|Fried butter on a stick - I bet there are more peroxides in the crust than in the butter beneath it.|
Oxidized frying oils and their effects on our health
The paper by Jaarin and Kamisah, two researchers from the Department of Pharmacology at the Universiti Kebangsaan Malaysi in Malaysia is certainly not the only, maybe not even the latest study dealing with this issue, what I like about it though, is the fact that they don't use some sort of standardized oxidized oil, but actually went through the following unquestionably not unrealistic procedure to obtain their frying oils:
"A kilogram of sweet potato slices were fried in a stainless steel wok containing two and half litres of palm oil or soy oil for 10 minutes at 180°C. Upon completion of the frying process, once heated oil was obtained. The process was repeated four times to obtain five times heated oil with a cooling interval of at least five hours. The food quantity was proportionately adjusted with the amount of vegetable oil left. No fresh oil was added between the frying processes to make up for the loss due to uptake by the frying materials." (Jaarin. 2012)After the oils had been heated, a small quantity was extracted and the peroxide value, fatty acid composition and vitamin E content measurements (another strength of the study, most other studies discard the fatty acid composition and vitamin E content).
|Figure 1: Peroxide levels (data expressed relative to fresh palm oil) of palm and soy oil after frying at 180°C for 10minutes once or five times (based on Jaarin. 2012)|
|Figure 2: Relative fatty acid composition of the fresh, once or five times heated oils (based on Jaarin. 2012)|
"[...t]he fresh soy oil contained about five times more polyunsaturated fatty acid compared to the palm oil. It seemed that five times heating had reduced about 10% of the polyunsaturated fatty acid content in the soy oil. The content of monounsaturated fatty acid in the fresh palm oil was higher than that of the fresh soy oil. Palm oil had a quite balanced ratio of saturated and unsaturated fatty acids, whereas more than 70% of soy oil fatty acid was unsaturated (polyunsaturated and monounsaturated). This unique fatty acid composition of palm oil renders its stability against oxidative insult." (Jaarin. 2012)Now, both the oxidation process during heating as well as the in vivo oxidative effects of the consumption of those oils does not depend on their fatty acid makeup and oxidation status, only, but is also affected by the amount of antioxidants, in particular vitamin E, the respective oil brings to the table.
Against that background, is should be obvious that the profound reductions in both tocopherols and tocotrienols will augment the negative effects, so that it is actually not that surprising that the amount of thiobarbituric acid reactive substances (TBARS = standard marker of lipid oxidation) in the blood of the rodents which consumed the diets with 15% of the five times heated palm and soy oil more than doubled.
|Figure 4: While olive oil is more stable than corn and soy, it's not 'oxidation proof either' what's also intriguing is the amount of oxidation that is induced just by exposing the oil to air or air and light for 30 days! (Naz. 2012)|
I assume that no one of you will use soy oil and only few will be cooking with palm oil, but I know that many people still use olive oil in a hot pan for several minutes. And while I do not have data from the same study, the data in figure 5 would suggest that olive oil is probably as susceptible to heat as palm oil (if you go by the relations of palm:soy in the study at hand and olive:soy in the study by Naz et al.). If we go by this rule of thumb estimate, baking your potatoes in olive oil may not kill you, but clearly isn't the best way either.
Coconut oil!? No, unfortunately even the wonder oil does not come to a rescue. At least if we go by the data Matthäus obtained in 2007, it produces about as much peroxides as palm oil during the frying process (Matthäus. 2007). That's better than soy, and way below the critical margin, but still not without consequences on the overall oxidative burden you are exposing yourself to, when you consume significant amounts of fried foods on an everyday basis - and since I know that you do only have your occasional piece of fried grass fed butter on a stick, you won't have to be afraid anyway ;-)
- Pimp my virgin olive oil - Discusses among other things how the polyphenols stabilize the vitamin E and render EVOO more heat stable than regular olive oil.
- Vitamin(s)! E - A brief reminder that there is more than alpha-tocopherol + some evidence that delta tocopherol is king, when it comes to protect dietary oils
- Matthäus, B, Use of palm oil for frying in comparison with other high-stability oils. Eur. J. Lipid Sci. Technol. 2007;109: 400–409.
- Jaarin K, Kamisah Y. Repeatedly Heated Vegetable Oils and Lipid Peroxidation. Intech. 2012.
- Naz S, Siddiqi R, Sheikh H, Sayeed SA. Deterioration of olive, corn and soybean oils due to air, light, heat and deep-frying Food Research International, Volume 38, Issue 2, March 2005, Pages 127–134.