The Effect Of Ph-shifting And Thermosonication On The Formation Of Calcium-aided Pea Protein Emulsion Filled Gels

Pea protein has become one of the functional components and structural builders of animal protein replacement products due to its advantages of rich nutrition,low allergenicity,wide planting,non-GMO,and environmental friendliness.80%of pea protein is globulin.Due to its compact structure,low water solubility,and less sulfur-containing amino acids,it has limited functions.Therefore,structural modification of pea protein combined with electrostatic cross-linking of metal ions was performed to improve its functional properties.In this study,the structure of pea protein was modified by pH-shifting and thermosonication,and the emulsion was prepared to explore calcium chloride（0,5,10,15,and 20 m M）-induced pea protein emulsion-filled gel system（16%w/v protein,10%v/v oil,pH 6.20）rheological and gel properties.The main research contents are as follows:The effects of pH-shifting(pH₁₂)and thermosonication（HU）modification treatments on the properties of calcium-induced pea protein（PPI）gels（16%w/v protein,pH 6.20）were first investigated.The study found that the pH₁₂and HU dissociated the tight protein subunits,exposed the hydrophobic groups,and formed a more uniform and dense gel network,which effectively improved the gel strength（1.9 times and 2.0 times,respectively）,and improved the water holding capacity（3.9%,7.8%increase,respectively）.When 5 m M Ca²⁺was added,the strength of PPI gel reached the maximum,at this time,the strength（1.5-fold and 2.0-fold increase,respectively）and water holding capacity（13%and 9%,respectively）of the pH₁₂and HU PPI gel were compared with those of the untreated PPI gel.Gel was significantly improved（P<0.05）.In contrast,the untreated PPI was only partially micro-expanded by the action of 5 m M Ca²⁺,and the protein molecules were not uniformly aggregated through electrostatic shielding,hydrophobic group aggregation,calcium bridge building,etc.,and the gel network skeleton became thicker,the structure is loose.The pH₁₂and HU PPI protein solution have higher viscosity,less collision probability between protein molecules,orderly binding with 5 m M Ca²⁺,and the gel network is more denser.In addition,the tryptophan fluorescence results of HU PPI showed a more obvious blue-shift phenomenon,indicating that it was more affected by calcium and the aggregation of protein molecules was more intense,so spherical pores appeared when the total volume of the gel remained unchanged.The results of gel force analysis showed that the number of intermolecular disulfide bonds in HU PPI increased significantly（71%）due to the close intermolecular distance between proteins by 5 m M Ca²⁺,which was also the reason for the highest gel strength.Based on protein gels,pH₁₂and HU PPI-stabilized emulsions were filled into untreated PPI dispersions to explore the rheological properties of emulsion-protein composites and emulsion-filled gel properties（16%w/v protein,10%v/v oil,pH 6.20）.The study found that the pH₁₂and HU modification treatment enhanced the emulsifying activity and emulsion stability of PPI,and the emulsion particle size was smaller and uniform,and the interfacial interaction was stronger.Compared with the untreated PPI gel,filling the untreated PPI emulsion significantly enhanced the gel strength（74%）and water holding capacity（27%）（P<0.05）,and the moisture was more tightly bound;Further unfolding at the interface exposes more sulfhydryl groups and enhances the number of disulfide bonds in the composite gel,so filling with pH₁₂and HU PPI emulsions can further improve gel strength（110%,80%increase,respectively）and water holding capacity（increased by 30%and 29%,respectively）.With the addition of Ca Cl₂,the gel strength reached a maximum at 15 m M Ca²⁺,and the composite gels filled with pH₁₂and HU PPI emulsions were compact and highly ordered.Ca Cl₂further enhanced the tightness of water binding,although some transfer of fixed water(T₂₁)to free water(T₂₂)occurred.Based on protein gels,pH₁₂and HU PPI-stabilized emulsions were filled into modified PPI dispersions to explore the rheological properties and emulsion-filled gel properties of emulsion-protein composites（16%w/v protein,10%v/v oil,pH 6.20）.It was found that the strength of emulsion-filled gels based on pH₁₂treated PPI was not significantly improved compared with pH₁₂treated protein gels,and did not change with calcium chloride concentration（P<0.05）.The strength of emulsion-filled gels based on HU PPI was significantly higher than that of HU protein gels（67%and 57%for gels filled with untreated PPI emulsion and HU PPI emulsion,respectively）,and at 10 m M Ca²⁺maximum strength.Further testing of HU PPI-based emulsion gels showed that the gels filled with untreated PPI emulsions had obvious droplet aggregation.In the presence of Ca^2+,the gel network wall thickened,the pore size decreased,and the structure was more uniform.For the gel filled with HU PPI emulsion,due to the small particle size and good stability of the emulsion,the emulsion droplets are stably filled in the matrix network,and the excessive aggregation caused by Ca²⁺can be effectively prevented,so the gel network is uniform and dense.After the heating stage,the calcium-induced emulsion gel filled with HU PPI emulsion had the highest elastic modulus,showing a strong hydrophobic aggregation phenomenon.However,it was also found that the number of disulfide bonds was reduced due to the aggregation of matrix proteins by Ca²⁺.In summary,pH₁₂and HU treated PPI further unfolded at the interface,and in the high-protein and low-fat complex solution,the emulsion droplets acted as active fillers embedded in the PPI matrix to enhance the interaction between the interface protein and the gel matrix（such as disulfide bonds）;milk droplets occupy the network space to reduce the protein molecular distance and enhance the interaction force,and through the steric barrier to induce calcium-induced matrix proteins to aggregate evenly,effectively preventing the poor aggregation of proteins caused by excess calcium,optimizing calcium Concentrations（from 5m M to 10 or 15 m M）further enhance gel strength.The research on the plant-based emulsion-filled gel system in this topic provides theoretical guidance for the diversified application of plant protein products.