As of January 1, 2006, all food products manufactured for sale in the United States is required to list trans fat on the Nutrition Facts panel.
A trans fat is a triglyceride with at least one unsaturated fatty acid that contain at least one double bond in trans configuration. This means in at least one of the three long carbon chains that help make up a fat molecule, at least one chain is kinked the wrong way. When a carbon chain contains a double bond instead of a single bond, it causes a bend in the chain. The positions of the hydrogen atoms around the bond determine how the chain bends. In cis formation, both hydrogens are on the same side and an angle is formed, while in the trans configuration, the hydrogens are on opposing sides resulting in a straight chain. In nature, almost all the unsaturated fatty acids are in cis configuration and trans fatty acids are rare. In the foods we buy, a significant portion of the fat can be trans fat because of our extensive use of partially hydrogenated oils (a way to convert liquid fats into solid fats through the forced attachment of more hydrogen atoms). (See the beginning of Kitchen Notes: Saturated Fats, Cholesterol, and Heart Disease for a brief discussion on this topic.)
Three grams of trans fat is definitely the most I've ever seen in any food product. So, I wanted to post a "challenge". What is the most amount per serving of trans fat you've ever seen listed on a food's nutrition information? Post your responses as comments to this article.}?>