Isolation and characterization of high molar mass water-soluble arabinoxylans from barley and barley malt

Abstract

Water-soluble arabinoxylans have been fractionated from barley and barley malt by size exclusion chromatography to obtain high molar mass arabinoxylans involved in the viscosity of wort and beer. In both cases, five fractions were obtained: AXb₁, AXb₂, AXb₃, AXb₄ and AXb₅ for barley and AXm₁, AXm₂, AXm₃, AXm₄ and AXm₅ for barley malt. For barley and malt fractions, Ara/Xyl ratio, molar mass and intrinsic viscosity decreased as their elution volume increased. $\text{M}\text{̄}{}_{\mathrm{<mtext>w</mtext>}}$ of barley fractions were higher than that of malt fractions, attesting arabinoxylans degradation during malting. NMR and methylation studies revealed that whatever the fractions, approximately 50% of the xylose residues were un-substituted. 2,3-Xyl level decreased from AXb₁ to AXb₅ and from AXm₁ to AXm₅, whereas 3-Xyl level increased at the same time. A non negligible amount of 2-Xyl was found, this level being almost the same (6–8%) for all the fractions. AXm₁, which exhibited the highest intrinsic viscosity, (240 ml/g) was considered as an interesting fraction with regard to brewing problems. In spite of its highly branched structure, AXm₁ was easily degraded by xylanases. Evidence for the presence of ferulic acid dimers in the water-soluble arabinoxylans from barley and barley malt is provided.