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.