All food will deteriorate over time. For example, fresh fruits and vegetables can ferment, and fats and oils will go rancid.
There are concerns that consuming rancid fats and oils is damaging to health. Whilst more research is needed to establish impact of long-term consumption of rancid fats and oils, this report discusses some of the current evidence, factors that contribute to an oil becoming rancid and common sources of rancid oils.