## 3 Results and Discussion
### Chemical Properties  
Apples contain significant amounts of polyphenols, which are plants' secondary metabolites. Those phytochemicals have strong antioxidant effects [42; 43; 44]. Total phenolic content (TPC) in fresh apple was equal to 797.5 \(\pm\) 44.0 milligrams of gallic acid (GAE) per 100 g of dry matter (d.m.). In turn, the content of those bioactive compounds in dried apples was in the range of 312.5-937.0 mg GAE/100 g d.m. TPC was significantly dependent not only on the distance between the infrared lamps and the apple slices (\(\eta^{2}\) = 0.828) and the specific energy input (\(\eta^{2}\) = 0.846) but also on the interaction of these two independent variables (\(\eta^{2}\) = 0.684). As can be seen in Figure 8, increasing the distance between the  
Figure 7: Values of the BI (browning index) 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). infrared lamps and the apple slices from 10 to 20 cm resulted in higher retention of TPC. Polyphenols are compounds sensitive to elevated temperatures [42]; therefore, more intense heating of the surface of apples being dried could have led to their higher degradation. Interestingly, a further increase in the distance between the infrared lamps and the samples (from 20 to 30 cm) resulted in a significant reduction of TPC for all samples. As can be seen from Figure 2, the drying of apples was the longest when the distance to the infrared lamps was equal to 30 cm. Longer drying means longer heating of the apple surface and therefore an increased risk of thermal degradation of polyphenols. Application of the PEF as a treatment prior to IR-CD resulted in obtaining apples with lower retention of TPC than that obtained for untreated ones. The electric field-induced damage to the cell membrane increased its permeability. This, in turn, could have led to the release of polyphenols from the apple's cellular structure, increasing the risk of their thermal degradation during drying. As mentioned above, PEF could also increase the activity of enzymes responsible for the oxidation of phenolic compounds, i.e., polyphenol oxidase or peroxidase [7; 19; 25; 32; 34; 35]. Moreover, milder-processed apples (1 kJ/kg) exhibited significantly lower retention of polyphenols after drying than other PEF-treated apples. As pointed out above, apples are a rich source of antioxidants. Therefore, the fresh raw material and the dried apples were analyzed for their ability to scavenge ABTS**+** and DPPH**+** free radicals. Considering that the results of those analyses were presented as the EC\({}_{50}\) coefficient, which means the lowest concentration of the sample extract required to reduce the initial content of ABTS**+** and DPPH**+** radicals by 50%, it is assumed that a lower EC\({}_{50}\) value indicates a higher antioxidant capacity. For fresh apple, the EC\({}_{50}\) of ABTS and EC\({}_{50}\) of DPPH were 0.37 \(\pm\) 0.03 and 0.79 \(\pm\) 0.08 mg d.m./mL, respectively. In turn, for dried apples, those values were in the ranges of 0.39-1.26 and 1.13-3.62 mg d.m./mL, respectively. EC\({}_{50}\) of ABTS and EC\({}_{50}\) of DPPH were significantly influenced not only by the distance between the infrared lamps and the apple slices (\(\eta^{2}\) = 0.840 and \(\eta^{2}\) = 0.910 for ABTS and DPPH, respectively) and the specific energy input (\(\eta^{2}\) = 0.884 and \(\eta^{2}\) = 0.933 for ABTS and DPPH, respectively) but also by the interaction of these two independent variables (\(\eta^{2}\) = 0.755 and \(\eta^{2}\) = 0.826 for ABTS and DPPH, respectively).