Effect of sucrose on starch conversion and glass transition of nonexpanded maize 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, 10, and 20% db. The degree of starch conversion in the extrudates was determined using X-ray diffraction, differential scanning calorimetry, and rapid viscosity analysis. Starch conversion was greater in the fine material compared with the coarse material and greater for wheat compared with maize. Sugar addition decreased starch conversion in all cases, but the effect was greater for maize compared with wheat and for the coarse material compared with the fine material. The thermal mechanical properties were studied by dynamic mechanical thermal analysis to determine the effect of sugar on the glass transition temperature (T-g) in the four different materials. As expected, the mechanically determined T-g was reduced by sugar addition. Water plasticized wheat semolina less than the other three materials. It was suggested that this was because the extruded semolina was entirely amorphous, whereas X-ray analysis showed some crystallinity in the other three materials. Die swell was much less for maize grits possibly because elasticity decreased with decreasing starch conversion. The implications for the role of both water and sugar on the behavior of directly expanded extrudates are discussed.