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

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 (<= 50 mM NaCl) in aqueous phase improved the oxidative stability of W/O emulsions containing WPI, whereas high levels of ionic strength (>= 100 mM NaCl) accelerated lipid oxidation in the emulsions. Free radical scavengers (e.g. Trolox and alpha-tocopherol) could increase the oxidative stability of W/O emulsions with the incorporation of Trolox in the emulsion inhibiting lipid oxidation more strongly than alpha-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.