Rheology and NMR studies of true gel formation and gelation mechanism for lentinan

True gel formation for lentinan, a β-glucan obtained from shiitake mushrooms, was investigated and the mechanism for gel formation was discussed. The extraction method presented in this study yielded lentinan with the same rheological properties regardless of which part of the shiitake mushroom was used. True gel formation was confirmed by the frequency dependence of the storage shear modulus G′ and loss shear modulus G″ in dynamic viscoelasticity measurements, and by gel fracture tests at 4 °C. G′ and G″ increased steeply around 6 °C upon cooling, and the mechanical loss tangent was below 0.1 at 4 °C, suggesting that gelation started at around 6 °C. This characteristic temperature was consistent with the conformational transition temperatures observed for lentinan and schizophyllan. This conformational transition has been considered to be a change from the triple helix, Helix II, to the more ordered Helix I. The NMR data obtained in this study suggested that the conformation of the side groups changed with the change from Helix II to Helix I. It was thought that Helix I with less steric hindrance of the side group partially associates with other Helix I molecules to form lentinan gels.