Grafting natural pigments onto chitosan via Mannich reaction for development of pH-responsive intelligent food packaging films

This study aimed to enhance the solubility and functional applicability of natural pigments in chitosan-based polymer matrices. Curcumin (CUR) and alizarin (AL), two natural pigments, were grafted onto chitosan via the Mannich reaction, producing modified chitosan pigments with enhanced solubility under acidic conditions, providing a potential strategy to overcome the low water solubility of these pigments. The successful grafting was confirmed through 1 H NMR, FTIR, UV-Vis, XPS, and XRD. The modified pigments exhibited notable antioxidant activity, with DPPH radical scavenging rates of 72.13% for CS-g-CUR and 64.42% for CS-g-AL, demonstrating potential for multifunctional applications. Subsequently, chitosan/gelatin composite films incorporating the grafted pigments were fabricated via solvent casting. Characterization and molecular docking analyses revealed strengthened intermolecular interactions, particularly hydrogen bonding between the grafted pigment moieties and the amino and hydroxyl groups of chitosan, leading to a more compact and stable polymer network. Consequently, the films exhibited improved mechanical performance (tensile strength up to 22.91 MPa) and enhanced thermal stability and compatibility. In addition, the composite films showed good biocompatibility (cell viability of 92.35%). The films also exhibited pH-responsive color changes, enabling visual monitoring of food freshness. Application tests on freshwater fish demonstrated that color changes of the films accurately reflected spoilage progression. Overall, this work provides an effective strategy for improving the dispersion and functional integration of natural pigments in biopolymer matrices, potentially expanding their applicability in intelligent food packaging and providing a simple approach for real-time food quality monitoring.