## 3 Results and Discussion
### Color  
Lightness (L*) in the CIELab color space indicates a coordinate for black (0) and white (100) [31]. For fresh apple, L* took a value of 77.4 \(\pm\) 0.3, while for all dried samples, values of that color parameter were in the range of 59.5-75.0. L* color parameter was significantly influenced not only by the distance between the infrared lamps and the apple slices (\(\eta^{2}\) = 0.093) and the specific energy input (\(\eta^{2}\) = 0.644), but also by the interaction of  
Figure 2: Drying time to MR = 0.02 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–C) 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). these two independent variables (\(\eta^{2}\) = 0.231). Nevertheless, increasing the distance between the infrared lamps and the apple slices from 10 to 20 cm and from 20 to 30 cm did not cause significant changes in the lightness of the samples (Figure 3). In general, all PEF-pretreated apples were darker 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)--polyphenol oxidase or peroxidase [7; 19; 25; 32; 34; 35]. It was also noticed that milder processing of apples with PEF (1 kJ/kg) significantly reduced their darkening (in relation to treatment with specific energy inputs of 3.5 and 6 kJ/kg). Another coordinate in the CIELab color space, a*, is associated with the red-green opponents. Positive values of this color parameter indicate red, while negative values represent green [36]. For fresh apples, a* was equal to \(-2.9\)\(\pm\) 0.4. In turn, for dried apples, values of that color parameter were in the range of \(-0.4\)-5.1. The a* color parameter was significantly affected not only by the distance between the infrared lamps and the apple slices (\(\eta^{2}\) = 0.197) and the specific energy input (\(\eta^{2}\) = 0.747), but also by the interaction of these two independent variables (\(\eta^{2}\) = 0.206). As can be seen from Figure 4, the highest distance between the apple slices and the infrared lamps during drying (30 cm) resulted in products with significantly lower redness (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]. It is worth mentioning that compared to PEF-pretreated samples, untreated apples exhibited negative values of a* after drying, which proves that they were green. The application of the PEF as a treatment prior to IR-CD resulted in obtaining apples with significant redness. The electric field-induced damage to the cell membrane increased its permeability. This, in turn, could have led to the release of non-enzymatic browning substrates from the cellular structure of the apple tissue, increasing its redness after drying.