Effects of glucose, mannose and konjac glucomannan on the gel–sol transition in gellan gum aqueous solutions by rheology and DSC

Abstract

Abstract Effects of glucose, mannose and konjac glucomannan with different molecular weights on the sol–gel transition in gellan gum aqueous solutions were studied by dynamic viscoelastic measurement and differential scanning calorimetry. Temperature dependence of the loss shear modulus G ″ for a 1% gellan gum solution without sugars showed a one-step-like change at a certain temperature, which was attributed to the helix–coil transition temperature T hc . On addition of sugar, storage shear modulus G ′ and loss shear modulus G ″ increased and T hc shifted to higher temperatures. With progressive addition of sugar, the cross-over of G ′ and G ″ was observed at a lower temperature than T hc , which was attributed to the sol–gel transition T sg . Moreover, in the presence of sufficient sugar, both moduli did not decrease remarkably during heating up to 60 °C, although both moduli showed a steep increase around 50 °C during cooling process. In the presence of sugar of fairly low concentration, DSC measurement showed that both exothermic and endothermic peak temperatures, T s and T m , slightly shifted to lower temperatures, and that exothermic and endothermic enthalpies, Δ H s and Δ H m , significantly decreased. In contrast, rheological measurement showed that elastic moduli and the relaxational strength Δ G significantly increased by the addition of a sugar of fairly low concentration. However, further addition of sugar shifted T s and T m to higher temperatures and increased Δ H as well as elastic moduli. In the presence of sufficient sugar, both cooling and heating DSC curves became broader; however, many small peaks were observed at both higher and lower temperatures. Gellan with sufficient sugar formed various junction zones with different thermal stabilities during cooling, and then these junction zones became markedly heat resistant. In the presence of konjac glucomannan with relatively lower molecular weight, the effective concentration of gellan gum increases by immobilising water molecules in gellan gum solutions, so that konjac glucomannan shifts the helix– coil transition temperature in gellan gum molecules to higher temperatures.