Structure and thermomechanical properties of extruded amylopectin–sucrose systems

The degree of heterogeneity in three non-expanded amylopectin–sucrose (88:12, 68:32 and 57:43) systems prepared by twin-screw extrusion was studied using FTIR microspectroscopy. The results obtained with a spatial resolution of 30 μm suggested a fluctuation in the compositions of these blends. As expected, the heterogeneous character of these blends was reflected in their thermomechanical behaviour as studied by DMTA. The thermograms of the blends containing 27–29% water (db) exhibited typically four transitions (tan δ peaks) which were assigned to a β-relaxation process, T1 (−44 to −24 °C), a glass-transition of a sucrose-rich phase, T2 (−20 to −6 °C), an ice-melting transition of the sugar-rich phase, T3 (−10 to −2 °C) and a glass-transition of an amylopectin-rich phase, T4 (3 to 22 °C) depending on the blends composition. The reduction of the water content by dehydration of these blends down to 9–13% (db) resulted in the expected disappearance of the ice-melting transition and the shift of the temperatures of the glass–rubber transitions to 11–23 °C for the sucrose-rich phase (T2) and to 58–87 °C for the amylopectin-rich phase (T4). Wide angle X-ray diffraction was used to monitor the degree of order particularly to verify the amorphous state of sucrose in the blends over the sugar and water content ranges of interest.