## 3 Results and Discussion
### Comparison of MW-HPCO\({}_{2}\) and CE Methods
The reducing sugar contents of MW-HPCO\({}_{2}\)-GPP and CE-GPP were 7.68 \(\pm\) 0.22% and 8.43 \(\pm\) 0.23%, respectively. Reducing the sugar content is necessary for jam as it can increase the strength of the gel. Additionally, sucrose added to jam partially turns into invert sugar. High reducing sugar content of pectin may reduce its susceptibility to the Maillard reaction and may also indicate deterioration of the pectin structure [41; 42]. The color of pectin is one of the important quality characteristics that determines its commercial value. Pectin color is significantly affected by its pigment content and phenolic compounds. Additionally, reducing the sugar content of pectin is another important parameter that affects its color through the Maillard reaction that occurs during the extraction process [35]. Pectin obtained with MW-HPCO\({}_{2}\) had a lighter color (higher luminosity, L*) and higher yellowness (b*) values than that obtained by CE (\(p\) < 0.05), but the difference in redness (a*) value of GPP obtained by MW-HPCO\({}_{2}\) and CE was not significant (\(p\) > 0.05). The dark color of CE-GPP can be explained by the long processing time that causes a browning reaction in CE. The apparent viscosity of pectin is significantly related to its molecular weight. Both apparent viscosity and molecular weight of pectin obtained by MW-HPCO\({}_{2}\) extraction were higher than those obtained by CE (Table 3). The difference in viscosity of GPP is associated with differences in extraction procedure, such as heating method, which results in different methoxyl content [26]. High molecular weight and high viscosity of pectin enhance emulsion stability [43]. Hence, MW-HPCO\({}_{2}\)-GPP can be useful as a thickener in foods or beverages. The viscosity of MW-HPCO\({}_{2}\)-GPP is considerably higher than the viscosity of citrus pectin and fig stalk pectin reported by [PERSON] and [PERSON] [44]. Also, [PERSON] et al. [45] conducted a comparison of the viscosities of pectin extracted from waste cirrus peel. Their results showed that grapefruit peel pectin exhibited a higher viscosity than other citrus pectins (lemon peel, orange peel, and pomelo peel pectin). SEM images of GP before and after pectin extractions are shown in Figure 2. It is clear to see that while untreated GP has a smooth flat surface structure, both CE and MW-HPCO\({}_{2}\) extraction processes caused a dramatic change to GP by creating porous structure and pits on the GP surface. However, MW-HPCO\({}_{2}\) extraction caused more deformation of the GP morphology with a wrinkled surface and numerous fragments. These results support the high extraction yield in MW-HPCO\({}_{2}\). Previously, [PERSON] et al. [21] studied the effect of microwave-assisted extraction combined with pulsed electric field on pectin extraction from jackfruit peels and reported that a major disintegration in the physical structure of jackfruit peel occurred after treatment, leading to an increase in the extraction yield. In this study, the greater fragmentation of GPs after MW-HPCO\({}_{2}\) treatment compared to the CE method supports the significant differences in pectin yield between two methods. The FTIR spectra of MW-HPCO\({}_{2}\)-GPP and CE-GPP revealed that both pectins had same characteristic peaks and no difference was observed between them (Figure 3). Generally, the FTIR band of MW-HPCO\({}_{2}\)-GPP was more intense than that of CE-GPP. The samples extracted by MW-HPCO\({}_{2}\) and CE techniques had typical absorption peaks of pectin, considering previous studies on pectin [32; 43]. The large absorption peak at around 3300 cm\({}^{-1}\) was caused by stretching of hydroxyl groups (O-H) and the peak at 2900 cm\({}^{-1}\) was attributed to the C-H stretching of the CH\({}_{2}\) groups. The absorption at 1743 cm\({}^{-1}\) represents the C=O stretching vibration of ester carbonyl, while the absorption around 1600 cm\({}^{-1}\) represents the C=O stretching vibration of the carboxyl group.