Effect Of Inorganic Salt On Solubility And Conformation Of Soybean Protein

Soybean protein is widely used in the processing of food and health products because of its low price,wide source,and ease of being digested and absorbed by the human body.Vegetarian meat is a common processed food product with soybean protein as the raw material.However,the quality of vegetarian meat is not ideal in actual production,which greatly affects the prospects of the vegetarian meat market.It is well known that Nanoparticles（NPs）in food proteins can affect the physicochemical properties of proteins,which provides a new direction to solve the above problems.The study of NPs in soybean protein has an active role in guiding the improvement of soybean protein processing properties.As an additive commonly used in food processing,salt also greatly affects the molecular structure and functional properties of proteins.Therefore,two kinds of soybean proteins were selected as the research objects in this study.Starting from the NPs in soybean protein,the influence of inorganic salt on the solubility of soybean protein was studied.Meanwhile,the effects of the salt type and its concentration on the conformation of soybean protein were further explored,which provided a theoretical basis for the selection of raw materials for soybean protein products and the optimization of salt treatment conditions.The main research contents and results are listed as follows:Firstly,two kinds of soybean proteins were used as research models.The existence of NPs in two kinds of soybean protein solution was confirmed by ultraviolet-visible spectrum,fluorescence spectrum,ζ-potential,light scattering and nanoparticle tracking analysis（NTA）techniques,and the effect of the dilution ratio on the structure and properties of NPs was studied.The results showed that soybean protein existed in the form of multi-size NPs in an aqueous solution.The dilution affected the interaction between the NPs and promoted the dissolution of NPs,leading to the decrease of hydraulic radius（R_h）,the increase of hydrophobicity in the microenvironment of tyrosine and tryptophan residues.The protein concentration,ζ-potential and fluorescence intensity of the solution decreased.In addition,the fractal dimensions（d_f）of soybean protein GN particles first decreased and then increased,and the spatial structure of soybean protein GN particles was first loose and then dense,which was approximately spherical.The d_fof soybean protein DN particles increased,and the spatial structure compactness of soybean protein DN particles became larger,which was approximately rod-like.Secondly,KCl,Ca Cl₂and two soybean proteins were used as research models.The effects of salt（KCl,Ca Cl₂）solutions at six temperatures（35,30,25,20,15 and 10°C）and three concentrations（0.1,0.5 and 1.0 M）on the solubility of soybean protein were studied with an ultraviolet spectrophotometer.The results showed that a temperature rise destroyed the disulfide bond and caused the spatial structure of soybean protein NPs to unfold,and the solubility of soybean protein increased.The solubility of soybean protein DN was higher than that of soybean protein GN,which was not affected by salt concentration or ion type.With the increase of KCl concentration,a salting-out effect occurred in both soybean protein solutions,which inhibited protein dissolution.With the increase of Ca Cl₂concentration,the solubility of two soybean proteins showed a non-monotonic change,and the protein solution showed salting-in and salting-out effects successively.In KCl and Ca Cl₂solutions,ΔH⁰>0 andΔH⁰>TΔS⁰indicated that soybean protein dissolution is an endothermic reaction and is controlled by enthalpy change;when T>300 K,ΔS⁰>0 showed that the chaos degree of the system increased and hydrophobic force played a major role.Soybean protein in Ca Cl₂solution had a much higher dissolution activation energy than soybean protein in KCl solution,indicating that the dissolution of soybean protein in Ca Cl₂solution was more difficult,and the dissolution rate was more temperature-dependent.Finally,the effects of different kinds（Na Cl,KCl,CH₃COONa,NH₄Cl,Ca Cl₂,Mg Cl₂and Mg SO₄）and concentrations（0.1,0.5 and 1.0 M）of salt solution on the spatial conformation of two soybean proteins were investigated by SDS-PAGE gel electrophoresis,fluorescence spectroscopy,ultraviolet spectroscopy,and light scattering techniques.The results showed that with the increase in monovalent salt concentration,the r values of the second-derivative UV spectra generally decreased,and the micro-environment polarity of tyrosine residues in protein particles also decreased.The decrease of R_hand increase of d_fof soybean protein GN particles indicated that the protein chain curled up and contracted to form small and dense protein particles.The R_hand d_fof soybean protein DN particles increased slightly showed that the protein chain contracted to form large and compact protein particles.With the increase in bivalent salt concentration,the polypeptide chain with tyrosine residues in soybean protein DN particles moved to the hydrophobic region in the protein particles,and the conformation of the protein particles changed.There were large protein particles in the two soybean protein salt solutions.The electrostatic shielding effect of salt ions led to different degrees of aggregation of protein particles.In salt solution,soybean protein GN particles showed spherical or monodisperse linear clusters,while soybean protein DN particles exhibited a two-dimensional object structure with a fractal surface.The results of this study provide a theoretical basis for the selection of raw materials and the optimization of the salt treatment conditions of soybean protein products.