Influence of structure on the physicochemical properties of wheat arabinoxylan

Abstract

A water-soluble arabinoxylan from wheat flour was purified and fractionated by a graded ammonium sulphate fractionation technique. The yields (based on the total amount of the material recovered) of the four polysaccharide fractions obtained at 60% (F60), 70% (F70), 80% (F80), and 95% (F95) saturation with (NH 4) 2SO 4 were 58·8%, 29·2%, 7·6%, and 4·4%, respectively. Monosaccharide, 1H-NMR, and 13C-NMR analyses revealed differences in structural characteristics among the fractions. F60 had the lowest arabinose:xylose ratio (0·58), followed by F70 (0·71), F80 (0·85), and F95 (0·88). The relative amount of Xyl p residues doubly substituted at O-2 and O-3 with Ara f increased substantially from F60 to F95, whereas the amount of unsubstituted and monosubstituted Xyl p decreased in the same order. The content of ferulic acid was much higher in F60 than in the remaining samples; F60 also showed the highest intrinsic viscosity (4·70 dl/g) followed by F70 (4·20 dl/g), F80 (3·16 dl/g), and F95 (1·90 dl/g). Calculated coil overlap parameter values ( c ∗[η]: 1·20-1·30 ) were much lower than those of other random coil polysaccharides, implying a rather stiff/extended chain conformation. Oxidative gelation of arabinoxylan fractions was probed by dynamic rheometry. Fraction F60 formed rigid gel networks, as assessed by the storage modulus ( G′) and tan δ ( G′' G′ ) values, while F70 and F80 exhibited poor gelling capacity. The arabinoxylan fractions were also effective in stabilizing protein foams against thermal disruption in the order of F60 ? F70 > F80.