Spanish team cooked extra-virgin oil to test how it fares under fire 03/09/2020
Two questions keep coming up with regard to extra-virgin olive oil or EVOO.
First, consumers wonder whether an olive oil is really extra-virgin grade or has been mixed with the fully refined or semi-refined grades respectively called pure and virgin.
That’s understandable, since an estimated 50% of the EVOO sold in America is adulterated either with other vegetable oils or with lower-grade olive oil.
Lesser grades of olive oil don’t taste as good as EVOO, while misleadingly named “pure” grade olive oil has virtually no antioxidants and virgin grade has far fewer antioxidants: see “How do you know what you’re buying is really EVOO?”, below.
According to the UC Davis Olive Center, a quality EVOO should smell and taste fresh, have aromatic notes — such as grass, apple, green banana, artichoke, and herbs — and will typically have bitter and spicy notes, which indicate healthy levels of antioxidants.
People also wonder whether cooking with EVOO degrades its flavor, healthfulness, and antioxidant levels — and at what temperatures those losses become substantial.
A new study from a Spanish-Brazilian team addresses the second question in considerable detail — see “Spanish-Brazilian study pinpoints what cooking does to EVOO”, below.
Before we get to those new findings, let’s quickly review the health qualities of EVOO versus lesser grades of olive oil and look at some prior research on the cooking-temperature question.
EVOO’s health benefits tied to its high antioxidant and low omega-6 content
Clinical and population studies suggest that Mediterranean-style diets — ones high in vegetables, fruits, fish, nuts, EVOO, and beans — improve cardiovascular and metabolic health, and many of those benefits can be attributed to EVOO.
That’s because the potent phenol-type antioxidants in EVOO — such as oleuropein, hydroxytyrosol, and tyrosol — deliver powerful cardiovascular, anti-inflammatory, and other health benefits: see Olive Oil Benefits Linked to EV Grade's Key Antioxidants and Extra Virgin Olive Oil Confirmed as Best Cardiac Prevention Choice.
The Italian authors of a recent evidence review came to this conclusion: “… regular consumption of extra virgin olive oil (EVOO) … is associated with a reduced risk of developing chronic degenerative disorders such as cardiovascular diseases, type 2 diabetes, and cancer.” (Santangelo C et al. 2018)
And the Italian team noted that lab and animal studies suggest EVOO’s antioxidant/anti-inflammatory effects can help alleviate autoimmune conditions such as rheumatoid arthritis, inflammatory bowel disease, multiple sclerosis, lupus, and psoriasis.
Better yet, olive oil of any grade is relatively low in the omega-6 fatty acids that occur to extreme excess in the standard American diet. You can read about the unhealthful consequences of the average American’s “omega imbalance” on our Omega-3/6 Balance page, where you’ll find our “Out of Balance” video featuring interviews with scientists expert in this field.
The cheapest, most commonly used vegetable oils at home and in restaurants — soy, corn, safflower, sunflower, peanut, and cottonseed — are very high in omega-6 fats, which readily oxidize when exposed to heat and air.
To minimize omega-6 intake and heat-related degradation of a cooking oil, the best choices are olive oil, canola oil, and macadamia nut oil, which are high in heat-resistant monounsaturated fats and low in omega-6 fats. (Among all common cooking oils, canola oil has the highest levels of omega-3 fats, which is good, but when they are exposed to heat and air, omega-3s oxidize even more quickly than omega-6 fats.)
Prior research on EVOO’s ability to withstand heat
Many consumer guides advise people to keep EVOO cooking temperatures under 250-320°F, but present no evidence for this claim, nor can we find any in the scientific literature.
Evidence we reported several years ago suggested that cooking with extra virgin olive oil (EVOO) at temperatures of 300-350°F doesn’t produce excessive oxidation of its fats and preserves much of the oil’s antioxidants.
In fact, studies show that EVOO is remarkably resistant to damage when heated as high as 400°F, as when frying foods — largely because its high antioxidant content helps the oil resist oxidation. That said, cooking with EVOO for more than a few minutes at temperatures above 300°F will substantially reduce the antioxidant content of an EVOO.
Interestingly, a Spanish study published four years ago found that cooking vegetables in extra virgin grade olive oil (EVOO) does two very good things:
• Boosts vegetables' antioxidant power
• Instantly improves people's blood sugar and fat/cholesterol profiles
For more on that study — and some other studies confirming the health benefits of EVOO — see When is Oil Better than Water?.
Spanish-Brazilian study pinpoints what cooking does to EVOO
The new findings come from researchers at Spain’s University of Barcelona and Brazil’s University of São Paulo (Lozano-Castellón J et al. 2020).
In short, the results of their experiment confirm that EVOO retains substantial levels of antioxidants when used to cook foods at fairly high temperatures.
The study’s lead author, Rosa M. Lamuela, PhD, noted that most prior investigations tested roasting foods with EVOO in a laboratory or industrial situation affected its antioxidant content.
But as she said, those artificial environments are “… far from the daily life of homes.” So, she and her team set out to simulate the cooking conditions of a home kitchen to see they affect the antioxidants in EVOO.
Specifically, they examined the effects of short and long cooking periods and of varying temperatures — ranging from 248°F (120 ° C) up to 338°F (170 ° C) — on the antioxidant content of EVOO.
And their experiment revealed that the antioxidant content of EVOO declined by 40% at 248°F (120° C) and by 75% at 338°F (170 ° C).
Interestingly, cooking with it for fairly long time periods at fairly low temperatures didn’t reduce the total antioxidant content of EVOO — although longer cooking time did reduce levels of certain antioxidants, such as hydroxytyrosol.
Importantly, despite the temperature-related declines in the antioxidant content of EVOO recorded by the researchers, they remained at levels the European Union (EU) considers healthy.
As study co-author Julián Lozano said, “Despite the decrease in the total concentration of polyphenols during cooking, this oil … [still] … has properties that protect against oxidation of LDL cholesterol particles.” (Oxidation of LDL cholesterol is considered a key promoter of atherosclerosis, which is the underlying cause of cardiovascular disease.)
According to the authors of this study, they next intend to analyze the effects of cooking food such as meats and beans in EVOO.
How do you know what you’re buying is really EVOO?
Your best assurance of getting real EVOO is to buy from a trusted retailer and/or pick an EVOO that’s certified organic. Organic certification requires that the farm(s) of origin, the production facility, and the supply chain undergo regular inspections.
Some claim that genuine EVOO will solidify under refrigeration — a claim refuted by research from the University of California Davis Olive Center.
Dr. Mehmet Oz touted the fridge-test theory on a 2013 episode of his Dr. Oz Show, which unfortunately reached more than three million viewers. While cautioning them that his method isn’t fool-proof, Dr. Oz. encouraged viewers 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,” said Olive Center executive director Dan Flynn.
This prompted the Olive Center to conduct their study. They refrigerated seven samples, including two EVOOs, a lower grade 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.
As the study's authors wrote, “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.”
UC Davis scientists tested seven olive oil samples under cold conditions over eight days and discovered that the “fridge test” is unreliable in detecting either the purity or quality of olive oil.
As the Olive Center's Dan Flynn said “None of our samples showed any signs of congealing after 60 hours in a laboratory refrigerator set to 40.5°F. Even after 180 hours, the samples never fully solidified.”
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