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 beta-glucans in solution and gel state were studied. The fine structure of the beta-glucan preparations was assessed by C-13-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 x 10(3), the values of limiting viscosity ([eta]), critical concentration (c**) and coil overlap parameter (c**[eta]) 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 beta-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 beta-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 similar to 25 -35 degreesC; for higher temperatures the I-E values decreased. The dependence of storage modulus on beta-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 degreesC. 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 DeltaH) 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 beta-glucans. Large deformation mechanical tests (compression mode) revealed an increase in strength and a decrease in brittleness of oat beta-glucan gels with increasing M-w. The true stress (sigma(TR)) and strain (epsilon(TR)) at failure of beta-glucan gels increased with increasing polysaccharide concentration reaching a maximum at a certain concentration; at higher concentrations the sigma(TR), and epsilon(TR) values decreased. (C) 2003 Elsevier Science Ltd. All rights reserved.