Effects Of Molecular Environment On Oxidation Stability Of Lipids In Water-in-Oil Emulsions

Oils and food products with high lipid content are susceptible to oxidation as a result of their unsaturated fatty acids, especially polyunsaturated fatty acids (PUFAs). Lipid oxidation results in production of rancid odors, unpleasant flavor and even compromising the safety of food due to the generation of toxic products. Lipids are found to exist as bulk oils, oil-in-water (O/W) or water-in-oil (W/O) emulsions. Among them, W/O emulsion is an important form of lipids in the food. In recent decades, many investigations have focused on lipid oxidation in bulk oils and O/W emulsions due to their more common forms, whereas the research about lipid oxidation in W/O emulsion was limited. Due to the introduction of aqueous phase and the inversion of oil and aqueous phase, the oxidative stability of W/O emulsions might differ from those of the bulk oil and O/W emulsions. Based on this case, the effects of the molecular environment including emulsifiers and their concentration, free radical scavengers, chelating agents, and proteins, and their possible mechanisms were investigated in this work by preparing W/O emulsions with walnut oil, which was rich in PUFA. The results gained from these studies could provide some knowledge that could be used to improve the oxidative stability of oils in W/O emulsions, especially those oils that are high in PUFAs like walnut oil.Walnut oil was stripped prior to emulsion preparation to remove minor polar compositions and the effects of the water-oil interface on the oxidation stability of walnut oil in W/O emulsions were studied by taking un-stripped walnut oil as the controls. In addition, the influence of the emulsifier concentration on lipid oxidation in W/O emulsions and its mechanisms were also explored by adding polyglycerol polyricinoleate ester (PGPR) into emulsions at varying concentrations. The results showed that the existence of the oil-water interface in W/O emulsions accelerated lipid oxidation. Also, increasing emulsifier concentrations improved the physical and oxidative stability of emulsions. W/O emulsion model was prepared with medium triglycerides (MCT) instead of walnut oil and the obtained results showed that lipid hydroperoxides in W/O emulsions interacted with excessive emulsifier, resulting in the removal of lipid hydroperoxides from the oil-water interface and entering into the oil phase, thereby preventing lipid hydroperoxides from in contact with aqueous phase transition metals. And as a result, the oxidation rates of lipids in W/O emulsions were reduced.PGPR was used as a basic emulsifier to form stable W/O emulsions. Dodecyl trimethyl ammonium bromide (DTAB), sodium dodecyl sulfate (SDS), Tween 20 were incorporated into aqueous phase as small molecular surfactants because they are cationic, anionic, and nonionic, respectively. Meanwhile, whey protein isolates (WPI) was employed as large molecular surfactants. The results showed that the oxidative stability of W/O emulsions was increased with increasing SDS concentration (0.1～0.2 wt % of emulsions). In contrast, the addition of DTAB in the aqueous phase reduced the rates of lipid oxidation and the concentration of SDS (0.1～0.2 wt % of emulsions) had a non-significant influence on lipid oixdation. In addition, the presence of Tween 20 in the aqueous phase did not significantly influence on the rates of lipid oxidation in W/O emulsions. The antioxidant property of WPI was observed in the study and their antioxidant capacity increased with increasing concentration (0.05-0.2 wt % of emulsions). Aqueous phase pH had an important impact on the antioxidant capability of WPI with higher pH improving their ability to inhibit lipid oxidation in emulsions. The interaction between WPI and DTAB in the aqueous phase could suppress the prooxidant effect of DTAB and aqueous phase pH significantly influenced their interaction on the lipid oxidation with the oxidative rates at pH 7.0 being lower than at pH 3.0. Similarly, the interaction could also be observed between WPI and SDS in the aqueous phase. In particular, there was a synergistic effect between them at aqueous phase pH 7.0, therein the oxidative stability of W/O emulsions being better compared to the presence of WPI and SDS combined at aqueous phase pH 3.0 and alone at both pH 3.0 and 7.0.The effects of free fatty acids (FFA) on lipid oxidation in W/O emulsions were investigated. The oxidative stability of lipids in W/O emulsions increased with increasing addition of oleic acid to the emulsions. 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 surface activity 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 W/O 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 W/O emulsions.The impacts of antioxidant polarity, chelator type, and the interaction between them on the oxidative stability of W/O emulsions were also examined. The ability of polar Trolox to inhibit lipid oxidation was greater than nonpolar α-tocopherol in W/O emulsions. It was evidenced that Trolox was mainly located in the water-oil interface where lipid oxidation was prevalent, whereas α-tocopherol was primary in lipid phase. Meanwhile, Trolox had a better antioxidant activity at aqueous phase pH3.0 than at pH 7.0. Incorporation of both EDTA and DFO decreased the rates of lipid oxidation in W/O emulsions, suggesting endogenous transition metals were important factors which promoted lipid oxidation in W/O emulsions. Moreover, the emulsions with added EDTA had the better oxidative stability than those with added DFO. This might be because EDTA could chelate transitions metals (e.g., iron and copper) both in the aqueous phase and lipid phase, whereas DFO only chelated iron in the aqueous phase. The addition of EDTA and Trolox or α-tocopherol combined made the emulsions oxidize more slowly than them used alone, suggesting there were synergistic effects between them. Furthermore, aqueous phase pH influenced the interaction between Trolox and EDTA with the synergistic effects being in the order of Trolox+EDTA(PH3.0)> Trolox+EDTA(PH7.0).The impact of aqueous phase proteins, such as WPI, soy protein isolate (SPI) and peach kernel protein isolates (PKPI), on the physical and oxidative stability of lipids in W/O emulsions was researched. It was shown that increasing concentrations (0.1-0.4 wt % of aqueous phase) of WPI, SPI, and PKPI enhanced the physical and oxidative stability of W/O emulsions to different degrees. At aqueous phase pH7.0, PKPI had the strongest antioxidant activity, followed by WPI and SPI. The better antioxidation of the three kinds of proteins was observed at aqueous phase pH7.0 than at pH3.0. The physical stability of W/O emulsions containing WPI, SPI, and PKPI improved with the increase of ion strength (0-200mM CaCl2). However, increasing ion strengths resulted in the decreased antioxidant activities of the three kinds of proteins. And the suppressing effect of ion strength on their antioxidation was in the order of PKPI> SPI> WPI.