Extraction, fractionation, structural and physical characterization of wheat β-d-glucans

A high purity wheat β-d-glucan (91.58%) was obtained from white wheat bran after alkali extraction and multi-precipitation with ammonium sulphate compared to previous reported purity of only 70%. The molecular weight (Mw) of the purified wheat β-d-glucan was 487,000 g/mol with a broad Mw distribution (polydispersity is Mw/Mn =1.65). By applying an ammonium sulphate gradient precipitation method, the purified wheat β-d-glucan was fractionated into six fractions with Mw ranged from 43,000 to 758,000 g/mol and much narrower Mw distribution (polydispersity is Mw/Mn=1.03 to 1.26). Structural analysis revealed that there were no significant differences between the six fractions and between the fractions and the original sample. This set of sample was used to investigate the effect of Mw on physical properties of wheat β-d-glucan. Dynamic rheometry and scanning calorimetric studies revealed that the gelation rate and the melting enthalpy (ΔH) of wheat β-d-glucan increased with the decrease of molecular weight indicating smaller wheat β-d-glucan molecules (must be above a minimum Mw) is easier to form junction zones and establish stronger three-dimensional network due to their high mobility and structural regularity (high ratio of tri/tetra). In contrast, the melting temperature of wheat β-d-glucan gels increased with the increase of molecular weight, suggesting a more extended structural network was formed for high Mw wheat β-d-glucans.