Surface modification of sodium caseinate films by zein coatings

The objective of this study was to improve the barrier ability of sodium caseinate (SC) films using corn zein via direct coatings and structural inversion approach. The effects of zein coatings on barrier properties (oxygen and water vapor) and surface properties (surface hydrophobicity, surface free energy and surface topography) of the films were extensively investigated. Zein coating via the structural inversion approach heightened surface hydrophobicity of SC films, and polar parameter (γAB) of surface free energy (γ) decreased from 11.5 to 3.4 mJ/m2, whilst apolar parameter (γLW) of γ increased from 25.3 to 32.1 mJ/m2. In contrast, direct coating of zein presented an opposite pattern. Zein coatings increased surface roughness of SC films, structural inversion approach led to apparent surface irregularities with high irregular projections, whereas the well-defined zein nanospheres were evenly distributed in film surface via direct depositing. Zein coatings produced via structural inversion approach enhanced the barrier of SC films against water vapor and oxygen, whereas the direct coating approach only decreased oxygen permeability (OP) of the films. The present data pointed to a possible arrangement and orientation of hydrophilic and hydrophobic moieties of zein on surface of SC films under different conditions. A schematic illustration of the formation pathway of zein coatings on SC films was proposed to relate it with barrier and surface properties of the films.