What happens when the fat and oil containing food materials are left outside for a long time?

When food materials containing fats and oils are left outside for a long time, they undergo a process known as rancidification. The primary cause of this deterioration is oxidative rancidity, where the fats and oils react with molecular oxygen from the air. This oxidation reaction specifically targets the double bonds in unsaturated fatty acids, leading to the formation of hydroperoxides, which further break down into volatile molecules like aldehydes and ketones [3]. These chemical changes result in characteristic off-flavors and unpleasant odors, rendering the food rancid [1]. While factors like light, heat, and moisture can accelerate this process, the fundamental chemical change occurring when exposed to air is the oxidation of lipid components. To prevent this, antioxidants are often added to food to retard autoxidation by scavenging free radicals [1].

Sources

1. [3] https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/rancidity
2. [1] https://repository.up.ac.za/bitstreams/f326b6f2-5c77-402f-af0a-71487502602b/download