Effect Of Ultrasound-assisted Ionic Liquid Pretreatments On The Structure And Enzymatic Hydrolysis Characteristics Of Soy Protein Isolate

Soy protein isolate（SPI）,as an excellent plant protein product,has high nutritional value and abundant resources.However,the natural SPI molecules often present a compact globular structure and its internal groups cannot be exposed,which resulted in unsatisfied functional properties.Therefore,it was crucial to explore new modification methods for improving the application value of SPI,which has profound economic value and social benefits.In this study,ultrasound-assisted ionic liquids（ILs）was used to induce the dissociation and aggregation of SPI molecules or subunit.Furthermore,we have investigated the relationship between the structure of regenerated proteins and their interfacial properties,and the functional properties of soy protein hydrolysates so as to provide theoretical guidance for application of protein.The main results are as follows:（1）The solubility of SPI firstly was chosen as an index to optimize the suitable ILs type and concentrations.Then the effect of ultrasound-assisted ionic liquid（ILs）pretreatment on the aggregation structure of SPI was investigated by circular dichroism spectroscopy,intrinsic fluorescence spectra and atomic force microscopy.It was found that 1-butyl-3-methyl imidazolium hexafluorate phosphate（[BMIM][PF₆]）and 1-butyl-2,3-dimethyl imidazolium chloride（[BDMIM][Cl]）could significantly improve the solubility of SPI（P<0.05）.The effect of ultrasound-assisted 1 mg/m L[BDMIM][Cl]on the protein solubility was better than the single ultrasound or ionic liquids treatment,which was increased by 71.79%as compared to control.Ultrasound assisted ionic liquids（[BMIM][PF₆]and[BDMIM][Cl]）with different concentrations could effectively change the aggregation state of SPI,and the synergistic effect of ultrasound assisted[BDMIM][Cl]with low concentration was the best.At this time,the content of sulfhydryl group and the exposure of hydrophobic groups were the highest.The content ofα-helix was the lowest,and the formed protein particle size was more uniform.（2）The structure and interfacial properties of regenerated SPI were studied by UV spectra,SDS-PAGE,thermal analysis and other equipments.These results showed that ultrasound-assisted ionic liquid（ILs）pretreatment could induce the extension of protein structure,and expose more hydrophobic and chromogenic groups embedded in the protein.However,there was no obvious change in the subunit composition of protein.Thermal analysis showed the regenerated SPI treated by ultrasound-assisted[BDMIM][Cl]had better thermal stability.Ultrasound assisted ionic liquid（ILs）pretreatments could significantly improve the foaming capacity and foaming stability of regenerated SPI.When the ratio of IL/protein was 1,compared with control,the foaming capacity and foaming stability of regenerated SPI treated by ultrasound-assisted[BDMIM][Cl]was increased by 69.23%and 20.52%,respectively.（3）The enzymatic characteristics of regenerated SPI and the functional properties of its hydrolysates were studied under the optimum conditions.Ultrasound-assisted ionic liquid pretreatments significantly improved the degree of hydrolysis（DH）of SPI,generated more peptide fragments with small molecular weight distribution and changed the content of hydrophobic amino acids.Under the condition of suitable ionic liquid concentration,ultrasound-assisted[BMIM][PF₆]pretreatment had a positive synergistic effect on the foaming property of SPI hydrolysates.However,ultrasound-assisted[BDMIM][Cl]pretreatment had no obvious effect on the foam property of SPI hydrolysates.Ultrasound-assisted[BMIM][PF₆]pretreatment significantly reduced the emulsion creaming index at the ratio of IL/protein at 0.1.When SPI was pretreated by ultrasound-assisted[BDMIM][Cl]with high concentrations,the calcium-binding ability and the DPPH radical scavenging activity of SPI hydrolysates were the highest,while the hydroxyl radical scavenging activity was not very ideal.