Trans Fatty Acids Are Not Formed by Heating Vegetable Oils


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One of the frequent questions I receive in my email concerns the formation of trans fatty acids in the typical cooking process. I was quite surprised the first time I received this question, for several reasons. I knew that there were several things that were necessary for the formation of the trans fatty acids. One was a tank of hydrogen; second was a closed container, which allowed an adequate vacuum to form; third, an appropriate catalyst was needed; and last, the heat that would allow the chemical changes to occur had to be sufficiently high in conjunction with the other components.

During my many years of analyzing foods for the presence of trans fatty acids, I had found numerous examples of used frying oil that had started out without being partially hydrogenated and did not have any trans fatty acids, and there was still never any trace of trans fatty acids in the used oil unless the oil had been used for frying foods that had been prefried in a partially hydrogenated oil.

I am not sure who started the rumor that frying or even just cooking or heating polyunsaturated oils would produce trans fatty acids in those oils; but it is just that, an untrue rumor. It was likely started by one of the many internet writers hired to fill space or by someone who thought he or she knew the reason that there was trans fat in a particular product.

The idea that cooking with heat damages the oils that are highly polyunsaturated is true and the warning against cooking or frying using fragile oils such as flaxseed oil is valid, but not because trans fats are formed. What is formed under harsh circumstances such as high-temperature cooking and frying is a polymerized oil, and this is because the heat has helped to form free radicals and then various breakdown products. (Flaxseed oil that is still in the ground seed can be heated in baking and it does not become damaged.)

A number of years ago, a dietitian/nutritionist told me about her experience trying to make trans fatty acids in an open pan on top of the stove. She wanted to make a video of the process to use for teaching purposes. She was unsuccessful with this venture, and she had contacted me to ask me why her project had failed. She had not actually known how the trans were formed to begin with and assumed from what she had been told that the raising of the temperature would cause the trans to form. The project had been undertaken in one of the laboratories in a local university, and the analysis was to be done by someone in the same laboratory who knew how to use the instrument for analyzing the oil.

Certain types of trans fatty acids could probably be formed from a highly polyunsaturated oil during deep fat frying in one of the new pressure cooker fryers, but these types of trans fats would be like those formed in high pressure deodorization. They would not be the broad range of trans fats with delta-6, 7, 8, 9, 10, etc. So far, none of the groups doing analysis have reported this. Very small amounts of trans fatty acids have been found in corn chip products formed by extrusion cooking. This is due to the high pressure and the presence of a type of alkaline catalyst; but those trans that are formed are reported only in trace amounts from omega-6 or omega-3 oils.

Those fats and oils that are appropriate for cooking or sautéing and will withstand fairly high temperatures are those that have been in use for thousands of years, including olive oil as well as the more stable saturated coconut and palm oils and the animal tallows. An oil such as sesame oil with its special heat-activated antioxidants can be blended with coconut oil and olive oil to form a very stable good cooking oil.

 

This article appeared in Wise Traditions in Food, Farming and the Healing Arts, the quarterly magazine of the Weston A. Price Foundation, Winter 2003.

3 Responses to Trans Fatty Acids Are Not Formed by Heating Vegetable Oils

  1. Steve reed says:

    Thank you for this article, the trusted info backed by science is always enjoyable to read especially when my m.o.v.e. Coach at the veterans clinic who is a certified nutritionist,insists olive oil and others can turn into trans fats when heated. Please continue posting good info for those of us who like to research.

    • amy says:

      Thank you for your article. I too like Steve run into this question all the time. I am a certified nutritionist (AFPA), teach nutrition and cooking classes and this can be a very “heated ” topic. I am going to have your article on hand when the question and “Heat Turns” up again. Thank you! Amy

  2. Chen says:

    Hi,
    according to these:
    http://www.sciencedirect.com/science/article/pii/S0308814610006084
    http://www.ncbi.nlm.nih.gov/pubmed/22668846

    There is a *slight* (yet not significant) change in trans fatty acids.

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