## 3 Results and Discussion
### Drying Kinetics and Drying Times  
Figure 1 presents the drying curves of untreated and PEF-treated apples dried by the microwave-convective method. At the beginning of the process, a significant efficiency of water removal was observed, which was related to the evaporation of easily available capillary and surface water [48]. As drying progressed, the efficiency of the process decreased due to increasing difficulties in removing water more bound to the material [49]. As can be seen, the course of microwave-convective drying of apples differed depending on the amount of energy supplied during the PEF treatment and the level of microwave power. The drying time of the apples untreated with PEF and dried at a microwave power of 100, 200, and 300 W equalled 200 \(\pm\) 0, 110 \(\pm\) 0, and 60 \(\pm\) 0 min, respectively. The quadratic model allowed the significant prediction (\(p<0.05\)) of the drying time of apples treated with PEF and dried using the microwave-convective method. Moreover, a very good fit of the model to the data obtained experimentally was observed, which was confirmed by the high value of the determination coefficient (R\({}^{2}\) = 0.9846) and the insignificant result of the lack of fit test (\(p>0.05\)) presented in Table 3. ANOVA showed that the drying time of the tested samples was significantly influenced (\(p<0.05\)) only by microwave power (linear and quadratic effects). Based on Figure 2, it can be seen that the drying time decreased with increasing the power of the microwave. The obtained tendency is consistent with the observations of other authors [50; 51; 52]. The drying time of apples subjected to PEF was considerably reduced (by 8-28%) compared to untreated samples. However, no significant effect of the amount of energy supplied during the pretreatment (1-6 kJ/kg) on this parameter was found (\(p>0.05\)). Such results suggest that application of lower specific energy input during PEF treatment may be justified, taking into account energy and environmental factors. Reduced drying time generally results in a decrease of energy consumption [40; 53; 54; 55], so the application of PEF, by intensifying the MW-CD of apples, probably led to a reduction in the amount of energy used. Based on Equation (9), where the factors should be specified in actual values, it is possible to calculate the drying time of PEF-treated and microwave-convective dried apples. The use of PEF pretreatment resulted in damage to the apple tissue (according to CDI) at the level of 0.1-0.4 (Table 2). This means that this process, to some extent, led to a break in the continuity of the cell membrane [46]. The pores in the cell membrane created as a result of electroporation lead to an increase in the mobility of various molecules, thanks to which the effectiveness of diffusion-based processes increases, e.g., drying [56; 47]. This may explain the achieved reduction in drying time for apples subjected to PEF treatment compared to untreated ones. During microwave drying, the radiation energy is absorbed by the material. The microwave energy is transformed into thermal energy, which heats the material in its entire volume [50; 57]. Increasing the microwave power results in an increase in the temperature of the material, which causes a growth in both internal pressure and the concentration gradient. For this reason, the evaporation of water is intensified, which reduces the drying time [51; 52].  
\[\begin{array}{l}Drying~{}Time~{}=~{}276.60526-1.33504\cdot\text{{Microwave Power}}-7.82456\cdot\text{{Specific Energy}}~{}\text{{Intake}}\\ +0.005\cdot\text{{Microwave Power}}\cdot\text{{Specific Energy}}~{}\text{{Intake}}+0.002211\cdot\text{{Microwave Power}}^{2}\\ +0.736842\cdot\text{{Specific Energy}}~{}\text{{Intake}}^{2}\end{array} \tag{9}\]