Edible oil rancidity is a process especially induced by oxygen in the air, when fatty acids are oxidized to aldehydes and ketones. Volatile compounds responsible for rancid taste and smell are unsaturated aldehydes such as acrylic aldehyde, also named acrolein, as well as crotonaldehyde. The main component responsible for the rancidity is acrolein that is easily perceivable due to its high volatility at room temperature.

Regardless of the type of oil, the main volatile compound is acrolein. Oils are mixtures of fatty acids (short-chain C2 to long-chain C22 fatty acids or even longer chain) but hydrolysis and free radicals attack generates unsaturated C3 fatty acids in all types of oils, then C3 unsaturated acids are oxidized to acrolein. There are also longer-chain unsaturated aldehides in old oils but they are not much volatile.

Thanks a lot for your answer Prof. Carmen Mateescu