Extra virgin olive oil is tasty and excellent for your health, but experts say as much as 70 percent of it sold in America is adulterated, or of a lower grade. How can you tell if your brand is the real deal?

One notion — the so-called fridge-test theory — says you can determine the purity of your extra virgin olive oil, or EVOO, by putting it in your refrigerator. If it solidifies, you can trust your EVOO is pure — or so the theory goes.

Is that scientifically accurate?

No, according to new research from the UC Davis Olive Center. Testing seven samples under cold conditions over eight days, researchers discovered the fridge test is unreliable in detecting either the purity or quality of olive oil.

Flynn

“None of our samples showed any signs of congealing after 60 hours in a laboratory refrigerator set to 40.5 degrees Fahrenheit,” said Dan Flynn, executive director of the Olive Center. “Even after 180 hours, the samples never fully solidified.”

The fridge-test theory surfaced on The Dr. Oz Show on Feb. 11, during an episode of the nationally syndicated television show that included Flynn as a guest on the topic of food fraud.

Physician-host Mehmet Oz, while cautioning that his method isn’t fool-proof, encouraged his audience to test the purity of EVOO by seeing if it solidifies in the fridge.

“After the show aired, we were swamped with calls from people who were concerned they were being ripped off,” Flynn said.

So the Olive Center conducted a study. They refrigerated seven samples, including two EVOOs, an olive oil, a canola oil, a safflower oil, and two blends. Some samples showed minor congealing at the bottom of the bottles, but none solidified completely.

“It’s true that waxes and long-chain fatty acids in extra virgin olive oil can lead to the oil solidifying in the cold, although relative amounts of these compounds vary from oil to oil,” the study said.

Olive oils are graded based on how the oil is extracted from olives and on chemical and sensory standards. True extra virgin olive oil is extracted from olives without heat or chemicals and must have no defective flavors such as rancidity. Extra virgin olive oil is more expensive than other oils and is therefore an attractive product for fraud.

Using sensory and chemistry testing, the Olive Center is working on dependable methods for detecting when an extra virgin olive oil is fraudulently labeled. In the meantime, the center advises consumers to choose an oil within 15 months of the harvest day (not the best-before date), look for a certification seal indicating that the oil passed chemical and sensory tests, and seek (and store) oils protected from light.

Diane Nelson is a writer for the College of Agricultural and Environmental Sciences.