DETERMINATION OF THE MICROSTRUCTURE OF THE FERMENTATION POLYMER XANTHAN

Although xanthan has long been the subject of wide-ranging investigations, it has not yet been possible to establish any clear link between the solution properties and the chemical and steric microstructure. This is due to the fact that large variations in the properties may occur between samples of different manufacturers. Hence more than fifty different samples in liquid and powder form were investigated. This involved the determination of pyruvate and acetate content for all the samples by means of enzymatic analysis and comparison with results from H-1 NMR spectroscopy. In addition, the contents of the side groups are for the first time given in mass and molar percentages. Low angle and multi-angle laser light scattering as well as viscometry were employed for the determination of the steric microstructure. Here it was ascertained that preliminary treatment of the sample has a crucial influence on the results of light scattering. A method of sample preparation was therefore devised which is aimed at avoiding the formation of microgels and preserving the structure of the native.xanthan. In addition, xanthan is used to illustrate a novel procedure which enables very readily interpretable Zimm plots to be obtained from one individual reading over a few minutes. In a comparison of the scattering curve extrapolated to c=0 with calculated model curves for semiflexible rods, the structure of native xanthan was determined to be more rigid than corresponds with the exponents of previously published [eta]-M(W) relationships.