EFFECT OF MOLECULAR-SIZE ON PHYSICAL-PROPERTIES OF WHEAT ARABINOXYLAN

A water-soluble wheat arabinoxylan was fractionated by molecular sieve chromatography into five polysaccharide fractions with limiting viscosities of 8.5 (F1), 6.2 (F2), 4.3 (F3), 3.8 (F4), and 3.4 (F5) dL/g. Compositional, C-13 NMR, and H-1 NMR analyses revealed very little structural variation among these fractions. The flow behavior of arabinoxylan fractions in dilute and concentrated solutions showed two critical concentrations (c* and c**). Oxidative gelation of the arabinoxylan fractions, involving intermolecular cross-linking via feruloyl residues, was followed by dynamic rheometry. Almost linear relationships between the storage modulus (G') of the gel network and polymer concentration were found for all fractions. However, while G' values of F1 and F2 increased very rapidly with increasing polymer concentration, F3, F4, and F5 exhibited much less concentration dependence of G'. Positive linear relationships between rigidity (G') of cross-linked arabinoxylan gels and intrinsic viscosity [eta] of arabinoxylan fractions were found. The effectiveness of cross-linking reactions, n(e(exp))/n(e(theor)) (ratio of cross-link density experimentally determined to the theoretical value), increased with increasing molecular size of the arabinoxylan fractions. Fractions that formed networks with a high density of cross-linking (F1 and F2) were the most effective in their ability to entrap and hold water. The arabinoxylan fractions were also effective in stabilizing protein foams against thermal disruption in the order F1 > F2 > F3 > F4 > F5.