By Vesanto Melina, RD
Oil can be heated up to a certain point with no significant change in chemical composition. The point at which it changes is called its “smoke point”, but this is different for each oil.
The smoke point is the temperature at which oil begins to break down and form a bluish smoke. Its flavor and nutrition are damaged.
The smoke contains acrolein that is irritating to the eyes and throat. The smoke point also marks significant changes in flavor and nutritional degradation.
“Cold pressed” oils contain heat-sensitive vitamins and phytochemicals. These oils are great for dressings. But the vitamins and phytochemicals they contain are vulnerable to heat damage. High temperatures turn them into contaminants.
That’s why these “cold pressed” and unrefined oils have lower smoke points than their refined counterparts. Refined oils have been stripped of these vitamins and phytochemicals.
When choosing an oil to use in cooking, stick with oils that are refined (to remove heat-sensitive vitamins and phytochemicals). Good choices are olive oil, or high oleic sunflower or safflower.
Saturated fats, such as coconut oil and ghee (for ovo-lacto vegetarians) are even better for frying because they’re less subject to oxidation.
When you fry (or stir fry), overheating or over-using the oil leads to formation of rancid-tasting products of oxidation, molecular changes, and toxic compounds such as acrylamide (from starchy foods). These changes may not be visible, evident, or obvious. But the flavor might change.
Which oils are best when served raw (for example in salad dressings)?
From a nutritional perspective, the best oil to use for salads dressing is flaxseed oil due to its particularly high content of Omega-3 fatty acids. Hempseed oil and walnut oil are less common, but also high in Omega-3 fatty acids. However, none of these should be used in cooking.,
For a list of popular oils and the temperature to which it can be safely heated, see the Smoke Point table.
Which oils are best for frying?
For frying, use any of the oils with asterisks in the Smoke Point table.
Which oils are best for baking / roasting?
You may use any of the oils with asterisks in the Smoke Point table. For high oven temperatures, choose an oil with an appropriately high smoke point.
You will find it helpful to keep in your home flaxseed oil (for salad dressings and as a source of essential omega-3 fatty acids) and refined coconut oil or olive oil (for stir fries and other heated menu items). You really don’t need any other oils.
However, some people like to use refined sesame oil for a slightly different flavor in stir fries. And some like the flavor of canola, safflower or sunflower oil in their baked goods and pancakes.
An even cleaner choice is to eat a raw or high-raw diet and make whole foods like avocado, olives, nuts, seeds, and their butters, and coconut your primary sources of dietary fat.
For the complete lesson on this topic — and 49 lessons like it — be sure to enroll in the Vegan Mastery Program or Vegetarian Mastery Program the next time enrollment is open to the public.
 Choe E, Min DB. Chemistry of deep-fat frying oils. J Food Sci. 2007 Jun;72(5):R77-86.
 Paul S, Mittal GS. Regulating the use of degraded oil/fat in deep-fat/oil food frying. Crit Rev Food Sci Nutr. 1997 Nov;37(7):635-62.
 Fats and Oils, Udo Erasmus. 1986.