By: Elna Botes van Schalkwyk
The two main things to consider when evaluating whether it is OK to heat extra-virgin olive oil (or any other oil for that matter) is the smoke point of the oil and the oil’s ability to becomes rancid.
The smoke point is the temperature at which visible gaseous vapor from the heating of oil becomes evident. (Safflower oil has indeed a very high smoking point.) It is traditionally used as a marker for when decomposition of oil begins to take place. Since decomposition incurs chemical changes that may not only result in reduced flavor and nutritional value but also the generation of harmful cancer causing compounds (oxygen radicals) that are harmful to your health, it is important to not heat oil past its smoke point. Inhaling the vapors can also be damaging. The more omega 3 fatty acids in the oil, the less suitable it is for cooking.
Rancidity is when the oil has begun to deteriorate, and the three biggest enemies of olive oil are heat, light, and oxygen. For example, adding heat in the production process, storing the oil in direct light, or exposing the oil to oxygen will all cause an oil to go bad. (A tightly capped bottle will help prevent your oil from being unnecessarily exposed to oxygen.) Since all of these factors can also speed up the rancidity process, protection from heat and light are also important when it comes to your food oils. With respect to light, your best bet is to purchase oils in bottles made from darker (tinted) glass (usually dark brown or dark green glass). You'll also want to store your oils in a cabinet that is lightproof. With respect to heat, many oils are best kept in the refrigerator where the temperature remains continuously low. Heating oils (almost all oils) actually starts to turn the oil into a partially hydrogenated oil. The burning process denatures the oil (starts to turn it into a trans fat) and also oxidizes it and makes it rancid.
Finally, it's important to note that any given oil's smoke point does not remain constant over time. The longer you expose an oil to heat, the lower its smoke point becomes. Also, when you're deep-frying food, little bits of batter or breading will drop off into the oil, and these particles accelerate the oil's breakdown, lowering its smoke point even more. So in general, fresher oil will have a higher smoke point than oil you've been cooking with for a while.
Below is a table that shows the smoke points for several of the most common cooking fats and oils. In some cases you'll see a range of temperatures rather than a single smoke point, because of different degrees of refinement among numerous brands of oils as well as other variations.
Smoke Points of Fats and Oils
Vegetable Shortening (Hydrogenated) : 325°F
Olive Oil: 325°F - 375°F
Coconut Oil: 350°F
Canola Oil: 425°F - 475°F
Sesame Oil: 445ºF
Sunflower Oil: 445°F
Palm Oil: 445°F - 475°F
Soybean Oil: 450°F - 475°F
Safflower- and Avocado Oil: 475°F - 500°F