Raw and extruded fibre from pea hulls. Part II: Structural study of the water-soluble polysaccharides

Abstract

The soluble fraction (9%) obtained by aqueous extraction of an extruded pea hull fibre gave 3 peaks on DEAE-Sepharose-6B. The first population (25%) was not bound to the gel and contained xylose, arabinose and glucose (15:6:1), the second one (12%), eluted for an ionic strength of 0·08 m, contained xylose, arabinose and glucuronic acid (18:1:1), and the third one (63%), eluted for an ionic strength between 0·3 m and 0·8 m, contained galacturonic acid, arabinose, xylose, rhamnose and galactose (5·3:3:1·8:1·7:1). The structure of these fractions was studied through enzymic degradation, methylation, 1H-NMR and 13C-NMR. The first one was composed mainly of a heteroxylan having a (1 → 4)-β- d-xylan backbone (average d.p. 103) with only very few branching points at O-2, and of a slightly branched arabinan. The second one was an acidic xylan. Both heteroxylans were acetylated. The third one was pectic in nature with a main chain consisting of (1 → 4) linked galacturonic acid, partly substituted at the O-2 and O-3 positions, and (1 → 2) linked rhamnose residues which were mainly substituted at the O-4 position. This acidic fraction was appreciably hydrolyzed by an endopolygalacturonase, indicating the presence of homogalacturonan regions in the molecules. Arabinose, galactose and xylose were present as side-chains.