Lipid Oxidation

Oxidative rancidity has long been recognized as a major cause of spoilage in food. In many products (potato chips, nuts, oil, cereals, etc) stored for varying lengths of time, the off-flavors and off-odors generated as a result of lipid oxidation are described as rancid, cardboardy, or painty. In meat that has been cooked, refrigerated, and then re-warmed, the off-flavors are described as warmed-over. In stored fish, the off-odors may be described as either rancid or fishy.

Lipid oxidation is a radical-driven process that begins with an activated oxygen species abstracting a hydrogen atom from an unsaturated fatty acid. The active oxygen species can be produced by light; or by interaction of the oxygen with enzymes such as lipoxygenase, peroxidase, or microsomal enzymes; or with transition metal ions, particularly iron. The oxygenated fatty acid, in turn, forms a hydroperoxide (primary reaction product) by abstracting a hydrogen atom from an adjacent fatty acid, which, in turn, can continue the process of abstracting hydrogen from a neighboring fatty acid. Hydroperoxides do not directly cause an adverse effect on the flavor and aroma of the food; rather, it is their breakdown to aldehydes and ketones (secondary reaction products). During this breakdown, additional free radical compounds may be generated that, in turn, could start another sequence of lipid oxidation reactions.

The susceptibility of a food to oxidize is dependent on both its chemical composition and its physical environment. The major chemical component in the food that will affect oxidation is the degree of unsaturation that exists in the fatty acids. The more double bonds in the fatty acid, the faster it will oxidize. For example, fish contain unsaturated fatty acids with one to six double bonds, and they will oxidize faster than chicken, which contains unsaturated fatty acids with one to four double bonds. Concentrations of catalysts in the food system are also important in determining the susceptibility of the food to oxidize. Of these catalysts, the nonenzymic catalysts (transition metal ions) play a key role, as they not only participate in the initiation process but also participate in the breakdown of hydroperoxides to low molecular weight off-flavors and off-odors. The fact that most foods, even immediately after harvest and slaughter, are believed to have hydroperoxides present, would lend credence to a dominant initiation role being played by transition metal-catalyzed hydroperoxide breakdown. Other compositional factors affecting the food's susceptibility to oxidize include pro-oxidants and antioxidants (tocopherol, flavonoids), as well as the microbial contamination. Illustrating this latter point, for instance, lipid oxidation was reduced in pork muscle stored at 2°C when lipolytic enzyme-producing bacteria were present (10). As for the physical environment, oxygen availability, temperature, and water activity play a role in the susceptibility of a food's lipid to oxidize.

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