Changes in the structure and dissociation of soybean protein isolate induced by ultrasound-assisted acid pretreatment

•Structure changes of soybean protein isolate induced by ultrasound and acid.•Solubility was not improved by ultrasound-assisted acid treatment.•Ultrasound-assisted acid induced unfolding of soluble aggregates.•Ultrasound and acid have synergistic effect on dissociation of soybean protein.

Structure and dissociation properties of soybean protein isolate (SPI) induced by ultrasound and acid were investigated. Results of solubility showed that ultrasound-assisted acid had no effect on the content of soluble aggregates in SPI. Increase of fluorescence intensity and red-shift of maximum emission wavelength indicated that acid induced molecular unfolding of SPI and exposure of hydrophobic groups. Circular dichroism spectra showed that ultrasound-assisted acid pretreatment resulted in increases in the α-helix content by 29.2% and random coils content by 8.3%, while β-sheet decreased by 13.4% (P < 0.05), as compared with those of control. Analysis of sodium dodecyl sulfate-polyacrylamide gel electrophoresis and atomic force microscope revealed that the contents of small subunits and particle increased significantly when SPI treated by ultrasound-assisted acid comparing with the SPI treated by single acid and ultrasound treatment. This study illustrated the ultrasound and acid have synergistic effect on the structure unfolding and dissociation of SPI.