Aggregation and emulsifying properties of soybean protein isolate pretreated by combination of dual-frequency ultrasound and ionic liquids

Abstract

The objectives of this study were to investigate the changes in the aggregation and emulsifying properties of soy protein isolate (SPI) pretreated with dual-frequency ultrasound and three different ionic liquids (ILs), tributyl methyl ammonium chloride ([TBMA]Cl), 1-butyl-3-methyl imidazolium chloride ([BMIM]Cl) and 1-butyl-3-methyl imidazolium tetrafluoroborate ([BMIM]BF4). Results showed that dual-frequency ultrasound and ILs could not change the primary structure of SPI. The changes in UV–vis spectra indicated the unfolding of SPI as pretreated by [TBMA]Cl and [BMIM]Cl combined with dual-frequency ultrasound, followed by formation of soluble aggregates. The thermal stability analysis of the regenerated SPI suggested that [BMIM]BF4 presumably broke most of the hydrogen bonds within adjacent SPI molecules, leading to the decrease of thermal stability. Transmission electron microscopy showed major changes in protein morphology upon protein–protein interactions. SPI emulsion pretreated with dual-frequency ultrasound and [BMIM]BF4 had a higher emulsion stability index than the other samples (P < 0.05). Furthermore, as compared to untreated protein, all ultrasound and ILs pretreatments decreased the creaming index of SPI emulsion. These results indicated that combination of dual-frequency ultrasound and ionic liquids could be a potential method to change the aggregation and emulsifying properties of SPI.