## 3 Results and Discussion
### Bioactive Compounds and Antioxidant Activity in Hybrid Dried Apples  
Bioactive components are crucial to human health, so it is important to consume fruits and vegetables that are their main sources. The preservation of antioxidant substances  
Figure 6: Microstructures of dried apple samples based on XRCT visualization (horizontal and vertical cross-sections on a black background), and SEM pictures (third picture for each sample, magnification of 220\(\times\)). The microchannels are marked on the images with the red arrows. in dried foods contributes to their stability during long-term storage [60]. Changes in bioactive compound content in the apple tissue were assessed based on the content of vitamin C, polyphenols, and flavonoids, whereas the antioxidant activity was evaluated with the DPPH assay (Figure 7). Untreated apples contained 143.2 \(\pm\) 5.0 mg of vitamin C, 1683 \(\pm\) 2 mg of polyphenols, and 715 \(\pm\) 8 mg of flavonoids per 100 g of dry matter, while the antioxidant activity was 4.33 \(\pm\) 0.06 mg TE/g d.m. Vitamin C was used as a criterion for nutritional quality estimation due to its high sensitivity to temperature, oxygen, and light [61]. As a result of US pre-treatment and hybrid drying, the vitamin C content changed and ranged from 107.5 \(\pm\) 0.2 mg/100 g d.m. Figure 7: Effects of the US treatment time and the US power applied during drying on (**a**) the vitamin C content of hybrid dried apples; (**b**) the total polyphenol content (TPC) of hybrid dried apples; (**c**) the total flavonoid content (TFC) of hybrid dried apples; and (**d**) the antioxidant capacity with DPPH radicals of hybrid dried apples. Blue dots on the figure shows research data. to 171.5 \(\pm\) 8.2 mg/100 g d.m. For all of the US power values applied during drying, when US pre-treatment lasted 10 min, the vitamin C content was significantly lower as compared to those with US pre-treatment lasting 20 min or 30 min (Figure 7a). This finding was in agreement with the Pareto chart (Figure S5a), according to which the US treatment time had a significant linear effect on the vitamin C content of dried apples. A different finding was reported by [PERSON] et al. [61]. The researchers used US pre-treatment for 10 min, 20 min, or 30 min before the hot-air drying of kiwifruit slices and observed the highest ascorbic acid content for the 20 min US pre-treatment, while its level did not differ significantly when the US treatment duration was 10 min or 30 min. In contrast, [PERSON] et al. [62] reported no significant differences in ascorbic acid content in convection-dried pineapples subjected to US pre-treatment in distilled water for 10 min, 20 min, or 30 min. Additionally, in the present study, for all variants with a US pre-treatment of 30 min and for the two variants with a US treatment of 20 min, when the US power during drying was 120 or 200 W, the vitamin C content was higher than for the fresh sample. This can be explained by the fact that the cavitation produced during US pre-treatment could eliminate the oxygen that might affect the degradation of vitamin C [63]. Polyphenols play an integral role in the antioxidant activity of foods by capturing active oxygen species [64]; however, they are thermolabile compounds [65]. As a result of processing, the total polyphenol content decreased in most cases. The exceptions were samples subjected to 20 min of US pre-treatment and US-assisted drying with 160 W (T_20\(D\)160W), 30 min of US pre-treatment and US-assisted drying with 200 W (T_30\(D\)200W), and 20 min of US pre-treatment and US-assisted drying with 200 W (T_20\(D\)200W), for which it slightly increased by 14.5 mg/100 g d.m., 40 mg/100 g d.m., and 98.3 mg/100 g d.m., respectively. This increase could be attributed to the release of more bound phenolic compounds due to the breakdown of cellular components. Although the breakdown of cell walls releases oxidative and hydrolytic enzymes that can degrade antioxidants, thermal processing may deactivate these enzymes, helping preserve phenolic acids [66].