Tempering time and air temperature relationships for real-scale paddy drying and their effects on the physical, physicochemical, and morphological qualities of polished rice

Tempering-drying time may affect the quality of polished rice. Hence, this study aims to evaluate the effect of drying-tempering time and air temperature relationships on the physical, physicochemical, and morphological qualities of polished rice. Herein, paddy with 20–24% (w.b.) moisture were subjected to drying and tempering with air temperatures of 75–115 °C, 115–130 °C, and 75–130 °C over 14 h to reduce their moisture content up to 12% (w.b.). With the increase in the paddy mass temperature to 49 °C, the whole grain yield and the percentages of protein, crude fiber, fat, and ash reduced; additionally, the morphology of starch and the composition of minerals were altered. The cumulative effects and prolonged time of the drying-tempering influenced the physical, physicochemical, and morphological qualities of the rice. In conclusion, paddy drying technologies with tempering periods should not be designed for temperatures not exceeding 75 °C and drying times not longer than 8 h for good system performance and rice quality.