Xanthan Samples Research Articles

Properties of xanthan solutions after long-term heat treatment at 90°C

The thermal stability of two xanthan samples has been studied using light-scattering and viscometry. The experiments were performed in NaCl solutions at concentrations of 1 g/litre, 5 g/litre and 50 g/litre and in a solution containing 5 g/litre NaCl + 1 g/litre CaCl 2. The samples were all O 2-free and the storage temperature was 90°C. Information has been obtained on the time-dependence of relative viscosity, intrinsic viscosity and average molecular weight for storage times up to 6 months. In the solvent containing 1 g/litre NaCl, a rapid decrease in relative viscosity was observed. At this salinity, the xanthan molecule is in its disordered form and the degradation is probably accelerated by chain splitting. In the solvent containing 5 g/litre NaCl, decreases in both the relative and intrinsic viscosities were observed during heat treatment although the degradation is less pronounced than at the lower NaCl concentration. At 5 g/litre NaCl/90°C, the order-disorder transition of xanthan is expected to occur, and degradation is induced when molecules are in the disordered form, which is less stable than the ordered one. The experimental [η] - M w correlation found for heated samples is consistent with the theoretical one for double helix molecules with persistence lengths of 120–150 nm. In the solvent containing 50 g/litre NaCl the transition temperature is notably higher than 90°C and thus the xanthan molecule is in its fully ordered conformation during the whole thermal treatment. Under this condition, no polymer degradation was observed. Both the relative and intrinsic viscosities were found to be unchanged or slightly higher after heat-treatment of several months. In the solvent containing 5 g/litre NaCl + 1 g/litre CaCl 2 the intrinsic viscosity characteristics were the same as for the 50 g/litre NaCl-solvent. The slight rise in intrinsic viscosity observed after 2–3 months of heat treatment might be attributed to an extension of the backbone of the xanthan molecule.

Acid hydrolysis and high-performance liquid chromatography of xanthan

A method for the routine determination of the hexose and organic acid content of xanthan preparations based on acid hydrolysis of the exopolysaccharide followed by high-performance liquid chromatography (HPLC) of the hydrolysate on two cation-exchange columns was investigated. A kinetic model was fitted to the recoveries of these components over a range of hydrolysis conditions using non-linear regression, affording values for the rate constants and the initial concentration of each compotent in the unhydrolysed polysaccharide. The ratio of glucose to mannose was greater than the assumed 1:1 ratio in unhydrolysed xanthan, suggesting that the polysaccharide may contain fewer terminal mannosyl residues than previously proposed. Quantification of glucuronic acid was complicated by its lactonisation and consequent elution as two peaks and by interference from its degradation products. The possibility of determining this component as its aldobiouronic acid [β- d-GlcA p-(1→2)- d-Man p] precursor was investigated. Acetic acid recovery was not significantly affected by the hydrolysis conditions over the range studied. The difference between the maximum recoveries and the fitted initial concentrations of each component indicate that compositional analyses of xanthan samples based on single time and acid strength conditions should be treated with caution as they are unlikely to yield quantitatively accurate values.

Behaviour of xanthan in cadoxen

The stability of xanthan versus time in cadoxen at 20°C has been studied using optical rotation, viscosimetry and light scattering. For xanthan samples with a high initial molecular weight, the relative and the intrinsic viscosities, the weight average molecular weight and the radius of gyration of the samples continuously decrease with time. These values were found to be compatible with those given by Sato et al. for partially depolymerised xanthan fractions, even if the interpretation is quite different. The decrease in molecular weight in cadoxen can be explained by a random process of depolymerisation when cadoxen is used as solvent. For lower molecular weight xanthans, no degradation is obtained in cadoxen, but the ratio M ̄ w NaCl M ̄ w cadoxen is found to be less than 2. Because of the depolymerisation observed for high molecular weight xanthan it is necessary to take great care when measuring and interpreting the properties of xanthan when cadoxen is used as a solvent.

