Abstract:Walnut oil is rich in polyunsaturated fatty acid (PUFA) and was employed as the oil phase in order to prepare water-in-oil (W/O) emulsions using a ultrahigh pressure microfluidizer. The nonpolar antioxidant α-tocopherol and its polar counterpart 6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid (Trolox) were added into the emulsions separately. These samples were stored in the dark at 45 ℃. The content of the primary oxidation products (lipid hydroperoxides) and the secondary oxidation products (headspace hexanal) were determined in order to evaluate the influence of antioxidant polarity on the lipid oxidative stability in W/O emulsions. Both α-tocopherol and Trolox (10~1000 μM) reduced the formation of lipid hydroperoxides and hexanal in the W/O emulsions. Trolox had a concentration-dependent antioxidant activity, whereas the inhibitory effect of α-tocopherol on the formation of lipid hydroperoxides was reduced at a relatively high concentration (500 to 1000 μM). The α-tocopherol concentration was lower than the polar oxidant Trolox for the partitioning in the oil-water interface of the emulsion, suggesting that the non-polar antioxidant α-tocopherol had lower activity than the polar antioxidant Trolox. The antioxidant activity of Trolox was higher when the pH of the aqueous phase of the emulsion was 3.0 compared to a pH of 7.0. The aqueous phase pH affected the antioxidant activity of Trolox through its partitioning in each phase of the W/O emulsion.
