Effect of innovative ultrasonic frequency excitation modes on rice protein: Enzymolysis and structure

Abstract Fixed frequency ultrasound (FFU), cyclic-sweeping frequency ultrasound (CSFU), and random-sweeping frequency ultrasound (RSFU) with the identical central frequency were constructed, and their effects on enzymatic hydrolysis and structural characteristics of rice protein (RP) were studied. FFU and RSFU enhanced the degree of hydrolysis (DH) and the angiotensin-I converting enzyme (ACE) inhibitory activity of RP hydrolysate significantly. CSFU increased the ACE inhibitory activity but had no significant effect on DH. The molecular weight distribution results showed that FFU and RSFU promoted the conversion of the hydrolysate to low molecular weight peptides (1000–2000 and 200–1000 Da). Meanwhile, FFU, CSFU, and RSFU pretreatment improved the surface hydrophobicity index H0 from 397.53 to 464.23, 436.37, and 477.6, respectively. Circular dichroism spectra indicated that the three ultrasonic frequency modes decreased α-helix but increased random coil content. Ultraviolet–visible spectra and atomic force microscopy showed that the three ultrasound frequency modes caused the exposure of hydrophobic groups and reduced the particle size, respectively. Therefore, the innovative ultrasonic frequency modes modified the structural characteristics of RP and enhanced the ACE inhibitory activity of its hydrolysate.