Warmed-over flavor is an unpleasant characteristic usually associated with meat which has been cooked and then refrigerated. The deterioration of meat flavor is most noticeable upon reheating. As cooking and subsequent refrigeration is the case with most convenience foods containing meat, it is a significant challenge to the processed food industry. The flavor is variously described as "rancid," "stale," and like "cardboard," and even compared to "damp dog hair." Warmed-over flavor is caused by the oxidative decomposition of lipids (fatty substances) in the meat into chemicals (short-chain aldehydes or ketones) which have an unpleasant taste or odor. This decomposition process begins after cooking or processing and is aided by the release of naturally occurring iron in the meat.