Food processing and lipid oxidation.

Food lipids are principally triacylglycerides, phospholipids and sterols found naturally in most biological materials consumed as food and added as functional ingredients in many processed foods. As nutrients, lipids, especially triglycerides, are a concentrated caloric source, provide essential fatty acids and are a solvent and absorption vehicle for fat-soluble vitamins and other nutrients. The presence of fat significantly enhances the organoleptic perception of foods, which partly explains the strong preference and market advantage of fat-rich foods. As a class, lipids contribute many desirable qualities to foods, including attributes of texture, structure, mouthfeel, flavor and color. However, lipids are also one of the most chemically unstable food components and will readily undergo free-radical chain reactions that not only deteriorate the lipids but also: (a) produce oxidative fragments, some of which are volatile and are perceived as the off-flavors of rancidity, (b) degrade proteins, vitamins and pigments and (c) cross-link lipids and other macromolecules into non-nutritive polymers. Free-radical chain reactions are thermodynamically favorable, and as a result, evolutionary selection has strongly influenced the chemistry, metabolism and structure of biological cells to prevent these reactions kinetically. However, the loss of native structure and the death of cells can dramatically accelerate the deteriorative reactions of lipid oxidation. The effects of all processing steps, including raw product selection, harvesting, storage, refining, manufacturing and distribution, on the quality of lipids in the final commodity are considerable. Certain key variables now known to influence oxidative processes can be targeted to increase food lipid stability during and after processing. Retention of or addition of exogenous antioxidants is a well-known consideration, but the presence and activity of catalysts, the integrity of tissues and cells, the quantity of polyunsaturated lipids and the structural properties of the final food product, including total surface area of lipids, and the nature of surfactant materials all play important roles in final product stability.