How Come Nobody Ever Told Me That? (The Truth About Cooking Oils)

Cooking Oil For as long as I’ve been studying diet, nutrition and healthy cooking, it seems like I should have know about this before: Even so-called healthy oils can be very, very bad for you. Like artery-clogging, cancer-causing, cholesterol-boosting bad. In some cases, the oil started out bad before you even got it home, but more likely, things began to go awry the moment you cracked the seal on the bottle. Turns out, all that hype over an oil’s heat rating or smoke point has to do with more than just how it makes your food taste.

It’s true that when an oil reaches its smoke point it begins to smell and taste bad, but those things are just side effects of a more sinister process. As oil begins to smoke, it undergoes a chemical breakdown that oxidizes the oil and generates a number of unhealthy compounds. A study published in the May 2010 issue of Food Chemistry identified 30 such compounds and found that the longer an oil (any oil) was heated and the higher the temperature it reached, the  greater the number and concentration of those harmful compounds. This means that even before you see or smell the smoke, your “healthy” oil is already undergoing a yucky chemical change that essentially turns it into something very much like trans fat. To avoid this toxic breakdown, here are a few precautions you can take:

  • Don’t use the same kind of oil for everything. Check the oil’s heat rating, often printed on the bottle. It will say something like “Medium-High” or “Great for Sautéing.” Certain oils are better for baking and sautéing, while only a very few are advised for deep-frying. If stored properly, most oils are safe for drizzling or mixing into salad dressing, etc.
  • Consider refined oils for higher-heat cooking. Refined oils have been processed in a way that allows them to be heated to a higher temperature without breaking down. They will also have been stripped of most of their flavor, so don’t expect them to add much complexity to your meal. This can be a benefit: I often sauté with refined coconut oil because of its higher smoke point, and I’m happy not to have the coconut flavor in my Mexican or Italian dishes. If you’re looking for that olive oil (or other delicate) flavor, you can always drizzle a bit on at the very end.
  • If your oil gets too hot, throw it out and start over. It’s poison. Don’t eat poison.

Complicating the matter further is the issue of shelf life and rancidity. Even oils with high smoke points that are relatively stable at room temperature over the short-term can slowly breakdown and go rancid if they’ve been sitting on your shelf for weeks or months. This is largely just a slower process of the same negative chemical breakdown as the smoke point, leading to the oxidative release of free radicals in the oil. To guard against this, it’s recommended that you:

  • Buy the freshest oil you can find. Look for oils with many months to go before their expiration date.
  • Buy the smallest bottle available. Replacing oil frequently safeguards against toxic breakdown.
  • Store all oils in your refrigerator. Ambient temperature, light and exposure to oxygen can all make an oil go rancid quickly.

All of this keeps bringing me back to one question: Should we really even be cooking with oil? I try to do it as little as possible, and I’ve found that with the right pan, I can often get away with using a little water or vegetable broth instead of oil to sauté and even sear or char vegetables. Baking is another matter, although there are plenty of oil-free recipes out there, an in my experience the taste is virtually the same. So if you’re willing to compromise a little on texture, you’ll be rewarded with healthier treats. One of the best resources for oil-free cooking I know of is the Happy Herbivore web site, and her cookbooks, which I own several of.


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