Effect of water uptake on amorphous inulin properties

Physical property changes of amorphous spray-dried inulin were investigated during water uptake at 20 °C. Modulated Differential Scanning Calorimetry (MDSC) and Wide Angle X-ray Scattering (WAXS) were used to investigate the evolution of the glass transition temperature (T_g) and the crystallinity index, respectively. The water content, crystallization and thermal properties relationship enabled the identification of three zones in the T_g–water content state diagram. Zone I delimited inulin in a glassy amorphous state, while zone II characterized inulin in a liquid amorphous state. Inulin crystallized and caked when T_g was below the storage temperature of 20 °C, but crystallization (zone III) was not spontaneous and was delayed by the defined zone II. The crystallization led to thermograms with an endotherm close to T_g. Temperature-Resolved WAXS allowed to correctly ascertain the MDSC endothermic peak as a melting peak because the crystallinity index was maximal at onset temperature of the transition, and dropped to zero at the endset temperature.