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 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.
Schematic demonstration of the impact of ionic strength on lipid oxidation in water-in-walnut oil emulsions containing 0.2 wt% WPI during storage at 45°C in the dark for up to 20 days. Low levels of ionic strength (≤ 50 mM NaCl) improved the oxidative stability of W/O emulsions containing WPI, whereas high ionic strength (≥100 mM NaCl) accelerated lipid oxidation in the emulsions.
The effect of free fatty acids (FFA) on lipid oxidation in water-in-stripped walnut oil emulsions was investigated. The formation of the primary oxidation products (hydroperoxides) and the secondary oxidation product (headspace hexanal) increased with increasing addition of oleic acid to the emulsions. However, oleic acid at high concentrations was primarily involved in accelerating hydroperoxide formation at early stage and then hydroperoxide decomposition rapidly after hydroperoxides reached the maximum point. The prooxidant effect of saturated FFA was dependent on their chain length with lipid oxidation rates being in the order of lauric acid>palmitic acid>stearic acid. The highest prooxidant activity of lauric acid among these FFA was probably due to its largest polarity and making the water droplet interface more negatively charged than the others when the aqueous phase pH was 7.0, which was above its pKa, thereby attracting prooxidant metals to the water droplet surface. The highest ability to promote lipid oxidation in water-in-oil emulsions was shown by linolenic acid, followed by linoleic and oleic acids, indicating that the oxidative capacity increased with increasing degree of unsaturation. The prooxidant effect of FFA with the cis double bonds was lower than those with the trans ones when oleic acid (18:1, cis) was compared to elaidic acid (18:1, trans) and linoleic acid (18:2, cis–cis) was compared to linoelaidic acid (18:2, trans–trans), which suggested that geometric isomeration of FFA influenced lipid oxidation rates of water-in-oil emulsions.
Practical applications: FFA promote lipid oxidation in bulk oil and oil-in-water (O/W) emulsions. From the present study, it became clear that FFA were also strong prooxidants in water-in-oil (W/O) emulsions. This research pointed out the importance of using oils and fats containing minimal FFA in W/O emulsion formulation to obtain the highest quality.
