Measurement errors in water vapor permeability of highly permeable, hydrophilic edible films

Abstract

Water vapor transmission WVT of films is commonly measured using modifications of the ASTM E 96 Standard Method (‘cup method’). A stagnant air layer exists between the underside of the film mounted on the cup and the surface of the desiccant, saturated salt solution or distilled water contained in the cup. The method considers the air gap resistance to be negligible to water vapor transport. When high water vapor-transmitting hydrophilic edible films are measured with the cup method, the resistance of the stagnant air layer can be significant and, if neglected, can lead to underestimation of water vapor transmission rates. Equations were presented in this study to correct WVT data for the air gap resistance. For both a methylcellulose and a corn zein film, water vapor permeabilities measured with air gaps of 1·0 and 1·5 cm were statistically significantly ( α = 0·05) different. Values corrected to account for air gap resistance were not statistically significantly ( α = 0·05) different. Literature data on water vapor permeability of other hydrophilic edible films were corrected to account for the air layer resistance. Underestimation of actual values ranged between 5 and 46%.