Thermodynamics of water vapor sorption of fiber-reinforced starch films

The importance of adding cellulose fibers to starch films is that they are used as reinforcement, reduce the interaction of films with water, and enhance stability at low water activity levels compared to films based on starch only, offering new options as materials intended for food packaging. The aim of this work was to analyze the thermodynamic properties of films composed of a) Commelina starch and b) Commelina starch added with cellulose fibers. The sorption isotherms were fitted to the Guggenheim-Anderson-De Boer equation (GAB). The thermodynamic analysis allowed allocating the minimum integral entropy value in the 0.23-0.55 range of water activity in both types of film. The lowest range of water activity was observed in films of Commelina starch added with cellulose fibers. According to the compensation enthalpy-entropy theory, at low water activity values (0.1 to similar to 0.39), the water vapor sorption process was mainly driven by enthalpy, while at high values of water activity (similar to 0.39 to similar to 0.90) it was mainly controlled by entropy. The behavior of films with or without the addition of cellulose fibers could be akin to that of other non-ionic, hydrophilic biopolymers. This characterization is important for the handling and application of films intended for food packaging and storage.