## 3 Results and Discussion
### Color
This was caused by a significant reduction in lightness and an increase in redness of PEF-pretreated apples, probably due to an increased rate of non-enzymatic browning reaction. In addition, there are studies indicating an increase in the activity of certain enzymes due to the action of the pulsed electric fields (including those responsible for enzymatic browning) [7; 19; 25; 32]. Moreover, milder-processed apples (1 kJ/kg) exhibited significantly lower \(\Delta\)E after drying than apples with the highest specific energy input (6 kJ/kg). Figure 5: Values of the b* color parameter of dried apples depending on the various process parameters (distance between the infrared lamps and the apple slices during infrared–convective drying, IR-CD: 10, 20, and 30 cm; specific energy input during pulsed electric fields pretreatment, PEF: 0, 1, 3.5, and 6 kJ/kg); the uppercase letters (A–B) represent homogeneous groups in terms of distance, but the lowercase letters (a–b) indicate homogeneous groups in terms of specific energy input (\(\alpha\) = 0.05). Figure 6: Values of the \(\Delta\)E (total color difference) of dried apples depending on the various process parameters (distance between the infrared lamps and the apple slices during infrared–convective drying, IR-CD: 10, 20, and 30 cm; specific energy input during pulsed electric fields pretreatment, PEF: 0, 1, 3.5, and 6 kJ/kg); the uppercase letters (A–B) represent homogeneous groups in terms of distance, but the lowercase letters (a–c) indicate homogeneous groups in terms of specific energy input (\(\alpha\) = 0.05). Considering the non-enzymatic and enzymatic browning processes occurring during drying, it seems crucial to calculate (based on the obtained values of L*, a*, and b* color parameters) the browning index (BI), which is a measure of the purity of the brown color [41]. For fresh apple, BI was equal to 22.5 \(\pm\) 2.9, while for all dried samples, it took values in the range of 34.3-53.6. Even a twofold increase in the BI value for dried apples in relation to fresh material confirms the occurrence of browning processes during water removal. BI was significantly affected by both the distance between the infrared lamps and the apple slices (\(\eta^{2}\) = 0.160) and the specific energy input (\(\eta^{2}\) = 0.246). As can be seen in Figure 7, the shortest distance between the apple slices and the infrared lamps during drying (10 cm) resulted in products with the highest values of browning index (compared to the distances of 20 and 30 cm). Intense heating of the samples in the proximity of infrared lamps could promote the occurrence of non-enzymatic browning due to the elevated temperature of the material [27; 32; 33]. PEF-pretreated apples exhibited higher BI after drying than untreated samples dried in the same way. Electroporation of the cell membrane could induce leakage of non-enzymatic browning substrates, e.g., sugars, from the internal environment to the extracellular fluid [32]. This, combined with the heating of the sample's surface in the proximity of infrared lamps [27], could promote the occurrence of non-enzymatic browning [33]. PEF could also increase the activity of the enzymes responsible for the oxidation of phenolic compounds to quinones (brown pigments), i.e., polyphenol oxidase or peroxidase [7; 19; 25; 32; 34; 35].