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 (Tg) and the crystallinity index, respectively. The water content, crystallization and thermal properties relationship enabled the identification of three zones in the Tg–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 Tg 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 Tg. 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.