Polyphenol-plasticized wheat gluten films via formic acid solubilization

Biopolymers like gluten are promising film-formers for sustainable food packaging, but brittleness and moisture sensitivity limit their use. Here, gluten was solubilized with formic acid (FA) and cast into films containing glycerol (10-20% w/w protein) and polyphenol-rich extracts (1-4% w/w protein), including green tea extract (GTE) and grape seed extracts of low (GSE-LP) and high polymerization (GSE-HP). At 10% glycerol, GSE-HP increased tensile strength ∼4×, whereas GTE better retained ductility. Polyphenol structure also decoupled moisture sorption from transport: water vapor permeability ranged 1.30-4.30 g·mm/m²·day·kPa, while water uptake remained low (3.5-6%). Fluorescence quenching and SDS-PAGE indicated predominantly non-covalent GTE-protein interactions vs stronger aggregate, covalent adduct formation with GSE, which explains the observed strength-flexibility trade-offs. Residual film formic acid was ≤20 mg/kg. Overall, polyphenol polymerization provides a handle to tune the strength-flexibility-hydration balance of FA-solubilized gluten films as a potential candidate for multilayer, bio-based packaging.