Q. How exactly is trans fat created? If I bake with some (say, 1/4 cup) oil in a batter, does that make trans fats?
A. We’ve all read a lot about the dangers of trans fats. Most of the trans fat in our diet comes from artificially hydrogenated oils that are still widely used in food manufacturing. These “Franken-fats” are polyunsaturated fatty acids (PUFAs) that have been twisted into an unnatural configuration called a trans-isomer. This means that the molecule contains the exact same number and type of atoms as it did before, but they are arranged in a different shape.
But what a difference that small change makes! Polyunsaturated fats and their trans-isomers produce completely opposite effects in the body. While PUFAs help to improve cholesterol profiles and reduce your risk of heart disease, trans fats contribute to heart disease by fueling inflammation, raising bad cholesterol and lowering good cholesterol.
What does it take to create a “Franken-fat”?
Artificial hydrogenation involves mixing polyunsaturated oils with hydrogen atoms under very high pressure–not something you could accidentally do at home! High heat can also cause PUFAs to trans-isomerize into trans fats, if it is applied for long enough. For example, you have to heat a polyunsaturated oil to 240 degrees F for 16 hours before it begins to trans-isomerize into trans fats–and even then, only a very small percentage (less than 1%) are converted.
As you can see, normal cooking temperatures and times are not nearly enough to create any significant amount of trans fats in the foods we cook at home.
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Reference: Wolff, R. L. 1993. Heat-induced geometric isomerization of alpha-linolenic acid: effect of temperature and heating time on the appearance of individual isomers. Journal of the American Oil Chemists Society 70(4): 425-430.
See also this episode of my podcast on Which Oils are Best For Cooking?