Protecting the nutrient richness of the World's Healthiest Foods is high up on my list of goals for each and every one of our health-supporting foods. But when it comes to extra virgin olive oil, I take an especially careful approach with respect to exposing it to excessive amounts of heat. All vegetable oils are susceptible to heat damage—much more so than the whole foods from which they were pressed or extracted. But in the case of extra virgin olive oil, the susceptibility is especially great. Here are some of the reasons that extra virgin olive oil requires special attention.
All types of olive oil (including extra virgin) contain a large amount of monounsaturated fat. In fact, 70-80% of the total fat found in olive oil is monounsaturated. This monounsaturated fat comes from the monounsaturated fatty acid (MUFA) called oleic acid. In comparison to all commonly used vegetable oils, olive oil is fairly unique in its high MUFA content. Canola oil comes close (60-70% MUFA), but many of the other commonly used vegetable oils, including sunflower, safflower, corn, and soybean oils naturally contain less than half as much MUFA as olive oil. It's worth noting here that cooking oil manufacturers sometimes create high-oleic version of these other oils, so that it is now possible to purchase high-oleic sunflower or safflower oil in many food stores.
In general, monounsaturated fat increases the stability of a vegetable oil in comparison to polyunsaturated fat. This increased stability is related to the chemical structure of monounsaturated fat. MUFAs have fewer "reactive spots" than PUFAs (polyunsaturated fatty acids) and it is more difficult for oxygen radicals to interact with these kinds of fat. However, despite this lower reactivity, olive oil and other vegetable oils containing a high amount of MUFAs (like canola oil) still have relatively low smoke points and cannot withstand a large amount of heat. Unless these high-MUFA oils have been refined or conditioned in a way that increases their smoke point, they typically cannot withstand heats of much greater than 200-250˚F (93-121˚C) without incurring damage (the temperature of stove-top frying is 375-525˚F, or 191-274˚C). So even though the high-MUFA composition of extra virgin olive oil increases its chemical stability, it does not protect this wonderful oil from most stovetop or oven cooking temperatures.
It is worth noting in this discussion of MUFAs that the oleic acid found in olive oil has been the subject of expanding research interest when it comes to insulin resistance, cancer, and cardiovascular disease. Recent studies have made it clear that oleic acid can directly alter the activity of certain cancer genes and appears to have anti-cancer effects that may be part of the Mediterranean diet's health benefits. This primary MUFA in extra virgin olive oil may also help to lower a person's risk of insulin resistance as well as favorably altering some of the blood fat patterns that can be associated with risk of cardiovascular disease.
Yet, to get these benefits it's important to enjoy extra virgin olive oil when its MUFAs are best protected, which means at heats below 250˚F (121˚C).
Research studies on the heat susceptibility of extra virgin olive oil tend to focus on higher heat ranges than we typically recommend at the World's Healthiest Foods. Heats between 320-374˚F (160-190˚C) are most often used to determine the changes that occur in extra virgin olive oil due to heating. Studies on this subject repeatedly show that heats as low as 320˚F (160˚C) can substantially damage the phenols in olive oil. Although studies tend to expose extra virgin olive oil to heat for prolonged periods of time involving 1-3 hours, some studies have looked at relatively short heat exposures—sometimes as brief as 10 minutes. In all studies, heat exposure destroys key phenol components of extra virgin olive oil. At temperatures close to 350˚F (177˚C), for example, about 60% of the dihydroxyphenols in extra virgin olive oil are lost after 10 minutes! Hydroxytyrosol is another extra virgin olive oil phenol that gets damaged very quickly by heat, including heat as low as 320˚F (160˚C) according to published research studies. Alongside of the phenol loss that occurs with heat exposure is also the loss of vitamin E.
While I have not seen research studies that explored damage to extra virgin olive oil's rich phenol mixture at temperatures lower than 320˚F (160˚C) I do not believe the extra virgin olive oil's phenols are safe at temperatures lower than this! In fact, it seems highly likely to us that research studies involving lower temperatures would confirm damage to hydroxytyrosol and dihydroxyphenol at these lower temperatures—just less damage than already demonstrated at 320˚F (160˚C).
Until I see studies indicating otherwise, I will choose to play it safe with regard to heating extra virgin olive oil. These extra virgin olive oil phenols are simply too important to risk potential damage through unnecessary heating. The 200-250˚F (93-121˚C) temperature range is the one I feel safest with when it comes to the heating of extra virgin olive oil and protection of its phenols. This temperature range will work well for making sauce or for the warming of a dish that has extra virgin olive oil added just before this warming stage (but not during the actual cooking or baking process).
There are, of course, many dishes that do not require heating that will allow you to enjoy extra virgin olive oil. Healthy sautéed vegetables can be drizzled with extra virgin olive oil before serving. Extra virgin olive oil can be blended together with garlic and beans in a food processor to make a delicious dip. Extra virgin olive oil can be used in place of butter on bread or rolls, and it can also be combined with garlic and added to potatoes that have been previously steamed to create a wonderful mashed potato side dish. There are also, of course, delicious salad dressings containing extra virgin olive oil that do not call for any heating whatsoever. In all of these circumstances, you'll get to enjoy the great flavor of extra virgin olive oil along with the full benefits of its amazing health-protective phenols.
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