Effects of ultrasound pretreatment on enzymolysis of sodium caseinate protein: Kinetic study, angiotensin-converting enzyme inhibitory activity, and the structural characteristics of the hydrolysates

Effects of ultrasound frequency and power density on enzymolysis of sodium caseinate (NaCas) and on the structure of the protein hydrolysate were studied. Enzymolysis kinetics, angiotensin-I converting enzyme (ACE) inhibitory activity, atomic force microscopy (AFM), scanning electron microscopy (SEM), intrinsic fluorescence and the circular dichroism (CD) were determined. Ultrasound significantly (p <.05) increased the degree of hydrolysis (DH), conversion rates of protein, ACE inhibitory activity and the apparent breakdown rate (kA) but decreased the apparent constant (kM). AFM revealed that sonication increased the protein particles diameters, and Rq and Ra values. Fluorescence spectra and SEM images show increases in surface areas of NaCas. CD spectra revealed increases in β-sheet, β-turn and random coil by 20, 5.33 and 5.91%, respectively, and decreased α-helix by 8.82%. Overall, ultrasound successfully improved enzymolysis of NaCas and release more ACE inhibitory activity due to the increases in specific surface area of protein molecule. Practical applications: NaCas is a functional ingredient widely used in foods industry, that is, it has beneficial effects on lowering blood pressure by producing ACE inhibitory peptides. Effects of ultrasound frequency and power density were performed to improve the enzymolysis of NaCas and the release of ACE inhibitory peptides. The mechanism of ultrasound on releasing of ACE inhibitory peptides was studied by monitoring the changes in protein structure. Generally, sonication remarkably provided a good reference about the enzymolysis of NaCas and the ACE inhibitory activity in polypeptides industries.