Effect Of Whey Protein Isolate-Tween 20 Interaction At The Interface On The Stability Of Walnut Oil-in-water Emulsions

Food emulsions are one of the most widely used systems in food industry.Protein and small molecule surfactants are important components of food emulsion and play a very central role in maintaining the stability of emulsions.Whey protein isolate?WPI?is a high-quality emulsifier,which exhibits excellent interfacial activity due to its typical amphiphilic structure and can lower interfacial tension.However,proteins are always impacted by these factors such as pH,temperature and ionic strength,which leads to the inability of emulsions coated by proteins.Therefore,small molecule surfactants are often used to prepare relatively stable emulsions.In the work,the competitive adsorption between WPI and Tween20 at emulsion interface,and the effect of this interaction on the stability of water-in-walnut oil emulsions were explored.The main contents and results are as follows:The effects of pH,temperature and ionic strength on the emulsification and emulsion stability of WPI were investigated.The results showed that p H greatly affected the emulsifying properties of WPI.The emulsification and emulsion stability of WPI were the lowest at pH 4.0⁵.0,which was close to the isoelectric point?pI?of WPI.When the pH was away from the isoelectric point,the emulsification and emulsion stability were improved obviously.The temperature and pH also affected the emulsifying properties of WPI.At pH3.0,the best temperature of the emulsification of WPI was 80?,while it was 60?at p H7.0.Lower ionic strength generally increased the emulsifying properties of WPI,whereas higher ionic strength decreased those of WPI.In the neutral condition,the endurance of WPI to ionic strength was improved by heat treatment.In this work,WPI acts as an emulsifier to prepare 5%walnut oil oil-in-water emulsions.The small molecule surfactant,Tween 20,with a variety of concentrations were introduced into the emulsion system to compete with WPI on the droplet surface,and the effects of this interaction on the physical properties of the emulsions were investigated.The results showed that increasing the Tween 20 level in the emulsions led to a significant decrease in the protein surface concentration,indicating that the proteins were progressively displaced by the surfactant.Complete displacement of the proteins occurred upon addition of 0.3%Tween 20.At the same time,the emulsion droplets size gradually decreased and the distribution of the droplets tended to be concentrated,which revealed that Tween 20 has a function of stabilizing the emulsion.This process also resulted in a decrease of the potential of WPI emulsion droplets,which varied at different pH values.A mixed pro-oxidants consisting of FeCl₃,H₂O₂ and V_C were added to the system to investigate the effect of the interfacial replacement under the pro-oxidant condition on the physical properties of WPI emulsions.The results showed that the prolonging of the oxidation time led to the coalescence of emulsion droplets,an increase of the emulsion droplet size and a loose and irregular distribution of the droplets.The absolute value of the droplet potential increased gradually with increasing addition of Tween 20,which was affected by pH values.Finally,the effect of interfacial dispacement with pro-oxidant on lipid oxidation and protein oxidation in WPI emulsions was explored.The results showed that the primary oxidation products?ROOH?decreased firstly and then increased,while the secondary oxidation products?TBARS?always increased.The carbonyl content gradually increased,the content of sulfhydry and the fluorescence intensity decreased.When the concentration of Tween20 increased from 0%to 0.5%,the total carbonyl content in the emulsion decreased,the content of sulfhydryl and the fluorescence intensity increased,which were also affected by pH values.Overall,the introduction of Tween20 accelerates lipid oxidation and attenuates protein oxidation.