Effect of Combination of Pulsed Electric Field and pH Shifting on Structure and Functional Properties of Soybean Protein Isolates

Soybean protein isolate (SPI) is an important vegetable protein source used in the food industry. Its functional properties are negatively impacted by its dense spherical aggregate structure, which reduces its molecular flexibility. In this study, pulsed electric field (PEF) and pH shifting techniques were applied to modify SPI structure. The effects of three different treatment strategies combining pulsed electric field and pH shifting (PEF first followed by pH shifting, pH shifting first followed by PEF, and simultaneous PEF and pH shifting) on the structure and functional properties of SPI were investigated. Among the three methods, simultaneous PEF and pH shifting achieved the most obvious effect on the aggregated structure of SPI. This treated SPI exhibited the highest solubility (90.23%), smallest turbidity (0.074), largest absolute ζ-potential value (44.4 mV), and smallest particle size (63.5 nm). Simultaneous PEF and pH shifting could induce partial unfolding of secondary and tertiary structures in SPI, converting the spherical aggregates into a "molten globule state" that exhibited lower α-helix and β-sheet content and higher random coil content. The overall treated protein structure was loose and disordered. Modified SPI using different methods showed better functional properties. The order of improvement in SPI functional properties was as follows: Simultaneous PEF and pH shifting>pH shifting first followed by PEF>PEF first followed by pH shifting. Compared with untreated SPI, simultaneous PEF and pH shifting treatment of SPI increased the emulsifying activity, emulsifying stability, foaming activity, and lutein binding constant by 119.24%, 39.33%, 59.03%, and 245.81%, respectively. In conclusion, different treatments combining pulsed electric field and pH shifting can significantly affect the structural and functional properties of SPI. Simultaneous PEF and pH shifting treatment can induce the greatest degree of unfolding of the advanced structure of SPI and most strongly improve its functional properties.