Composite films with properties improved by increasing the compatibility of sodium caseinate and zein in a heated 60% ethanol solvent

To improve the mechanical and barrier properties of zein films, sodium caseinate (CN) was combined with zein to fabricate composite films, by achieving co-dispersion of the two proteins in a newly developed film-forming solvent mixture (60% v/v aqueous ethanol, at 65 °C). The synergistic interactions between CN and zein at various ratios in the films were evaluated. Incorporation of CN into zein-based films increased elongation at break, tensile strength and water contact angle, and decreased water vapor permeability. The film with a CN/zein ratio of 0.75:1 had the lowest water vapor permeability (2.53 ± 0.07 × 10−12 g/(m·s·Pa)), and that with a ratio of 1.25:1 had high elasticity, with an elongation at break 4.11 times that of a pure zein film. CN addition also increased the thermal stability of the composite films, such that films with a CN to zein ratio of 0.75:1 had the highest glass-transition temperature (105.3 °C). Infrared spectral analysis confirmed the high compatibility and strong interactions between CN and zein, and scanning electron microscopy indicated that the microstructure of the composite films was more compact and homogeneous than pure zein films. However, atomic force microscopy revealed that the composite films had a rougher surface. This study provides new perspectives for the development and application of protein-based films as food packaging materials.