Polysaccharides from Sunflower Stalk Pith: Chemical, Structural, and Partial Physicochemical Characterization
Polysaccharides from sunflower stalk pith (SSP) were extracted by hot water and sodium hexametaphosphate (SHMP). The chemical analyses suggested both fractions dominated in galacturonic acids with small amount of neutral sugars, indicating their identity as pectins. The yield of water-extractable pectin (WEP) and SHMP-extractable pectin (SEP) were 1.8% and 11.0%, respectively. The detailed structure of each fraction was investigated by high performance size exclusion chromatography (HPSEC), FT-IR spectroscopy, methylation analysis and 1D & 2D NMR spectroscopy. The weight average molecular weight and intrinsic viscosity for WEP and SEP were 57.2 kDa, 0.48 dL/g and 23.1 kDa, 0.50 dL/g, respectively. FT-IR spectra confirmed WEP and SEP had characteristic pectin-like structures and the degree of esterification (DE) was calculated to be 92% and 64 %, respectively. Methylation analysis demonstrated that 1,4-GalpA residues dominated all the sugar linkages, while other linkage types including 1,2-Rhap, 1,2,4-Rhap, 1,4-Galp and 1,5-Araf were also present in WEP and SEP. NMR spectroscopy further revealed WEP and SEP mainly homogalacturonan (HG) structure and a small fraction of rhamnogalacturonan-I (RG-I) structure was also in evidence. WEP and SEP showed limited surface activity within a concentration up to 1 % (w/w), compared with commercial citrus pectin. Residue fraction (RF) after pectin extraction, which was mainly composed of insoluble cellulose and hemicellulose, had the best water holding and oil binding capacities among all the examined fractions.