Study On The Emulsifying Properties Of Soy Protein And Its Enzymatic Hydrolysates Complexed With Gum Arabic

Soy protein is a kind of high quality plant protein,which has been widely used in food emulsification system.However,the stability of soy protein emulsion was sensitive to the external conditions(pH,processing method,temperature and so on).Particularly,the solubility decreased under acidic or slight acidic condition due to the pH close to the isoelectric point of soy protein,which restricts the application of soy protein in acidic dairy products.Limited enzymatic hydrolysates of soy protein were prepared by Alcalase and their emulsifying properties were studied.Then gum arabic was used to increase the solubility of soy protein/limited enzymatic hydrolysates at acidic pH through coacervation interaction.The emulsifying properties of soy protein/limited enzymatic hydrolysates complexed with gum arabic under acidic condition were further discussed to provide the theoretical basis for the extended application of soy protein in food industry.Firstly,limited enzymatic hydrolysates were prepared by hydrolyzing soy protein using Alcalase and the properties of enzymatic hydrolysate emulsions was studied.The results showed that DH 5% and DH 7% hydrolysates had no macromolecular aggregates.The solubility of soy protein was only 8.07% and 4.36% at pH 4 and pH 5.The solubility of of hydrolysates was significantly higher than that of soy protein under the conditions of pH 4 and pH 5.The emulsifying activity and emulsion stability of DH 5% hydrolysates were higher than other hydrolysates.And compared with soy protein(SP)emulsion,the particle size,the degree of flocculation and the viscosity of DH 5% hydrolysate(SPH)decreased under acidic condition,but the stability of emulsion was not significantly improved.Secondly,the emulsifying properties of soy protein complexed with gum arabic was studied.The solubility and the elastic modulus of oil-water interface of SP/GA complexes increased at pH 4~5.And the interfacial tension of SP/GA complexes decreased obviously.Different preparation ways had obvious influence on the properties of emulsions.The first way was to adjust different pH values after mixing gum arabic and protein,then added oil and emulsified to form “mixed layer emulsion”.The second way was to emulsify soy protein,then added Arabia gum and adjusted different pH values to form “layer-layer emulsion”.The particle size,the degree of flocculation,and creaming index of the emulsions(“layer-layer emulsion”)prepared by the second way were lower than those("mixed layer emulsion")prepared by the first way during pH 4~5.SP emulsion prepared by the second way showed better stability at pH approaching the isoelectric point of soy protein with the creaming index of 11.43%~14.28%.The results of the thermal stability of emulsion showed that SP and SP/GA emulsions had higher heat-stability at pH 7.At pH 4,the unstability of SP/GA emulsion incerased,however,the stability of SP emulsion increased with the increase of heat treatment temperature,but the emulsion viscosity increased significantly compared to that of unheated emulsion,from 3.28 mPa·s(unheated emulsion)to 29.8 mPa·s(heated emulsion).The increased viscosity might be the main reason for the high stability of SP emulsion.Finally,the emulsifying properties of limited enzymatic hydrolysates complexed with gum arabic was studied.The soluble complexes formed by SPH/GA at pH 4 increased the elastic modulus of the SPH interface significantly,and the elastic modulus was also higher than those of the SP/GA soluble complexes.The result showed that SPH/GA had strong interfacial film strength at oil-water interface.The experiment also used two ways to prepare the emulsions.The results showed that the particle size and flocculation degree of emulsion prepared by the first way were lower than those by the second way.The two ways had no significant effect on the adsorption rate of SPH.GA interface adsorption rate of the first way was 20%~30% higher than that by the second way at pH 3~5.At pH 4,SPH/GA emulsion showed better stability,the creaming index was only 2.86% after 14 days storage.The results of the thermal stability of emulsion showed that SPH and SPH/GA emulsions with or without heat treatment were not divided into two layers at pH 7.The emulsions prepared by SPH/GA with mass ratio 1:1 and 1:2 had better thermal stability at pH 4,and the viscosity of SPH/GA emulsion after heat treatment slightly increased,from 2.60~2.71 mPa·s(unheated emulsion)to 3.07~3.36 mPa·s(unheated emulsion).It is indicated that the high strength interfacial film formed by SPH/GA at the oil-water interface may be play an important role in maintaining the stability of the SPH/GA emulsion,which provides the potential for the application of soy protein in the dilute acidic dairy products.