A comparative study on structure–function relations of mixed-linkage (1 → 3), (1 → 4) linear β-D-glucans
A. Lazaridou, C. G. Biliaderis, M. Micha-Screttas and B. R. Steele
Food Hydrocolloids Volume 18, Issue 5 , September 2004, Pages 837-855
2004
บทคัดย่อ
The effects of fine structure and molecular size on the rheological properties of six mixed-linkage (1→3), (1→4)-b-D-glucans (b-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 (Mw) of the peak fraction of the main eluting peak calculated as ~200×103 for an oat, a barley, and a wheat b-glucan and ~100×103 for an oat and a barley b-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)-b-D-glucan. Following lichenase digestion of b-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 b-glucan fine structure. All b-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 (IE=[d log G′/dt]max) increased. Differential scanning calorimetry (DSC) showed that cereal b-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 ΔH) 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 b-glucans; the Tm for lichenan was ~89 °C and for cereal b-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 b-glucan gels with increasing DP3/DP4 ratio and molecular size of the polysaccharide.