Does frying in oil produce trans fats?

Short Summary

Most of the harmful trans fatty acids in our diet come from partially hydrogenated oils, which involves high temperatures and high pressure. But does heating up vegetable oils during cooking also generate those dangerous substances? What are the cooking methods that generate the highest amounts of trans fats? For a quick answer click here.

Explanation

It has been broadly proven that Trans fatty acids (TFAs), or trans fats, cause many negative health effects, such as increased risk of cardiovascular disease and various cancers. Therefore, they should be avoided. (read more..)

Some countries, such as Denmark, have introduced strict limits on the contents of TFAs in foods and on the usage of partially hydrogenated oils (PHOs). These measures successfully resulted in a drastic reduction of the consumption of TFAs, below the recommended upper levels. (read more..)

Nevertheless, the question remains whether home food preparation techniques, which use high temperatures, can generate these harmful substances.

How are trans fatty acids formed?

Trans fatty acids are formed in the process of lipid oxidation. High temperatures accelerate the rate of this oxidation and, therefore, speed up the formation of trans fatty acids.

Unsaturated fatty acids, such as vegetable oils, contain carbon double bonds in “cis” formation that are unstable.  This means that they are more prone to spoilage. When heated up, the atoms get agitated and a hydrogen atom is removed from the “cis” configuration, forming a lipid radical. To create a more stable configuration, a hydrogen atom is then added to the other side of the damaged chain, forming what we know as “trans” fatty acids. (read more..)

This is the most common process of partial hydrogenation and it is how the majority of the dietary trans fats are formed. ⁽¹⁾Choe E, Min DB. Mechanisms and Factors for Edible Oil Oxidation. Volume 5, Issue 4. September 2006. Pages 169–186. Available here.

Can you generate trans fats while cooking?

Since the partially hydrogenated process, that generates high amounts of industrial trans fatty acids (iTFAs), results from heating unsaturated fats, can the same happen during cooking at home or in restaurants?

Several studies were performed on different types of oils and cooking methods, such as baking, stir frying, pan frying, deep frying and roasting.

Stir frying – study 1
The following are the main conclusions of a study on trans fat generation, using various methods of cooking with corn oil.

They found evidence that stir frying at a temperature of 170 degrees Celsius (338 Fahrenheit) increases the trans fatty acid contents, when compared to raw oil before cooking.  Other cooking methods did not show an increase in Trans fats.

The study concluded that the formation of trans fats in stir-frying was simultaneously due to high temperatures and stirring, causing an increase rate of oxidation of fatty acids. ⁽²⁾Song J, Park J, Jung J, Lee C, Gim SY, Ka HJ, et al. Analysis of Trans Fat in Edible Oils with Cooking Process. Toxicol Res. 2015 Sep; 31(3): 307–312. Available here.

Deep frying – study 2
Another study on deep frying, showed that frying potato chips in a reheated oil for 10 times at the temperature of 180 degrees Celsius (356 Fahrenheit), generated more trans fatty acids than heating oils to the same temperature for 4 hours without chips.

When potatoes were fried in an oil, which was reheated 10 times, the total fat content in fried potatoes was 9%. The trans fatty acids contents were, however, within the U.S. health recommendations of <0.5g per serving. 100g of fried potatoes contained only 0.1g of trans fatty acids, which represented about 1% of total fat contained in fried potatoes. ⁽³⁾Tsuzuki W, Matsuoka A, Ushida K. Formation of trans fatty acids in edible oils during the frying and heating process. Food Chemistry. Volume 123, Issue 4, 15 December 2010, Pages 976–982. Available here.

Cocoa bean roasting – study 3
Another study showed a negligible increase of trans fatty acids contents in cocoa butter by roasting cocoa beans at up to 135 degrees Celsius (275 Fahrenheit) for 15 minutes. ⁽⁴⁾Zyzelewicz D, Budryn G, Krysiak W, Oracz J, Nebesny E,Bojczuk M. Influence of roasting conditions on fatty acid composition and oxidative changes of cocoa butter extracted from cocoa bean of Forastero variety cultivated in Togo. Food Research International. Volume 63, Part C, September 2014, Pages 328–343. Available here.

Conclusion

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Stir frying noticeably increases the amount of Trans Fatty acids, significantly more than any other method. Deep frying in high temperatures and in the oils used multiple times also produces some trans fats. Other cooking methods either don’t raise trans fat content or only raise it a negligent amount.

Although the generated trans fats through stir frying and deep frying in reheated oils are still within the recommended safe limits, it is important to note that frequent consumption of these foods adds up during the day and may exceed the maximum limits.

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