Physicochemical characterization of grifolan: Thixotropic properties and complex formation with Congo Red

Grifolan is a branched (1,3)-β-glucan extracted from the fungus Grifola frondosa. Its physicochemical properties were investigated by studying the visible spectrum of Congo Red–glucan complexes and the rheological behavior. The analysis of the visible spectrum of Congo Red–glucan complexes indicated that the alkali-extracted grifolan was in a triple-helix conformation. The triple-helix structure remained unaltered in the temperature range studied (5–25 °C) or when varying the sucrose content (0–20%), but disappeared at a urea concentration of 5.0 M. The alkali-extracted grifolan formed a gel at subambient temperatures. The thixotropic viscosity profile of grifolan gels was fitted by a structural kinetics model, and the thixotropy was attributed to the breakdown of aggregates of triple helices under shear. The rate of structure breakdown was found to increase both with increasing shear rate and temperature. The amount of gel structure available for shear-induced structure breakdown was shear-rate independent, but decreased with increasing temperature. The presence of sucrose or urea in grifolan gels resulted in significant changes in thixotropy, which were attributed to the enhancing effect of sucrose on the strength of aggregates of triple helices and the loss of triple-helix structure due to the breaking of intermolecular hydrogen bonds by urea.