Factors Affecting Stability Of Walnut Oil-in-water Emulsions Stabilized By Whey Protein Isolate And Casein

Emulsion system is one of the most common dispersed systems in the food,pharmaceutical,cosmetic,personal care and other industries,which usually consists of continuous phase,oil phase,proteins and/or small-molecule surfactants.The interaction of each part constitutes the unique properties of the emulsion.The emulsion is athermodynamically unstable system,and the change of environmental conditions leads to the emulsion flocculation,coalescence,creaming,lipid oxidation,and other physicochemical instabilities.In present work,two kinds of milk proteins including whey protein isolate(WPI),and casein(CS)were employed as emulsifiers to prepare reasonably stable walnut oil-in-water emulsions and the effects of various factors on physical and oxidative stability of protein emulsion were investigated.At the same time,small-molecule surfactant Tween20 was added into the emulsions coated by WPI and CS,respectively,the effects of the interaction between protein and small molecule surfactant on the properties of the emulsion were also investigated.The main results were as follows:Firstly,the effects of different factors(e.g.the concentration of WPI and CS,continuous phase pH,the concertration of NaCl and CaCl2)on the physical stability of emulsions were investigated.The results showed that,the physical stability of emulsion increased with the increasing protein concertration.The minimum concentration of protein to stabilize the emulsion was related to the pH value.At pH 7.0,the minimum concentration of WPI and CS was 0.3% and 0.9%,respectively.At pH 3.0,the minimum concentration of WPI and CS was 0.7% and 1.5%,respectively.pH had different effects on the stability of emulsion.When p H approached to the proteins' isoelectric point(pI),some unstable phenomena would occur,such as flocculation,coalescence,creaming,et al.The increase of salt ionic concertration gave more unstable emulsion,so did the salt ionic strength.Secondly,the effects of protein concertration,continuous phase pH,the concertration of NaCl on oxidative stability of emulsion were evaluated based on physicaly stable emulsions.The results showed that increasing protein concertration led to a decrease in lipid hydroperoxide(ROOH)and thiobarbituric acid-reactive substances(TBARS).The study further proves the antioxidant activity of proteins.pH had a great influence on lipid oxidation.At pH 3.0(droplets were positively charged),lipid oxidation rates were lower than those of p H 7.0,8.0 and 9.0(droplets were negatively charged).The effects of salt on oxidative stability of emulsion were concerned about concentration.Lower NaCl concentrations improved the antioxidant activity of proteins,and reduced the lipid oxidation.However,higher NaCl concentrations had the opposite effects.Usually,in the actual food system,proteins and small-molecule surfactants were used together to stabilize emulsions.Therefore,in the work Tween20(non-ionic small-molecule surfactant)was added into the emulsions stablised by WPI and CS,respectively,and the effects of interaction of WPI/CS and Tween20 on the physical characteristics of emulsion were also studied.The results showed that the surface protein concentration decreased to zero with the increasing Tween20 concertration;the emulsion droplets size decreased,approaching to that of Tween20-stabilized emulsion.At the same time,the emulsion ?-potential absolute value decreased.These results above suggested that the competitive adsorption between proteins and Tween20 occurred at emulsion interface,and total displacement of proteins was observed when Tween20 concertration was high enough.In the WPI-stablised emulsion,0.1% Tween20 began to displace interfacial proteins at pH 7.0 and 3.0.And 0.4% Tween20 completely displaced WPI from imterface.For CS emulsion,at pH 7.0,0.2% Tween20 began to displace CS,and 0.5% Tween20 completely displaced interfacial CS;whereas at pH 3.0,0.2% Tween20 began to displace CS,and 0.4% Tween20 completely displaced interfacial CS.Finally,the effects of interaction of WPI/CS and Tween20 on the lipid and protein oxidation as well as the relationship between them were researched.The lipid oxidative reaction was monitored by the measurement of ROOH and TBARS and the protein oxidative reaction was monitored by the measurement of carbonyl and sulfhydry content,and intrinsic tryptophan fluorescence.The results suggested that the increasing Tween20 concertration led to a decrease in the lipid oxidative stability,and CS had a better antioxidant activity than WPI.During the oxidation of emulsion,the carbonyl content increased,the sulfhydry content decreased,the fluorescence intensity weakened,suggesting proteins having been oxidized when inhibiting lipid oxidation.But the increase in Tween20 concertration led to the lowered degree of the carbonyl content decreased,the sulfhydry content increased,the fluorescence intensity strengthened,suggesting the decrease in the antioxidant activity of proteins induced by interfacial displacement mediated by Tween20.