Impact of thermal processing on the antioxidant mechanisms of continuous phase β-lactoglobulin in oil-in-water emulsions

The influence of native and thermally (50-95 °C) denatured β-lactoglobulin (β-Lg) on the oxidative stability of surfactant-stabilized menhaden oil-in-water emulsions (pH 7.0) was evaluated. β-Lg (500 μg/g oil) heated at 95 °C for 30 min provided the best protection against lipid oxidation, inhibiting the formation of lipid hydroperoxides and thiobarbituric acid reactive substances (TBARS) by 87% and 88%, respectively, following 7 days of storage. The possible mechanisms of antioxidant activity of native and heated β-Lg were evaluated by measuring peroxyl radical scavenging and iron chelating capacities of the protein treatments, as well as reactive sulfhydryl concentrations and tryptophan fluorescence (a marker of protein conformation changes). The aforementioned in vitro assays only partially corroborated the results from the oxidizing emulsion system since β-Lg heated at 95 °C exhibited the lowest iron chelation capacity and free sulfhydryl concentration, yet displayed the highest peroxyl radical scavenging capacity and inhibition of lipid oxidation in oil-in-water emulsions of all treatments tested. The results of this study demonstrate the feasibility of proteins as a natural class of antioxidants in food emulsions, and further elucidate the possible mechanisms by which proteins inhibit lipid oxidation.