A comparative study on structure–function relations of mixed-linkage (1→3), (1→4) linear β- d-glucans

Abstract

The effects of fine structure and molecular size on the rheological properties of six mixed-linkage (1→3), (1→4)-β- d-glucans (β-glucans) in the solution and gel state were studied. Molecular size characterization was carried out with high-performance size exclusion chromatography combined with a refractive index detector. Samples were divided into two groups according to the values of apparent molecular weight ( M w) of the peak fraction of the main eluting peak calculated as ∼200×10 3 for an oat, a barley, and a wheat β-glucan and ∼100×10 3 for an oat and a barley β-glucan, and a lichenan sample. All polysaccharides analyzed by 2D NMR spectroscopy and high-performance anion-exchange chromatography of the cellulosic oligomers released by the action of lichenase showed the typical fine structure of mixed-linkage linear (1→3), (1→4)-β- d-glucan. Following lichenase digestion of β-glucans, the molar ratios of tri- to tetrasaccharides (DP3/DP4) were found to follow the order of lichenan (24.5)>wheat (3.7)>barley (2.8–3.0)>oat (2.1). Differences in critical concentration (c ∗ ∗), viscosity, viscoelastic and shear thinning properties among samples were dependent mainly on differences in molecular size of the polymeric chains as well as on the β-glucan fine structure. All β-glucan isolates were able to form gels, as probed by dynamic rheometry; with decreasing molecular size and increasing DP3/DP4 ratio, the gelation time decreased and the gelation rate (I E =[ d log G′/ dt] max ) increased. Differential scanning calorimetry (DSC) showed that cereal β-glucan gels exhibit rather broad endothermic gel→sol transitions at 55–80 °C, while lichenan gels give a sharper transition, implying a more cooperative process. The DSC kinetic data showed similar responses to that from dynamic rheometry; the rate of development of the endotherm increased with increasing DP3/DP4 ratio of the polysaccharide. Furthermore, the storage modulus ( G′) and the apparent melting enthalpy values (plateau Δ H) increased with decreasing molecular size and with increasing DP3/DP4 ratio. The melting temperature of the gel network, as determined by DSC and dynamic rheometry, was found to increase with the molecular size and the DP3/DP4 ratio of β-glucans; the T m for lichenan was ∼89 °C and for cereal β-glucans varied in the narrow range of ∼65–72 °C. Large deformation mechanical tests (compression mode) up to failure revealed an increase in strength and a decrease in brittleness of mixed-linkage β-glucan gels with increasing DP3/DP4 ratio and molecular size of the polysaccharide.