| Protein is usually used in food,medicine and cosmetics as natural emulsifier.Natural plant proteins have much dense large molecular aggregates,which tend to produce strong steric hindrance and adsorption energy barrier at the oil-water interface so that cannot exhibit good interfacial activity.Consequently,exploring new ways of plant protein modification has important meaning.In this study,we intend to use ultrasound and ionic liquids to induce the dissociation and re-aggregation of soybean protein isolate（SPI）molecules and subunits,and explore the treatment conditions of SPI dissociation and re-aggregation.We will study the relationship between protein aggregation structure and emulsifying properties so that provide theoretical guidance for production of protein macromolecules with various emulsifying activity.The main results are as follows:（1）The effects of ultrasonic and acid treatments on the structure of SPI aggregates were investigated.The results showed that appropriate ultrasonic time at0.001 M HCl was beneficial to the increase of solubility of SPI.The ultraviolet spectra indicated that ultrasonic treatment caused hydrophobic groups and chromophores to be exposed,which resulted in increase of absorbance and red shift of the maximum absorption wavelength.Circular dichroism（CD）analysis showed that when the ultrasonic treatment time exceeded 10 min,the content ofα-helix increased rapidly but the content of random coil decreased.The results of laser particle size analyzer and atomic force microscope displayed that 10 min ultrasonic treatment under 0.001 M HCl could collapse large aggregation of SPI and then form aggregates with small particle size and uniform distribution.（2）Inthisexperiment,theeffectsofenergy-gatheredultrasound,energy-divergent ultrasound and slit ultrasound treatment on the structure and emulsion stability of SPI were compared and analyzed with dual frequency conditions.The results revealed that the solubility and surface hydrophobicity of SPI were improved obviously when treated by energy-gathered ultrasound.The solubility of SPI reached the maximum value at 100 W/L power for 30 min.Under these conditions,the surface hydrophobicity index was 715.33,which was 34.67%higher than that of the control group.Fluorescence spectroscopy showed that the fluorescence emission intensity of SPI reached the maximum at 100 W/L for 40 min,40 min and 60 min,respectively,treated by energy-gathered,energy-divergent and slit ultrasound.The content of free sulfhydryl（-SH）increased firstly and then decreased with the prolongation of ultrasonic time and the increase of power density.The three kinds of ultrasonic treatment improved the emulsifying activity of SPI.（3）We analyzed the effects of the three ionic liquids coordinated dual-frequency ultrasound treatment on the structure and emulsifying properties of SPI.In terms of structure,the synergistic of ultrasound and 1-butyl-3-methylimidazolium chloride caused theα-helix content of SPI to decrease by 15.89%but the random coil content to increase by 6.49%.The crystallinity of SPI increased by the combination of ultrasonictreatmentand1-butyl-3-methylimidazoliumtetrafluoroborateor1-butyl-3-methylimidazolium chloride.The three kinds of ionic liquids combined with ultrasonic treatment improved the numerical average molecular weight and weight average molecular weight of SPI.In terms of emulsifying properties,methyltributylammonium chloride and 1-butyl-3-methylimidazolium tetrafluoroborate in coordination with ultrasonic treatment could improve protein emulsifying stability.The three ionic liquids in combination with ultrasonic treatment decreased the particle size of SPI emulsion.When the shear rate is low,the apparent viscosity of SPI emulsion treated by ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate and ultrasound reached the highest value,and the viscosity is 169.00%higher than that of the control group at shear rate of 10 s-1. |
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