Physicochemical and bio-functional properties of walnut proteins as affected by trypsin-mediated hydrolysis

The walnut proteins were trypsin-hydrolyzed for 0, 15, 30, 60 and 120 min and the effects of hydrolysis on the structural, functional, antioxidant, bioactive-loading, and angiotensin converting enzyme (ACE)-inhibitory properties of walnut protein were studied. The trypsin-mediated hydrolysis of walnut proteins was monitored using sodium dodecyl sulfate-polyacrylamide gel electrophoresis and an O-phthaldialdehyde spectrophotometric assay. The unfolding of walnut protein structure during the hydrolysis process was confirmed using intrinsic fluorescence spectroscopy which increased the surface hydrophobicity of protein. The solubility of walnut protein was increased from 17 to 63% after 2 h of trypsin-mediated hydrolysis. The walnut protein hydrolysates showed higher surface activity compared to the native non-hydrolyzed protein, which significantly improved their foaming and emulsifying properties. The hydrolysates also showed significant enhanced antioxidant activity, namely, ABTS radical quenching activity and reducing power due to the release of active antioxidant sequences during the enzymatic hydrolysis. The ability of walnut proteins to inhibit the activity of ACE and their capacity to load hydrophobic bioactive molecules including curcumin and quercetin were also considerably improved by treating with trypsin. The results of this study suggested that the trypsin-mediated hydrolysis could be used as an efficient technology to produce bioactive multi-functional hydrolysates from walnut proteins.