## 3 Results and Discussion
### Physicochemical Properties
As shown in Table 2, the \(\Delta\)E values ranged from 9.0 to 17.9, which indicates a significant colour deviation, visible even by an untrained observer [71]. Such high \(\Delta\)E values obtained for dried apples could be the result of the methodology used, where the light reflected from a porous surface, devoid of a significant amount of water (dried materials), is compared to the surface containing water (fresh material) [30]. Despite the lack of significant differences (\(p\) > 0.05), an increase in the values of the \(\Delta\)E parameter was observed as the microwave power increased during the drying of untreated and PEF-pretreated apples (specific energy intake: 3.5 and 6 kJ/kg). This could be due to the supply of more microwave energy to the samples, thus increasing their temperature. Such conditions conduce not only the caramelization of sugars but also the formation of Maillard reaction products, which could lead to nonenzymatic browning of dried materials [50; 51; 58]. Additionally, increasing the amount of energy supplied during the PEF pretreatment resulted in a higher, but statistically insignificant (\(p\) > 0.05), colour deviation of dried apples (at the microwave power of 200 and 300 W). The calculated CDI values indicated that the more energy was supplied, the more ruptured the apple tissue was (Table 2), which could also mean increased cell membrane permeability. As a result of electroporation, the content of cells could be released, including, for example, substrates for polyphenol oxidase, which could result in enzymatic browning of the analysed samples [21]. With the increase in microwave power at a specific amount of PEF energy supplied equalling 1 kJ/kg, a decrease in the values of \(\Delta\)E was observed, which led to the material (MW-CD300_PEF1) with the lowest total colour difference (\(p\) < 0.05). Perhaps the damage to the apple tissue at the level of CDI = 0.1 ensured the most optimal permeability of the cell membrane, thanks to which both the mass transport during drying and the colour of the obtained dried material were more homogeneous [72]. Moreover, such an amount of specific energy intake could be conducive to partial inactivation of enzymes responsible for enzymatic browning of food [1]. High diversity of the optical properties is also related to the native distribution of enzymatic browning substrates, which is not equal, and the percolation associated with the electroporation. Drying and other methods of food processing lead to changes in the physical and chemical properties of the treated materials [73]. The total phenolic content (TPC) of the apples untreated with PEF and dried at a microwave power of 100, 200, and 300 W equalled 1003.09 \(\pm\) 43.30, 1379.26 \(\pm\) 29.89, and 1649.48 \(\pm\) 25.04 mg GAE/100 g d.m., respectively. With the quadratic model, it is possible to significantly (\(p\) < 0.05) predict the values of TPC of apples pretreated with PEF and dried in the microwave-convective dryer. Additionally, based on the relatively high value of the determination coefficient (R\({}^{2}\) = 0.8755) and the insignificant result of the lack of fit test (\(p\) > 0.05), a good fit of the model to empirical data was found (Table 3). Based on the analysis of variance, it can be concluded that the TPC was significantly influenced only by the microwave power with linear and quadratic effects (\(p\) < 0.05). One can apply Equation (14) to obtain the specific TPC for PEF-treated and microwave-dried apples, however, its factors must be based on actual values. According to the response surface shown in Figure 5, the TPC values decreased with increasing microwave power to a level of about 170 W, and from this value, an increase in TPC was observed to reach a maximum with a microwave power of 300 W. Despite no significant impact of PEF treatment on this parameter (\(p>0.05\)), an increase in TPC was observed with a decrease in specific energy intake. Supplying the highest amount of specific energy (6 kJ/kg) resulted in a CDI of approximately 0.4 (Table 2).