Impacts of sonication and high hydrostatic pressure on the structural and physicochemical properties of quinoa protein isolate dispersions at acidic, neutral and alkaline pHs

Herein, 1 wt% quinoa protein isolate (QPI) was exposed to sonication using a 20 kHz ultrasonicator equipped with a 6 mm horn (14.4 W, 10 mL, up to 15 min) or high hydrostatic pressure (HHP, up to 600 MPa, 15 min) treatments at pH 5, pH 7, and pH 9. The changes to physicochemical properties were probed by SDS-PAGE, FTIR, free sulfhydryl group (SH), surface hydrophobicity (H-0), particle size and solubility. As revealed by SDS-PAGE, substantial amounts of 11S globulin participated in the formations of aggregates via S-S bond under HHP, particularly at pH 7 and pH 9. However, protein profiles of QPI were not significantly affected by the sonication. Free SH groups and surface hydrophobicity were increased after the sonication treatment indicating protein unfolding and exposure of the embedded SH and/or hydrophobic groups. An opposite trend was observed in HHP treated samples, implying aggregation and reassociation of structures under HHP. HHP and sonication treatments induced a decrease in ordered secondary structures (random coil and beta-turn) accompanied with an increase in disordered secondary structures (alpha-helix and beta-sheet) as probed by FTIR. Finally, the sonication treatment induced a significant improvement in the solubility (up to similar to 3 folds at pH 7 and similar to 2.6 folds at pH 9) and a reduction in particle sizes (up to similar to 3 folds at pH 7 and similar to 4.4 folds at pH 9). However, HHP treatment (600 MPa) only slightly increased the solubility (similar to 1.6 folds at pH 7 and similar to 1.2 folds at pH 9) and decreased the particle size (similar to 1.3 folds at pH 7 and similar to 1.2 folds at pH 9). This study provides a direct comparison of the impacts of sonication and HHP treatment on QPI, which will enable to choose the appropriate processing methods to achieve tailored properties of QPI.