If you’ve ever put some oil in a skillet to heat and then gotten distracted by a phone call or something on TV, you very likely ended up with a kitchen full of acrid black smoke and a wailing smoke alarm. Of course, you never want to let oil get so hot that it starts smoking. Not only is the oil then unusable, but the smoke that’s produced when oil is overheated contains harmful chemicals that you don’t want to be breathing in.
Ideally you don’t even want it to get hot enough to start changing color, because that is an indication that the oil has started to chemically degrade. When oil starts to break down, free radicals form, along with other harmful compounds.
That’s why it’s important to choose a cooking oil with a high smoke point. The smoke point is determined in part by the type of oil. Walnut oil, for example, starts to smoke when it reaches about 320°F, but grapeseed oil can be heated to 475°F before it begins to smoke. The type of oil is just one factor, however.
The refining process, which removes impurities from oil, also raises the smoke point. For example, extra virgin olive oil may smoke at 325°F, but refined (or “light”) olive oil can usually be heated to 450°F or higher. So, here’s a tip that will make your cooking healthier and save you money at the same time: Save your extra virgin olive oil for off-heat uses such as salad dressings and choose a refined olive oil for sautéing vegetables.
But Smoke Point Is Only Half the Story
Avoiding smoking oil isn’t all you need to worry about. Oils that are high in polyunsaturated fats (such as corn, soy, sunflower and other seed oils) form a harmful compound called HNE when they’re heated. That happens even in oils that are highly refined and have a high smoke point. HNE ends up in the foods that are cooked in the oil and is taken up in the body, where it can have damaging effects.
Now, I don’t want to over-state the dangers. Your body has an impressive capacity to defend itself against harmful compounds. Good thing, because it’s impossible to completely avoid them. Then again, given the realities of modern life, your defense and detox systems are probably getting a pretty good workout. So, why not minimize our exposure to toxins wherever we can? One way to minimize your exposure to HNE is to avoid cooking with highly polyunsaturated oils like corn, soy and all-purpose “vegetable” oil (even if they have a high smoke point).
Why You Should Never Reuse Frying Oil
It’s not a good idea to reuse cooking or frying oil. For one thing, heating up oil lowers its smoke point so it will start smoking at a lower temperature the next time it is used. And the longer or more frequently oil is heated, the more HNE it will contain. Most restaurants not only fry foods in polyunsaturated oils, but reuse the oil over and over again. Consider that one more reason to limit your consumption of fried foods.
What Causes Oil to Get Rancid?
Unfortunately, even if it never gets near the heat, oil will eventually get rancid just sitting around in your cupboard. In addition to smelling and tasting nasty, rancid oil contains harmful free radicals and shouldn’t be consumed. The more polyunsaturated fat there is in an oil, the faster it will spoil. For that reason, buy nut and seed oils – which are high in polyunsaturated fat – in small quantities and store them in the fridge. Oils that are lower in polyunsaturated fats, such as like olive, canola, and coconut oil, are fine stored in a cool cupboard.
What Oils Should You Use?
Taking all of this into consideration, here are my recommendations for the best oils to use in the kitchen:
For frying, searing, grilling, stir-frying, or roasting, I suggest light (or refined) olive oil, avocado oil, clarified butter (also known as ghee), refined palm, or coconut oil. All have a high smoke point and are low in polyunsaturated fats.
For gentle sauté, stewing, baking, or braising, any of the above would work fine. For extra flavor, you could also choose a filtered olive oil.
If you're making a salad dressing or drizzling over a finished dish, choose unfiltered extra virgin olive oil, or unrefined or toasted nut and seed oils for maximum flavor and health benefits.
Read more from board-certified nutritionist Monica Reinagel, author of Nutrition Diva’s Secrets for a Healthy Diet.