The drying kinetics of cannabis leaves dried by convective drying (CD) (a), vacuum–microwave drying (VMD) (b) and combined drying (CD-VMD) (c) are shown in Figure 1, while Table 1 presents models used to fit the experimental data.

Three models were used, namely the modified Page, logarithmic and Midilli models. All of them presented a very good fit, determined on the basis of high values of the coefficient of determination (R² > 0.9) and low values of RMSE. The modified Page and Midilli models were previously used to describe the drying kinetics of hemp leaves in the study by Chasiotis and Filios [16]. As can be seen in Table 2, VMD480 drying was poorly described by the logarithmic model (R² > 0.9527, RMSE = 0.0816), while both modified Page and Midilli models proved to be more suitable to describe more intense drying process, as depicted by the drying kinetics of VMD480. Overall, the Midilli model was selected as the best choice to describe the drying kinetics of hemp leaves.

In all models, the value of parameter a was equal to 1, as it shows the starting point for CD and VMD, while in the case of combined drying it was significantly lower, pointing to the beginning of the vacuum–microwave finishing drying. Parameter k in the models represented a drying rate. During CD, the k parameter was lower than during VMD and increased with an increase in the temperature of the hot air. This is consistent with the studies on the drying of hemp leaves [16]. Higher temperatures during CD accelerated the water evaporation from the surface of the material, leading to a shorter time of drying. An increase in temperature from 50 °C to 70 °C resulted in a significant reduction in drying time, from 300 min for 50 °C to 150 min in the case of 70 °C (Table 3). The VMD method proved to significantly reduce drying time compared to CD despite the power of magnetrons used. Vacuum–microwave drying has been reported for its ability to significantly reduce the drying time compared to other methods such as convective and freeze-drying [17]. This is due to the volumetric heating occurring when the microwaves are applied. As a result, the material is heated from the inside, and an additional pressure gradient between the material and the environment increases the water evaporation, leading to a shorter time for the process. The application of higher power during VMD increases the drying rate, which can be seen as a higher k parameter (Table 1). A higher power of magnetrons also increases the temperature of the material, which can reduce the amount of thermolabile compounds in the dried hemp leaves. The obtained results are consistent with the previous studies on hemp flowers [14]. On the other hand, it is worth noting that the highest temperatures of the material were obtained during CD70 (T = 70 °C), and an increase in power during VMD resulted in still lower temperatures of the material. However, the temperatures in the study were measured on the surface of the material, and the actual temperature in the inside might be different. The combination of CD as a pre-treatment with VMD has been reported for its ability to enhance the quality of the products with the following mechanism: pre-drying lowered the unbound moisture of fresh material without any impact on the bioactive compounds of the material, followed by vacuum–microwave drying that brought the moisture content to a certain desired level [18]. In the present study, the application of combined drying reduced the drying time as compared to CD; however, it still resulted in a longer time than when only VMD methods were applied.

According to the decrease in the moisture ratio (Figure 1), it can be observed that only a falling rate period can be distinguished in the convective drying of hemp leaves. This behavior indicates that drying kinetics is controlled by internal liquid diffusion governed by internal heat and mass transfer [19]. This is consistent with previous studies on sage [20] and hemp leaves [16]. On the other hand, during VMD, a constant rate period could be identified, followed by a falling rate period. This can be due to the high initial moisture content in the materials, which led to high microwave absorption resulting in intense water evaporation at the beginning of drying. Then, when the process progressed, the falling rate period could be reported when the drying rate decreased as a result of the dominance of the internal diffusion of bound water, which is consistent with the previous studies [8].

The effects of drying methods and parameters of the process on the color of dried hemp leaves was assessed and is presented in Table 2. Samples treated by CD were significantly brighter (L*) than hemp leaves dried by VMD and CD-VMD techniques. Increases in both temperature during CD as well as the power of magnetrons during VMD led to the increase in the b* parameter, which means a shifting towards yellowness. VMD and CD-VMD had the lowest values for yellowness (b*) compared to the yellowness of the leaves that were treated with the CD treatment. Color degradation commonly occurred during the drying process due to thermal exposure; thus, minimizing thermal exposure during the drying process is expected to lower the color degradation [18]. The same result was observed wherein a higher temperature of CD damaged the color of thyme leaves [21].