## 3 Results
### Optical Properties of Red Pickly Pear Juice
The thermal treatments significantly affected (\(p<0.05\)) the antioxidant activity, decreased by 56.73 and 58.13% for HTST and UHT on day 0, respectively. Although phenolic compounds and betalains are considered thermolabile, they are components that present antioxidant activity. A decrease of 37.9 and 40.79% was obtained for HTST and UHT after 20 days of storage, respectively. This could be due to the partial regeneration of betalains when exposed to optimal storage conditions [39]. Due to storage under stable conditions, antioxidant activity stabilized after five days. Whit respect to pigments, betacyanin content in untreated juices was 16.65 and 15.66 mg/L at pH 3.6 (Figure 4a) and 7 (Figure 4b), respectively. UV-C treatment at day zero presented values of 13.87 mg/L (31.87 mJ/cm\({}^{2}\)) to 15.22 mg/L (9.81 mJ/cm\({}^{2}\)). HTST treatment at day zero presented a value of 14.18 mg/L in juice at pH 3.6. UV-C treatment at day zero presented values of 12.4 mg/L (31.87 mJ/cm\({}^{2}\)) to 15.01 mg/L (9.81 mJ/cm\({}^{2}\)) and UHT treatment of 5.87 mg/L in juice at pH 7. UV-C irradiation significantly affected (\(p<0.05\)) the pigment content at pH 7. The betalain transformation in the presence of light (UV and visible range) leads to electronic excitation, characteristic of the betalain chromophore [2]. Additionally, in these juices, it was observed that both pigments, betacyanins (red-purple pigments) and betaxanthins (orange-yellow pigments), were significantly affected (\(p<0.05\)) by storage time. The treatments at pH 7 (Figure 4b) showed  
Figure 2: Effect of UV-C irradiation and thermal treatment (HTST or UHT) on total polyphenols (mg GAE/L) in red prickly pear juice: (**a**) juice at pH 3.6 (**b**) juice at pH 7. -- Thermal treatment --- Juice untreated; -- UV-C (31.87 mJ/cm\({}^{2}\)), -- UV-C (15.13 mJ/cm\({}^{2}\)), -- UV-C (9.81 mJ/cm\({}^{2}\)). that betacyanin contents decreased as storage time for UV-C treatments. The betalains may be altered/transformed/degraded during storage since they are sensitive to several factors such as pH, and the optimal pH for betalains stability ranges from 5.5 to 5.8 [2]. All UV-C treatments in juices at pH 3.6 showed betacyanin (Figure 4a) stability. Citric acid, which was used to acidify the samples, is considered a stabilizer of these pigments [35]. Changes were noted at doses of 15.13 and 31.78 mJ/cm\({}^{2}\), and immediately after the process, the reductions were 11.11 and 16.69% for betacyanin in juice at pH 3.6 (Figure 4a) and 19.66 and 20.81% for juice at pH 7 (Figure 4b), respectively. Values similar to the 22% reduction were reported in the Aloe vera and pitaya blend processed by UV-C light [30]. Contents observed at lower doses were similar to those of non-treated juice. At a dose of 9.87 mJ/cm\({}^{2}\), betacyanin reductions of 8.58 and 4.15% were observed in juice at pH 3.6 and 7 (Figure 4a,b), respectively. Thermal treatments significantly affected (\(p<0.05\)) both betalains. Both HTST and UHT treatments resulted in diminished pigment content. HTST treatment (80 \({}^{\circ}\)C/30 s) reduced (\(p<0.05\)) the 14.83% of betacyanins and 14.39% of betaxanthins compared to the untreated juice. UHT also significantly decreased (\(p<0.05\)) betacyanin and betaxanthin contents by 63.09 and 45%, respectively, in comparison with untreated juice. Processing at high temperatures causes hydrolysis, generating betalamic acid and cyclo-dopa-5-O-glucoside [5]. However, betalains increased from day 5, caused by betalain regeneration due to the hydrolysis occurring due to the high temperature, which could be partial in the bond of betalamic acid, and therefore this bond can be conjugated with precursor amino acids, such as proline [45]. Additionally, after storage, the content of pigments at pH 3.6 reaches the UV-C treatments by the partial regeneration of betalains when exposed to optimal storage conditions [39]. [PERSON] et al.