Isolation options for non-cellulosic heteropolysaccharides (HetPS)

The isolation of non-cellulosic heteropolysaccharides (HetPS) from barley husks (Hordeum spp.) and yellow poplar wood chips (Liriodendron tulipifera) was accomplished using mild steam explosion followed by extraction with water and ultrafiltration. The generally low yields, low purity, and low degree of polymerization (DP) improved when the HetPS were isolated following either alkali extraction of hammermilled or disk-refined biomass, or from holocellulose preparations generated by the conventional chlorite method or by organosolv delignification. Several purification methods were examined including precipitation using methanol; treatment with hydrogen peroxide (H2O2) or activated carbon (C) followed by precipitation with methanol; and H2O2-treatment followed by ultrafiltration. The isolation protocols were judged based on product yield, xylan content, and DP. The results indicate that, although steam explosion is effective in removing HetPS from the fiber source, virtually none remain in polymeric form. By contrast, alkali extraction succeeds in separating polymeric HetPS from the fiber source; and HetPS purity increases and polydispersity decreases with fiber prehydrolysis and delignification. Significant processing difficulties were attributed to the intimate association of HetPS with lignin which was effectively disrupted by acid-catalyzed pretreatment and treatment with H2O2. Ultrafiltration of H2O2-treated HetPS solutions represents the best procedure for isolating a xylan-rich polymer in high yield, with high DP and with high purity. Aqueous HetPS solutions can be spray- or freeze-dried into powderous products.