Ultrasonic assisted preparation of covalent bonding pea protein and polyphenol conjugate in emulsion delivery system

Protein modification can improve the bioavailability of polyphenols, while polyphenols can also improve the functional properties of pea protein. Four representative polyphenols (gallic acid, ferulic acid, catechin and kaempferol) were covalently combined with pea protein by ultrasonic assisted grafting (free radical grafting, laccase method and alkali treatment), and the structure and functional properties of the conjugated compounds were characterized. The interaction was evaluated by surface hydrophobicity, free sulfhydryl content determination and SDS-PAGE. Fluorescence spectra and Fourier transform infrared spectra revealed that polyphenol binding reduced the α-helix content of pea protein and changed its structure. In addition, the antioxidant activity of the conjugate was significantly improved compared to natural pea protein (DPPH^˙by 7.5 to 20.2 times, ABTS^˙+by 2.8 to 4.3 times, FRAP by 1.4 to 4.1 times). At the same time, the introduction of polyphenols also optimized the thermal stability of pea protein, and increased the denaturation temperature of pea protein (113.1 °C) by 0.19-6.63 °C. The emulsifying properties were also enhanced by covalent modification, and the emulsifying properties of the ultrasound-assisted conjugates were better. In summary, the pea protein-polyphenol conjugate provides a new solution for improving the bioavailability of polyphenols and preparing polyphenol delivery systems for complex emulsions.