RELATIVE-HUMIDITY AND TEMPERATURE EFFECTS ON TENSILE-STRENGTH OF EDIBLE PROTEIN AND CELLULOSE ETHER FILMS

The effect of relative humidity and temperature on tensile strength of two types of protein-based [corn zein (CZ) and wheat gluten (WC)] and two types of cellulosic [methylcellulose (MC) and hydroxypropyl cellulose (HPC)] hydrophilic edible films was investigated. A central composite response surface design was used. Studied ranges of relative humidity and temperature were 23 to 75% and 5 to 45 degrees C, respectively. For all four types of films, tensile strength (TS) decreased with relative humidity and increased with temperature. Ranges of mean tensile strength values among the nine different combinations of the two variables were 5.7 to 23.6 MPa, 2.7 to 21.4 MPa, 61.9 to 104.4 MPa, and 11.1 to 35.0 MPa for CZ, WG, MC, and HPC, respectively. A second-order polynomial model was fitted to the data with least squares regression. A regression model linear in relative humidity and quadratic in temperature showed a very good fit to tensile strength data of CZ (R(2) = 0.93) and MC (R(2) = 0.98) films. A regression equation linear with respect to both relative humidity and temperature satisfactorily fitted (R(2) = 0.75) TS data of HPC films. A best fitted model for TS data of WG films, that included relative humidify and temperature, the square of temperature, and the cross-product of the two variables, had a poor fit (R(2) = 0.67).