“Melt-in-the-mouth” gels from mixtures of xanthan and konjac glucomannan under acidic conditions: A rheological and calorimetric study of the mechanism of synergistic gelation

The effect of acidification on a typical commercial xanthan and on pyruvate-free xanthan (PFX), alone and in gelling mixtures with konjac glucomannan (KGM), has been studied by differential scanning calorimetry (DSC) and small deformation oscillatory measurements of storage modulus (G0) and loss modulus (G00). For both xanthan samples, progressive reduction in pH caused a progressive increase in temperature of the disorder–order transition in DSC, and a progressive reduction in gelation temperature with KGM. This inverse correlation is interpreted as showing that synergistic gelation involves disruption of the xanthan 5-fold helix, probably by attachment of KGM to the cellulosic backbone of the xanthan molecule (as proposed previously by a research group in the Institute of Food Research, Norwich, UK). Higher transition temperature accompanied by lower gelation temperature for PFX in comparison with commercial xanthan at neutral pH is explained in the same way. However, an additional postulate from the Norwich group, that attachment of KGM (or galactomannans) can occur only when the xanthan molecule is disordered, is inconsistent with the observation that gelation of acidified mixtures of KGM with PFX can occur at temperatures more than 60 �C below completion of conformational ordering of the PFX component (as characterised by DSC). Increase in G0 on cooling for mixtures of commercial xanthan with KGM at pH values of 4.5 and 4.25 occurred in two discrete steps, the first following the temperature-course observed for the same mixtures at neutral pH and the second occurring over the lower temperatures observed for mixtures of KGM with PFX at the same values of pH. These two ‘‘waves’’ of gel formation are attributed to interaction of KGM with, respectively, xanthan sequences that had retained a high content of pyruvate substituents, and sequences depleted in pyruvate by acid hydrolysis. At pH values of �4.0 and lower, gelation of mixtures of KGM with commercial xanthan followed essentially the same temperature course as for mixtures with PFX, indicating extensive loss of pyruvate under these more strongly acidic conditions. Mixtures prepared at pH values in the range 4.0–3.5 gave comparable moduli at room temperature (20 �C) to those obtained at neutral pH, but showed substantial softening on heating to body temperature, suggesting possible applications in replacement of gelatin in products where ‘‘melt-in-the mouth’’ characteristics are important for acceptability to the consumer.