All fats are combinations of fatty acids. Nutritionists characterize a dietary fat or oil as saturated, monounsaturated, or polyunsaturated depending on which fatty acids make up the largest portion of the fat or oil:
Foods such as butter, which are high in saturated fatty acids, are solid at room temperature and get harder when chilled.
Foods such as olive oil, which are high in monounsaturated fatty acids, are liquid at room temperature; they get thicker when chilled.
Foods such as corn oil, which are high in polyunsaturated fatty acids, are liquid at room temperature and stay liquid when chilled.
So how come margarine, which is composed primarily of unsaturated fatty acids, is solid? Because its fatty acids have been artificially saturated with extra hydrogen atoms. This process, called hydrogenation, turns an oil, such as corn oil, into a solid fat — margarine.
Hydrogenated fats are sometimes called trans fatty acids, but no matter what you call them, these fatty acids raise — rather than lower — cholesterol levels. As a result, these days most margarines boast “no trans fats” right on the label. I know your mother told you not to toot your own horn, but these guys have earned the right. So when you’re shopping, pick them.