Study On The Interaction And Rheological Properties Of Deamidated Soy Protein Isolate And Chitosan

Proteins and polysaccharides are two important components in food.Rational utilization and control of its interaction have far-reaching significance in optimizing food formulations and building new food ingredients.Soy protein isolate?SPI?has received widespread attention due to its balanced amino acid ratio,high bioavailability and extensive sources.However,SPI has poor solubility in neutral and weakly acidic systems and heating,salt instability,which severely limits its application in semi-solid foods and liquid beverages.The purpose of this study is to modify SPI using citric acid,by changing the distribution of charge to improve its multiple functional features.Then the interaction between deamidated soy protein isolate?DSPI?and chitosan?CS?and the structure and functional properties of its complexes are explored.What's more,the influence of temperature,ionic strength and pH on the rheological properties of DSPI-CS mixtures are investigated.The results are as follows:?1?The influence of deamidated degree on DSPI and CS interactions was studied by?-potential,turbidity and scanning electron microscope.Firstly,three types of DSPI were prepared and their degree of deamidation were 35.60%,52.13%,64.75%,respectively.SDS-PAGE showed that DSPI had a certain decrease in molecular weight compared to SPI.As the degree of deamidation increased,the isoelectric point of SPI dropped from pH 4.68 to pH 4.42,4.19,4.03,respectively.The charge density of protein-polysaccharide complexes decreased gradually and phase transition points moved to lower pH so the pH range of the complex formation expanded.SEM showed that SPI had smooth and dense sheet-like structure while the structure of DSPI became loose and large pores appeared.The structure of DSPI-CS complex was more dense and smooth than SPI-CS complex.?2?Effects of protein conformation and rheological properties of complexes before and after the interaction between DSPI with different degree of deamidation and CS were studied by ultraviolet spectra,fluorescence spectra,fourier transform infrared spectra?FTIR?,differential scanning calorimeter?DSC?and rotational rheometer.The results indicated that:as the degree of deamidation increased,part of tryptophan residues of DSPI were re-hidden and some tyrosine residues were exposed and the protein-polysaccharide complexes generally became more hydrophilic.The SPI-CS complexes was mainly driven by the carboxyl groups of SPI(at 1654.99 cm^-11 and 1534.76 cm^-1)and the amino groups of CS(at 1598 cm^-1).As the degree of deamidation increased,the thermal denaturation temperature and viscoelasticity of DSPI-CS complexes increased with varying degrees.In addition,at lower concentrations,DSPI-CS complexes existed linear viscoelastic zone?LVR?,while SPI-CS complexes had not it.?3?The effects of temperature,ionic strength and pH on the rheological properties of DSPI and CS mixtures were investigated.The results showed that proper heating increased the viscoelasticity of DSPI-CS mixtures.When the pre-heated temperature raised to 95?,the viscoelasticity decreased,which might reduce the binding sites of DSPI and CS due to hydrophobic aggregation or thermal decomposition to reduce the entanglement of DSPI and CS molecular chains.Adding 20-200 mM NaCl to DSPI-CS mixtures,viscoelasticity of the mixtures showed different degrees of salt ion enhancement effect.After DSPI was preheated at 85?,the viscoelasticity of mixtures showed an increase in low salt ion concentration and a decrease in high salt ion concentration,indicating that heating changed the ability of DSPI to bind to salt ions.At different pH systems,the viscoelasticity of DSPI-CS mixtures were ranked:pH 5.0>pH 6.0>pH 4.0>pH 7.0 and 85? DSPI-CS mixtures had a similar order.