Which cooking oils pose harm when over-heated?

Some oils turn rancid in very high temperatures, so it pays to know which to use for what purpose.

Some oils turn rancid in very high temperatures, so it pays to know which to use for what purpose.

Are you confused about which oils to use in high heat and what risks it might pose to your health? You're not alone.

In response to a column about tossing vegetables in olive oil and roasting them in a 200 degrees Celsuis* oven, a reader writes: "My wife and I often roast vegetables as you described. But we recently heard that olive oil, when used with high heat, creates [compounds] that are definitely not good for us. We are now concerned about using olive oil in cooking and baking and are somewhat confused as to what we should do. Do you have any knowledge or information about such issues?"

High heat can definitely damage the integrity and nutritional value of cooking oils, say food chemists. Especially when the oil is heated beyond its "smoke point". How do you know when it reaches this level? It emits a harsh odour and sets off your smoke alarm. When oils begin to smoke, they release volatile and sometimes dangerous compounds into your food.

Oils vary in their smoke point. Extra virgin olive oil, for example, disintegrates at a lower temperature than refined olive oil. And the practice of reusing the same oil over and over – such as in deep fat-frying – causes it to become rancid much more quickly.

Studies conducted on "thermally stressed culinary oils" show they can produce toxic substances known to harm health. And valuable omega-3 fats are destroyed when vegetable oils are heated above their smoke points.

Exposure to light and oxygen can accelerate the deterioration of cooking oils as well, so where we store them becomes important. Here are some other guidelines:


Avoid consuming fried foods as much as possible. Duh. When you do cook in oil, use high monounsaturated fats such as olive and canola oils. These may be better able to resist oxidative damage when heated than oils that are high in polyunsaturated fats such as corn or safflower oils.

Use extra virgin unrefined oils for salad dressings and lightly sauteed dishes. Use more refined oils for cooking at higher temperatures. Extra virgin olive oil, for example, has a smoke point of about 160C, whereas refined olive oil begins smoking at around 210C.

Use nut oils like peanut, walnut, sesame and grapeseed for dishes that require higher cooking temperatures. These oils generally have smoke points of 200C or higher.

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Save your cherished flaxseed, hemp and poppyseed oils for recipes that require little if any heat. These oils have a high propensity to become rancid even at low cooking temperatures.

Who needs to char foods with ghastly high temperatures anyway? According to a technical paper on this topic from Oklahoma State University, most foods cook well at temperatures between 160C and 190C. Maybe we could even turn the heat down a bit with our olive oil-tossed roasted vegetables.

Barbara Quinn is a registered dietitian and certified diabetes educator at the Community Hospital of the Monterey Peninsula.

*Fahrenheit temperatures in the original article converted to rounded Celsius.

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 - MCT


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