## 3 Results and Discussion
### Sugar Content and Fourier-Transform Infrared Spectroscopy  
The total sugar content (TSC) in the fresh material was equal to 36.71 g/100 g of dry matter (4.94 g of sucrose/100 g d.m., 24.80 g of fructose/100 g d.m., and 6.11 g of glucose/100 g d.m.) (Figure 8), whereas the TSC in dried fruits ranged from 23.81 g/100 g d.m. to 31.64 g/100 g d.m. The US pre-treatment and US power applied during the drying process resulted in a reduction in the sugar content. The water-soluble saccharides may have been washed out by the water during the US pre-treatment. The obtained effect could also be influenced by the phenomenon of cavitation erosion, which increased the access of water to sugars and increased their outflow outside the tissue [70]. The results showed that the use of shorter ultrasound pre-treatment times and medium US powers during drying resulted in a higher sugar content in the material. Similarly, the use of shorter US pre-treatment times and the lower and medium US powers resulted in the highest fructose content in the material. The largest percentage of sugars in the dried apple tissue was fructose, respectively, in the range of 15.86 g/100 g d.m. to 21.85 g/100 g d.m. The more ripe the analyzed fruit was, the more simple sugars and less sucrose it contained [71; 72], whereas the glucose content in the dried materials ranged from 2.63 g/100 g d.m. to 5.24 g/100 g d.m. and the use of average US pre-exposure times and average US powers during drying resulted in higher glucose content. As mentioned earlier, a decrease in sugars in dried apples compared with raw ones was found. Hot-air drying had an effect on the content of glucose and fructose in the dried material. At high temperatures, reducing sugars reacted with amino acids, peptides, or proteins containing a free amino group. Formed products of the Maillard reaction decreased the content of monosaccharides in the dried apple tissue. Reducing the sugar content may also have been related to the caramelization process [73]. Furthermore, the smallest amounts of sucrose occurred with the middle US pre-treatment time and the lower US power used during drying. This may have been related to the fact that sucrose had broken down into simple sugars (glucose, fructose), which were observed in large numbers in this area. Higher glucose and fructose contents in dried fruits can be explained by sucrose hydrolysis. In addition, the amount of sucrose was affected by the degree of apple ripeness and storage time [71; 72]. On the basis of FTIR analysis, the chemical changes of the dried material were determined (Figure 9). The dried materials exhibited almost identical FTIR spectral patterns, with slight differences in the heights of the transmittance peaks across different wavenumber ranges. Almost identical infrared spectral patterns indicated the presence of the same functional groups in the analyzed fruits. The first peak, in the wavenumber range of 3000-3500 cm\({}^{-1}\), indicated O-H bond stretching, suggesting the presence of phenols, carboxylic acids, and alcohols in the analyzed fruits [74]. A broad peak with relatively high absorbance confirmed a significant content of polyphenols and flavonoids in the dried material. Another peak, within the wavenumber range of 2800-3000 cm\({}^{-1}\), corresponded to the C-H stretching vibrations of aliphatic methyl and methylene groups (-CH\({}_{3}\) and -CH\({}_{2}\)) present in the dried fruit material [75; 76]. Additionally, a peak in the range of 1300-1500 cm\({}^{-1}\) indicated the presence of these compounds in the apple tissue, specifically related to C-H bond bending. The absorbance in this range indicated the presence of proteins, organic acids, and polysaccharides (e.g., pectin) in the apple tissue [77]. The peak in the wavenumber range of 1700-1750 cm\({}^{-1}\) indicated C=O bond stretching and the presence of aldehydes, ketones, and carboxylic acids in the dried material.