Rancidity of Oils and Fats

Rancidity is the general name for the unpleasant smell and disagreeable taste that develops in oils and fats after they have been kept for a long time, and the chapter identifies two distinct chemical routes by which it happens. The first is hydrolytic rancidity. Fats are chemically triesters -- triglycerides -- formed from long-chain carboxylic acids (fatty acids) and glycerol (propane-1,2,3-triol): each of glycerol's three -OH groups is joined to a fatty-acid chain (R1, R2, R3) through an ester (-O-CO-R) linkage. In the presence of water, and especially in the presence of certain microorganisms whose enzymes catalyse the reaction, this triester bond is hydrolysed: Triglyceride + 3 H2O -> Glycerol + 3 free fatty acids (each written H-O-CO-R). The particular fatty acids released determine the particular off-flavour that results: rancidity in milk and butter comes from the release of short-chain butanoic (4-carbon), hexanoic (6-carbon) and octanoic (8-carbon) acids; chocolate develops an oily, fatty off-flavour from released palmitic, stearic and oleic acids; and lauric acid, released from coconut oil, gives it a soapy flavour. The second route is oxidative rancidity, caused not by water but by molecular oxygen in the air attacking the C=C double bonds present in the fatty-acid part of mono- or poly-unsaturated fat molecules. This oxidative attack breaks the unsaturated fat molecule down into volatile aldehydes and carboxylic acids, and it is these small, volatile breakdown products that carry the characteristic unpleasant rancid taste. The reaction is a free-radical chain reaction, and it can be initiated either by light (in which case it is specifically called photo-oxidation) or catalysed by enzymes or by metal ions. Because more C=C double bonds mean more sites vulnerable to this attack, p …