Effects of ultrasound on the structure and physical properties of black bean protein isolates

In this study, our aim was to compare the effects of low-frequency (20 kHz) ultrasonication applied at various powers (150, 300, or 450 W) and for different durations (12 or 24 min) on the functional and structural properties of black-bean protein isolate (BBPI) dispersions. In SDS-PAGE analysis, we detected no marked changes in protein electrophoretic patterns. However, secondary-structure analysis performed using circular dichroism indicated that all samples except Sample E (300 W, 24 min) showed a decrease in the α-helix proportion and an increase in β-sheets content in the BBPI after ultrasonic treatment. Moreover, emission-fluorescence spectra revealed that the tertiary structure of black-bean proteins changed after ultrasonic treatment, and scanning electron microscopy of ultrasonicated BBPI samples showed that BBPI microstructure had changed and it contained larger aggregates when compared with the untreated BBPI sample. When medium-power ultrasonication was applied for 24 min, the particle size was minimized and the absolute zeta potential was maximized. Surface hydrophobicity and protein solubility of the BBPI dispersions were enhanced after ultrasonication, which increased the destruction of internal hydrophobic interactions of protein molecules and accelerated the molecular motion of proteins to cause protein aggregation. However, medium-power ultrasound treatment disrupted BBPI dispersions into small soluble protein aggregates by means of cavitation forces that induced increases in surface hydrophobicity and solubility. High-power ultrasound treatment caused a restructuring of BBPI aggregates, which led to an increase of particle size but a decrease in the absolute zeta potential.