## 3 Results and Discussion
### Physicochemical Properties  
Water activity is one of the most important parameters in assessing food quality and safety. Sufficiently low water activity limits the growth of microorganisms and reduces  
Figure 1: Drying curves of microwave-convective drying of apples untreated or PEF-pretreated. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Response** & **Model** & **R\({}^{2}\)** & **CV [\%]** & **_p_-Value (Model)_ & **_p_-Value (Lack of Fit)** \\ \hline Drying Time & Quadratic & 0.9846 & 6.51 & 0.0002 & 0.3464 \\ Water Activity & Quadratic & 0.9471 & 4.60 & 0.0033 & 0.4502 \\ \(H_{72h}\) & Quadratic & 0.9541 & 0.4477 & 0.0023 & 0.4658 \\ RR & Linear & 0.5437 & 5.43 & 0.0433 & 0.3580 \\ SSL & Linear & 0.5753 & 3.55 & 0.0325 & 0.2979 \\ TPC & Quadratic & 0.8755 & 15.52 & 0.0259 & 0.1358 \\ \(EC_{50}\) ABTS & Reduced Quadratic & 0.7966 & 17.02 & 0.0285 & 0.4117 \\ \(EC_{50}\) DPPH & Reduced Quadratic & 0.7608 & 14.26 & 0.0449 & 0.6957 \\ \hline \hline \end{tabular}  
* The _p_-Value (model) \(<\) 0.05 and _p_-Value (lack of fit) \(>\) 0.05 indicate a significant fitting of the model.  
\end{table} 
Table 3: The results of statistical tests of fitting the equations of the response surfaces to the values obtained experimentally (\(\alpha\) = 0.05); R\({}^{2}\) (determination coefficient), CV (coefficient of variation). Figure 2: Response surface of drying time to MR = 0.02 of apples treated with PEF prior to microwave-convective drying, depending on the microwave power (factor A) and specific energy intake (factor B). the activity of enzymes [58]. The water activity of the apples untreated with PEF and dried at a microwave power of 100, 200, and 300 W equalled 0.239 \(\pm\) 0.014, 0.205 \(\pm\) 0.014, and 0.200 \(\pm\) 0.002, respectively. With the quadratic model, it is possible to significantly (\(p\) < 0.05) predict the values of water activity of apples pretreated with PEF and dried in the microwave-convective dryer. Additionally, based on the high value of the coefficient of determination (R\({}^{2}\) = 0.9471) and the insignificant result of the lack of fit test (\(p\) > 0.05), a very good fit of the model to empirical data was found (Table 3). Based on the analysis of variance, it can be concluded that the water activity was significantly influenced by the microwave power (linear and quadratic effects), specific energy intake (linear effect), and the interaction between these two factors (\(p\) < 0.05). As shown in Figure 3a, in most cases, increasing the microwave power and reducing specific energy intake resulted in a reduction in water activity. The quadratic effect was manifested by a slight increase in the water activity of apples treated with the PEF treatment, during which energy was supplied in the amount of 1 kJ/kg, and then dried with a microwave power of 300 W. The water activity of PEF-treated and microwave-convective dried apples can be determined via Equation (10), in which factors should be expressed in actual values. The effectiveness of the drying process depends, among others, on the properties of the dried material and the applied process parameters [49]. The dry matter content in all obtained dried apples varied between 90.7% and 97.1% (Table 2), which shows that they contained 2.9-9.3% of water. The achieved dry matter content in dried apples is consistent with the results (88.5-95.0%) obtained by other authors [59; 60; 61; 62]. With such a low water content, the water activity in the range of 0.188-0.292 ensured the safety of the obtained dried materials [63]. As it is visible in Table 2, apples dried at the lowest microwave power exhibited the lowest dry matter content. Under such drying conditions (100 W, 30 \({}^{\circ}\)C), the drying process was not very intensive, which is confirmed by the shape of the drying curves shown in Figure 1. Therefore, it would have been probably impossible to remove more water from the material dried at such process parameters during a reasonable time.