## 3 Results and Discussion
### Physicochemical Properties
\[\begin{array}{l}Water\;Activity\;=\;0.272047-0.000748\cdot\mathit{Microwave\;Power}+0.007208 \cdot\mathit{Specific\;Energy\;Intake}\\ \effect on the RR of the tested samples. The RR of PEF-treated apples dried via microwave method can be determined by applying Equation (12), on the condition that factors within it are described with actual values. The values of RR increased with increasing microwave power (Figure 4a), which is consistent with the literature data [52, 57, 70]. Despite no significant impact of PEF treatment on this parameter (\(p>0.05\)), an increase in RR was observed with a decrease in specific energy intake. Perhaps the higher amount of energy supplied during the PEF treatment resulted in an overprocessing of the material, making it impossible to fully replicate the initial water content. On the other hand, damage to apples at the level of CDI = 0.1 (specific energy intake: 1 kJ/kg) could lead to optimal porosity, limiting shrinkage of the obtained dried apples and thus an improved ability to absorb water [33, 52]. Drying of apples at low microwave power (100 W) lasted the longest (Figure 1), which could lead to the highest shrinkage of the samples and thus a reduction in the water absorption ability. The use of higher microwave power could lead to an increase in the porosity of the material and the rapid creation of a barrier surface layer, which, by preserving the volume of dried apples, could reduce their shrinkage and thus lead to an increase in the RR [52, 70].  
As a result of immersing the dried material in water during the rehydration process, apart from water absorption and swelling of the material, the leaching of soluble solids undoubtedly occurs [52, 67]. The relative dry matter content (SSL) in the apples untreated with PEF and dried at a microwave power of 100, 200, and 300 W equalled 0.7545 \(\pm\) 0.0177, 0.6956 \(\pm\) 0.0327, and 0.7337 \(\pm\) 0.0017, respectively. Based on the linear model, it is possible to significantly (\(p<0.05\)) predict the SSL values of microwave-convective dried apples with PEF pretreatment. This was confirmed by the insignificant (\(p>0.05\)) result of the lack of fit test (Table 3) and the value of 7.1714 obtained in the adequate precision test. Unfortunately, a relatively weak fit of the selected model to the data obtained experimentally (R\({}^{2}\) = 0.5753) was found. According to the results of the analysis of variance, only the microwave power (linear effect) had a significant (\(p<0.05\)) impact on the SSL. To evaluate the SSL of PEF-treated and microwave-convective dried apples Equation (13) can be used, but both factors should be specified in actual values. Figure 4b shows that the SSL values increased with the decrease in microwave power, which is synonymous with the decrease of the loss of soluble  
Figure 4: Response surfaces of: (**a**) rehydration ratio (RR); (**b**) relative dry matter content (SSL) in apples treated with PEF prior to microwave-convective drying, depending on the microwave power (factor A) and specific energy intake (factor B).  
solids. Despite the lack of a significant effect (\(p\) > 0.05), an increase in the value of SSL was observed with increasing the amount of energy supplied during the PEF treatment. Based on the previously described results for the RR parameter, it can be concluded that the lower the amount of energy supplied during PEF treatment and the higher the microwave power used during drying, the higher the ability of dried apples to absorb water (the highest RR values) and the higher the risk of significant leaching of soluble solids (the lowest SSL values).  
\[RR = 2.00682+0.001203\cdot\mathit{Microwave Power}-0.033101\cdot\mathit{Specific Energy}\ \mathit{Intake} \tag{12}\]  
\[SSL = 0.787578-0.000296\cdot\mathit{Microwave Power}+0.008321\cdot\mathit{Specific Energy}\ \mathit{Intake} \tag{13}\]  
Using the measured values of the colour parameters: L* (58.2-79.7), a* (-3.2-8.7), and b* (18.5-37.3) of dried apples, the total colour difference (\(\Delta\)E) of the obtained materials was calculated in relation to fresh apple (L* = 77.4 \(\pm\) 0.3, a* = -2.9 \(\pm\) 0.4, b* = 17.7 \(\pm\) 1.9).