Effect of pH on Antioxidant Activity of α-Tocopherol and Trolox in Oil-in-Water Emulsions

The pH had a more significant effect on the antioxidant activity of Trolox than α-tocopherol in corn oil emulsified with Tween 20 at 60 °C. Although with increasing pH the oxidative stability of corn oil emulsions decreased, the antioxidant activity of α-tocopherol increased. Initially, Trolox and α-tocopherol were similar in inhibiting hydroperoxide formation with or without phosphate buffers at pH 3−7, but at later stages α-tocopherol was more effective than Trolox at pH 3, 4, 5, and 7 but not at pH 6. In contrast, α-tocopherol inhibited hexanal formation better than Trolox during the entire oxidation period. Trolox was more stable than α-tocopherol in bulk corn oil but less stable in Tween 20 solutions at different pH. Both antioxidants in Tween 20 solutions were most stable at pH 3 and least stable at pH 7. Although more Trolox partitioned into the oil phase and the oil−water interfaces at pH 3−4, its antioxidant activity was lower than at pH 5−6. The overall antioxidant activities of α-tocopherol and Trolox depend on their hydrogen-donating ability, relative stability, and distribution in emulsions.