| Whey protein isolate(WPI)as food ingredients play a major role in the sensorial properties due to it’s special thickening,gelling,emulsifying,foaming texturizing and water-holding.It has been commonly used as important functional protein additives in the production of dairy products.The application in the food processing is limited,because they are sensitive to the physical and chemical conditions,especially for the β-lactoglobulin(βlg).It is the main protein in WPI(>60%),and its conjugated properties,such as polysaccharide,polyphenol and ligands had important influence on the application of WPI.λ-carrageenan(λcar)is the most-used polysaccharide in food processing with the biological activity of lowering blood cholesterol and triglyceride.The addition of λcar in semisolid dairy products as fat replacers would cause an increase in viscosity,thickness,creaminess,and smoothness.Because of the interaction of protein-polysaccharide is a very complicated process,especially of βlg-λcar complex,this study is focused on the interaction of βlg.WPI and λcar through the aspects of thermodynamic properties,binding affinity,functional properties to comprehensive understand the mechanism of protein-polysaccharide conjugation and expand the range of applications of WPI in food systems.The main research results are as follows:(1)The interaction between βlg and λcar in an aqueous solution was studied at the condition of varies pH(4.0-7.0).Isothermal titration calorimetry(ITC)was used to determine the change in enthalpy and entropy caused by the interaction of βlg and λcar.Fluorescence quenching(FQ),dynamic light scattering(DLS)and Fourier transform infrared spectroscopy(FTIR)were used to mintor the changes in fluorescence emission,hydrodiameter and secondary structure.The results showed that the interaction was spontaneous exothermic and the binding affinities(Ka)decreased from 9.0 ± 1.3 ×10~5 to 1.3 ± 0.8 ×105 M-1,when the pH increased from 4.0 to 7.0,as well as the stoichiometry,which indicated the binding of βlg and λcar was considered a moderate binding affinity.Approximately 60 βlg molecules were adsorbed on a single λcar molecule,and the diameter was approximately 800 nm at pH 4.0.The decrease of intrinsic fluorescence emission intensity of βlg,and the blue shift of maximal fluorescence emission after addition of λcar also indicated λcar interacted with βlg at lower pH.β-sheet and α-helix components are the main secondary structures of βlg.The interaction of βlg and λcar interaction resulted in a decrease in the β-sheet content and an increase in the α-helical content,wherase,almost no changes in β-turn contentand random coil.(2)Turbidity method was used to study phase separation behavior and formation of WPI-λcar complex in the different pH,salt concentration,temperature and the mass ratio.The changes in zeta potential and average particle size were also determined.The driving force of WPI and λcar interactions was analyzed through fluorescence spectra,FTIR and laser confocal.The results showed that the WPI-λcar complex exhibited four critical pH points(phase points)as the decreasing of pH.From high to low,these four points were pHc 5.5,pH? 5.3,pHopt 4.5,pHd 2.5,respectively.The turbidity reached the maximum(0.512 ± 0.11),and the average particle size reached 2.387 nm,and zeta potential was zero at pH 4.5.The high concentration of Na Cl(100 m M)inhibited the formation of the complexes,then decreased the turbidity of the mixture,moreover,phase region of insoluble complexes became narrow.Low temperature(<25 °C)shifted the critical pH to higher,while,high temperature(>25 °C)shifted the critical pH to lower.The formation of WPI-λcar complex was mainly driven by electrostatic attraction,as well as,hydrogen bonding and hydrophobic interactions.The increased mass ratio of WPI/λcar facilitated the formation of the complex,the critical pH shifted toward higher pH,and pHopt moved toward the isoelectric point of the WPI as the mass ratio increased.The maximum turbidity appeared at the mass ratio of 10:1(WPI/λcar).The presence of polysaccharides altered the spatial structure of the protein,and moved the tryptophan residues into a more hydrophobic environment.The addition of a polysaccharide under acidic conditions led to a decrease in surface hydrophobicity and an increase in hydrophilicity of protein.(3)On the basis of theoretical research,the emulsifying activity and emulsifying stability of the double-layer emulsion were further adressed under acidic conditions.The effects of pH,polysaccharide concentration,salt ion concentration and temperature on emulsion were studied by the aspects of zeta potential,average particle size,emulsification,emulsification stability and microstructure.The results showed that there was electrostatic attraction between the λcar and WPI emulsion droplets at pH of 3.0 and 4.0.The WPI/λcar double-layer emulsion had good emulsifying activity at the condition of 0.1% λcar and pH 4.0.WPI/λcar double-layer emulsion had good emulsifying stability at the condition of 0.1% λcar and pH 6.0.The salt ion concentration and temperature had hardly effect on the stability of double-layer emulsion.(4)The effects of pH,ionic strength,polysaccharide concentration,and temperature on foaming capacity and foaming stability of WPI-λcar soluble complex were studied.The results showed that the soluble complex formed at pH 5.5 had high foaming capacity and foaming stability,and the WPI-λcar soluble complex foam obtained by heat treatment had high foaming capacity and foaming stability.The low concentration of Na Cl(≤50 m M)significantly improved the solubility,foaming capacity and foaming stability of the mixed system of protein and protein-polysaccharide.λcar at the addition of 0.15% improved the solubility,foaming capacity and foam stability of WPI. |
PDF Full Text Request