Effects of sonication on the physicochemical and functional properties of walnut protein isolate

The objective of this study was to investigate the impact of high-intensity ultrasound treatment (sonication) on the molecular, physicochemical, and functional properties of walnut protein isolate. Aqueous walnut protein suspensions were sonicated at varying power levels (200, 400 or 600 W) and times (15 or 30 min), and then any alternations in protein structure and properties were determined. SDS-PAGE demonstrated that there were no changes in protein electrophoretic patterns, indicating that sonication did not break covalent bonds. Circular dichroism spectroscopy indicated a small change in protein secondary structure after sonication, with a decrease in α-helix and increase in β-sheet, β-turn, and random coil content. There was an increase in surface free sulfhydryl (SH) groups and a decrease in fluorescence intensity after sonication, indicating that appreciable changes in tertiary structure occurred. Ultrasound reduced the size of the particles in aqueous walnut protein dispersions as confirmed by static light scattering and scanning electron microscopy, suggesting that sonication dissociated protein aggregates. Moreover, the water-solubility (+22%), emulsifying activity index (+26%), and emulsifying stability index (+41%) all increased after sonication. These results suggest that sonication is a valuable tool for improving the functional attributes of walnut proteins.