Assessing Interactions between Lipophilic and Hydrophilic Antioxidants in Food Emulsions

Dietary lipids containing high concentrations of polyunsaturated fatty acids are considered to be beneficial to human health, yet their incorporation within formulated foods is complicated by their susceptibility to oxidation. Lipid oxidation in foods is inhibited through the incorporation of antioxidants, yet the list of antioxidants approved for food use is small, and consumers frequently demand foods without synthetic additives. As a consequence, food processors are now tasked with improving the efficacy of approved, "natural" (i.e., nonsynthetic) antioxidants; a rational strategy for doing so involves localizing the antioxidants at the interface where oxidation usually occurs and regenerating the consumed antioxidants after the oxidation event has occurred. The present study describes a procedure to evaluate antioxidant interactions in oil-in-water food emulsions, which is based on controlled oxidation reactions induced in the dispersed oil phase by the lipophilic radical generator, 2,2'-azobis(2,4-dimethylvaleronitrile). The extent of lipid oxidation is measured spectroscopically by following the loss of an oxidatively labile, lipophilic probe (methyl eleostearate), the synthesis of which is described here. Using this procedure, the ability of various aqueous phase solvated antioxidants (ascorbic acid, gallic acid, (-)-epicatechin, (-)-epigallocatechin-3-gallate) to regenerate lipid phase solvated α-tocopherol was evaluated. In all cases, the test compounds were able to inhibit oxidation reactions; however, these effects were not profoundly synergistic, and the maximum synergistic interaction observed was only ∼ 3% using ascorbic acid.