By Harold McGee
It turns out that the hydrogenation process straightens a certain proportion of the kinks in unsaturated fatty acids not by adding hydrogen atoms to them, but by rearranging the double bond, twisting it so that its bend is less extreme. These molecules remain chemically unsaturated—the double bond between two carbons remains—but they have been transformed from an acutely irregular cis geometry to a more regular trans structure (see illustration). Cis is Latin for “on this side of,” and trans for “across from”; the terms describe the positions of neighboring hydrogen atoms on the double bond between carbon atoms. Because the trans fatty acids are less kinked, more like a saturated fat chain in structure, they make it easier for the fat to crystallize and so make it firmer. They also make the fatty acid less prone to attack by oxygen, so it’s more stable. Unfortunately, trans fatty acids also resemble saturated fats in raising blood cholesterol levels, which can contribute to the development of heart disease. Manufacturers will soon be required to list the trans fatty acid content of their foods, and they’re beginning to implement other processing techniques that harden fat consistency without creating trans fatty acids.