## 3 Results
### Optical Properties of Red Pickly Pear Juice
Stable pH values achieved through UV-C radiation or thermal treatment were also reported for pomergnanate [15]. Color measurement is an important evaluation of juice quality. Color changes in red prickly pear juice are mainly due to the betalain pigments (betacyanins or betaxanthins). \(L^{*}\) is a parameter that measures the luminosity (0 = black, 100 = white) of the juice. It is observed that \(L^{*}\) values ranged from 51.96 to 59.34 (Table 4). A difference was noted in \(L^{*}\) values between UV-C doses, thermal treatment, and pH. The juice has less turbidity at a pH of 3.6 (Table 1), which will consequently result in a high \(L^{*}\) value. Juices at both pH levels (3.6 and 7) treated with the highest UV-C radiation dose and HTST showed the same tendency in luminosity due to the loss of compounds such as betalains and polyphenols. In this study, there was a significant change (\(p<0.05\)) in \(L^{*}\) during storage of processed juices, which increased with the number of days compared to the control (untreated juice). Juice treated with UV-C radiation showed a significantly lower \(L^{*}\) value (\(p<0.05\)) during storage compared to UHT treatment, indicating the degradation of red pigments with this thermal treatment. [PERSON] et al. [6] evaluated a beet-based drink that was pasteurized at 96 \({}^{\circ}\)C at different times (540, 720, and 900 s) and stored at 27-30 \(\pm\) 2 \({}^{\circ}\)C for 180 days, evaluating the juice every 15 days, reporting a significant difference (\(p<0.05\)) in the color parameters due to betalain degradation. The color parameter \(a^{*}\), which represents the trend from green (\(-\)) to red (\(+\)), presented values ranging from 31.38 to 63.98 (Table 4), indicating a trend towards red (Figure 1). Color degradation was minimal for UV-C treatments, preserving the red prickly pear juice color, but differed significantly (\(p<0.05\)) during storage. Juices with a pH of 3.6 presented stable \(a^{*}\) values during storage, indicating betalain stability at an acidic pH. The acidification of juice using citric acid is advantageous due to its chelating property, which improves the stability of betalains against factors such as temperature, light, and the presence of oxygen [35]. Regarding the juice at pH 7, \(a^{*}\) values remained stable in untreated juice due to the pH decrease during storage resulting in greater stability (Table 3), while \(a^{*}\) values decreased slightly during storage for UV-C treatments. Conversely, thermal treatment (UHT 130 \({}^{\circ}\)C/3 s) significantly increase (\(p<0.05\)) \(a^{*}\) values during storage in juice at pH 7. Temperature is the main factor affecting betalain stability, changing its hue value from a characteristic red to brownish-red tonality [36]. [PERSON] et al. [6] reported that betacyanin changes to yellowish-brown from a deep-violet-red in beet-based drinks pasteurized at 96 \({}^{\circ}\)C at different times (540, 720, and 900 s). The color parameter, \(b^{*}\), represents the color trend from blue (\(-\)) to yellow (\(+\)), which ranged from 9.98 to 33.92 (Table 4), with a tendency to yellow color. This parameter differed significantly (\(p\) < 0.05), with the highest values recorded in the juice at pH 7.0. A significant difference (\(p\) < 0.05) between UV-C radiation and thermal treatments (HTST and UHT) was observed. Increased color degradation was observed at higher temperatures due to the thermolability of the color compounds. The juices at pH 3.6 treated with UV-C radiation presented higher values of the parameter \(b^{*}\) compared to the thermal treatment. However, at pH 7, an increase in the \(b^{*}\) value is observed in UV-C and thermal treatments, so we can observe that the UV-C treatments show an increase during storage, while the thermal one presents an opposite behavior.