How Popular Cooking Oils Stack Up in Terms of Oxidation

July 06, 2023

Updated:

January 16, 2024

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This is part 4 in “Debunking the Fat Dogma”

In this series, we will break down the actual health effects of vegetable oils and whether they are better alternatives to saturated fats. Follow this series to find out whether what you are using to cook with is really the best option.

Vegetable oils have been recommended as a healthier alternative to saturated animal fats for decades. But emerging data suggest the opposite—at least when it comes to the heavily refined, denatured seed oils most people are consuming.

Most vegetable oils consist primarily of monounsaturated and polyunsaturated fats that are more prone to oxidation than saturated animal fats. To explain that in molecular detail, the susceptibility of individual fatty acids to oxidation depends on their length and the presence or absence of double bonds.

Saturated fats have no double bonds, making them the least prone to oxidation. Monounsaturated fats have one double bond, making them less stable. Polyunsaturated fats can have two or more double bonds, making them even less stable.

This oxidation can lead to inflammation and the formation of cancer-causing byproducts, such as aldehydes, according to professor Martin Grootveld from De Montfort University in the UK, who specializes in bioanalytical chemistry and chemical pathology.

How Quickly Oil Produces Harmful Substances

Oxidation in oils can be measured by calculating the stability of heated oil over time, according to Grootveld. This process determines how quickly an oil produces potentially harmful compounds (pdf).

To measure its stability, oil is heated to a specific temperature, usually about 230 degrees Fahrenheit (110 degrees Celsius), triggering oxidative processes and the formation of oxidants. These oxidants dissolve into water-filled conductivity cells, altering the water’s conductivity. The test continues until the water’s conductivity changes significantly, indicating significant oxidation.

A longer oxidative stability time signifies higher oxidation resistance. Oils with longer stability times can withstand higher temperatures and prolonged heating without producing significant amounts of oxidants.

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The following list ranks oils based on oxidation stability, although other metrics measure oxidation tendency.

Main Cooking Oils Ranked From Least to Most Stable

1. Sunflower and Soybean

Sunflower and soybean oils are prone to oxidation because of their high polyunsaturated fatty acid (PUFA) content. Sunflower and soybean oils contain 69 and 61 percent PUFAs, respectively. According to an article in Inform Magazine by Min Hu, a principal scientist at DuPont Nutrition and Health, these oils become highly unstable when heated for a long time and quickly produce oxidants at high temperatures.

Efforts have been made to enhance the oxidative stability of sunflower and soybean oils by increasing oleic acid levels. Oleic acids are more stable monounsaturated fatty acids. Research is exploring genetic editing, selective breeding, and refinement processes to make these oils more oxidation-resistant.

Increased oleic acid levels have successfully improved soybean oil’s oxidative stability. Sunflower and soybean oils with higher oleic acid content exhibit enhanced resistance to oxidation when heated.

2. Canola

Canola oil is predominantly composed of monounsaturated fatty acids—about 60 percent. The oil exhibits varying levels of oxidation stability.

Canola oil has an induction hour (another term for oxidative stability) of about six to eight hours. However, Grootveld’s studies suggest that when heated to temperatures of 365 degrees F (180 degrees C) or higher, oxidant production can occur as early as 60 to 90 minutes. Further research is needed to understand canola oil’s complete oxidative stability profile.

3. Olive and Avocado

Both olive and avocado oils are rich in monounsaturated fatty acids, which constitute about 60 to 80 percent of their respective compositions. The high monounsaturated fat content contributes to their heat resistance.

A laboratory study showed that at 230 degrees F (110 degrees C), extra-virgin olive oil exhibits an induction period of about 32 hours, while avocado oil has an induction period of about 10 hours (pdf). However, when subjected to higher temperatures, about 356 degrees F (180 degrees C), both oils can begin to oxidize within approximately 90 minutes, according to Grootveld’s studies.

Studies suggest that unrefined extra-virgin olive oil and avocado oil (pdf) tend to have better resistance against oxidation than their refined counterparts. Refinement processes may remove natural antioxidants that protect against oxidation.

When purchasing extra-virgin olive oil and avocado oil, it’s essential to check that it isn’t adulterated. Because of the high prices of both types of oil, the industry has faced fraud issues. Previous studies of avocado oil found that 82 percent of avocado oil was adulterated or rancid.

One way to verify the authenticity of extra-virgin olive oil is by sipping it. Genuine extra-virgin olive oil contains oleocanthal, which can give you a tickling or coughing sensation when you taste the oil.

4. Palm

Palm oil, derived from the flesh of the palm fruit, consists of approximately 50 percent saturated fat. It exhibits good resistance to oxidation, with an oxidative stability score of around 16 hours (pdf).

Red palm oil, in particular, is abundant in vitamin E and beta-carotene, a precursor to vitamin A. These antioxidants contribute to the oil’s resistance to oxidation and its vibrant orange-red color. Prolonged heating destroys carotene (pdf), leading to a color loss in the oil.

5. Coconut and Palm Kernel

Both coconut and palm kernel oil have a saturated fat content of more than 80 percent, making them highly resistant to oxidation.

However, the length of the saturated fats also influences their heat and oxygen resistance. Since both oils consist mainly of medium-chain fatty acids, they have lower melting and smoke points than other saturated fats.

The laboratory study on coconut oil mentioned above revealed that when the oil is heated to 230 degrees F (110 degrees C), it takes approximately 51 hours for the oil to generate significant amounts of oxidants. While the oxidative stability of palm kernel oil isn’t known, it’s chemically very similar to coconut oil, making them suitable substitutes for one another.

6. Tallow, Butter, and Ghee

Tallow is the rendered fat from beef or lamb and contains more than 50 percent long-chain saturated fatty acids. Both clarified butter (ghee) and regular butter are high in saturated fat, with saturated fat accounting for more than 70 percent of their total fat content.

Tallow exhibits high stability, as evidenced by the Inform Magazine report indicating an oxidative stability score of 69 hours for beef tallow. This means that when heated to 230 degrees F (110 degrees C), it took the oil 69 hours to produce significant amounts of oxidants.

Debunking the Fat Dogma