Effects of Drying Methods on the Structural, Rheological, and Gelation Properties of Lemon Pectin

This study compared the differences in structural, rheological, and gelling properties between pectin extracted from fresh (SF) and dried (GF) lemon peel. The results indicated that drying treatment significantly altered the molecular structure of pectin. SF pectin exhibited higher galacturonic acid content (308.94 mg/g), weight average molecular mass (343.67 kDa), and esterification degree (64.88%), with a structure predominantly composed of linear homogalacturonan (HG). In contrast, GF pectin showed a higher proportion of neutral sugars, greater branching (R₃ = 0.127), lower weight average molecular mass (212.62 kDa), and a reduced degree of esterification (56.57%). Small-angle X-ray scattering analysis revealed that SF pectin possessed a larger radius of gyration (R_g = 22.31 nm) and a smaller cross-sectional radius (R_c = 8.18 nm), indicating an extended, slender, and more rigid rod-like conformation. Conversely, GF pectin exhibited a smaller R_g (22.23 nm) and a larger R_c (8.66 nm), suggesting a more compact molecular conformation with a thicker cross-section. Rheological analysis demonstrated that both types of pectin exhibited shear-thinning behavior and their flow curves were fitted by the power-law model. Due to its more extended molecular conformation, SF pectin exhibited higher viscosity and viscosity coefficient (k). Dynamic oscillatory tests further indicated that SF pectin had higher storage modulus (G') and loss modulus (G”), exhibiting superior gel strength, thermal stability, and mechanical properties (such as hardness) compared with GF pectin. As temperature increased, the viscosity of the system decreased, and the fluid behavior approached that of a Newtonian fluid. SF pectin demonstrated higher flow activation energy, indicating greater temperature sensitivity. This study, from the perspective of molecular conformation, elucidated the mechanism by which drying treatment affected pectin functionality by causing its degradation and structural reorganization, providing a theoretical basis for selecting processing methods for raw materials in lemon pectin extraction.