Triacylglycercols (triglycerides) are the major component of most foods, typically making up more than 95 to 99% of the total lipids present. Triacylglycerols are esters of three fatty acids and a glycerol molecule. Triglycerides are too large to be volatile (and hence have aroma) but free fatty acids, particularly the small ones, can be volatile enough to have characteristic smells. Unsaturated (particularly polyunsaturated) fatty acids are prone to oxidation. The significant end products of lipid oxidation are rancid-smelling flavor aldehydes derived from the fatty acids. Lipid oxidation follows a free radical mechanism that starts with the abstraction of a hydrogen atom from the fatty acid. The lipid radical form (R●) rapidly reacts with oxygen to form a peroxy radical via a free radical chain reaction. The peroxy radical (ROO●) can gain a hydrogen atom to form a lipid hydroperoxide (ROOH) which is relatively stable and exists in significant quantities in many natural fats. The lipid hydroperoxide has no off-flavor but rapidly break down (particularly in the presence of heat and a metal catalyst) to form rancid flavors
The hydroperoxide breakdown starts with the loss of a hydroxy radical (●OH) to form a lipid alkoxy radical (RO●).
The alkoxy radical rearranges and breaks the molecule into two parts including a volatile and rancid-smelling aldehyde. The aldehydes formed are often so odor active that even a few ppm can lead to a food being unacceptable.