Does oil oxidation create trans fats?

Question

I am considering switching to beef tallow for my high heat cooking, as it has a high smoke point and is a saturated fat (which means, that it does not oxidize easily). Also recent randomized control trials suggest, that saturated fat on its own is not causally linked to heart disease problems.

From what I have learned, is that unsaturated fats, even those with high smoke points, oxidize easily, because of their chemical property of being unsaturated, which means, that they have "space" for other electrons and react easily. From the unsaturated fats, the polyunsaturated fats have twice as much potential for reaction and oxidization as the monounsaturated fats, because they have two "arms".

With beef tallow for example, it seems to be the other way around: It has a high smoke point, but even more important: It's a saturated fat and it doesn't make new bonds easily, because it has all its electrons.

Now, my question is, how are trans fats being created? Are they the product of oxidized polyunsaturated oils? Where do trans fats come from? Or are companies intentionally putting them in food for economic reasons?

I am aware, that trans fats are banned in the US, but they are not banned in EU yet.

Answer 1

First, a little chemistry primer on what unsaturated fatty acids look like (this is petroselinic acid):

By Yikrazuul (talk) - Own work, Public domain, Link

You can see the double bond near the "middle" of the molecule. The "rest" of the molecule is attached to the same side of the double bond axis on both ends, making this a "cis" fatty acid. Rotation around double bonds requires a relatively high amount of activation energy, so this "cis" configuration is stable, at least at low-ish temperatures.

An example of a "trans" fatty acid would be elaidic acid:

By Benjah-bmm27 - Own work, Public domain, Link

Here you can see that the molecule continues on opposite sides of the double bond axis ("trans"). At higher (e.g. frying pan) temperatures, unsaturated fatty acids more readily isomerize between cis and trans, and since trans is energetically favorable, more unsaturated fatty acids will be in their trans configuration after being heated to a high temperature.

So to answer the question(s): No, trans fatty acids are not generated by oxidation, but the same conditions that favor oxidation (high temperature) also favor the generation of trans fats from cis fats. Also, the bacteria in the digestive tracts of ruminants (cattle, sheep, but also deer etc.) produce a significant proportion of trans fatty acids. In addition, the simplest (and cheapest) industrial processes to saturate ("harden") unsaturated fats produce a relatively high amount of trans fats. So yes, not adopting other processes can be economical, if this can be considered "intentional"...your call.

A more extensive explanation can also be found in the accepted answer to this question: Does preparation of food change the nutritional content with respect to fat type?