Systematical Characterization Of Functional Properties And Mechanisms Ofthermal-induced Polymerized Whey Protein

Whey proteins are produced as byproducts from cheese manufacture and are mixtures of globular proteins.?-Lactoglobulin generally accounts for half of the total whey protein.Its functional activity is limited because of its low molecular weight.Modification of whey protein can further improve and enhance its functional properties and expand its application in food industry,which is of great significance for the effective utilization of natural protein.In this paper,whey protein was modified by heat treatment to form polymerized whey protein.The properties,functional properties,antioxidant properties and denaturation mechanism of polymerized whey protein were systematically studied.The specific research contents and results were as follows:(1)The factors of pH,protein concentration,heating temperature and heating time in the preparation of PWP were investigated.The particle size,potential,free sulfhydryl group,the viscosity and denaturation of PWP were determined.The results show that:Heating whey protein above 75 ~oC at pH7 or 8 resulted in denaturation of80–90%whey protein.pH variation had a remarkable influence on particle size of samples(P<0.05),where as heating temperature and time did not generate significant changes.Zeta potential of PWP samples fell in the range of-30 to-40mV.Free sulfhydryl group content of PWP samples decreased with increasing level regarding each factor.Time sweep test revealed that increasing protein concentration,heating temperature or time led to higher apparent viscosity.Flow behavior of PWP samples approached Newtonian character as protein concentration,heating temperature or time decreased.(2)The emulsifying property,emulsifying stability,foaming property,foaming stability and solubility of PWP solution were measured and analyzed.The following conclusions were drawn:After heat treatment,the solubility,emulsifying ability and emulsifying stability of all PWP samples decreased.The denaturation and polymerization of whey protein increase its foamability and foaming stability.(3)The antioxidant activity of PWP samples was evaluated mainly by scavenging DPPH/ABTS free radicals and its oxygen radical absorption capacity(ORAC).The results showed that compared with natural whey protein samples,the scavenging capacity of PWP samples were significantly reduced,however,the ABTS free radical scavenging ability of PWP samples is higher than that of control samples.The effects of different pH,protein concentration,heating temperature and heating time on ORAC were not very different.(4)The mechanism of formation and change of PWP was preliminarily explored.The effects of pH,protein concentration,heating temperature and heating time on the formation of PWP were observed and analyzed.The results showed that the degree of dissociation of unheated whey protein molecules was low.Compared with protein concentration,heating temperature and heating time,the change of pH between 6 and8 had less influence on the secondary structure of PWP.The ellipticity of all PWP samples was lower than that of the control samples,and the zero boundary point of the curve moved to the left,indicating that heating increased the secondary structure of whey protein,and the content of?-LA and?-LG also changed.Surface hydrophobicity analysis showed that samples at higher pH or concentration became less hydrophobic,and increasing heating temperature or time resulted in higher hydrophobicity index.