The predominant polymer in the water-soluble pentosans (Fig.3) consists of a straight chain of anhydro-D-xylopyranosyl residues linked β-1,4 with an anhydro-L-arabinofuranosyl residue at the 2,3 position. Pentosans are generally classified on the basis of solubility in cold water, which is determined by the extent of arabinose branching of the xylose chain. Fincher
and Stone (1974) clarified the situation by obtaining two distinct fractions when treating water-soluble pentosans with saturated (NH4)2SO4. The soluble fraction was found to be arabinogalactan covalently linked to a peptide. The precipitate, arabinoxylan contaminated with free protein, was later shown by Yeh et al. (1980) to contain ferulic acid (Fig.4).