## 3 Results and Discussion
### Color Changes after Pretreatments and Drying  
Color is an important attribute of a food product that serves to assess its quality and gain consumer preference. Subjecting fruits and vegetables to heat treatments may lead to the non-enzymatic browning and destruction of the pigment. These phenomena can result in color changes in dried tissues [47]. [PERSON] et al. [48] stated that the use of an appropriate US pretreatment duration could not only accelerate the drying process but also effectively reduce color change. The raw beetroot was characterized by the value of the L* parameter amounted to 29.3 \(\pm\) 1.5 (Figure 3a). Samples subjected to US, with parameters other than 35 kHz and 20 min, had a statistically significant increase in the L* parameter, compared to the untreated tissue. However, no significant effect of the time and frequency of US pretreatment used on the brightness of the beetroot tissue was noted. Soaking for 10 min also increased the brightness of the beetroot, while blanching as well as soaking for longer than 10 min did not significantly change the value of this parameter. Other researchers also observed that beetroot samples (juice and pomace) after 10 or 15 min of US pretreatment were lighter than untreated samples, while samples after thermal steam pretreatment for 10 or 15 min did not differ significantly from the untreated samples [49]. The drying process increased the brightness of the dried materials (Figure 3b). A similar effect was obtained by [PERSON] and [PERSON] [50] for the beetroot dried with hot air drying, where the lightness of the beet increased throughout the entire drying duration. Meanwhile, [PERSON] et al. [51] stated that the brightness increases according to the increase in hot air drying temperatures. The lowest growth of the L* parameter was recorded for the untreated and blanched samples, amounting to 1.1 and 1.9, respectively. In the other variants for soaked and US-treated, the growth ranged from 5 to 11. The tissues treated with US were significantly brighter, compared to the untreated one, which is visible also in the macroscopic photographs of beetroot dried with the infrared-hot air method presented in Figure 4. Furthermore, it was found that there was an increase in the L* parameter as the time of ultrasonic treatment increased. An increase in the brightness of dried materials could be a result of the leakage of soluble solids, including betalain pigments, during pretreatment processes from the beetroot tissue to the water [52]. When a pretreatment of at least 10 min was used, the values of the L* parameter were noticeably greater as the leakage lasted longer. Additionally, the changes in brightness may be related to the leakage of pigments from the tissue. However, the colorimeter measures only surface color and does not assess the actual pigment content [50]. In addition, the higher values of the L* parameter in the dried fruit may have been due to the method of measuring, in which radiation reflected from the sample is recorded. In the undried beetroot samples, the water content was higher, so that part of the radiation was absorbed, and a smaller amount was reflected from the surface and measured by the device [53]. Similar results were obtained in the study of [PERSON] et al. [52], in which pretreatment in the form of blanching did not affect the lightness of dried beetroot, whereas US pretreatment resulted in an increase in brightness by over 50% in relation to dried material without any pretreatments. The value of the a* parameter, which accounts for the proportion of the green (\(-\)) and red (\(+\)) color, for the raw tissue was 18.7 \(\pm\) 2.2 (Figure 3c). As a result of blanching, the value of the a* parameter decreased statistically significantly by 30% to a value of 13.1. This could be related to the temperature sensitivity of the red pigment as betacyanin [50].