HETEROGENEITY IN THE STRUCTURE OF WATER-SOLUBLE ARABINOXYLANS IN EUROPEAN WHEAT FLOURS OF VARIABLE BREAD-MAKING QUALITY

The contents of water-soluble, enzyme-extractable, and total nonstarch polysaccharides (NSPs) were determined in six European wheat flours ( Apollo, Slejpner, Sperber, Camp Remy, Minaret, and Soissons) with diff erent mixing time and baking absorption characteristics and, hence, wi dely varying bread-making capacities. The NSP contents varied between 0.42 and 0.69, 1.20 and 1.73, and 1.35 and 2.25%, respectively. The wa ter-soluble NSPs were extracted and fractionated by ethanol precipitat ion into arabinoxylans and arabinogalactans. In the arabinoxylan fract ions, beta-glucan occurred as a minor contaminant. The L-arabinose to D-xylose ratios of such arabinoxylans varied between 0.51 and 0.61, wh ereas the ratios of di- to monosubstituted D-Xylose residues (estimate d by H-1 nuclear magnetic resonance) varied between 0.80 and 1.81. A m ajor proportion of the xylan residues (63-66%) were not substituted by L-arabinose, whereas 13-21% and 16-24% of the D-xyloses were mono-or disubstituted, respectively. From the H-1 nuclear magnetic resonance s pectral data, it could be concluded that the disubstituted D-xyloses w ere either isolated or occurred as pairs in the arabinoxylan chain. Sh odex B-806 chromatography yielded, for all arabinoxylan fractions, thr ee peaks with estimated molecular weights between 8.5 X 10(5) and 5 X 10(3), with only the relative proportions of the peaks, but not their molecular weights, varying with the variety. No apparent relationship was found between the molecular weight profiles and the L-arabinose su bstitution pattern in the arabinoxylans. Correlation coefficients betw een the analytical data obtained for the NSP content in the six Europe an wheat flours and the baking absorption and mixing time of the corre sponding flours increased when the structural variation in the arabino xylans was taken into account, showing that increased knowledge of the arabinoxylan structure may lead to an enhanced understanding of the r ole of NSP in breadmaking.