MEASUREMENT ERRORS IN WATER-VAPOR PERMEABILITY OF HIGHLY PERMEABLE, HYDROPHILIC EDIBLE FILMS

Water vapor transmission WVT of films is commonly measured using modif ications of the ASTM E 96 Standard Method ('cup method'). A stagnant a ir layer exists between the underside of the film mounted on the cup a nd the surface of the desiccant, saturated salt solution or distilled water contained in the cup. The method considers the air gap resistanc e to be negligible to water vapor transport. When high water vapor-tra nsmitting hydrophilic edible films are measured with the cup method, t he resistance of the stagnant air layer can be significant and, if neg lected, can lead to underestimation of water vapor transmission rates. Equations were presented in this study to correct WVT data for the ai r gap resistance. For both a methylcellulose and a com zein film, wate r vapor permeabilities measured with air gaps of 1.0 and 1.5 cm were s tatistically significantly (alpha = 0.05) different. Values corrected to account for air gap resistance were not statistically significantly (alpha = 0.05) different. Literature data on water vapor permeability of other hydrophilic edible films were corrected to account for the a ir layer resistance. Underestimation of actual values ranged between 5 and 46%.