| Summary: | This study was conducted to investigate the drying kinetics and internal and external mass transfer characteristics of walnuts for an understanding of the drying mechanism. The drying characteristics, mass transfer characteristics, and color of walnut during hot air drying (<i>HAD</i>) were investigated under different initial moisture content (<i>IMC</i>) (0.35, 0.39, and 0.43 g water/g wet mass) and drying temperatures (50, 60, 70, and 80 °C). The results indicated that the <i>IMC</i> and drying temperature both have significant effects on the drying process of walnut, showing the higher the <i>IMC</i>, the longer the preheating time, the smaller the effective moisture diffusivity (<i>D<sub>eff</sub></i>) and mass transfer coefficient (<i>h<sub>m</sub></i>), and the longer the drying time, but reverse results for drying temperature. The values of <i>D<sub>eff</sub></i> and <i>h<sub>m</sub></i> for walnut ranged from 4.94 × 10<sup>−10</sup> to 1.44 × 10<sup>−9</sup> m<sup>2</sup>/s and 1.24 × 10<sup>−7</sup> to 3.90 × 10<sup>−7</sup> m/s, respectively. The values of activation energy for moisture diffusion and mass transfer ranged from 21.56 to 23.35 kJ/mol and 28.92 to 33.43 kJ/mol, respectively. Multivariate linear prediction models were also established for estimating the <i>D<sub>eff</sub></i> and <i>h<sub>m</sub></i> as a function of the <i>HAD</i> process parameters. The drying temperature has a greater effect on the walnut kernel lightness than the <i>IMC</i>. The Verma et al model could be used to describe the <i>HAD</i> process of the walnut. The findings contribute to the understanding of moisture transfer mechanisms in walnuts and have practical value for the evaluation and improvement of drying systems.
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