## 3 Results and Discussion  
### NaDES Screening for the Extraction of NEPs from Mangosteen Peel  
After the conventional extraction of EPPs from mangosteen peel, the recovery of NEPs from the extraction residue was studied by employing seven different NaDES in combination with UAE (see Table 1 and Figure 1). These NaDES were selected according to different studies found in the literature about the extraction of phenolic compounds from natural sources ([PERSON] et al., 2011; [PERSON] et al., 2012; [PERSON] et al., 2013). The selection of NaDES was of great importance, considering their properties such as viscosity, polarity, physicochemical interactions, and solubility ([PERSON] et al., 2011). For instance, the relatively high viscosity of NaDES at room temperature may decrease the mass transfer rates from the matrix to the solvent. Thus, in the solvent screening, 30% (\(p/v\)) water content was set to reduce solvent viscosities. In addition, UAE was the extraction technique employed because it was based on a phenomenon called cavitation. The ultrasonic waves generated by the ultrasound probe were compressed and stretched, providing small gas bubbles to the liquid medium. These bubbles increased their volumes, achieving a critical size in which the gas phase could not be kept in the bubble because the ultrasound energy was not enough, and then the bubbles imploded, discharging a lot of energy. Therefore, bubble cavitation allowed the separation of the bonds among molecules generating radicals and thus increased the extraction of the compounds ([PERSON] et al., 2013). The UAEconditions employed to select the best NaDES to release NEPs from the extraction residue of mangosteen peel are described in Section 2.4. Total PA content and antioxidant capacity of the extracts obtained with the seven different NaDES based on choline chloride and acids (lactic, citric, and formic acids), polyalcohols (ethylene glycol, sorbitol, and glycerol), and a nitrogen-containing substance (urea) were studied, and the results are shown in Table 2. NaDES composed of choline chloride-sugars were not employed because several studies reported the poor extraction efficiency of these NaDES for the recovery of phenolic compounds from natural matrices [48; 49; 50]. The total PA content that remained in the extraction residue of mangosteen peel was established using two spectrophotometric methods, DMAC and butanol/HCl. At present, for the quantification of PAs, the DMAC assay is favored over others such as the vanillin assay. Despite having similarities, DMAC is considered to have higher sensitivity and in addition, light absorbance is measured at 640 nm in the DMAC assay, reducing the possibility of interference from anthocyanins and being more specific to determine monomers [51]. On the other hand, the butanol/HCl assay involves depolymerization of the polymer in acid and conversion of the monomers to anthocyanin, being more specific when determining polymers than the DMAC assay, which can be spectrophotometrically quantified [52]. According to Table 2, similar results were obtained using both assays, DMAC and butanol/HCl, the extracts achieved with ChCl:LA being the ones with the highest PA content using the DMAC and butanol/HCl assays (25.4 and 2447 mg epicatechin/100 g of sample, respectively). In the DMAC assay, the extracts that also showed high PA content were those collected with ChCl:FA, followed by ChCl:gly and ChClCA (15-18 mg epicatechin/100 g of sample). Nevertheless, the butanol/HCl assay also showed that these NaDES allowed the release of a high total PA content to the extracts with no significant differences (\(p\geq 0.05\)) between the ChCl:FA extract (1767 mg epicatechin/100 g of sample) and the ChCl:Gly and ChCl:CA extracts (1954 and 1493 mg epicatechin/100 g of sample, respectively) (Table 2). In both assays, the extracts with the lowest PA values were obtained with the NaDES ChCl:EG, ChCl:sorb, and ChCl:urea.