Moisture sorption isotherm and isosteric heat of sorption characteristics of starch based edible films containing antimicrobial preservative

Moisture sorption isotherms (MSI) of antimicrobial (benzoic acid, potassium sorbate, and sodium propionate) incorporated cornstarch based self-supporting edible films have been determined at 25, 35, and 45°C following isopiestic vapour transfer technique. Antimicrobials show positive treatment effect on the equilibrium moisture content. The MSI of these films are best described with modified Oswin model among the three models (GAB, modified Oswin and Peleg) attempted. MSI at 25°C but at 35 and 45°C deviates more from that of control (no antimicrobial) - showing lower water activity with benzoic acid incorporated films. While the binding sites on the film has been found to be heterogeneous, temperature shows direct effect on moisture sorption with potassium sorbate and sodium propionate. The net isosteric heat of sorption decreases exponentially as the film moisture content increases, and at lower moisture level it follows the order: potassium sorbate sodium propionate benzoic acid control.