Effects of dual-frequency ultrasound with different energy irradiation modes on the structural and emulsifying properties of soy protein isolate

The effects of dual-frequency ultrasound pretreatments with different energy irradiation modes, including energy-gathered ultrasound (EGU), energy-divergent ultrasound (EDU) and slit ultrasound (SU) on the solubility and emulsifying properties of soy protein isolate (SPI) were investigated. The surface hydrophobicity, intrinsic fluorescence spectra, and circular dichroism spectra of SPI were determined. Results revealed that all ultrasonic pretreatments increased the solubility of SPI significantly (P ≤ 0.05). Ultrasonic pretreatments with EDU and SU could not improve the emulsifying activity (P > 0.05). The emulsion stability index of SPI treated by EGU was higher than those of EDU and SU with the same ultrasonic time and power density. Changes in surface hydrophobicity and intrinsic fluorescence showed the unfolding of protein and exposure of hydrophobic groups as a result of action of ultrasound. The CD spectra of SPI pretreated by optimal EGU, EDU and SU showed that dual-frequency ultrasound induced the decrease of β-sheet contents and increase of α-helix and random coil contents significantly (P ≤ 0.05). In conclusion, the influence of EGU on the structural and emulsifying properties of SPI was more significant than those of EDU and SU.