Humidity stability of tapioca starchepullulan 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 0e20% 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 (2e10%) 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.