What is high-phenolic olive oil? It pops up more and more on social media, websites, and news articles, yet it is usually not clearly defined. A Greek scientific team seeks to change that. They analyzed more than 5700 olive oil samples over 11 years, then proposed a definition of “high-phenolic olive oil” partly based on how long its healthy phenols last.

This is important to olive oil consumers, producers, and everyone between them in the marketing and distribution chain, because the phenols naturally found in olive oil have been associated with numerous health benefits. Many scientific studies suggest that olive oils with higher phenolic content offer consumers much more than what is expected from a monounsaturated fat. In 2012, this led the European Food Safety Authority to authorize a specific health claim for olive oil.

As Panagiotis Diamantakos and his coauthors point out in the journal Molecules, European Regulation 432/2012 “recognizes that olive oil polyphenols contribute to the protection of blood lipids from oxidative stress” in the case of olive oil that contains at least 5 mg of hydroxy-tyrosol and its derivatives (oleuropein complex and tyrosol) per 20 g of olive oil. (Those derivatives include oleacein and oleocanthal.) This claim means live oil polyphenols can help prevent many serious illnesses. 

The phenolic compound oleocanthal has been shown to have anticancer activity, anti-inflammatory activity much like ibuprofen, and the ability to help prevent Alzheimer’s disease. Oleacein has anti-inflammatory, antioxidant, neuroprotective, and antiatherosclerotic (against hardening of the arteries) properties. Scientific studies have shown that higher phenolic olive oils offer more health protection than lower phenolic oils.

To get to the bottom of what “higher phenolic” means, as their article explains, Diamantakos and his colleagues at the University of Athens “performed a large-scale screening and statistical evaluation of 5764 olive oil samples from Greece coming from >30 varieties for an eleven-year period with precisely measured phenolic content by qNMR,” a method for analyzing the compounds in olive oil that was developed by Professor Prokopios Magiatis and his team and published in 2012.

The scientists discovered “a large variation among the different cultivars,” or olive varieties. They found that overall “the mean concentration of total phenolic content was 483 mg/kg. The maximum concentration recorded in Greece reached 4003 mg/kg.” The harvest time, olive variety, and olive oil production method tended to make a difference in the level of phenols in the oil.

To help consumers get the benefits they seek, Diamantakos and his colleagues went one step further to consider how long high levels of phenols remain in olive oil under normal storage conditions.They “found an average loss of 46% in 12 months.”

Considering the European health claim regulation, their own statistical evaluation of thousands of samples, and their discovery that nearly half an olive oil’s phenols tend to be lost in one year, Diamantakos and his team came up with this proposal: “the term high-phenolic should be used for olive oils with phenolic content > 500 mg/kg that will be able to retain the health claim limit (250 mg/kg) for at least 12 months after bottling.” Moreover, they suggest, “the term exceptionally high-phenolic olive oil should be used for olive oil with phenolic content > 1200 mg/kg (top 5%).”

The scientists also explored the question of how storage conditions affect phenolic content over time. When refrigerated for 6 months at 4°C, samples retained approximately 92% of their phenolic content. They did even better in the freezer: at −18°C, 97% of the phenols remained after 6 months. In fact, frozen samples lost only 9.5% of their phenolic content in an entire year.

The statistical analysis also revealed other interesting points. An oleocanthal content of more than 500 mg/kg was rare, although upper limits of oleocanthal exceeded 2000 mg/kg. Oils from the Kalamon olive variety tended to yield especially high oleocanthal content. The Olympia variety was richest in total phenols, with Zakynthou olive oil not too far behind.

However, the olive variety isn’t everything: “although Koroneiki variety showed a mean value a little lower than the general mean, it also showed a big variability ranging from 0 to 2637 mg/kg. This finding highlights the role of factors such as the olive mill conditions or the pedoclimatic influence,” although conclusive results on these factors were not obtained during this study.

Regarding the total content of the measured phenols, 74% of the thousands of analyzed Greek samples contained enough to exceed requirements for the EU health claim (250 mg/kg or 5 mg/20 g) at the time of measurement. “About 38% and 17% of the samples had phenol concentration two and three times higher, respectively,” than required for the health claim. “In addition, 8% of the samples had phenolic content more than 1000 mg/kg and about 5% of high-quality samples contained phenols over 1180 mg/kg.”

As Diamantakos and his colleagues point out, “this observation, based on a long-term recording and for a large number of samples,” proves that the average Greek olive oil “fulfills international requirements” for olive oil with a certified health claim related to phenolic content.

That is no accident: after the EU health claim for olive oil was approved in 2012, many Greek producers began striving to increase their olive oil’s phenolic content, for example by harvesting their olives earlier. The Molecules article shows that harvest time does make a difference, with “a decreasing trend of the phenolic content from September to January,” and the important phenolic compound oleocanthal following that same path.

“To our knowledge,” Diamantakos and his coauthors write, “this is the first attempt to define the term high phenolic olive oil based on robust statistical data.” They point out that other studies considering the level of phenolic compounds in olive oils used a smaller number of samples, studied them over a shorter time period, and utilized several different methods for measuring phenolic content as well as different ways of expressing results. The studies they cite yielded results ranging from 20 to 4497 mg/kg of phenols, but the variations in measurement and reporting make a meaningful comparison of those results difficult.

The eleven-year study in the Laboratory of Pharmacognosy and Natural Products Chemistry in the Department of Pharmacy at the National and Kapodistrian University of Athens was noteworthy for its length, consistency of methodology (1D qNMR spectroscopy), and sample size. “To date,” the authors assert, “the current work is the largest statistical study of the phenolic content of olive oil and aims to be a reference study for the future.”

In February 2021, Panagiotis Diamantakos and his colleagues at the Laboratory of Pharmacognosy and Natural Products Chemistry in the Department of Pharmacy at the National and Kapodistrian University of Athens and at the Laboratory of Sylviculture, Forest Genetics and Biotechnology at the Institute of Mediterranean and Forest Ecosystems, part of the Hellenic Agricultural Organization “Demeter,” published the article titled “A New Definition of the Term ‘High-Phenolic Olive Oil’ Based on Large Scale Statistical Data of Greek Olive Oils Analyzed by qNMR” in the journal Molecules. The article’s authors are Panagiotis Diamantakos, Kostas Ioannidis, Christos Papanikolaou, Annia Tsolakou, Aimilia Rigakou, Eleni Melliou and Prokopios Magiatis.

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