## 3 Results and Discussion  
### Optimization of US-Assisted Extraction Conditions Through Taguchi Design  
#### 3.1.1 TPC and Antioxidant Capacity of OM Peel and Pulp Extracts  
The optimization of extraction parameters was crucial to enhance efficacy and ensure a more sustainable extraction process [29; 30]. The solvent composition was considered essential, as its features significantly influence chemical extraction [31]. Water is an ecological solvent, renowned for its remarkable ability to extract polar molecules (including the water-soluble pigments betalains), as well as for its affordability and non-toxic properties for human consumption [27]. On the other hand, ethanol, which is also a food-grade solvent, was utilized to extract betaxanthins [32]. Water was compared with ethanol as a solvent for the optimal extraction of these compounds, therefore endorsing the food-grade applicability of the extracts [33]. The Taguchi design was employed to examine the extraction conditions using as few as nine design points to identify the optimal approach to generate high-value extracts. In addition, a conventional STE (water as solvent at 40 \({}^{\circ}\)C for 2 h) was used as a means of comparing the effectiveness of the UPAE technique. The STE conditions were obtained from a previous study by [PERSON] et al. [20]. The authors optimized the pigment extraction procedure of _Opuntia_ species in which critical extraction parameters, such as temperature and duration, were studied. They found that the optimum condition to extract total betalains was to use water as a solvent for 115 min at 40 \({}^{\circ}\)C. [PERSON] et al. [34] also stated that water was found to be beneficial in the extraction of bioactive compounds from several _Opuntia_ species, including _O. macorrhiza_, when compared to a 50% hydroalcoholic mixture. Regarding the extraction temperature, several studies [2; 17; 18] have emphasized that the ideal extraction temperature should not exceed 40 \({}^{\circ}\)C, since higher temperatures may result in the disintegration of betalains and the depigmentation of the extracts, an observation confirmed in our preliminary experiments. Finally, the rationale for this comparison between UPAE and STE was that the target molecules can be successfully extracted using cavitation, in addition to the environmental sustainability of the method compared to conventional extraction [35]. The results of the TPC, FRAP, and DPPH assays across OM peels and pulp are presented in Table 4. The vast impact of each extraction parameter was observed in all assays. The TPC results revealed a fivefold difference among the samples, as it ranged from 2.77-12.44 in peels and 5.51-13.47 mg GAE/g in pulps, with design points 8 and 9 having the lowest values in both cases. Similar results were observed in the study by [PERSON] et al. [34]. The authors used pure water or 70% aqueous methanol solution as extraction solvents, using a US bath to extract bioactive compounds from OM fruit (peels and pulp). The results revealed that water was a more efficient solvent than the hydromethanolic solution to recover polyphenols, as peels yielded 10.9 and pulp yielded 12.1 mg GAE/g dw. In a more recent study by [PERSON] et al. [36], bioactive total polyphenols from Southern African _O. ficus-indica_ peels and pulp were explored using acidified water, ethanol, acidified methanol, or hexane. The yielded polyphenols widely ranged in terms of both pulp (4.25-14.83 mg GAE/g dw) and peels (9.10-16.51 mg GAE/g dw). Water acidification was not found preferable as it yielded the lowest possible total polyphenols in both cases. Concerning the antioxidant capacity, the range of both OM peels and pulp in the FRAP assay varied from 6.16-39.06 and 22.00-49.22 umol AAE/g dw, respectively. The corresponding values in the DPPH assay had a range of 13.09-42.98 and 23.56-54.73 umol AAE/g dw, respectively. It could be observed that pulp had higher values not only in TPC but also in both antioxidant capacity assays.