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

Abstract

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 ( M w) of the peak fraction of the main eluting peak ranging between 35 and 250×10 3, 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 M w, were able to form gels, as revealed by dynamic rheometry; with increasing M w the gelation time increased and the gelation rate (I E =[ d( log G′)/ dt] max ) decreased. The gelation rate also increased with increasing concentration and gel curing temperatures reaching a maximum at ∼25–35 °C; for higher temperatures the I E values decreased. The dependence of storage modulus on β-glucan concentration ( C) followed power law relationships; G′ varied as C 7.2–7.5. 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 Δ H) 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 M w. The true stress ( σ TR) and strain ( ε TR) at failure of β-glucan gels increased with increasing polysaccharide concentration reaching a maximum at a certain concentration; at higher concentrations the σ TR, and ε TR values decreased.