Influence of sucrose and water content on molecular mobilityin starch-based glasses as assessed through structureand secondary relaxation

Abstract : Molecular mobility is known to be a key parameter in controlling the physical properties of materials and thus their quality and performance. Beyond glass transition related changes, attention should be called to the impact of local motions remaining in the glassy state. Gelatinized waxy maize starch at different sucrose contents (0–20% solids) was equilibrated between 0 and 14% water and sorption isotherms determined at 25°C. The effect of water and sucrose content on the molecular mobility of glassy starch was investigated by differential scanning calorimetry through enthalpy relaxation studies and dynamical mechanical thermal analysis. The existence of sucrose–starch interactions was suggested by the sorption isotherms not following the expected additivity of the single component sorption curves. Contrary to the glass transition or associated α relaxation, water and sucrose affected differently the secondary relaxations. Indeed, the β relaxation observed around −15°C was shifted to lower temperature upon increasing hydration, and to higher temperature when sucrose content increased, suggesting a hindering of these local motions. Enthalpy relaxation of the ternary mixtures was studied following aging up to 668 h at Tg −15°C. Ternary mixtures exhibited an enthalpy relaxation upon aging lower than starch alone as a sign of lower polymer mobility in the presence of small molecules, contrary to the free volume theory. Relaxation kinetics were characterized with the Cowie–Ferguson model and compared to literature data. The extent of the enthalpy relaxation appeared to be controlled by the distance between the aging temperature and the β relaxation temperature.