Trans fats form under temperatures much higher than those reached while cooking. While some trans fat formation may occur during cooking, it is too modest to be of any significance. To put things in perspective, the use of very high heat in cooking may yield an increase in trans fats of roughly 3%, whereas, industrial hydrogenation (which uses a catalyst, in addition to high heat) will yield 25% to 45%.
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Przybylski R, Aladedunye FA. Formation of Trans fats: during food preparation. Canadian Journal of Dietetic Practice and Research. 2012;73(2):98-101.