Abstract: In this paper, the three-dimensional mathematical model for heat and mass transfer that we previously developed was applied to simulate the isothermal drying-tempering process of paddy. The pattern of moisture distribution inside paddy grains was analyzed and compared with that in the simple drying process. The results showed that compared with the simple drying process at 60 ℃, the tempering operation could reduce the maximum moisture gradient, which was related to the drying stress and cracking, and consequently reduced the drying time. Tempering could also greatly decrease the final moisture gradient of paddy grains by about 50%. Besides, it was found that tempering temperature, drying/tempering ratio, and tempering time were important process parameters. The optimal drying-tempering process was obtained involving three stages: shorter-time tempering, drying and longer-time tempering, which could reduce effectively the maximum and final moisture gradient as well as the whole tempering time and number. The findings obtained in this paper provide theoretical and technical support to the drying-tempering operation practice.

Key words: paddy, drying, tempering, mathematical model, moisture gradient