Factors That Influence the Microwave Expansion of Glassy Amylopectin Extrudates.

The microwave expansion of glassy, unexpanded amylopectin pellets was studied. Amylopectin was extruded at three levels of specific mechanical energy (483, 809, and 846 kJ/kg), and 35-40% moisture content, without expansion at the die. Glassy pellets were obtained by drying and equilibrating the extrudates at five water activities (a(w) 0, 0.11, 0.33, 0.67, and 0.75). The pellets were characterized by measuring volume, porosity, and moisture content. The pellets were then expanded in a constant power microwave oven to determine the degree of expansion. When subjected to microwave heating, regardless of extrusion condition and initial a(w), the pellets expanded from the center where the highest temperature was recorded and then expansion advanced in the whole volume. Maximum expansion was reached after 30 sec of heating, after which samples started to burn from the center. Samples simultaneously expanded and lost moisture, both processes being faster and more intense for pellets of higher initial a(w). No expansion was observed for the pellets stored at a(w) 0, while collapse was observed for pellets stored at a(w) 0.73. A linear correlation between pellet expansion temperature and glass transition temperature was obtained. A hypothesis for the microwave expansion of glassy extrudates was formulated and represented on a state diagram.