Effect of sucrose on starch conversion and glass transition of nonexpandedmaize and wheat extrudates

Nonexpanded "half products" were prepared by twin-screw extrusion of maize and wheat of fine and coarse particle size in three levels of sucrose, 0, 1 0, and 20% db. The degree of starch conversion in the extrudates was determ ined using X-ray diffraction, differential scanning calorimetry, and rapid viscosity analysis. Starch conversion was greater in the fine material comp ared with the coarse material and greater for wheat compared with maize. Su gar addition decreased starch conversion in all cases, but the effect was g reater for maize compared with wheat and for the coarse material compared w ith the fine material. The thermal mechanical properties were studied by dy namic mechanical thermal analysis to determine the effect of sugar on the g lass transition temperature (T-g) in the four different materials. As expec ted, the mechanically determined T-g was reduced by sugar addition. Water p lasticized wheat semolina less than the other three materials. It was sugge sted that this was because the extruded semolina was entirely amorphous, wh ereas X-ray analysis showed some crystallinity in the other three materials . Die swell was much less for maize grits possibly because elasticity decre ased with decreasing starch conversion. The implications for the role of bo th water and sugar on the behavior of directly expanded extrudates are disc ussed.