Stability Of Hydrolyzed Walnut Protein Isolate Emulsions Complexed With Polysaccharide

Walnuts are a major crop in China,and walnut protein is a good source of protein.However,its low solubility limits its applications in food processing.Research on walnut proteins is mainly focused on the functional properties and the structural properties,and there are few studies on the enzymatic modification of walnut protein.In this study,walnut meal was the raw material used to extract a walnut protein isolate(WPI).A limited enzyme hydrolysis was done using trypsin and the hydrolysates' structural and functional properties were evaluated.Emulsions were prepared from WPI and different degree of hydrolysis(DH),and the stability of the emulsions were studied.Polysaccharides were used as an emulsion stabilizer to investigate the effect of polysaccharide concentration and ionic strength on emulsion stability.The main research results were as follows:(1)Functional properties of WPI and different degree of hydrolysates(1,3,7.1%)were investigated.The results indicated that limited enzyme hydrolysis could improve the functional properties of WPI.Hydrolysis increased the solubility of the WPI and a DH of 7.1%had the highest solubility of 74.8%.The emulsifying stability was also improved.The foaming and foam stability were increased and then decreased with increased DH.Hydrolysates of WPI showed a higher oil hold capacity and lower water hold capacity compared to WPI.The amount of hydrophobic amino acid like proline in the soluble fraction increased after enzymatic hydrolysis.(2)With the increased DH,the molecular weight of WPI gradually decreased,while the free sulfhydryl groups increased.Surface hydrophobicity and endogenous fluorescence showed an increasing trend.The structure of WPI was more flexible after enzymatic hydrolysis.The estimate of the a-helix content increased from 17.5 to 18.8%,while the estimate of ?-sheet content decreased from 54.6 to 51.4%.In addition,the microscopic morphological observations showed that the enzymatic hydrolysis destroyed the flaky loose structure of the WPI,the structure of the enzymatic hydrolysate was loose,and honeycomb depressions were observed on the surface.(3)Emulsions were prepared using WPI and its hydrolysates.They were tested for emulsification,and for pH,ionic strength and thermal stabilities.The effect of pH on emulsion stability was not significant.In the same ion concentration range,the average particle size of the 3%hydrolyzed WPI emulsion formation was not changed significantly,and it was stable with neutral conditions(pH 7-8)and temperature<90?(pH 7,0 mM NaCl).(4)The effects of polysaccharide type and polysaccharide concentration on the stability of WPI and its hydrolysates' emulsions were studied.The emulsion system showed a better stability when the concentration of the polysaccharide was 0.2%.The effect of xanthan and flaxseed gums on emulsion stability were compared.The flaxseed gum emulsions were more stable.(5)The effect of ionic strength(0?400 mM NaCl)on stability of WPI and hydrolysates with polysaccharides showed that the average particle size of the emulsion increased as the ionic strength increase.The viscosity of the WPI-polysaccharide emulsion system increased,while the hydrolysates-polysaccharide emulsion systems decreased.However,the xanthan gum emulsion system remained stable at higher NaCl concentrations.