Stoichiometry and Conformation of Xanthan in Synergistic Gelation with Locust Bean Gum or Konjac Glucomannan: Evidence for Heterotypic Binding

Abstract

Synergistic gels of xanthan or deacetylated xanthan (DX) with locust bean gum (LEG) or konjac glucomannan (KM) melt and set at similar to 60 degrees C, with no thermal hysteresis, Gelation occurs with the xanthan component in either its ordered or its disordered form and, with KM as cosynergist, is accompanied by large enthalpy changes (Delta H) in DSC. Gel modulus (G') and Delta H increase linearly with increasing ratio of KM:DX up to similar to 1:1, with little further change at higher ratios. Liquid-like character (tan delta) passes through a sharp minimum at about the same composition. Mixed gels of KM with unmodified xanthan show similar behavior, but the maximum value of Delta H is lower, and the proportion of KM required to achieve this maximum is higher. The heat changes (per gram of xanthan or DX) depend only on mixing ratio, not on total concentration, arguing strongly for stoichiometric binding rather than an exclusion mechanism. With LEG in place of KM, the sol-gel transition is much wider and gives no discernible peaks in DSC. The minimum in tan delta with varying composition, however, is still evident, again arguing for a binding process, and the moduli are higher (similar to 3x). Gels incorporating KM show evidence of structural rearrangement after their initial formation (maxima in the temperature dependence of G''; shoulders in tan delta and in DSC); no such effects are seen for LEG. In the light of previous X-ray diffraction studies in the condensed phase, it is suggested that initial gelation involves heterotypic junctions between xanthan or DX and KM or LEG, with both components in a 2(1) conformation, but that junctions involving KM convert to a more compact 6-fold arrangement at lower temperature.