Effect of bound water on thermal behaviors of native starch, amylose and amylopectin

Thermal behavior of dry native starch, amylose and amylopectin originating from different plants was investigated using Thermogravimetry (TG) and Differential Scanning Calorimetry (DSC). The decomposition temperature increased as the molecular weight increased and a sudden increase was observed at a molecular weight of 50,000 that can be the critical value. DSC analyses verified that the amorphous phase in the specimens was swollen by bound water within a temperature range of 323-428 K. The change in swelling enthalpy divided by the peak temperature or swelling depended linearly on the volume fraction of bound water, regardless of the origin and the composition or purity of the specimens. Furthermore, bound water depressed the melting points of crystallites in the specimens, which agreed well with the prediction of Flory's lattice theory.