Structural Comparison of Arabinoxylans from Two Barley Side-Stream Fractions

The structures of barley (Hordeum vulgare) arabinoxylans isolated from two industrial side fractions, barley husks (BH) and barley fiber (BF), were characterized. Arabinoxylans were extracted with saturated barium hydroxide after enzymatic pretreatment. Barium hydroxide was selective toward arabinoxylans, and only a minor amount of glucose-containing material was coextracted. Acid methanolysis followed by gas chromatography, 1H NMR spectroscopy, and specific enzymatic treatments followed by anion exchange chromatography with pulse amperometric detection (HPAEC-PAD) revealed that the chemical structure of barley husk arabinoxylan (BHAX) clearly differed from that of barley fiber arabinoxylan (BFAX). BFAX was more branched, containing more β-d-xylopyranosyl (β-d-Xylp) residues carrying α-l-arabinofuranosyl (α-l-Araf) units at both O-2 and O-3 positions. BHAX, on the other hand, contained more 2-O-β-d-Xylp-α-l-Araf substituents than BFAX. BHAX and BFAX also differed with respect to the hydrodynamic properties investigated with multidetector size exclusion chromatography. BFAX had a higher weight-average molar mass and larger hydrodynamic volume, the latter indicating less dense conformation than BHAX. M n, M w /M n, R h, and the Mark−Houwink a value were also determined for both arabinoxylans.