Primary Structural Transformations of Water-Soluble Polysaccharides in the Granulated Juice Sacs of Citrus changshanensis

Granulation is a commonly seen internal disorder of citrus fruit, characterized by decreased juice yield, thickened cell wall, and abnormal hardened juice sac. The present study compared the juice sacs of Citrus changshanensis at different granulation degrees (healthy, gelatinous, moderately granulated, and severely granulated) regarding their soluble polysaccharide content and structural changes of water-soluble polysaccharides. The contents of water-, CDTA-, Na₂CO₃-, 1 mol/L KOH-, and 4 mol/L KOH-soluble polysaccharides increased with the granulation degree. The water-soluble polysaccharides were further fractionated into water eluent (WA), 0.05 mol/L NaCl eluent (0.5S), 0.1 mol/L NaCl eluent (1S), and 0.2 mol/L NaCl eluent (2S) by DEAE-Sepharose chromatography. The detailed structure of these fractions was analyzed using high-performance liquid chromatography, Fourier transform infrared spectroscopy (FT-IR), and nuclear magnetic resonance spectroscopy. Structural changes were evident in the granulated samples compared to the healthy samples, especially in the 0.5S and 1S fractions. As granulation progressed, the degree of methyl esterification in homogalacturonan (HG) regions decreased, and the proportion of rhamnogalacturonan I (RG-I) regions increased. Concurrently, the branching side chains of RG-I, composed of arabinans and arabinogalactans, increased significantly. In line with these structural changes, the viscosity of the water-soluble polysaccharide fractions also increased with granulation degree.