Abstract
► Detailed methodology for the characterisation of xanthan using GPC-MALLS by taking into account the insoluble highly associated fraction. ► Characterisation of a commercial xanthan following relevant industrial treatments using various techniques on the same sample. ► Method to produce xanthan low molecular weight fraction with tunable conformations. ► Summary of all work to date on xanthan conformation and proposal of a model to reconcile the conflicting reports.
The structure and conformation of xanthan in aqueous solution following various processing treatments typically encountered in its application were investigated in this study. Treatments such as heating, autoclaving, high pressure homogenisation and irradiation were subjected to the same sample. Parameters such as weight average molecular weight (Mw), polydispersity index, root mean square radius of gyration, intrinsic viscosity and Huggins constant were used to monitor the effect of these treatments. Additionally, we have quantified the mass recovery of samples examined by gel permeation chromatography and light scattering to properly account for all fractions present in xanthan solutions. Atomic force microscopy (AFM) images together with height measurements confirmed that xanthan conformation is double helical ordered renatured state (pre-heat treated by the manufacturer) in dilute solution conditions and random coil conformation in very dilute solution. The ordered (renatured) conformation is shown to have partially molten double helix, with more flexibility than the perfectly ordered native double helix. Heat treatment for 2h at 85°C reduces the Mw of xanthan to half its initial value, and mass recovery measurements indicate that it completely overcomes its associative nature. Thermally treated xanthan solution in the dilute region leads to an order–disorder transition, as determined by contour length per unit mass. Similarly, irradiation of xanthan solution results in an order–disorder transition together with the production of single strand low molecular weight molecules. Autoclaving and high pressure homogenisation treatments cause degradation of xanthan. The results from treated xanthan solutions following high pressure homogenisation and irradiation confirm that xanthan does not reassociate. A revised summary of xanthan conformation in solution together with schematic models following the various treatments are proposed.