## 3 Results and Discussion
### Dry Matter, Water Activity, and Tissue Density Changes after Pretreatments and Drying  
The applied pretreatments utilized led to a mass alteration, which correlated with changes in the dry matter content of the material (Table 1). Initially, fresh beetroot exhibited a dry matter content of 14.9%. Traditional blanching and soaking methods induced weight losses of approximately 14% and 12-16.6%, respectively. These losses were attributed to the removal of dry substance components, including water-soluble betalain dyes, during the processes [40; 41]. Tissue subjected to ultrasound at a frequency of 21 kHz for durations of 10 and 30 min showed no statistically significant difference in dry matter content compared to untreated tissue. A slightly statistically significant decrease was observed in samples treated with ultrasound for 20 min at 21 kHz frequency. The most significant decrease occurred in samples exposed to ultrasound at a frequency of 35 kHz for 30 min, resulting in a reduction in dry matter content by 17.5% compared to untreated samples. However, the duration of ultrasound exposure and the applied frequency showed no significant effect on changes in the dry matter content of the material. Dried beetroot obtained through convective drying supported by infrared radiation, when untreated, exhibited a dry matter content of 92.0%. The pretreated dried material was characterized by similar or slightly higher dry matter content in the range of 92.9-95.5%. Statistically, neither the duration nor the frequency of ultrasound operation exhibited a significant impact on the dry substance content in dried fruit. Figure 1: Drying kinetics of beetroot subjected to pretreatment: (**a**) untreated, blanched, and soaked, (**b**) ultrasound at a frequency of 21 kHz, and (**c**) ultrasound at a frequency of 35 kHz; a———letters next to the drying time are homogeneous groups; different letters indicate statistical significance (\(p<0.05\)). Raw beets exhibited a high water activity of 0.997, leading to the low microbiological durability of the material [42]. Following pretreatment, there was no statistically significant decrease in water activity, and this decrease did not significantly impact product durability. The water activity of dried beetroot ranged from 0.294 to 0.377, a range that inhibits the growth of microorganisms. Pretreatment had a positive effect on reducing water activity slightly, with significant reductions observed in samples exposed to ultrasound at a frequency of 35 kHz for 30 min. Similar results for dried beetroots were obtained by [PERSON] et al. [43]. Apparent density is a parameter that reflects changes in the mass of the sample relative to its volume. The preliminary treatments resulted in a reduction in this parameter for the material slices. Except for soaking, other preliminary processes did not cause statistically significant changes in this parameter compared to the raw material. Soaking for 10 min decreased the material density by 4.3%, while extending the soaking time to 30 min led to a 7.5% decrease compared to the untreated material. Furthermore, as the ultrasonic exposure time increased, the reduction in density became less noticeable compared to the untreated raw material. However, statistical analysis did not reveal a significant effect of processing time and the applied ultrasound frequency on changes in the density of the raw material exposed to ultrasound. The drying process resulted in an increase in the density of the beetroot material. Significant differences were observed in the density of the material depending on the type of pretreatment applied to the beet. Beetroot soaked for 10 min exhibited the lowest apparent density. For ultrasound treatments at a frequency of 21 kHz, the apparent density decreased with increasing treatment duration, while for ultrasound treatments at a frequency of 35 kHz, the relationship was reversed. None of these relationships were observed in soaked samples.