By Harold McGee
Saturated fats are also more stable, slower to become rancid than unsaturated fats. The double bond of an unsaturated fat opens a space unprotected by hydrogen atoms on one side of the chain. This exposes the carbon atoms to reactive molecules that can break the chain and produce small volatile fragments. Atmospheric oxygen is just such a reactive molecule, and is one of the major causes of flavor deterioration in foods containing fats. Water and metal atoms from other food ingredients also help fragment fats and cause rancidity. The more unsaturated the fat, the more prone it is to deterioration. Beef has a longer shelf life than chicken, pork, or lamb because its fat is more saturated and so more stable.