Effects of Different Extraction Methods on the Structure, Physicochemical Properties, and Functional Activity of Pectin from Citrus limon (L.) Burm. f. Peel

In this study, the effects of different extraction methods on the structure, physicochemical properties, and functional activities of lemon peel pectin were determined. Pectin was extracted from lemon peels using microwave-assisted acid (MA), ultrasonic-assisted acid (UA), ultrasonic-assisted microwave (UM), and ultrasonic microwave-assisted acid (UMA) extraction procedures and the effects of these four extraction methods on the structural characteristics, physicochemical properties, and functional activities of the pectin were compared. The results showed that, pectins extracted using all four methods had a predominantly rhamnogalacturonan I (RG-I) structure, among which the pectin extracted using UMA was found to have the highest yield, whereas that extracted using UM retained alarger proportion of the RG-I structure. The Fourier transform infrared spectroscopy revealed that the pectins obtained using all four procedures were characterized by typical polysaccharide absorption peaks and lacked a triple helix structure, whereas scanning electron microscopy analysis revealed that pectin extracted using the MA method had a looser structure. Furthermore, compared with the pectin obtained using other extraction processes, that extracted using the UA approach was found to have significantly (P<0.05) higher water retention (22.98±0.24 g/g), oil retention (4.56±0.22 g/g), and emulsifying properties (40.00%±0.10%), whereas the pectin obtained via UM extraction had stronger DPPH· (0.813±0.17 mg/mL), ·OH (0.800±0.12 mg/mL), and ABTS+·(0.731±0.02 mg/mL) radical scavenging properties, and showed superior inhibitory activity against α-glucosidase (3.245±0.07 mg/mL) and α-amylase (6.868±0.22 mg/mL), all significantly (P<0.05) higher than the pectin obtained by enzymatic methods. Collectively, these findings indicates that pectins extracted using the four assessed methods all have hypoglycemic potential, although using different extraction methods can yield pectins with varying functional characteristics (such as water retention, emulsification, antioxidant properties and so on). The findings of this study will provide a valuable reference for the comprehensive utilization of lemon peels.