Water vapor permeability of wheat gluten and soy protein isolate films

Abstract

Protein films were produced from wheat gluten and soy protein isolate. The water vapor permeability of the films was evaluated at 5, 15, 25 and 35°C and at 100-50% and 100-70% relative humidity gradients across the films. For all tested temperatures, estimated water vapor permeability constant values were greater at the 100-70% relative humidity gradient than at the 100-50% relative humidity gradient indicating a pressure dependence of the permeability constant. Similar behavior has been documented in the literature for other types of plastic and biopolymer hydrophilic films. Most likely, water sorption by the hydrophilic protein films was higher when the outer film surface was exposed to 70% rather than to 50% relative humidity. Increased sorbed water plasticized protein films facilitating water vapor permeation and yielding greater water vapor permeability constant values. At both tested relative humidity gradients over the studied temperature range, the water vapor permeability constant of the two protein films decreased with increasing temperature. This can also be explained by increases in water sorption and film plasticization, since water sorption by proteins is thermodynamically favored at lower temperatures.