## 4 Discussion
On the contrary, in the present study, in most of the samples with the increasing drying temperature, a higher vitamin C content was observed; this result could be due to the reduced treatment time (necessary to reach \(\mathrm{a_{w}}=0.2\)) and thus to a shorter exposure to the highest temperature. Moreover, the lower number of processing steps such as in the case of the control sample (C) or of those subjected to a single pre-treatment (OD and PEF samples)allowed a better retention of vitamin C and probably even other valuable compounds. In fact, OD and PEF kiwifruit chips dried, respectively, at 70 and 50 \({}^{\circ}\)C showed ascorbic acid values significantly similar to those of C samples. These results clearly showed that the number of processing steps together with an oxygen-rich environment could significantly reduce the ascorbic acid content. The AA, assessed with both DPPH and ABTS methods, revealed different results owing to the different capabilities of the two assays to detect certain valuable components. In fact, DPPH is more prone to identify flavonoids compounds, while the ABTS method seems to be more suitable for vitamin C [34]. Indeed, a higher retention of bioactive compounds by the DPPH assay was observed for C and OD chips dried at 60 \({}^{\circ}\)C, which is in line with the TPC results observed in the same dried samples (R\({}^{2}\) of 0.77 and 0.70, respectively). The AA results obtained with the ABTS method were higher in the kiwifruit chips pre-treated only with PEF and dried at all three temperatures and with the combination of PEF/OD after drying at 70 \({}^{\circ}\)C. These findings were not affected by the observed values of vitamin C content, expect for the PEF-treated chips dried at 50 \({}^{\circ}\)C (R\({}^{2}\) of 0.70). Moreover, the changes in the cellular membranes may have induced a greater release of bounded antioxidant compounds, making them more accessible during the extraction step before the assays of both methods. Similar results were reported by [16], who investigated the application of PEF (2.8 kV/cm, 750 pulses) and OD (55 \({}^{\circ}\)C, 60 min) prior to the drying process at 60 \({}^{\circ}\)C in a goji berry product. [PERSON] et al. [35] reported a high retention of bioactive compounds assessed by the DPPH method in apricot pre-treated with PEF (at 1.25 kV/cm, 30 s, 100 Hz and pulse width of 20 \(\upmu\)s) and subsequently subjected to soaking in a 0.2% sodium sulfite solution (fruit/solution ratio of 2:5) for 1 h and finally hot air-dried at 45 \({}^{\circ}\)C.  
As for the intensity of the PEF treatment, [PERSON] et al. [24] observed a significant increase, around 15.7%, in the antioxidant activity assessed by DPPH in strawberry tissues treated at 200 V/cm and 10 \(\upmu\)s (total treatment time of 10 s and specific energy of 1.92 kJ/kg), while for kiwifruits, lower intensities of PEF (100 V/cm, 0.96 kJ/kg) were found more beneficial on the retention of antioxidant compounds (7%). However, the combination of PEF at 100 and 200 V/cm and OD with trehalose solution (40%) caused lower antioxidant activity in both strawberry and kiwifruit samples compared to related untreated tissues. The application of different pre-treatments caused a reduction in the drying time. This reduction was higher when the lowest temperature of 50 \({}^{\circ}\)C was used (62-68%), while for the samples dried at 60 and 70 \({}^{\circ}\)C, the reduction in drying time was of 25-50% and 28-57%, respectively. A reduction in drying times of 30-60% was also observed by [PERSON] et al. [36] and [PERSON] et al. [37], respectively, for ginger slices and kiwiberry. Moreover, OD treatment can reduce the total energy consumption (two-three times) if compared to convective drying without any pre-treatment [38]. Similarly, PEF pre-treatment, due to the electroporation that takes place in the whole treated product, can also cause the enhancement of mass transfer and consequently a reduction in the drying time [15; 16].