INTERACTION IN XANTHAN-GLUCOMANNAN MIXTURES AND THE INFLUENCE OF ELECTROLYTE

The interaction of xanthan and glucomannan (konjac mannan, KM) has been monitored using differential scanning calorimetry (DSC), electron spin resonance (ESR), and mechanical spectroscopy. The DSC and ESR results indicate that interaction occurs in water immediately following xanthan side chain-backbone association. The process is thermodynamically driven and occurs to reduce xanthan-water contacts. In the presence of electrolyte, both techniques show that the xanthan conformational change shifts to higher temperatures and depends on the nature of the cation. Divalent cations give rise to a greater shift than monovalent cations. The results indicate that xanthan/KM interaction occurs at temperatures much lower than the conformational transition. The rheological properties (i.e., storage (G') and loss (G'') moduli and gelation temperature, T(gel)) are greatly influenced by the presence of electrolyte. In all instances, G', G'', and gelation temperature are reduced when electrolyte is present, with divalent cations having a greater effect than monovalent cations. The reduction in G', G'', and T(gel) follows the lyotropic series with increasing effects in the order K+ approximately Cs+ < Na+ approximately NH4+ approximately Ba2+ much less than Mg2+ approximately Ca2+. These observations are explained by the fact that electrolyte promotes xanthan self-association at the expense of xanthan/KM interaction.