The necessity of walnut proteolysis based on evaluation after in vitro simulated digestion: ACE inhibition and DPPH radical-scavenging activities

This study aimed to evaluate the necessity of enzymatic hydrolysis for walnut peptide preparation based on a novel evaluation approach. Defatted walnut meal hydrolysate (DWMH) was prepared by hydrolyzing defatted walnut meal (DWM) with alcalase, and gastrointestinal digestion of DWM and DWMH was simulated in vitro using pepsin and pancreatin. The peptide and free amino acid (FAA) contents, angiotensin-I-converting enzyme (ACE) inhibitory activity, 2,2-Diphenyl-1-picrylhydrazyl (DPPH) radical-scavenging activity and molecular weight distributions of DWM, DWMH and their gastric and gastrointestinal digestive fluids were compared. Results showed that DWM could be well digested. High peptide content (21.66 mg/mL) with MW < 3000 Da and more FAAs (8.09 mg/mL) were observed in DWM digests. In addition, DWM digests had high ACE inhibitory activity (42.9%) and DPPH radical-scavenging activity (62.58%), which showed no significant difference when compared with DWMH digests. The above results indicate that enzymatic hydrolysis seems unnecessary for the production of walnut peptides; at the least, hydrolysis with alcalase was unnecessary for producing peptides with significant ACE inhibitory and DPPH radical-scavenging activities.