Study On The Effect Of Lipid And Protein Co-oxidation On Stability Of Emulsion And Protein Functional Properties In Oil-in-water Emulsion

Oil-in-water emulsion system is one of the most common dispersed systems in the food.Lipids are highly susceptible to oxidative degradation in emulsion food system,resulting in the changes of food nutrition and sensory qualities.Proteins in the food system are also vulnerable to the oxided lipids.In order to explore the co-oxidation properties of lipids and proteins in emulsion systems,two kinds of milk proteins including whey protein isolates(WPI)and casein(CN),and two kinds of soy proteins including soy protein isolate(SPI)and soy protein concentrate(SPC)were used as emulsifiers to prepare protein-stabilized oil-inwater emulsions in this study with the emulsion system of proteins in the aqueous phase(Tween20+WPI,Tween20+CN)as a control.Then the differences of the physical chemistry stabilities of system and the functional properties of proteins during oxidation were analyzed.The research revealed the interaction mechanism between lipids and proteins in complex colloidal systems,and provided theoretical support for the development of high stability protein emulsion systems.The main results of the research were as follows:Firstly,the effects of co-oxidation of lipids and proteins on the physical stability of the emulsion were studied.The distribution state of the droplets was observed by laser confocal scanning microscopy.The microscopic observation of the oxidized proteins showed significant large lipid globule aggregates while there was slightly aggregates and no flocculation were observed in Tween20+WPI emulsion and Tween20+CN emulsion.It was found that the emulsions stabilized by WPI,CN,SPI and SPC,respectively showed lower physical stability than the Tween20+WPI,Tween20+CN emulsion systems.The changes of the emulsion rheology were also studied during the oxidation process by determining shear rheology and interfacial tension.With the prolongation of oxidation time,the viscosity and interfacial tension increased.Meanwhile,the particle size and the absolute value of the ζ-potential also increased.The results showed that all formulated emulsions lost their stability.In addition,compared to the emulsions with milk proteins,the particle size distribution of emulsions with soy proteins changed significantly,indicating that the emulsions stabilized by milk proteins showed better physical stability than those stabilized by soy proteins.Secondly,the effects of oxidation products of lipids on protein oxidation and emulsion oxidation stability were studied.The results showed that the the primary oxidation product(ROOH)and secondary oxidation product of lipid(TBARS)increased.Compared with the oxidation of the small molecule surfactant(Tween20),which revealed that proteins inhibit lipid oxidation by oxidizing itself.The carbonyls in the emulsion stabilized by WPI and CN increased by(0.411±0.06)nmol/mg and(0.462±0.12)nmol/mg,respectively.The carbonyls in the emulsion systems of proteins in the aqueous phase(Tween20+WPI,Tween20+CN)increased by(0.391±0.27)nmol/mg and(0.450±0.35)nmol/mg,respectively,indicating that the oxidation stability of the proteins was lower at the water-oil interface than that in aqueous phase.The carbonyl content of emulsion stabilized by SPI and SPC increased by(0.266±0.05)nmol/mg and(0.475±0.23)nmol/mg,respectively,SPI showed stronger antioxidant than that of SPC.During the oxidation of emulsions,the content of sulfhydry decreased gradually,the fluorescence intensity weakend.The content of Schiff base used as protein-lipids polymer increased.According to the analysis of protein components by electrophoresis,it was found that the protein was more susceptible to oxidation in the cream phase than in the aqueous phase.Finally,the proteins in the non-oxidized and oxidized emulsions were extracted to study the effects of emulsion oxidation on their functional properties such as protein solubility,water-and oil-holding capacities and foaming.The research showed that the emulsion oxidation had different effects on the functional properties of the protein,the co-oxidation of lipids and proteins resulted in a decrease in protein solubility,water-and oil-holding capacities.The solubility of SPC was affected by co-oxidation with the greatest degree of decline by 60.4%,the solubility of WPI was affected by co-oxidation with the lowest degree of decline by 8.5%.And the order of reduction of water holding capacity of four types proteins was SPI > WPI> SPC> CN,the order of reduction of oil-holding capacity was WPI>CN>SPC> SPI.In addition,co-oxidation showed a negative impact on the emulsifying ability and emulsion stability of the protein,the emulsifying of WPI,CN,SPI and SPC weakened by 14.9%,14.7%,7.4%,8.3%,respectively,while the foaming and foam stability of the protein changed irregularly during the co-oxidation of lipids and proteins.