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Like many people around the world, Katerina Karaiskos SPH ’25 uses olive oil every day. 

“I have Cypriot roots, so it’s been a cooking staple for as long as I can remember,” Karaiskos said.

Now, Karaiskos says she is shopping with renewed confidence in the olive oil industry. In a report led by Tassos Kyriakides GRD ’99, an assistant professor of biostatistics at Yale and an olive oil sommelier, the pair documented the purity of commercial olive oils.

The report, released in August and backed by the North American Olive Oil Association, or NAOOA, conducted a randomized sampling of 158 representative extra virgin olive oils and 58 olive oils across the market. It found that only two samples, fewer than 1 percent, were adulterated and failed the purity test.

“The message to the consumer is that what you’re buying off the shelf is olive oil. There’s nothing else in it,” Kyriakides said.

Kyriakides oversaw the study as an independent consultant for the NAOOA, which represents olive oil companies and stakeholders in the industry. To de-identify the samples, a third-party organization purchased bottles, placed samples into identical vials and shipped them to a laboratory in Spain, where most of the testing was done. The lab reviewed the chemical composition of the oils and compared them to the industry-established standards.

The lab staggered the release of results as it conducted testing. With each result, Kyriakides and Karaiskos blew a sigh of relief.

“I would get results from the lab on a regular basis every week, so I didn’t know the cumulative until we pulled it together,” Kyriakides said.

Pure olive oil is important because of its health benefits and was even one of the first foods to receive a health claim from the European Food Safety Authority. Extra virgin olive oil, the purest form of olive oil, increases beneficial HDL cholesterol while decreasing bad LDL cholesterol, improving cardiovascular health and promoting longevity.

Researchers are even looking into effects on cognition, ultraviolet light protection and autoimmune disease. And purity of the oil is essential to reap the full benefits.

John Dinkler, a cardiologist and the associate head of Timothy Dwight College, wrote to the News that mixing seed oils “has the potential to dilute the protective benefits of olive oil by reducing the percentage of anti-inflammatory polyphenols.”

According to Dinkler, some seed oils may even have negative effects by promoting unwanted inflammation. To attain additional healthy heart benefits, Dinkler also recommends a predominantly plant-based diet to his patients.

While the results are encouraging, the researchers could not conduct a taste quality panel for the oils. Factors such as shipping time, fermentation, or oxidation could negatively affect quality without being detected by chemical tests.

These results do not address the growing pressure within the olive oil industry. According to Kyriakides, climate change has been decreasing production and forcing prices higher. Although olive trees are well-adjusted to warm, dry temperatures, drastic changes to their growing conditions could worsen harvests and yield. Planting more olive trees could help stave off these changes, as olive trees can absorb carbon dioxide from the air.

“All in all, even with climate change affecting harvest seasons and prices, the results of this project show that the authenticity of olive oil remains strong,” Karaiskos said. “Buyers can be reassured that when they purchase olive oil, they are getting what is on the label.”

The seventh Yale International Symposium on Olive Oil and Health will be held from Dec. 4 to Dec.  7 in New Haven.