Study Of Effects Of Oil-water Interfacial Protein Properties On The Stability Of Water-in-oil Emulsions

The water-in-oil（W/O）emulsion system is a crucial form of lipids in the food industry,known for maintaining the unique lubricating taste and flavor of lipids.However,the practical application of W/O emulsions is limited by its poor physical stability,intense lipid oxidation,and an unclear stability mechanism.Therefore,this manuscript shifted the focus from macroscopic emulsions to the oil-water interface to address these limitations.In this work,protein is added to W/O emulsions for oil-water interface design（including interfacial layer design,interfacial layer modulation and interfacial antioxidant modulation）to enhance the physicochemical stability of W/O emulsions.The key control sites and influence mechanisms of protein properties at the oil-water interface to enhance the stability of W/O emulsions were also clarified.The main research contents and results are as follows:First,the preparation conditions of W/O emulsions（oil to water ratio was 75 wt%:25 wt%;6 wt%polyglycerol polyricinoleate（PGPR）,three times homogenization at 70 MPa）were determined.The significance of the oil-water interface for the physicochemical stability of W/O emulsions was clarified from the perspective of interface generation and droplet size（interface area）.The smaller particle size（322 nm）was conducive to the physical stability of W/O emulsions.For chemical stability,the formation of oil-water interface accelerated the lipid oxidation.The initial oxidation rate was independent of the droplet size（interface area）,while the long-term oxidation stability was dependent on the droplet size（interface area）.In addition,the’non-linear hypothesis’was shown to be applicable to W/O emulsion systems,and the action sites of highly interfacial active antioxidants（octyl gallate,C8）were thought to be at both the oil-water interface and the oil-air interface.Second,in order to verify the feasibility of protein-based interface design,the structure,physicochemical properties,functional properties of different types of proteins（sodium caseinate,CAS;whey protein isolate,WPI;chickpea protein isolation,CPI;pea protein isolate,PPI）and their interactions with hydrophobic emulsifiers（PGPR）were characterized.The results showed that plant-based proteins（CPI and PPI）had high interfacial activity due to their high surface hydrophobicity（H₀）and small particle size.The high antioxidant activity of WPI and PPI was inferred to be related to the content of active sulfhydryl（8.43μmol/g for WPI and4.83μmol/g for PPI）.In addition,proteins were shown to be able to collaborate with PGPR to rapidly form viscoelastic discontinuous double-layer interface layers at the oil-water interface,confirming the feasibility of protein-based oil-water interface design.Third,different types of proteins were added to W/O emulsions to enhance the physicochemical stability of W/O emulsions.The results showed that the mixed emulsifier reduced PGPR use by at least 2 wt%.In addition,effects of proteins on the physical stability of W/O emulsions were thought to be related to their H₀ and particle size.For instance,the smaller particle size（108 nm）of CPI and higher H₀（281.96）resulted in the W/O emulsion with the highest physical stability（Turbiscan stability index was 1.37 lower than control group）.Proteins also could reduce droplet collisions by increasing the viscosity of W/O emulsions.In terms of chemical stability,sulfhydryl contents of proteins were related to their antioxidant properties.For example,WPI and PPI with higher sulfhydryl content reduced the production of primary oxidation products in W/O emulsions by 28.74%and 36.09%,respectively.In addition,the physical barrier formed by proteins and their ability to bind metal ions also influenced the results of lipid oxidation.Fourth,to further enhance the physicochemical stability of W/O emulsions,the four selected proteins were treated with ultrasound and used to improve their limitations in enhancing the physicochemical stability of W/O emulsions.The improvement of functional properties of proteins depended on the degree of structural modification.The enhanced functional properties of proteins improved the physicochemical stability of W/O emulsions（e.g.,the Turbiscan stability index of the W/O emulsion added with 500 W-treated CAS was reduced by 2.33;The primary and secondary oxidation products of lipid oxidation decreased by 16.96%and 17.31%,respectively）.However,the small particle size（108 nm）of CPI might limit the efficiency of ultrasound treatment,and its structural and functional properties and effects on the physicochemical stability of W/O emulsions were also not significant.Finally,to improve the limitations of CPI,a protein with high interfacial activity,low antioxidant activity and insensitive to ultrasound treatment,in enhancing the physicochemical stability of W/O emulsions,it was chemically modified using alkali treatment and combined with gallic acid（GA）at different p H conditions to modulate interfacial antioxidants.The results showed that alkali treatment significantly affected the structure and functional properties of CPI and further improved the physicochemical stability of W/O emulsions.In addition,the mild alkali treatment（p H 9）promoted covalent binding of CPI and GA.Cross-linking sites might involve Met,Glu,His,Lys,Arg,and Trp residues.Due to the accumulation of GA at the interface caused by CPI,the CPI-GA conjugate significantly inhibited the lipid oxidation of W/O emulsions（primary and secondary oxidation products decreased by 57.03%and 61.34%）.