Impact of extraction methods on the physicochemical and structural characteristics of pomelo peel pectin

Four methods (acid extraction (HAE), acid–ultrasound extraction (US–HAE), alkaline extraction (BE), and alkaline–ultrasound extraction (US–BE)) were used to prepare pomelo peel pectins. The effects of these different extraction methods on the physicochemical and structural properties of pomelo peel pectins were investigated. As a result, HAE and US–HAE showed higher yields (9.93% and 11.72%) and higher molecular weights (M w s 1,756.33 and 1,641.43 kDa), while BE and US–BE showed lower yields (5.28% and 6.79%) and smaller M w s (912.61 and 806.67 kDa). The purity of US–HAE pectin was 7.62% higher than that of HAE, and the purity of US–BE was 3.74% higher than that of BE. Acid-extracted pectins (HAE and US–HAE) exhibited a high degree of esterification (DE), high M w , predominantly homogalacturonan-rich structures, more linear conformations, and a loose lamellar morphology. By contrast, alkali-extracted pectins (BE and US–BE) displayed low DE, low M w , and high branching, which were characterized by more complex rhamnogalacturonan I side chains and a rough, porous surface. The apparent viscosity, gel strength, and emulsifying properties of the acid-extracted pectins were significantly higher than those of alkali-extracted pectins ( p < 0.05). Ultrasound-assisted extraction increased the yield and purity of pectin while reducing its DE and M w without significantly affecting apparent viscosity or gel strength. Ultrasound treatment promoted GalA accumulation in the primary chain, enhancing linearity and increasing the degree of structural fragmentation. Overall, this study provides a theoretical basis for the high-value utilization of pomelo peel pectin and the design of function-oriented extraction processes.