Ionic strength, antioxidants and chelators affect whey protein isolates' antioxidantion in water-in-walnut oil emulsions

Abstract

The objective of this research was to evaluate the impact of factors, such as ionic strength, free radical scavengers, and chelators, on lipid oxidation in water-in-walnut oil (W/O) emulsions containing whey protein isolates (WPI). The incorporation of WPI in aqueous phase could retard lipid oxidation in W/O emulsions, indicating WPI were antioxidants in the emulsions. Low levels of ionic strength (≤50mM NaCl) in aqueous phase improved the oxidative stability of W/O emulsions containing WPI, whereas high levels of ionic strength (≥100mM NaCl) accelerated lipid oxidation in the emulsions. Free radical scavengers (e.g. Trolox and α-tocopherol) could increase the oxidative stability of W/O emulsions with the incorporation of Trolox in the emulsion inhibiting lipid oxidation more strongly than α-tocopherol. This indicated that polar antioxidants were more effective at retarding lipid oxidation than non-polar antioxidants. The addition of WPI and free radical scavengers combined made the emulsions oxidize more slowly than either substance used alone, suggesting there were synergistic effects between them. The use of both EDTA and citric acid could inhibit lipid oxidation in W/O emulsions, indicating that endogenous transition metals were the major prooxidants in W/O emulsions. The combination of WPI and EDTA or CA could further lower the rates of lipid oxidation in W/O emulsions. Practical applications: Whey protein isolates inhibit lipid oxidation in oil-in-water (O/W) emulsions. From the present study, it became clear that WPI could also retard lipid oxidation in water-in-oil (W/O) emulsions. However, the antioxidant activity of WPI was affected by factors such as ionic strength, free radical scavengers, and chelators. This research pointed out that the incorporation of whey proteins in W/O emulsions, such as spreads, chocolate, and butter, could increase the oxidative stability of the emulsions. Moreover, the appropriate choice of ionic strength, free radical scavengers, and chelators could promote the antioxidation of WPI in W/O emulsions.