The effect of pyruvate on viscosity properties of xanthan

It is shown that the viscosity of concentrated (∼ 1%) solutions of high-pyruvate xanthan in water is increased by the addition of salt. On decreasing the concentration of polymer to ∼ 0·4% a cross-over in this behaviour is observed, i.e. addition of salt decreases the viscosity. In low-pyruvate xanthan samples the salt effect is greatly reduced, confirming the predictions of Smith et al.

Biodegradation of xanthan by salt-tolerant aerobic microorganisms

Three salt-tolerant bacteria which degraded xanthan were isolated from various water and soil samples collected from New Jersey, Illinois, and Louisiana. The mixed culture, HD1, contained aBacillus sp. which produced an inducible enzyme(s) having the highest extracellular xanthan-degrading activity found. Xanthan alone induced the observed xanthan-degrading activity. The optimum pH and temperature for cell growth were 5–7 and 30–35°C, respectively. The optimum temperature for activity of the xanthan-degrading enzyme(s) was 35–45°C, slightly higher than the optimum growth temperature. With a cell-free enzyme preparation, the optimum pH for the reduction of solution viscosity and for the release of reducing sugar groups were different (5 and 6, respectively), suggesting the involvement of more than one enzyme for these two reactions. Products of enzymatic xanthan degradation were identified as glucose, glucuronic acid, mannose, pyruvated mannose, acetylated mannose and unidentified oligo- and polysaccharides. The weight average molecular weight of xanthan samples shifted from 6.5·106 down to 6.0·104 during 18 h of incubation with the cell-free crude enzymes. The activity of the xanthan-degrading enzyme(s) was not influenced by the presence or absence of air or by the presence of Na2S2O4 and low levels of biocides such as formaldehyde (25 ppm) and 2,2-dibromo-3-nitrilopropionamide (10 ppm). Formaldehyde at 50 ppm effectively inhibited growth of the xanthan degraders.

Modified xanthan—its preparation and viscosity

Xanthan with various pyruvic acid and acetate contents has been prepared from a single commercial polysaccharide sample using optimised chemical conditions (acid and alkali hydrolysis, respectively) for removal of acetal and acyl groups. The only significant change found on analysis of the modified xanthans was loss of pyruvic acid and/or acetate; no low moleculur weight carbohydrate-containing material was released. Contrary to some previous reports, evidence is presented to show that the pyruvic acid acetal and o-acetyl contents of xanthan do not affect solution viscosity. The viscosities of native, pyruvate-free and pyruvate/acetate-free xanthan solutions (0·3% w/v) were similar at shear rates 8·8–88·3 s −1 in both distilled water and 1% KCl. Over the concentration range 0·2-1·5%, the viscosities of native and pyruvate-free xanthan at 10 s −1 were similar. The viscosity increase on addition of 1% KCl to salt-free xanthan solutions was independent of pyruvic acid acetal substitution. Our results suggest that xanthan samples with various pyruvic acid acetal and o-acetal contents, prepared under different fermentation conditions of Xanthomonas campestri should not normally be used for assessing the contribution of these groups to solution viscosity.

Molecular weight of xanthan polysaccharide

The molecular weight ( M w ) and molecular-weight distribution of the extracellular polysaccharide xanthan, synthesized by the bacterium Xanthomonas campestris, have been determined from measurements of the sedimentation coefficient, s 20,itw, and the intrinsic viscosity, [η], with the aid of the Mandelkern-Flory-Scheraga equation. The sedimentation coefficient of native xanthan was measured by band-sedimentation of polysaccharide molecules that had been tagged with a fluorescent group; the fluorescent label permits the use of very low concentrations of polymer. A typical, native-xanthan sample has M w  15 x 10 6; the polydispersity index M w/M n is 2.8. Measurement of s and [η] for a homologous series of five xanthan samples having M w ranging from 0.40 to 15 X 10 6, prepared by sonication of native xanthan, shows that, for low molecular weight, the intrinsic viscosity [η] obeys the relation [η]  KM 1.35. The high value of the Staudinger exponent in this relation demonstrates that xanthan is a rod-like molecule having stiffness similar to that of native DNA, which has a Staudinger exponent of 1.32. Moreover, the absolute values of [η] suggest that xanthan has a mass per unit length of about 1900 daltons/nm, which is twice the mass per unit length of the single-stranded structure proposed from X-ray work.