Roles of proanthocyanidins and stearic acid in modulating interfacial structure, physicochemical properties, and controlled release behavior of gelatin composite films.

Gelatin films have inherently high hydrophilicity and show insufficient antioxidant capacity, which greatly limit their practical applications in active food packaging. Herein, gelatin-based composite films were fabricated by incorporating a fixed concentration of proanthocyanidins (0.1%, w/v) and varying concentrations of stearic acid (0.25%-1.25%, w/v). The interfacial interactions, physicochemical characteristics, and functional properties of all film samples were systematically investigated. The results indicated that films containing both proanthocyanidins and stearic acid exhibited strong ultraviolet‑blocking ability, enhanced surface hydrophobicity (water contact angle > 100°), and reduced water vapor permeability and water solubility. Proanthocyanidins endowed the matrix with strong antioxidant activity (84% for 2,2-Diphenyl-1-picrylhydrazyl and >99% for 2,2'-azino-bis (3-ethylbenzothiazoline-6-sulfonic acid)) and simultaneously enhanced tensile strength, whereas stearic acid acted as an effective plasticizer to increase film flexibility. Kinetic analysis revealed that proanthocyanidin release followed Fickian diffusion, with the release rate modulated by stearic acid in a concentration‑dependent manner. In a fresh‑cut apple preservation assay, the optimized films containing 0.75%-1.0% stearic acid effectively delayed surface browning, suppressed polyphenol oxidase activity, and reduced weight loss, confirming potential of these films as active packaging materials. This combined strategy effectively modulated interfacial structure and functional properties, offering a promising route for developing bio-based packaging materials.