Mechanisms underlying the effect of walnut pellicle extracts and its four representative polyphenols on in vitro digestion of walnut protein isolate

Walnut pellicle extracts (WPE) is rich in phenolic acids. The effects of WPE and its monomeric phenolics (ellagic acid (EA), (+)-catechin (CA), chlorogenic acid (CLA), and epigallocatechin gallate (EGCG)) on in vitro digestion of walnut protein isolate (WPI) were studied. The underlying mechanisms on WPI enzymatic digestion through WPE and pure phenolic compounds with digestive enzymes (pepsin and trypsin) therein using Michaelis-Menten kinetics, fluorescence quenching, molecules docking, and surface plasmon resonance (SPR). The in vitro digestibility of WPI decreased significantly by the addition of WPE. WPE had the strong inhibitory effect on WPI digestion and pepsin and trypsin activities. The effects of polyphenols on WPI digestibility were mainly manifested in the interaction between polyphenols and digestive enzymes. SPR as well as fluorescence quenching indicated that WPE bound to pepsin and trypsin was the strongest. Among the four phenolic compounds abundant in the tested WPE, EGCG inhibited pepsin and trypsin more strongly than the other three compounds. SPR showed that EGCG was bound more tightly to pepsin and trypsin than CLA, CA, and EA. In vitro WPI digestion was strongly inhibited by WPE, mainly due to its high content of EGCG, CLA, CA, and EA causing strong inhibition of pepsin and trypsin. The study investigated the influence of plant protein by phenolic acids in the human digestive system and providing a theoretical basis for the processing and manufacturing of polyphenol-rich nut-based functional foods.