## 3 Results and Discussion
### Bioactive Compounds and Antioxidant Activity in Hybrid Dried Apples
Also, there was no noticeable correlation between the TPC and antioxidant activity (DPPH), which means that other compounds had a more substantial effect on antioxidant activity, e.g., vitamin C content. According to the response surface plot (Figure 7b), TPC reached the highest values for the longest US treatment time and the highest power of US during drying. This finding was in contrast with the results obtained by [PERSON] et al. [15]. They stated that apples subjected to drying with the application of ultrasound resulted in the loss of the TPC, around 20-39% of the polyphenols in the raw material. Furthermore, the application of a higher power of ultrasound resulted in a higher loss of TPC, especially when the drying temperature was 50 or 70 \({}^{\circ}\)C.  
Flavonoids are one of the major groups of polyphenols in fruits and vegetables [60]. Similarly to TPC, TPC was lowered by the processing methods employed to values varying from 471 \(\pm\) 21 mg/100 g d.m. to 701 \(\pm\) 29 mg/100 g d.m. The highest TPC was achieved by samples subjected to 30 min of US pre-treatment and US-assisted drying with 160 W (T_30\(D\)160W), and the TFC usually decreased when the US treatment time decreased or the power of US during drying increased or decreased (Figure 7c). Nevertheless, for both polyphenol and flavonoid contents, neither the US treatment time nor the power of US during drying had a statistically significant linear or quadratic effect on the contents of the mentioned bioactive components (Figure S5b,c). Likewise, when the sonication in distilled water was applied for 10 min, 20 min, or 30 min prior to the hot-air drying of pineapple, the TPC and TFC were not significantly different to the duration of US application [62]. As regards the power of US during drying, [PERSON] et al. [15] conducted a study that allowed them to note statistically significant changes--when apples were hot-air-dried at 70 \({}^{\circ}\)C, the highest values of TPC and TFC were achieved without the use of ultrasound,while the higher the power of ultrasound, the greater the loss of the discussed bioactive components. The authors suggested that the highest US power could lead to the most deteriorative treatment and cellular damage. The antioxidant activity of dried apples was lowest for the middle values of both US treatment time and US power during drying (Figure 7d). Although antioxidant activity increased for both increases and decreases in US treatment time and US power during drying, it can be stated from the Pareto chart (Figure S5d) that only the power of US during drying had a significant quadratic effect on antioxidant activity as measured by DPPH radical scavenging. In the study of [PERSON] et al. [66], hot-air-dried and freeze-dried onion slices were subjected to 1 min, 3 min, or 5 min of US pre-treatment. Although there were no significant differences in antioxidant activity between freeze-dried samples subjected to sonication at different times, in the case of hot-air-dried samples, the antioxidant activity was lower for hot-air-dried samples treated with US for a longer time. [PERSON] et al. [15] stated that better antioxidant activity was observed at higher drying temperatures without the use of ultrasound due to shorter drying times, whereas at a higher acoustic density of 30.8 kW/m\({}^{3}\), lower drying temperatures were more effective in minimizing the degradation of these quality parameters. Furthermore, when the sonication was applied during convective drying, [PERSON] et al. [67] noted that a medium intensity (902.7 W/m\({}^{2}\)) of US enables the obtaining of dried garlic slices of higher antioxidant capacity than lower (216.8 W/m\({}^{2}\)) or higher (1513.5 W/m\({}^{2}\)) intensity. This finding was shown using ABTS and FRAP methods. [PERSON] et al. investigated the effect of US-assisted air drying at 70 \({}^{\circ}\)C on broccoli florets.