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Structural Variation and Levels of Water-Extractable Arabinogalactan-Peptide in European Wheat Flours

¹Katholieke Universiteit Leuven, Laboratory of Food Chemistry, Kardinaal Mercierlaan 92, B-3001 Heverlee, Belgium. ²Corresponding author. E-mail: annemie.loosveld@agr.kuleuven.ac.be ³Department of Food Science, Wageningen Agricultural University, P.O. Box 6700 EV, Wageningen, The Netherlands. ⁴Center for Surface Chemistry and Catalysis, Kardinaal Mercierlaan 92, B-3001 Heverlee, Belgium.

Water-extractable arabinogalactan-peptides (WE-AGP) were isolated from flour of eight different wheat cultivars (harvest year 1996). Little structural variation in WE-AGP of flour of the different wheat cultivars was observed. The arabinose-to-galactose (A/G) ratio of WE-AGP varied between 0.66 and 0.73. Methylation analysis showed that the proportion of β-1,3-galactopyranosyl residues is almost equal for the different WE-AGP samples (10.2–11.1%). More variation was observed for the proportion of the β-1,6-galactopyranosyl residues (9.4–13.0%) and β-1,3,6-galactopyranosyl residues (76.0–80.1%). The ¹H-nuclear magnetic resonance spectra (D₂O, 85°C, 300 MHz) were comparable, and gel permeation analysis consistently yielded a narrow peak with an apparent molecular weight between 5.0 × 10⁴ and 10.0 × 10⁴. Interpretation of the results was facilitated by α-L-arabinofuranosidase debranching of WE-AGP. For flour samples of 18 wheat cultivars (10 from 1994 harvest, eight from 1996 harvest), the variation in percentage of water-extractable arabinoxylan (WE-AX) (0.31–0.78% of dry matter) was much larger than the variation in percentage of water-extractable arabinogalactan (WE-AG) (0.24–0.33%). The ratio of WE-AX to WE-AG for flour samples of different wheat cultivars varied between 1.00 and 2.44.