Synergistic binding of konjac glucomannan to xanthan on mixing at room temperature

Mixtures of 0.5 wt% xanthan with 0.5 wt% konjac glucomannan (KGM) in water and in 30 mM KCl were prepared by mixing stock solutions of the individual polymers at 20 °C, and characterised rheologically by low-amplitude oscillatory measurements, and by creep–recovery experiments using applied stresses of 0.2, 0.4, 0.8, 1.6, 3.2, 6.4, 12.8, 25.6, 51.2 and 102.4 Pa. Both mixtures gave gel-like mechanical spectra, and showed typical elastic response to stresses in the range 0.2–12.8 Pa, with recoverable strain increasing in direct proportion to applied stress. Fracture and flow occurred at higher stress (25.6 Pa for the mixture prepared in water; 102.4 Pa for the network formed in 30 mM KCl). On heating from 20 to 95 °C, both mixtures showed a typical melting process, seen as a sigmoidal reduction in G′ and G″, with G′ dropping below G″, and an accompanying endothermic transition in DSC (differential scanning calorimetry). Rheological and thermal transitions were observed on cooling to 20 °C and reheating to 95 °C, over the same temperature-range as those observed on initial heating. As anticipated, the networks formed after heating and cooling were stronger and more cohesive than those obtained on initial mixing at 20 °C. The order–disorder transition of 0.5 wt% xanthan in water, as characterised by DSC, was centred at ∼50 °C, within the temperature-range of the gelling/melting transition of the xanthan–KGM mixtures prepared in water. Conformational ordering of xanthan in 30 mM KCl, however, was complete by ∼70, 50 °C above the mixing temperature of 20 °C used in preparation of the mixtures with KGM. The transition enthalpy (ΔH) of the DSC endotherm observed on initial heating of the mixture prepared in 30 mM KCl was virtually identical to the values obtained on cooling and re-heating (6.11 J/g xanthan, in comparison with −6.00 J/g on cooling and 6.14 J/g in the second heating scan), indicating the same nature and extent of intermolecular association in mixtures prepared at 20 °C as in those cooled from high temperature. We therefore conclude that mixing at low temperature inhibits formation of a cohesive gel by disruption of network structure during mixing, rather than by a requirement for the xanthan molecules to be disordered before association can occur, as has been proposed previously.