Molecular size effects on rheological properties of oat β-glucans in solution and gels

The effects of molecular size on rheological properties of highly pure oat β-glucans in solution and gel state were studied. The fine structure of the β-glucan preparations was assessed by 13C-NMR spectroscopy and high-performance anion-exchange chromatography (HPAEC) of the cellulosic oligomers released by the action of lichenase. The tri- and tetra-saccharides accounted for 90.8–96.4% of the total oligomers analyzed and the calculated molar ratios of trimers/tetramers varied between 1.95 and 2.30. Molecular size characterization was carried out with high performance size exclusion chromatography (HPSEC) combined with a refractive index detector; for samples with apparent molecular weight (Mw) of the peak fraction of the main eluting peak ranging between 35 and 250×103, the values of limiting viscosity ([η]), critical concentration (c∗∗) and coil overlap parameter (c∗∗[η]) were within 0.67–3.83 dl/g, 0.70–2.10 g/dl and 1.41–2.67, respectively. The shear thinning and viscoelastic behavior of fresh β-glucan solutions were typical of a random coil type polysaccharide, and dependent on the molecular size and concentration of the polymer. On the other hand, time-dependent rheological behavior was revealed by thixotropic loop experiments for the lowest molecular size sample, implying the formation of intermolecular networks. All β-glucan samples, except the preparation with the highest Mw, were able to form gels, as revealed by dynamic rheometry; with increasing Mw the gelation time increased and the gelation rate View the MathML source decreased. The gelation rate also increased with increasing concentration and gel curing temperatures reaching a maximum at ∼25–35 °C; for higher temperatures the IE values decreased. The dependence of storage modulus on β-glucan concentration (C) followed power law relationships; G′ varied as C7.2–7.5. A two-step melting behavior was observed for gels cured under conditions favoring a quick gelation. Differential scanning calorimetry (DSC) showed that gels exhibit rather broad endothermic gel→sol transitions at 55–75 °C. The DSC kinetic data showed similar responses to those from dynamic rheometry; the rate of endotherm development increased with decreasing molecular size of the polysaccharide. Also, the apparent melting enthalpy values (plateau ΔH) increased with decreasing molecular size. The melting temperature of the gel network, as determined by DSC and dynamic rheometry, was found to increase with the molecular size of β-glucans. Large deformation mechanical tests (compression mode) revealed an increase in strength and a decrease in brittleness of oat β-glucan gels with increasing Mw. The true stress (σTR) and strain (εTR) at failure of β-glucan gels increased with increasing polysaccharide concentration reaching a maximum at a certain concentration; at higher concentrations the σTR, and εTR values decreased.