Thin carboxymethyl cellulose-shellac bilayer films with asymmetric water vapor transport for grape preservation

Thin (~30 μm) bilayer films were fabricated via layer-by-layer casting by varying the relative thicknesses of a hydrophilic carboxymethyl cellulose (CMC) inner layer and a hydrophobic shellac (SHL) outer layer, aiming to achieve direction-dependent water vapor transport and grape preservation. Structural analyses (FT-IR, XRD, SAXS, SEM) confirmed distinct bilayer morphology with a dense, pore-free microstructure and good interfacial adhesion. The CMC and SHL layers retained their characteristic structural features, suggesting limited interlayer diffusion stabilized by hydrogen bonding and van der Waals forces. Increasing SHL layer thickness reduced mechanical strength (from 27.49 ± 0.87 MPa to 3.74 ± 0.63 MPa) and elongation at break (from 39.92 ± 2.99% to 2.53 ± 1.55%). The optimal bilayer (12 μm CMC/18 μm SHL) exhibited low water vapor permeability (1.27 ± 0.04 × 10^-11 g·m·m^-2·s^-1·Pa^-1) and reduced gas permeability (O₂: 7.74 ± 0.22 × 10^-13 g·m·m^-2·s^-1·Pa^-1; CO₂: 2.91 ± 0.74 × 10^-12 g·m·m^-2·s^-1·Pa^-1). Soil burial tests demonstrated rapid weight loss and disintegration within 20 days. Applied to grape preservation, the CMC-SHL bilayer film reduced weight loss, maintained firmness, and minimized nutrient loss, showing comparable performance to polyethylene film. These findings demonstrate that CMC-SHL bilayer films are promising eco-friendly packaging materials with tunable barrier properties and effective postharvest preservation capability.