Oxidation of fats, generally known as rancidity, is caused by a biochemical reaction between fats and oxygen. In this process the long-chain fatty acids are degraded and short-chain compounds are formed. One of the reaction products is butyric acid, which causes the typical rancid taste.

Rancidity is dependent on temperature; at low temperatures, as in the refrigerator, the reaction occurs, but at a very low rate. In cooled fresh ready-to-eat meals bacterial spoilage will generally occur before rancidity can be observed.