The aggregation, structures and emulsifying properties of soybean protein isolate induced by ultrasound and acid.

The effects of ultrasound and acid on the aggregation, structures and emulsifying properties of soybean protein isolate (SPI) were investigated. Results of solubility showed that ultrasonic treatments at 0.001 M HCl increased the content of soluble SPI. The particle size of soluble aggregates subjected to ultrasonication and acid was initially decreased and then increased with increasing ultrasonic time. Secondary structure analysis, by circular dichroism, indicated lower a-helix and higher random coil amounts in SPI treated with short ultrasonic time, in contrast to the higher a-helix and lower random coil in SPI treated with longer time (more than 20 min). Emulsions prepared with SPI by 10 min of ultrasonication demonstrated significantly (P < 0.05) small droplet sizes and long term stability in comparison with their untreated counterparts. These results highlight that the emulsifying properties of SPI can be significantly improved by the synergistic effect of ultrasound and acid.