## 3 Results and Discussion
### Betalain Content Changes after Pretreatments and Drying  
An important indicator of the quality of products derived from beetroot is the content of betalain pigments that are responsible for the reddish-purple color of the vegetable and are its essential bioactive component. Betalain pigments are divided into betanin and vulgaxanthin, which are red and yellow pigments, respectively [56]. Betalains are water-soluble compounds that are sensitive to light and elevated temperature. Therefore, by analyzing the content of betalain pigments, it can assess the degradation resulting from technological processing. Moreover, beetroot extract is a food coloring additive with the symbol E162 and is often used for instant drinks, cakes, or meat products [57]. The quantity of red and yellow betalain content in raw beetroot is presented in Figure 5a,c, respectively. The content of betalain pigments in raw material was 899.2 mg betanin and 668.7 mg vulgaxanthin per 100 g d.m. In the case of fresh material, the application of each form of pretreatment tested in this study caused some variation in the betanin content. However, soaking and US treatment at both 21 and 35 kHz that were carried on for 20 min resulted in a significant increase in this value (15, 18, and 68%). Red colorant content in other samples varied (from \(-\)23 to +4%), but it did not differ in comparison to the untreated sample. In the case of vulgaxanthin content, blanched, soaked for 10 min, and US-treated at 21 kHz for 10 min and at 35 kHz for 30 min samples were characterized by a significantly lower quantity of the compound by up to 10%. There was also an increasing tendency in vulgaxanthin content noticed for the same samples subjected to 20 min long treatments, regardless of the type of treatment used. Usually, higher frequencies make the compression and rarefaction cycle more difficult, making it more difficult to induce cavitation bubble growth due to the shorter time intervals. Conversely, lower frequencies may influence the formation of transient cavitation bubbles [58; 59]. However, in our study, such a trend was not observed. Such observations might have been a consequence of technological treatment, but the diversity of the biological material could be a reasonable explanation as well [60]. Especially considering that ultrasounds are commonly used to improve the extraction yield of bioactive compounds, among others, reduction in betalain content in the US-treated material is therefore expected due to losses during the treatment [61]. [PERSON] et al. [49] found no significant impact of either steaming or sonication times on red and yellow pigments in freeze-dried beetroot juice and pomace. However, in that study, the reduction in betalain content after pretreatment was associated with a temperature increase in the steaming and washing out of the water-soluble dyes. IR-HA drying contributed to a remarkable loss in red (Figure 5b) and yellow (Figure 5d) betalain content. Betanin retention in dried beetroot slices was in the range of 49-74% in relation to the initial content in the pretreated material before dehydration. The highest retention was observed in variants that contained notably more dye before drying. The dried control sample contained 377 mg betanin/100 g d.m., which indicates a degradation of 58% of red dye due to IR-HA drying. In samples treated with 10 min soaking, and 21 and 35 kHz US, a 50, 50, and 42% decrease was observed, respectively, compared to the material before drying. The highest decrease in red dye content was obtained for tissue treated for 20 min with 21 and 30 min with 35 kHz ultrasound (a decrease of about 69%). In the case of yellow pigments, the raw tissue was characterized by its content of 668.7 mg vulgaxanthin/100 g d.m., while after the drying process, the content of this parameter decreased significantly, reaching a value of 334.2 mg vulgaxanthin/100 g d.m. for dried tissue without pretreatment.