Physicochemical properties of edible films formed by self-stratification of zein and hydroxy propyl methyl cellulose
Department of Food Science, University of Guelph, 50 Stone Road East, Guelph, Ontario, Canada N1G 2W1Self-stratification during film-forming is an innovative and effective method for forming multi-layer films in a one-step formulation. The phenomenon overcomes the drawbacks of preparing multi-layer film systems by applying different layers consecutively. The incompatibility of polymers in the coat-forming solution is a prerequisite to induce phase separation that drives the self-stratification process. To produce a homogeneous solution, two incompatible polymers are dissolved in a common solvent or in a mixture of several solvents, and then phase separation can occur as the solvent evaporates. In the present study, two incompatible polymers, i.e., zein and hydroxy propyl methyl cellulose (HPMC) were dissolved in 70% (w/w) aqueous ethanol at 5% (w/w) total polymer concentration at different ratios (0:5, 1:4, 2:3, 1:1, 3:2 zein: HPMC). The solutions were cast and dried at 20 ℃ and 75% relative humidity to obtain films of various morphology and physiochemical properties. The cross-section of pure HPMC film, as revealed in scanning electron microscopy (SEM), displayed a uniform and smooth morphologies. With the addition of zein, submicron zein particles were formed in the film. The average size of the particles increased from 497 nm to 1222 nm as zein content increased from 20% to 60%. The spherical particles were homogeneously distributed in the film. Attenuated total reflectance- Fourier transform infrared spectroscopy showed the characteristic absorbance peaks of zein and HPMC, with no new absorbance peak nor frequency shift in the composite films. The addition of zein did not cause significant changes (p > 0.05) in the average film thickness, which ranged between 22.40 μm (HPMC) and 23.30 μm (2:3 zein: HPMC). Pure HPMC film presented the highest water vapor permeability (1.89 10-5 g·mm/m2·h·kPa), which decreased to from 1.7910-5 to 1.4510-5 g·mm/m2·h·k·Pa as zein content increased from 20% to 60%. The opacity of HPMC film was 1.61 mm-1, which increased significantly (p < 0.05) as zein content increased. The pure HPMC films showed the lowest value of (0.113), which increased significantly (p < 0.05) with increasing zein content. The tensile stress decreased significantly (p < 0.05) from 62.35 to 33.65 MPa, while the elongation at break decreased from 14.88% to 1.88%, for HPMC and 3:2 zein: HPMC films, respectively. Overall, SEM analysis confirmed the existence of type-III self-stratification during the formation of the composite films, and intermolecular interaction was not detected between zein and HPMC after phase separation during the drying process.