An anthocyanin-grafted chitin film with enhanced stability prepared via Mannich multicomponent reaction for food freshness monitoring.

Anthocyanins are widely used as natural indicators for food freshness monitoring. However, their practical applications are limited by insufficient stability. In this study, a sustainable chitin-based intelligent indicator film was developed by covalently grafting anthocyanins onto chitin matrix via an in situ Mannich multicomponent reaction. The grafting behavior and structural properties of the resulting chitin-anthocyanin (Ch-A7) film were systematically investigated. The results demonstrated that anthocyanins were successfully immobilized onto the chitin matrix, and the apparent grafting degree varied with the reaction conditions. The Ch-A7 film exhibited distinct color responses over a wide pH range and showed high sensitivity toward ammonia with a detection limit of 0.2 ppm. Compared with the chitin loaded anthocyanin (ChLA) film, Ch-A7 demonstrated improved thermal durability, photostability, long-term storage stability, reversible responsiveness, and leaching resistance, consistent with stronger retention of anthocyanin-derived components within the chitin matrix. In addition, three distinct color states corresponding to different shrimp freshness levels were observed, enabling visual freshness evaluation. This study provides an effective strategy for improving the stability of anthocyanin-based freshness indicator labels and promotes the high-value utilization of chitin-derived biomass materials.