Nuclear Magnetic Resonance and Methylation Analysis-Derived Structural Features of Water-Extractable Arabinoxylans from Barley (Hordeum vulgare L.) Malts Winok Debyser,* ,† Margot E. F. Schooneveld-Bergmans, ‡ Guy Derdelinckx, § Piet J. Grobet, | and Jan A. Delcour † Laboratory of Food Chemistry, Center for Malting and Brewing Science, and Center for Surface Chemistry and Catalysis, Katholieke Universiteit Leuven, Kardinaal Mercierlaan 92, B-3001 Heverlee, Belgium, and Department of Food Science, Wageningen Agricultural University, P.O. Box 6700 EV, Wageningen, The Netherlands Water-extractable arabinoxylans (WEAX) were precipitated with ethanol (65%, v/v) in water extracts of six barley malts. The precipitates consisted of 82-89% arabinoxylan (carbohydrate basis) with arabinose to xylose ratios from 0.60 to 0.76. The WEAX recovered represented up to 50% of all WEAX present in barley malt (43-49.5%). Methylation analysis of these WEAX showed a low proportion of O-3 monosubstituted xylose residues (5.1-7.1%), a high proportion of disubstituted xylose residues (24.8-28.0%), and the presence of O-2-monosubstituted xylose residues (5.1-6.8%). Relative proportions of unsubstituted, monosubstituted, and disubstituted xylose resulting from the methylation analysis were confirmed by 1 H-nuclear magnetic resonance ( 1 H-NMR) spectral data. Calculations from 1 H-NMR spectral data appear to overestimate the level of O-2-monosubstituted xylose residues. Size exclusion chromatography could not reveal significant differences in WEAX molecular weight profiles between the six malts. The molecular weight profiles showed a peak at about 38 kDa. Keywords: Arabinoxylan; barley malt; structure; nonstarch polysaccharides INTRODUCTION Arabinoxylans constitute 4-10% of the barley grain (Henry, 1986; Lehtonen and Aikasalo, 1987). They comprise respectively 20-25% (Fincher, 1975; Ballance and manners, 1978) and 85% (McNeil et al., 1975) of the cell wall polysaccharides of endosperm and aleurone layers. The detailed study of barley arabinoxylans is recent, and contradictory results concerning the impor- tance in the brewing process (Cach and Annemuller, 1995; Schwarz and Han, 1995), more particullary with regard to wort viscosity and/or filtration (Ducroo and Frelon, 1989; Vie ¨tor et al., 1993) and beer haze forma- tion (Coote and Kirsop, 1976), necessitate further work in this area. For barley, structural features of both water- and alkali-extractable arabinoxylans have been reported in the past (Vie ¨tor et al., 1992, 1994; Oscarsson et al., 1996). Vie ¨ tor et al. (1992, 1994) studied structural features of alkali-extractable arabinoxylans of barley malt, but structural data of water-extractable arabi- noxylans (WEAX) from barley malt are, to the best of our knowledge, not available. Arabinoxylans consist of a backbone of -(1f4)-D- xylopyranosyl residues (Xyl), substituted mainly with R-L-arabinofuranosyl residues at O-2 (2-Xyl), O-3 (3- Xyl), or both O-2 and O-3 (2,3-Xyl) (Figure 1). Vie ¨tor et al. (1992, 1994) elucidated the structure of water- insoluble cell wall (WIS) material extracted with barium hydroxide solutions from dehusked barley and malt. Graded ethanol precipitation of WIS resulted in frac- tions in which the arabinose to xylose (Ara/Xyl) ratio increased with ascending ethanol concentration. Cleemput et al. (1995) found similar properties for wheat WEAX. In line with findings for rye (Vinkx, 1995), Vie ¨tor et al. (1992) found a strong correlation between the Ara/Xyl ratio of malt WIS fractions and the relative levels of 2-Xyl and 2,3-Xyl. Arabinoxylans were partly degraded during malting. Malt WIS tended to precipitate at somewhat lower ethanol concentrations than barley WIS, but there was no significant difference in molecular weight distribution between barley and malt WIS. Recently, Oscarsson et al. (1996) studied the WEAX isolated from 16 covered or naked barleys. * Author to whom correspondence should be ad- dressed [tel, (+32) 16 32 15 82; fax, (+32) 16 32 19 97; e-mail, jan.delcour@agr.kuleuven.ac.be]. † Laboratory of Food Chemistry. ‡ Department of Food Science. § Center for Malting and Brewing Science. | Center for Surface Chemistry and Catalysis. Figure 1. Structural elements of arabinoxylans from barley: (A) unsubstituted xylose residue, (B) xylose residue substituted at O-2 with arabinose, (C) xylose residue substituted at O-3 with arabinose, and (D) xylose residue substituted at O-2 and O-3 with arabinose. 2914 J. Agric. Food Chem. 1997, 45, 2914-2918 S0021-8561(97)00001-0 CCC: $14.00 © 1997 American Chemical Society