pH-responsive bilayer Indicator film based on Gelatin/polydextrose/curcumin/blueberry anthocyanins for monitoring the freshness of shrimp meat.

In this study, using gelatin and polydextrose as the matrix materials, a bilayer intelligent indicator film was fabricated by incorporating zinc oxide (ZnO) and a formulated combination of curcumin (Cur) and blueberry anthocyanins (BB) for real-time monitoring of food freshness. The outer layer (gelatin/polydextrose/ZnO) provides ultraviolet shielding, mechanical reinforcement, and moisture barrier functions. The inner layer (κ-carrageenan loaded with Cur-BB composite pigments) enables a broad pH-responsive (pH 2-13) high-contrast color transition (orange-red to yellow-brown), effectively addressing the limitations of single pigments, including low sensitivity in acidic environments and a narrow indication range. The results demonstrated that ZnO nanoparticles significantly enhanced the mechanical properties of the film (tensile strength >90 MPa), hydrophobicity (water contact angle >90°), and barrier performance (water vapor transmission rate < 550 g/m2·24 h). The composite pigments (Cur:BB = 1:2) exhibited high sensitivity toward ammonia and trimethylamine (TMA). Comparing different GP-ZnO film compositions, GP-ZnO-2:1 exhibited superior hydrophobicity and structural stability in refrigerated, high-humidity environments. Its ΔE), Standard RGB values (SRGB), as well as shrimp TVB-N and pH levels all showed good linear correlations, with correlation coefficients generally exceeding 0.93. During refrigerated storage of shrimp at 4 °C, the color transition of the films (green to gray-green) was consistent with changes in total volatile basic nitrogen (TVB-N ≥ 30 mg/100 g) and pH values. Combined with smartphone-based RGB analysis, this system achieves accurate assessment of spoilage status, providing a novel strategy for the application of intelligent packaging in perishable food monitoring.