Abstract: The thin layer hot-air drying characteristics of rough rice as a function of hot-air temperature, initial moisture content and air velocity were investigated by using an orthogonal array design and the applicability of 10 mathematical models for the hot-air drying process of rough rice was compared. The results showed that no apparent constant-rate drying period existed in the hot-air drying process, and moisture removal mainly occurred in the falling-rate drying period; hot-air temperature was the main factor that affects the hot-air drying, followed by air velocity. The optimal hot-air drying conditions for rough rice were determined as follows: initial moisture content, 20%; hot-air temperature, 50 ℃ and air velocity, 1.4 m/s. The Page Model was found to be the best mathematical model for describing the drying characteristics of rough rice under these conditions; tempering could effectively inhibit the occurrence of fissuring, and a better effect was observed by raising tempering temperature and prolonging tempering time. Under the conditions of initial moisture content of 24% and air velocity of 1.4 m/s, the relative mean deviation between the experimental and predicted results were 1.563% and 1.474%, respectively. The predictive drying curves fitted well the experimental data. With temperature increase from 40 ℃ to 60 ℃, the effective moisture diffusion coefficient of rough rice increased from 9.69 × 10-10 to 10.77 × 10-10 m2/s, and the activation energy for rough rice drying was 47.1 kJ/mol.

Key words: hot-air drying, mathematical model, effective moisture diffusion coefficient, fissure ratio, activation energy, rough rice