Expansion of rice pellets: Examination of glass transition and expansion temperature

Rice pellets were prepared by single-screw extrusion cooking at an in-barrel water content of 45 to 55 wt. %. The glass transition (T-g) and expansion (T-e) temperature of rice pellets has been determined by differential scanning calorimetry (DSC) and non-contact infrared thermometer, respectively. T-g and T-e had similar dependence on equilibrium moisture that acted as plasticiser. T-e increased linearly with T-g with a correlation coefficient (r(2)) of 0.95. T-e was 20-100 degrees C higher than T-g, which indicated that the expansion of pellets occurred at the rubbery region. Maximum expansion occurred at 10% equilibrated moisture due to the transition of pellet from brittle to ductile. The addition of dextrin reduced the average molecular weight as well as T-g, but increased the expansion ratio due to decreasing the viscosity. Gordon Taylor equation fitted well (r(2)=0.89) T-g of the pellets without dextrin. A model has been developed to estimate T-g for the pellet consisting of rice flour, water and dextrin. (C) 2000 Academic Press.