Interacting Mechanism Between Chitosan Hydrochloride And Soy Protein Isolate And Texture Regulation On The Electrostatic Complexes

As the main constituents of food,protein and polysaccharide are crucial to the texture characteristics of food.A better understanding on how the environmental conditions affecting the texture characteristics will contributes to food texture or function design,which is a new challenge for all food developers in the future.Soy protein isolate(SPI),originate from soybean,has not only economic sources,but also high protein absorption rate.15S fraction of SPI also has good gelling properties which made SPI a good base material for texture design.Chitosan hydrochloride(CHC)is a derivative of chitosan which has been solubility fortified by hydrochloric acid.Similar with SPI,chitosan is one of the most widely distributed polysaccharides and it is a strong cationic polysaccharides.Interaction interval between cationic polysaccharide and protein happens to close to the near-neutral environment of our daily food.Studying the interaction between SPI and CHC has a good guiding significance for food texture design.Therefore,this paper selected SPI and CHC as material and studied the thermodynamic mechanism of phase behavior and formation process of protein-polysaccharide complex under different pH,mixing ratio and different species and concentrations of salt ions.Physicochemical properties of the protein-polysaccharide complexes produced under different conditions were also investigated.Main findings of this article are as follows:(1)Formation process and phase behavior of SPI-CHC complex under different pH conditions were studied by ?-potential,microstructure observation,and shear viscosity measurement.The pH will not only affect the degree of carboxyl ionization on SPI,but also affect both the protonation process of the amidogen on SPI and CHC.?-potential measurement results of SPI and CHC under the mixing ratio of 1:1(v/v)shows that the addition of CHC will significantly improve the ?-potential of SPI before pI,while after the pI,complex could be seen clearly to be electro-neutralized products of SPI and CHC.Completely neutralization of SPI and CHC happens at the pH of 6.5,at which SPI and CHC carry an equal but opposite charges.Microstructure observation of the complex shows that the SPI and CHC formed the largest block around pH of 6.5.And it also founds that the complex forms tiny particles first and follows a cross-linking network,then aggregates forms quickly when pH increases,the observation process.shows that the formation process of SPI-CHC complex accords with nucleation and growth mechanism.Shear viscosity of SPI-CHC complexes showed that the shearing thinning phenomenon,maximum shear viscosity could be located at the pH of 6.5.(2)Effect of pH on SPI-CHC complex has been further investigated by isothermal titration calorimetric(ITC),Fourier Transform Infrared Spectroscopy(FTIR)and differential scanning calorimetric(DSC).The results of ITC test showed that the interaction of SPI and CHC is a spontaneous process since the Gibbs free energy is negative in both pH 4.0 and pH6.0 conditions.And compares with pH 6.0,binding sites and binding affinity of SPI and CHC is smaller,and the driving force at pH 4.0 is mainly hydrophobic interactions compares to electrostatic interaction and hydrogen bonding at pH 6.0.Secondary structure of SPI-CHC complex had been identified by fitting the amide I band data of FTIR spectrum,the result showed that the bonding of SPI and CHC will change the original secondary structure of SPI,and higher the degree of binding is,ordered structure(?-helix and ?-sheet)content of SPI-CHC complex is higher.Results of the DSC test found that combination between SPI and CHC will enhance the thermal stability of SPI,and the most thermal stable SPI-CHC complex is produced under the pH of 6.0.(3)Effects of SPI/CHC mixing ratios on the phase behavior and protein structure of the complex were investigated by turbidity,UV-second derivative spectra,fluorescence spectra,and synchronous fluorescence spectra respectively(??=15 nm and 60 nm).The result of turbidity shows that the critical points(pHc,pH?1,pHopt)which indicated the formation of the soluble/insoluble complex shift to low pH when R(SPI:CHC)is high while shifted to higher pH values when R is lower.UV-second derivative spectrum of SPI-CHC in the pH of 4.0 shows that the peaks representing phe and Trp residues has blue shift which indicated that phe and Trp are exposed to the solvent since the addition of CHC.While in the pH of 6.0,the peak representing Trp slightly red shifts from 290 nm to 291 nm,Trp prefer to settle in the hydrophobic part of proteins since the different R.Fluorescence spectrum under pH 4.0 is consistent with the results of the UV-second derivative spectrum.There is a red shift in the peaks of the three amino acid superposition spectra,indicating that the amino acid microenvironment generally has an increased polarity.The synchronous fluorescence spectrum(??= 15 nm)shows that Tyr has a blue shift at this time,and the opposite is that Trp(??= 60 nm)has no significant peak shift at pH 4.0.Fluorescence spectrums of different mixing ration under pH 6.0 show a blue shift which is also consistent with the results of UV-second derivative spectrum.Synchronous fluorescence spectroscopy(??= 15 nm)showed a redshift at Tyr,while Trp(??=60 nm)showed an unstable redshift at pH 6.0.(4)Effects of different species and concentrations of salt ions on SPI-CHC complexes were investigated by turbidity and dynamic rheological tests including stress sweep and frequency sweep.In the turbidity diagram under the influence of pH.different salt(NaCl,KCl,KBr)concentrations will not only decrease the complex content at pHopt,but also finds out that the concentration order of decreasing the turbidity follows Hofmeister series.To further discuss the effect of different salt ions on the SPI-CHC complex,the divalent salt ions(Mg2+,Ca2+,Ba2+)were added and the experiments were carried out by a new turbidity measurement method.By analyzing the turbidity curves under different salt ion concentrations,critical concentrations were determined by the zero points of the second derivative turbidity curve.The sequence of the critical points follows Hofmeister series too.In the dynamic rheological experiment(strain sweep and frequency sweep),the presence of salt ions enhances the viscoelastic properties of the SPI-CHC complex.However,at 50mM and 100mM,enhancement effect of ions could be simply put in an order:Na+>K+,Cl->Br-,Ca2+>Mg2+,except Ba2+,the order follows an inverse Hofmeister series.