Evolution and Modelling of the Moisture Diffusion in Walnuts during the Combination of Hot Air and Microwave–Vacuum Drying
Xiaolan Man | Long Li | Xiuwen Fan | Hong Zhang | Haipeng Lan | Yurong Tang | Yongcheng Zhang
To understand the moisture transfer mechanism of walnuts during the combination of hot air (<i>HA</i>) and microwave–vacuum (<i>MV</i>) drying (<i>HA-MVD</i>) process, the drying characteristics and moisture diffusion characteristics of walnut during <i>HA-MVD</i> were investigated. The results indicated that the <i>HA-MVD</i> of walnuts occurred mainly in the falling-rate stage. The value of effective moisture diffusivity (<i>D<sub>eff</sub></i>) dropped continuously with the decrease in moisture content (<i>MC</i>) during the <i>HA</i> drying, while switching to <i>MV</i> drying could truncate the decrease in <i>D<sub>eff</sub></i> and still maintain a high value until the end of drying. The <i>HA</i> temperature, <i>MC</i> of the transition point, microwave power, and <i>MV</i> thermostatic temperature have significant effects on the moisture diffusion characteristics of walnuts. The values of <i>D<sub>eff</sub></i> for walnuts ranged from 2.33 × 10<sup>−9</sup> m<sup>2</sup>/s to 6.89 × 10<sup>−8</sup> m<sup>2</sup>/s. The third-order polynomial prediction model of <i>D<sub>eff</sub></i> related to the sample <i>MC</i> and drying conditions was established to describe the dynamic change in the <i>D<sub>eff</sub></i> of walnuts during the <i>HA-MVD</i> process. The application of <i>MVD</i> in the final stage of drying could rapidly increase the internal vapor pressure of the walnuts, accelerate the diffusion speed of the internal moisture, and re-enhance the drying rate. The findings have practical value for the development of efficient and energy-saving drying methods in the walnut industry.
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