Moisture Diffusivity Characteristics and Model Fitting of Jerusalem Artichoke(Helianthus tuberosus L.) during Microwave Vacuum Drying

To explore the law of moisture variation in Jerusalem artichoke during microwave vacuum drying process, the effects of microwave intensity on the drying characteristics of Jerusalem artichoke were investigated. The Weibull distribution function and Dincer model were used to fit the drying curves, and the heat and mass transfer mechanism during drying process were analyzed by using the model parameters, namely scale parameter (α), shape parameter (β), lag factor (G) and drying coefficient (S). The results showed that there were three stages of rising rate, constant rate and falling rate under all the microwave intensity levels, except for the intensity level of 1.28 W/g. The higher the microwave intensity, the larger the maximum drying rate, and the shorter the duration of rising rate stage. The shape parameter (β) ranged from 1.314 to 2.175, which indicated that the drying process of Jerusalem artichoke was not completely controlled by internal water diffusion. The lag factor (G) ranged from 1.043 to 1.188, and decreased with the microwave intensity increased. The values of Biot number (Bi) located in the range of 0.179 and 5.762, which indicated that the temperature changes of sample in the drying process were controlled by internal thermal conductivity and boundary convective heat transfer. Effective moisture diffusion coefficients calculated by Weibull distribution function, Diner model and Fick’s second law were (D_cal=5.922×10⁻⁸~2.717×10⁻⁷ m²/s), (D_eff =7.570×10⁻⁷~1.799×10⁻⁵ m²/s), and (D^*_eff=2.353×10⁻⁹~7.546×10⁻⁹ m²/s), respectively. Under the same microwave intensity, their values were in order: D_eff>D_cal >D^*_eff. The maximum value of brightness (L^*) for dried samples was 69.05 under the microwave intensity level of 2.32 W/g. Both color parameters a^* and b^* decreased with the increasing of microwave intensities. SEM figures showed that the cell structure in dried Jerusalem artichoke was regular, and exhibited obvious pore characteristics when applying suitable microwave intensity. The tissue structure of dried samples shrank obviously at microwave intensity of 1.28 W/g, while part of the tissue structure collapsed and the intracellular material condensed at higher microwave intensities of 2.70 and 3.04 W/g.