Ca²⁺/Zn²⁺-dual-crosslinked Gelatin/poly(γ-glutamic acid)–pullulan/alginate Bilayer Films with Terminalia chebula Extract: Mechanical, Barrier, Migration, UV Protection and Active Packaging Properties for Extending the Cold-storage Quality of Spinach and Cherry Tomatoes

The aim of this study was to develop and characterize active bilayer films based on a gelatin/poly(γ-glutamic acid)–pullulan/alginate system, modified by Ca²⁺/Zn²⁺ ionic crosslinking and the addition of Terminalia chebula extract. The chemical structure, morphology, mechanical properties, swelling behavior, barrier properties, optical and antioxidant properties, migration, and material stability were evaluated. The most favorable physicochemical properties were obtained for the CaZnF film (containing CaCl₂ and ZnCl₂). This film showed a tensile strength of 30.50 ± 0.76 MPa, an elongation at break of 29.26 ± 0.82%, and the lowest swelling degree of 1464 ± 5%. It also exhibited the lowest oxygen permeability, at 0.184 ± 0.013 cm³(STP)·m⁻²·d⁻¹·0.1 MPa⁻¹, and a water vapor transmission rate (WVTR) of 228.7 ± 13.5 g·m⁻²·d⁻¹. The addition of T. chebula extract increased antioxidant activity and UV protection. Ca²⁺/Zn²⁺ crosslinking also improved the optical stability of the extract-containing film, reducing ΔE* from 15.74 to 3.90. The lowest total migration was observed for the ECaZnF film (containing T. chebula extract and crosslinked with CaCl₂ and ZnCl₂), at 51.8 mg·dm⁻². Storage tests showed that the films reduced mass loss in fresh products, particularly in spinach. For spinach, the CaF film (crosslinked with CaCl₂) reduced mass loss by approximately 41% compared with the unpackaged control after 22 days. The results indicate that the combination of bilayer architecture, Ca²⁺/Zn²⁺ crosslinking, and T. chebula extract enables the development of multifunctional materials for the active packaging of fresh plant products.