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Quinn on Nutrition: When heating cooking oils creates possible hazards

April 20, 2015

In response to a column about tossing vegetables in olive oil and roasting them in a 400 degree oven, a reader from Connecticut 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 odor 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 sautéed dishes. Use more refined oils for cooking at higher temperatures. Extra virgin olive oil, for example, has a smoke point of about 320 degrees F. whereas refined olive oil begins smoking at around 410 degrees.

Use nut oils like peanut, walnut, sesame and grapeseed for dishes that require higher cooking temperatures. These oils generally have smoke points of 400 degrees or higher.

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 325 and 375 degrees F. Maybe we could even turn the heat down a bit with our olive oil tossed roasted vegetables.

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