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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, which primarily consist of polyunsaturated and monounsaturated fats, are more prone to oxidation than saturated animal fats.
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 United Kingdom, who specializes in bioanalytical chemistry and chemical pathology.
Oxidation in oils can be measured by calculating the stability of heated oil over time, according to Mr. 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 F (110 C), 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.
The following list ranks oils based on oxidation stability, though other metrics measure oxidation tendency.
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—around 60 percent. The oil exhibits varying levels of oxidation stability.
Canola oil has an induction hour (another word for oxidative stability) of about six to eight hours. However, Mr. Grootveld’s studies suggest that when heated to temperatures of 365 F (180 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 F (110 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 F (180 C), both oils can begin to oxidize within approximately 90 minutes, according to Mr. 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 F (110 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 F (110 C), it took the oil 69 hours to produce significant amounts of oxidants.
Limited studies exist on the oxidative stability index of butter. However, research has shown that both ghee and regular butter produce low oxidation during the first 20 hours when heated to 230 t0 250 F (110 to 120 C). Ghee demonstrates greater stability when heated at higher temperatures, up to 284 F (140 C) (pdf). For ghee to possess superior preservation characteristics to tallow, it must be heated to temperatures of 266 to 284 F (130 to 140 C).
Tallow is highly resistant to high-heat cooking. In a study comparing tallow with lard and canola, soybean, and peanut oils heated to 365 F (185 C) for five hours, tallow produced the lowest peroxide-related oxidants(pdf).
Choosing the right oil for cooking needs is a complex process. But that’s not all—there’s more to explore.
Next: The next article in this series will explore how saturated fat affects heart diseases.