Introduction
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
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