| Oil-in-water(O/W)emulsions are widely applied in food and pharmaceutical products as delivery systems containing unsaturated fatty acids,liposoluble functional factors,and other components.In O/W emulsions,lipids typically oxidize faster than in bulk oils due to the increased oil-water interface region.The methods to control lipid oxidation include oxygen removal,modification of lipid structure and addition of antioxidants,among which the addition of natural antioxidants is preferred because of their safety and health properties.Previous studies have found that combinations of lipid concomitants result in different types of antioxidant interactions in different systems.Tocopherols are usually used as the primary antioxidants,and the distribution of antioxidants in O/W emulsions has a significant effect on their antioxidant activity,but the mechanism is not yet clear.Therefore,this study aimed first to investigate the effect of phospholipids on the interfacial effect of tocopherols in peony seed oil-in-water emulsions,and then to analyze the antioxidant interactions between tocopherols and phospholipids.Subsequently,the mechanism of the synergistic interaction between tocopherols and phospholipids was described by quantum chemical methods.The main study findings were as follows:Firstly,O/W emulsions were prepared using peony seed oil as the oil phase by highpressure homogenization method,and the effects of individual and compounded tocopherols and phospholipids on the physical and oxidative stability of O/W emulsions were investigated.The changes in particle size,polydispersity index,colour,and microstructure of O/W emulsions were not significant after the addition of tocopherols and phospholipids.The ζ-potential in O/W emulsions increased after the addition of phospholipids.Tocopherols could inhibit the formation of lipid hydroperoxides and malondialdehyde in O/W emulsions,and could extend their oxidation lag time to 8 and 10 days.However,the inhibition of lipid hydroperoxides in O/W emulsions by phospholipids was weak and could only extend the oxidation lag time of lipid hydroperoxides to 2 days,and could hardly inhibit malondialdehyde.The combination of tocopherol and phospholipids improved the oxidative stability of O/W emulsions,and the oxidation lag times of lipid hydroperoxides and malondialdehyde could be extended to 12 and16 days,respectively.The interaction index calculated by the King’s Q-value method showed that phospholipids and tocopherols exhibited synergistic antioxidant effects in O/W emulsions.Meanwhile,the combination of tocopherols and phospholipids in O/W emulsions showed a decreasing trend of phospholipid content and a decreasing trend of tocopherol content followed by an increasing trend during the storage process.Secondly,the phase distribution of tocopherols and phospholipids in O/W emulsions were studied by laser confocal microscopy and other techniques.Their interfacial adsorption effects were analyzed by measuring the dynamic interfacial tensions of tocopherols and phospholipids and the interaction forces between them.Tocopherols were mainly distributed in the oil phase,and the rest were located in the aqueous phase and interfacial layer.After the addition of phospholipids,the tocopherols in the oil phase gradually migrated to the interfacial layer.Phospholipids were mainly distributed at the interfacial layer.After the addition of tocopherols,the distribution of phospholipids was not affected.The interfacial tension decreased from 1.73 m N/m to 0.27 m N/m when the concentration of phospholipids was increased from 200 mg/kg to 1000 mg/kg.Tocopherols had almost no effect on the interfacial tension.The interfacial tension of phospholipids decreased from 1.64 m N/m to 0.25 m N/m after the addition of tocopherols.The concentration of the combination affected the diffusion,permeation,and rearrangement rates of tocopherols and phospholipids in interfacial adsorption.Therefore,the combination favored the interfacial adsorption of both.The data from fluorescence quenching method indicated that tocopherols and phospholipids interacted in the O/W emulsion.The interaction forces between them were hydrophobic interaction,and phospholipids drive the migration of tocopherols to the oil-water interface through these interaction forces.Finally,the synergistic antioxidant mechanism of tocopherols and phospholipids in O/W emulsions was further elucidated using a multifaceted combination of electron spin resonance,cyclic voltammetry and density functional theory.The data of electron spin resonance monitoring of free radical changes showed that α-tocopherol and δ-tocopherol could inhibit the production of alkoxy and hydroxyl radicals,while phosphatidylethanolamine(PE)and phosphatidylcholine(PC)did not.The PE/PC and α-/δ-tocopherol combination resulted in a slower reduction of alkyl radicals and a slower increase of alkoxy and hydroxyl radicals in the O/W emulsion during accelerated oxidation.The PE-containing combination was better than the PC-containing combination.The results of cyclic voltammetry showed that the oxidation potentials of α-tocopherol and δ-tocopherol in PBS and ethanol were 381.00±6.68 m V,383.33±1.25 m V and 613.67±4.64 m V,839.67±3.86 m V,respectively.Thus,α-tocopherol andδ-tocopherol can be used directly as free radical scavengers.However,the cyclic voltammetric curves of PE and PC in both systems did not show cathodic and anodic peaks.Therefore,they cannot be used directly as free radical scavengers.The results combined with the changes of tocopherol and phospholipid contents during storage indicated that tocopherol was used as a primary antioxidant to disrupt chain transfer during oxidation.Phospholipid was used as a secondary antioxidant to participate in the regeneration of tocopherol radicals into tocopherols.The structures of tocopherols and phospholipids and their related products were analyzed by quantum chemical methods to further explain the mechanism of synergistic antioxidant effects between them. |
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