Isolation and characterisation of cell wall polymers from the heavily lignified tissues of olive (Olea europaea) seed hull

Abstract

Cell wall material (CWM) was prepared from olive seed hull, which is heavily lignified and very tough. The material was cryomilled and delignified with chlorite/acetic acid for 9 h to give the holocellulose. Polymers were solubilised from the holocellulose by sequential extraction with cyclohexane-trans-1,2-diamine-NNN'N'-tetra-acetate (CDTA, Na salt), DMSO, 0.5, 1 and 4 m KOH and 4 m KOH + borate to leave the α-cellulose residue. The suspension of α-cellulose on neutralisation released a small amount of pectic material virtually free of xylan to give α'-cellulose. The polymers from the various extracts were fractionated by graded precipitation with ethanol prior to anion-exchange chromatography, and selected fractions were subjected to methylation analysis. During delignification, glucuronoxylans with relatively low degrees of polymerisation (DP) and xylan-pectic polysaccharide complexes linked to degraded lignin were solubilised. A proportion of the xylan-pectic polysaccharide complexes were solubilised by 0.5 m KOH. The major hemicellulosic polysaccharides of the olive seed hulls are glucuronoxylans, which occur as highly branched short chains, with DP of 30–60; or slightly branched chains with DP of 90–110. Partial acid hydrolysis of the major acidic xylan, gel-filtration chromatography and methylation analysis allowed us to propose a tentative structure for the major glucuronoxylan in which one residue of GlcpA occurs in each 14 continuously linked Xylp residues in a regular structure.