Biodegradable films of chitosan and tea polyphenols catalyzed by laccase and their physical and antioxidant activities

The effects of laccase crosslinking chitosan (CS) and tea polyphenols (TP) on the structure, physical and antioxidant properties of CS-based biodegradable films were investigated in this study. Laccase crosslinking significantly increased the particle size and zeta-potential of CS + TP particles. A simple blending of CS and TP decreased the elongation at break significantly, while the elongation at break of biodegradable films significantly increased after laccase crosslinking. Furthermore, TP addition increased the tensile strength (TS) of CS based films, while laccase crosslinking promoted the increase, making it more suitable for application. The addition of TP significantly changed the CS-based films from white and transparent to light brown. NMR spectra showed that no C=N bond was observed in CS + TP particles, which could result from the inhibition of Schiff base reaction due to the big steric hindrance of TP. Right shift of amide I peak to lower wavelength in laccase catalyzed films indicated the conversion of some primary amines into secondary amines caused by Michael addition reaction. It was proposed that laccase oxidized polyphenol to o-quinones, which reacted with nucleophilic groups of chitosan, leading to the increase of secondary amines. Furthermore, the antioxidant capacity of CS + TP films crosslinked by laccase increased significantly compared with CS and CS + TP blended films. In summary, laccase catalysis could be applied to manufacture biodegradable films with improved mechanical property and antioxidant activity.