Oxidised tannic acid as a bio-derived crosslinker for densifying amorphous tuna protein films: a chemistry–structure–property approach

Tuna-processing by-products represent an abundant yet underutilised resource for sustainable biomaterials. This study introduces oxidised tannic acid (OTA) as a bio-derived crosslinker to reinforce protein-based films from tuna myofibrillar proteins and bone gelatin, offering a novel strategy to valorise fish-processing waste and improve film performance. OTA, produced by alkaline oxidation, was incorporated during film formation to maximise interchain coupling. Mechanical, Fourier Transform Infrared Spectroscopy (FTIR), X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), and barrier/hydration analyses linked chemistry and microstructure to performance. OTA increased tensile strength and lowered extensibility across matrices; in bone-gelatin films, strength rose from 6.30 ± 0.42 to 8.23 ± 0.62 N mm−2, while elongation fell from 85.16% ± 7.26% to 58.21% ± 2.39%. FTIR showed red-shifted –OH/–NH bands and intensified C=O/C–O signals, indicating stronger hydrogen bonding and quinone-mediated covalent linkages. XRD revealed tighter amorphous halos (15° to 25° 2θ) without emergent crystallinity, consistent with denser short-range packing. SEM showed more cohesive, less porous surfaces. Barrier/hydration properties improved in parallel (water vapour permeability decreased ~18% for MI-OTA and ~27% for BG-OTA; moisture content and water absorption also declined). Overall, OTA is an effective bio-derived crosslinker that densifies amorphous tuna protein networks, yielding stronger, less extensible, and more water-resistant films for sustainable packaging.