Chitosan–fucoidan encapsulating cinnamaldehyde composite coating films: Preparation, pH-responsive release, antibacterial activity and preservation for litchi

In this study, an edible composite film with pH-responsive release was prepared by the formation of Schiff-base imine bonds between chitosan (CS) and oxidized fucoidan (CS–FU) and encapsulating cinnamaldehyde (CA). Fourier-transform infrared, 1 H nuclear magnetic resonance, X-ray photoelectron spectroscopy and gel permeation chromatography confirmed the formation of CS–FU. The result showed that, oxidation degree of FU, degrees of substitution, average molecular weight and yield of CS–FU were 25.57 %, 10.48 %, 23.3094 kDa and 45.63 ± 0.64 %, respectively. Scanning electron microscopy revealed that CA was encapsulated within the CS–FU matrix. Increasing the CA content could improve the mechanical properties and ultraviolet and visible-light resistances of the CS–FU coating films but enhance their water vapor permeabilities. The release of CA increased as the pH decreased, and the antibacterial rate at pH 5 was 2.3-fold higher than that at pH 7, indicating good pH-responsive release and antibacterial properties in mildly acidic environments. Owing to their excellent properties, the CA/CS–FU-0.1 coating films maintained the appearance and quality indices of litchis for at least eight days. Hence, multifunctional composite coating films are prospective eco-friendly and intelligently responsive controlled-release packaging materials for fruit preservation.