Preparation and Physicochemical Properties of Phenolic Acid-Citrus Pectin Copolymers by Free Radical-Mediated Grafting

In this study, phenolic acid-citrus pectin copolymers was prepared by a free radical-mediated grafting method, and their structure and physicochemical properties were analyzed and compared. The results showed that among five phenolic acid-citrus pectin copolymers, the grafting rate was syringic acid-citrus pectin copolymer was the highest, (74.2 ± 1.38) mg/g, followed by gentisic acid-citrus pectin copolymer, (67.24 ± 1.55) mg/g. Compared with pectin, the molecular mass distribution of the graft copolymers was more homogeneous and decreased significantly. The molecular masses of gentisic acid-citrus pectin and syringic acid-citrus pectin copolymers decreased from (109.98 ± 0.05) kDa to (65.11 ± 0.02) and (39.83 ± 0.05) kDa, respectively, the degrees of esterification increased from (51.62 ± 1.46)% to (70.83 ± 1.64)% and (72.73 ± 2.18)%, respectively, the contents of galacturonic acid increased from (39.18 ± 1.08)% to (52.42 ± 1.36)% and (53.88 ± 1.19)%, respectively, and the solubility increased from (39.34 ± 1.08)% to (54.40 ± 1.36)% and (59.87 ± 1.21)%, respectively. In addition, through Fourier transform infrared spectroscopy and scanning electron microscopy, it was found that the monosaccharide type of phenolic acid-citrus pectin copolymer was identical to that of pectin, and phenolic acid was mainly covalently grafted onto the molecular chain of citrus pectin, resulting in a decrease in its thermal stability, and causing the structure of citrus pectin to change from rough, dense, blocky to flaky with a relatively smooth surface.