Humidity stability of tapioca starch–pullulan composite films

Pure and composite films based on tapioca starch and pullulan were investigated in their stability against dry and humid storages (RH 23 and 85%). The films were prepared by casting aqueous dispersions of pullulan and starch mixtures at different ratios: 5% starch and 0–20% pullulan. For the pure pullulan film, more than 10% solids were required to produce a homogeneous film, whereas a rigid and homogenous film could be produced with starch even at 5% concentration. The pullulan film (20% solids) was highly susceptible to the exposure to the humid condition (RH 85%), whereas the composite films containing starch (5%) and pullulan (2–10%) exhibited improved stability with reduced moisture absorption and less deformation during the ambient storage at 85% RH for 2 weeks. As the starch incorporation increased, the films became more rigid showing increases in the tensile strength but decreases in the elongation at break. By using a proper composite ratio (5% starch, and 2 or 5% pullulan), the film could be optimally prepared to exhibit both mechanical strength and storage stability against humidity. However, the starch tended to decrease the water solubility of the composite films.