from each location were tested. The findings and the names of the brands evaluated are available online at http://olivecenter.ucdavis.edu/.

Of the California-made brands tested, all bottles of Corto Olive, California Olive Ranch, McEvoy Ranch Organic and Lucero met the extra-virgin criteria.

Of the imports, only Kirkland Organic passed all the extra-virgin standards with samples from all three locations. Samples of Bertolli, Pompeian, Carapelli, Mezzetta and Mazola failed from all locations.

Working with Australian olive experts, the UC Davis scientists used U.S. Department of Agriculture and international standards for evaluating extra-virgin olive oil, considered the premium and most expensive on the market. By definition, extra-virgin oil must be extracted from the olive without heat or solvent and meet specific criteria for quality, smell and taste.

But many of the oils tested did not meet those standards due to oxidation, adulteration with cheaper refined olive oil or poor-quality oils made from damaged and overripe olives, processing flaws or improper oil storage. Oxidation also may be caused by improper storage, including exposure to high temperatures and light as well as aging.

With rising interest in healthy cooking, the United States now represents the world's third-largest market for olive oil.