Leather waste collagen/purple cabbage composite film: a pH indicator for real-time meat freshness assessment

Real-time monitoring of meat freshness is essential for ensuring food safety and reducing waste; however, conventional methods are often complex and environmentally unsustainable. In this study, a fully biodegradable smart indicator film was developed by incorporating purple cabbage powder (PCP) into a collagen matrix extracted from leather waste. The film was covalently crosslinked with microbial transglutaminase (TG) and physically reinforced with κ-carrageenan (κC) to enable rapid pH-responsive color changes. Increasing the PCP content reduced the tensile strength and elongation at break, but enhanced surface hydrophilicity, thereby facilitating the rapid pH response of the film. The film exhibited a visible color transition from red to green in the pH range of 2-8, and when applied to pork stored at 25 °C, it underwent a distinct color change as the total volatile basic nitrogen (TVB-N) content-a key indicator of protein decomposition-reached the spoilage threshold of 15.21 mg/100 g. This work provides a sustainable strategy for the valorization of leather solid waste while offering an effective and eco-friendly solution for real-time monitoring of meat.