Physicochemical properties of zein-based edible films and coatings for extending wheat bread shelf life

The physicochemical properties of edible films from aqueous-ethanol dispersions of zein and zein-sunflower oil (10 and 25% dry basis) emulsions were investigated. Incorporation of sunflower oil in zein-based films decreased the water vapor permeability, equilibrium moisture contents and monolayer moisture values, estimated by the BET and GAB sorption-isotherm models. Glass transition temperatures of the composite zein films, determined by Differential Scanning Calorimetry (DSC) over a moisture content range, revealed a plasticization effect of sunflower oil on the films. Moreover, large deformation mechanical tests revealed a large decrease in tensile Young's modulus and strength with increasing level of sunflower oil in zein films. Breads coated by the zein-based coatings, using either brushing or spraying, following storage (4-days, 25 °C), exhibited retardation in moisture migration from crumb to crust compared to their uncoated counterparts. Additionally, products with zein coatings containing 25% sunflower oil showed a lower rate and extent of crumb staling compared to uncoated breads as evidenced by Texture Profile Analysis and DSC measurements of starch retrogradation. Starch re-ordering was less pronounced for breads coated by brushing which also had increased β-sheets and aggregate structural elements in the crumb protein matrix as probed by FTIR spectroscopy, suggesting dehydration due to ethanol vapor diffusion. These modifications were also confirmed by sensory evaluation which revealed a better crumb texture after 4 days storage for breads coated by both methods than their uncoated counterparts, with those coated by spraying being most preferable by assessors as they did not exhibit an ethanol- and other off-flavors.