By Harold McGee
The terms “saturated” and “unsaturated” fats are familiar from nutrition labels and ongoing discussions of diet and health, but their meaning is seldom explained. A saturated lipid is one whose carbon chain is saturated—filled to capacity—with hydrogen atoms: there are no double bonds between carbon atoms, so each carbon within the chain is bonded to two hydrogen atoms. An unsaturated lipid has one or more double bonds between carbon atoms along its backbone. The double-bonded carbons therefore have only one bond left for a hydrogen atom. A fat molecule with more than one double bond is called polyunsaturated.