Changes in physicochemical properties of rice in response to high-temperature fluidized bed drying and tempering

The effects of high-temperature fluidized bed drying and tempering on pasting, gelatinization, crystalline and microstructural properties of long-grain Vietnamese rice cultivar A10 were examined. The freshly harvest paddy (33% wet basis) was dried at two drying temperatures (80 and 90°C) and two drying durations (2.5 and 3.0 min) using a custom-made lab-scale batch fluidized bed dryer. The dried rice was immediately tempered in sealed containers at 75 and 86°C for various tempering durations (0, 30, 40, and 60 min), followed by thin-layer drying at 35°C to reduce moisture level to 14% wet basis. Characterization of pasting, gelatinization, crystallinity, and microstructure of the dried rice was undertaken using rapid visco analyzer, differential scanning calorimetry, X-ray diffraction (XRD), and environmental scanning electron microscopy (ESEM), respectively. Pasting properties and gelatinization enthalpy decreased with higher drying temperature and longer tempering time while pasting and gelatinization temperatures increased. Degree of crystallinity changed slightly, however, a shift from A-type to A+V-type XRD pattern was observed. ESEM observation demonstrated some degree of collapse of the rice microstructure. The kernel integrity appeared to be enhanced by the gel network formed during partial gelatinization of starch, corroborating with the changes in pasting and gelatinization properties of the fluidized bed dried and tempered rice.