Walnut phenolic compounds: Binding with proteins and antioxidant activities

Abstract: Walnut (Juglans regia L.) is rich in phenolic compounds (POHs) which show profound antioxidant capacity. But the extraction of POHs could not be completed as they usually interact with walnut proteins. In this study, different amount of urea or NaCl were added to walnut protein solutions in order to analyze the interactions between the POHs and proteins. Results showed that 77.7% total phenolic compounds and 34.6% total condensed tannins (at pH 3.0), 65.4% total phenolic compounds and 59.4% condensed tannins (at pH 8.0) bonding with proteins through various non-covalent bonds, among which hydrophobic bonds were predominant. Besides, the antioxidant activities of POHs were investigated. It showed that the POHs bonding with proteins through ionic bonds could scavenge 5.7% DPPH radical and chelate 5.9% Fe2+ at pH 3.0 while scavenge 10.9% DPPH radical and chelate 11.5% Fe2+ at pH 8.0, demonstrating the strongest antioxidant activities. In addition, the efficiency of 70% (v/v) ethanol and 60% (v/v) methanol in extracting POHs and condensed tannins from WPI at pH 3.0 and 8.0 were tested. Results showed the above two commonly used extractants could only slightly promote the extraction of POHs and were not efficient in extracting the POHs with higher antioxidant capacity.