Why do lipid-rich foods become rancid when exposed to oxygen for too long

Fats and oils that are in contact with moist air at room temperature eventually undergo oxidation and hydrolysis reactions that cause them to turn rancid, acquiring a characteristic disagreeable odor. One cause of the odor is the release of volatile fatty acids by hydrolysis of the ester bonds.

Explanation:

the wires in the toaster turning red and the toast crisping and turning brown