Interactive plasticizing–antiplasticizing effects of water and glycerol on the tensile properties of tapioca starch films

The effects of water and glycerol as diluents on physical properties of glassy tapioca starch films were studied. Water and glycerol depressed the calorimetric glass transition temperature (Tg) of the tapioca starch films. In glycerated and non-glycerated starch films, the tensile modulus (E) was plasticized by moisture. Glycerol acted as an antiplasticizer on film modulus when present at a low concentration (2.5%) in drier films (aw≤0.22). Both strain-at-break and toughness of starch films increased to a maximum as aw was increased to a critical level (0.22–0.32) before decreasing with further humidification. In contrast, minima in those two properties were observed in films containing 10–15% glycerol at aw=0.11 and 0.22. The presence of small amounts of glycerol was able to exert classic antiplasticizing effects on possibly all tensile properties studied, but only in relatively drier systems. On the other hand, antiplasticization by water manifested itself only in mechanical properties determined at high deformation (i.e. tensile strength and toughness).