Dynamic viscoelastic behavior of triple helical Lentinan in water: Effects of concentration and molecular weight

The dynamic viscoelasitc behavior of Lentinan, one triple helical β-(1 → 3)-d-glucan from the fruiting body of Lentinus edodes, in water was investigated as a function of concentration and molecular weight at 25 °C by using dynamic rheology. It was revealed that the shear storage moduli (G′), viscous loss moduli (G″), and the dynamic complex viscosity (η∗) exhibited strong dependence on concentration and molecular weight. At low concentrations, the Lentinan/water systems displayed liquid-like behavior with G′ lower than G″ at low frequencies and crossing-over at high frequencies. With increasing concentration, the elastic response of the Lentinan/water system was stronger than the viscous response, leading to the conclusion that the Lentinan/water systems displayed a predominantly solid-like behavior. The gel point (cgel) was determined from Winter-Chambon method (frequency-independence of tan δ). The most important point is that the cgel was much lower than some synthesized polymers and other flexible polysaccharides, which may be attributable to the high stiffness of triple helical Lentinan and strong intra- and intermolecular interactions among polysaccharide chains. Furthermore, a decrease in molecular weight leads to a sharp increase of cgel. The dynamic strain sweep measurements proved that the gelation of Lentinan in water is induced by the extremely entangled and stiff triple helices forming continuous network, and the Lentinan gel is structurally more like a solution that is unable to flow within a timescale of usual observation.