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Edible films and coatings are developed to improve quality and to extend shelf-life of food products. In this study, soy protein isolate was obtained from defatted soy flour by alkaline extraction and acid precipitation. Then edible films were prepared from soy protein isolate. The effects of protein concentrations and two plasticizers of glycerol and sorbitol on some physical and mechanical properties of the films were evaluated, using response surface methodology (RSM). The results showed that  tensile strength, opacity, thickness and water vapor transmission rate (WVTR) were increased, while elongation of the films decreased by protein concentration. Increasing the concentration of glycerol and sorbitol increased WVTR, elongation, total color difference (ΔE), thickness and opacity, while declined tensile strength of the films. The model developed by RSM for the physical and mechanical properties of the films had high coefficient of multiple determination (R² >0.87) and significant F values. The models suggested that the concentration of soy protein and the type and concentrations of the plasticizers are the most influential factors with significant effects on the properties of soy protein isolate films.

Keywords: Edible film, Soy protein isolate, glycerol, Sorbitol, Response surface method

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