What is rancid olive oil? What is the rancidity of olive oils?

Olive oil that has developed a wax or crayon-like stale smell is called rancid olive oil. Rancidity develops when oil undergoes oxidation caused by prolonged exposure to air and light, with heat during extended storage further accelerating the process.

Chemically, hydroperoxides are formed due to the photo- and thermal- oxidation of olive oil (oxidative rancidity). 

The hydroperoxides are very unstable and further break down to small molecules, such as aldehydes, ketones, hydrocarbons, alcohols, and esters, which are responsible for the undesirable “rancid” odor of the oil.

Oils can also become rancid through hydrolytic and microbial pathways. In both cases, water or microorganisms cause the fatty acid chains to break away from the glyceride structure, releasing free fatty acids into the oil. 

These free fatty acids create unpleasant flavors and aromas, signaling a loss of quality and freshness.