The influence of charge on the multiple thermal transitions observed in xanthan

Helix-coil transitions in xanthans occur at lower temperatures when the pyruvate group is charged, destabilising the polymer chains. Increasing salt content increases the transition temperature by reducing the effective charge on the pyruvate. A simple equivalent mass action model predicts how transition temperatures change as a function of salt concentration. The functional form of the change in transition temperature (1/T) versus natural log (salt concentration) is approximately linear and similar to more traditional polyelectrolyte theories. Transition temperatures in xanthans containing nominally homogeneous pyruvate contents show biphasic transitions, this is because the phases contain different pyruvate levels, however the transitions approach one another in temperature and eventually merge as salt content is increased. It is proposed that pyruvate groups, despite being present at a lower concentration relative to glucuronic acid, dominate the charge interactions due to their location on the outside of the helices

New insights into xanthan synergistic interactions with konjacglucomannan: a novel interaction mechanism proposal

The interactions of xanthans containing precise acetate and pyruvate concentration with Konjac gluco-mannan (KGM) were studied at different sodium chloride and polymer concentrations. A new unifiedmodel of the interaction is proposed, taking into account previous models in the literature. This study sug-gests that the interactions occur by two distinct mechanisms dependent on xanthan conformation. These interactions are not mutually exclusive and may co-exist and hence produce complicated traces. Consequently two types of gel which melt at different temperature ranges can be formed. Depending on the xanthan helix coil transition temperature, one or both of the synergistic states may exist in the hydrocolloid blend. The proposed model has been tested rheologically and using differential scanning calorimetry by varying salt concentration and using samples containing different functional group concentrations

Comparison of xanthans by the relative abundance of its six constituent repeating units

Five xanthans were hydrolyzed to their repeating units using cellulases. Hydrophilic interaction chromatography with online electrospray ionization ion trap mass spectrometry and evaporative light scattering detection was used to analyze the oligomers released. It was concluded that six different pentamer repeating units (RUs) exists within a xanthan sample. The most abundant RU shows acetylation on the inner mannose and pyruvylation on the outer mannose. The second most abundant RU shows acetylation on both the inner and the outer mannose. It becomes clear that more variations in the xanthan structure exist than generally recognized. Comparison of five different xanthan samples revealed that, although the molecular composition of xanthan samples can be exactly the same, the ratio in which the RUs occur can differ significantly. It is, therefore, concluded that xanthan samples should be characterized for both, their molecular composition and the relative abundance of the RUs presen

The influence of the primary and secondary xanthan structure on the enzymatic hydrolysis of the xanthan backbone

Differently modified xanthans, varying in degree of acetylation and/or pyruvylation were incubated with the experimental cellulase mixture C1-G1 from Myceliophthora thermophila C1. The ionic strength and/or temperature of the xanthan solutions were varied, to obtain different xanthan conformations. The exact conformation at the selected incubation conditions was determined by circular dichroism. The xanthan degradation was analyzed by size exclusion chromatography. It was shown that at a fixed xanthan conformation, the backbone degradation by cellulases is equal for each type of xanthan. Complete backbone degradation is only obtained at a fully disordered conformation, indicating that only the secondary xanthan structure influences the final degree of hydrolysis by cellulases. It is thereby shown that, independently on the degree of substitution, xanthan can be completely hydrolyzed to oligosaccharides. These oligosaccharides can be used to further investigate the primary structure of different xanthans and to correlate the molecular structure to the xanthan functionalities

A novel approach to the determination of the pyruvate and acetate distribution in xanthan

The present work demonstrates a quantitative relation between the physical properties of the order disorder transition in xanthan, such as transition temperature and modulus, with the acetate and pyruvate content. Models have been constructed which reflect simple linear dependence of transition temperature and the logarithm of the modulus on acetate, pyruvate, xanthan and salt concentration. Biphasic transitions in xanthan detected using Differential Scanning Calorimetry (DSC) are proposed to reflect phases containing different acetate and pyruvate levels in each phase. The respective acetate and pyruvate contents in these phases have been estimated. 13C Cross Polarisation Magic Angle Spinning (CPMAS) Nuclear Magnetic Resonance measurements further suggest that the distribution of functional groups measured in solution is reflected in a distribution of environments in solid powders. This could take the form of heterogeneity along an indvidal chain (intra-chain heterogeneity) or uniform but different levels of functional groups in individual chains (inter-chain heterogeneity). Support for the intra-chain proposal or “blocky” distribution is provided by samples which have been reported to exhibit different transition temperatures despite having apparently the same overall acetate and pyruvate content. This is ascribed to the values of transition temperature being different along an individual chain