Fabrication and characterization of biodegradable anthocyanin-infused bilayer packaging film for real-time colorimetric detection of fish spoilage.

In response to increasing global concerns regarding food safety and plastic pollution, this study presents the fabrication and characterization of a novel biodegradable bilayer film that simultaneously amalgamates two important packaging functionalities. Unlike conventional single-layer films that focus solely on preservation or spoilage indication, this bilayer system combines two specialized and coactive layers, which exhibit intelligent and active packaging capabilities. While the inner layer, composed of a κ-carrageenan/pectin (Κ-C/P) matrix incorporated with red cabbage anthocyanin (RCA), offers real-time pH-responsive colorimetric indication of food freshness, the outer layer, formulated from bio-based polyurethane (PU) synthesized from mahua oil-derived polyol, chitosan (CS), and nanoclay (5NC), imparts antimicrobial activity, enhanced barrier properties, and UV protection. This has created a unique packaging system that effectively augments both food quality monitoring and preservation, making it particularly suitable for highly perishable food items, such as fish. ATR-IR analysis of the films confirmed strong H-bonding, which accounted for the improved mechanical properties, reduced water vapor permeability, and lowered oxygen transmission. The inner layer (Κ-C/P/RCA) facilitated distinct color transitions - from pink to colorless- within 12 h in response to spoilage-induced pH changes. Meanwhile, the outer layer (PU/CS/5NC) exhibited strong hydrophobicity of 92.2° and demonstrated excellent performance in terms of UV shielding, antimicrobial efficacy against Escherichia coli and Staphylococcus aureus, and biodegradation up to 70 % in 18 days. The outer film showed a water vapor transmission rate (WVTR) of 146 g/m2/day, an oxygen transmission rate (OTR) of 1879.95 cm3/m2/day, a moisture content of 16.3 %, and a swelling capacity of 1.44 %, all of which attest to the superior barrier properties of the film. Overall, the synergistic integration of colorimetric indication and preservation functions, positions this bilayer film as a multifunctional, biodegradable packaging material ideal for real-time application in smart food packaging systems.