Fractional isolation and physico-chemical characterization of alkali-soluble polysaccharides from sugar beet pulp

Abstract

Pectin rich polysaccharide fractions were extracted with 2% sodium hydroxide at 45°C for 0.5, 1, 2, 3, 4 and 5 h, respectively, from sugar beet pulp. The neutral sugar rich polysaccharide preparations were obtained by treatment of the alkali extracts with pectinase. The pectin rich alkali-soluble polysaccharide–lignin complexes contained 36.7%–46.7% galacturonic acid, 13.7%–16.0% neutral sugars, 9.2%–14.8% protein, 3.6%–9.1% lignin and 18.9%–22.9% ash. The hemicellulose–lignin complex preparations were composed mainly of neutral sugars, in which galactose, rhamnose and arabinose were detected as the major sugar constituents. The weight-average molecular weights of the alkali-soluble polysaccharide–lignin complex fractions (38400–184900 Da) were much higher than those of the hemicellulose–lignin complex preparations (21100–26400 Da). The acetyl groups and methyl esters were found to be fully saponified during the alkali treatments, while feruloyl and p-coumaroyl esters appeared to be more resistant to the alkali conditions. Besides ferulic and p-coumaric acids, six other phenols were also firstly identified in the nitrobenzene oxidation products from all the isolated 12 fractions. These findings indicated that coumaryl, coniferyl and sinapyl alcohols are tightly associated with hemicelluloses and pectin in sugar beet pulp cell walls.