## 3 Results and Discussion
### Physicochemical Properties
With the growth of microwave power, the diffusivity of water molecules also increases [51; 57; 64; 65]. Using the highest power of microwave (300 W) resulted in obtaining dried apples with the  
Figure 3: Response surfaces of: (**a**) water activity; (**b**) hygroscopicity after 72 h (\(H_{72h}\)) of apples treated with PEF prior to microwave-convective drying, depending on the microwave power (factor A) and specific energy intake (factor B). highest dry matter content. This was due to the increased intensity of water evaporation, thus a significant amount of it was removed from the material. Based on the hygroscopic properties of dried materials, it is possible to determine their storage stability [66], as well as to assess the structural changes caused by a specific drying method or the applied preliminary treatment [67]. The mass gain (\(H_{72h}\)) after 72 h of water vapour adsorption (taking place in an environment with a\({}_{\mathrm{w}}\) = 0.75) of the apples untreated with PEF and dried at a microwave power of 100, 200, and 300 W equalled 1.2079 \(\pm\) 0.0007, 1.2357 \(\pm\) 0.0020, and 1.2481 \(\pm\) 0.0004, respectively. The quadratic model allowed us to significantly predict (\(p\) < 0.05) the values of this parameter for 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\({}^{2}\) = 0.9541) and the insignificant result of the lack of fit test (\(p\) > 0.05) presented in Table 3. ANOVA showed that the hygroscopic properties of the tested samples significantly were influenced (\(p\) < 0.05) by microwave power (linear and quadratic effects) and specific energy intake (linear effect). Based on Figure 3b, the values of \(H_{72h}\) decreased with increasing the amount of energy supplied during PEF treatment and with reducing the power of microwave from 250 to 100 W (between 300 and 250 W a slight growth of this parameter was observed). Equation (11) allows for quantifying the values of \(H_{72h}\) of PEF-treated and microwave-convective dried apples, provided its factors consist of actual values. The hygroscopic properties depend, among others, on the structure of a particular material, but the more it is damaged, the lower these properties are, which is important from the point of view of the storage stability [67]. According to the calculated CDI values (Table 2), the more energy that was supplied to the apples during the PEF treatment, the more disintegrated the tissue of this material was, which could result in a reduced ability to water vapour adsorption. Additionally, as a result of intense electroporation, changes in sugar distribution could have occurred [68]. In turn, during drying, together with the migration of water, there may be the transport of soluble solids directed towards the surface [69]. Stiffening of the external layer of the material due to drying could be a barrier to the adsorption of water vapour. As can be seen from Figure 1, the lower the microwave power, the longer the drying time of the apples. At lower drying rates, higher material shrinkage [52; 70] is observed, which may also negatively affect the hygroscopic properties.