Mechanical and barrier properties of beta-lactoglobulin (beta-Lg) and whey protein isolate (WPI) films were studied using sorbitol(S) as a plasticizer. The films were cast from heated aqueous solutions and dried in a climate chamber at 23 degrees C and 50% relative humidity for 16 h. The multivariate analysis used has proved to be a valuable tool for evaluating and quantifying the influences of the variables in the specified experimental domain. Two identical factorial designs were applied to evaluate the influence of the concentration of beta-Lg and WPI, the concentration of S, and the pH. The two materials, beta-Lg and WPI, show similar results, which can be attributed to the dominating protein beta-lactoglobulin. At pH 9, Young's modulus and stress at break are not affected when the concentration of beta-Lg, WPI, or S varies. At pH 7 and 8, Young's modulus and stress at break increase when the concentration of beta-Lg and WPI increases, and they decrease when the concentration of S increases. Strain at break increases when pH increases from 7 to 9, a more pronounced effect being observed for the WPI films. Water vapor permeability (WVP) decreases and increases for pH 7 and 9, respectively, as the concentration of beta-Lg and WPI increases. This contrast in behavior at different pH values is probably due to a structural difference that occurs above pH 8. Moisture content and WVP increase when S increases. Here a clear distinction can be observed between the two film materials: the beta-Lg films show higher values for both moisture content and WVP measurements.
