pH-responsive color indicator film based on gelatin/chitosan cross-linking with anthocyanin-Fe2+ chelate for pork freshness monitoring

In this study, the film matrix was formed through polycondensation reactions between citric acid (CA) as a cross-linking agent and bovine bone gelatin (BG) and chitosan (CS). Using blueberry anthocyanin-iron ion chelate (BA-Fe2+) as a pH-sensitive indicator, we aimed to develop a highly sensitive indicator film to monitor the freshness of pork effectively. The study found that adding 50% CS and 7.5% CA maximizes BG's mechanical strength and transmittance. In contrast, chelation with 4 mg/mL blueberry anthocyanin (BA) and 0.05 mg/mL FeSO4·7H2O significantly increases the film's sensitivity to ammonia vapor from 47.23% to 64.49%, without compromising its film-forming ability. Fourier Transform Infrared Spectroscopy and X-ray Diffraction analyses confirmed that the BA-Fe2+ chelate was successfully incorporated into the crosslinked matrix without disrupting its inherent structure. This integration was achieved through hydrogen bonding between the hydroxyl groups of the chelate and the hydroxyl, amino, and ester bonds of the crosslinked matrix. Using scanning electron microscopy and atomic force microscopy, the analysis revealed that the surface of the crosslinked matrix exhibits the advantages of being both dense and highly porous. The colorimetric parameters of the film in different pH buffer solutions indicate that the BA-Fe2+ films generally exhibit greater color differences, making them more easily discernible by the naked eye compared to the films with BA alone. Additionally, the film's thermal stability, barrier properties, antioxidant and antimicrobial activities, and the controlled release capability of BA, have all been significantly enhanced (p