Thermal stability and chain conformational studies of xanthan at different ionic strengths

Abstract

The aim of the present study was to determine the influence of the ionic strength on the thermal stability of xanthan, i.e. xanthan resistance to chain breaking at high temperatures. Xanthan solutions of various ionic strengths were kept at 80, 90 and 95°C for periods up to 95 h. The thermal stability was determined by measuring the intrinsic viscosity after the heating periods. The experiments showed a critical ionic strength for the thermal stability of xanthan between 10 and 100 m m NaCl or KCl in this temperature range. Below the critical ionic strength the intrinsic viscosity was rapidly reduced, whereas above the critical ionic strength the intrinsic viscosity was virtually unaffected by heating. We then looked for a possible correlation between thermal stability and secondary structure of xanthan. The transition ionic strength (I m ) of xanthan solutions, i.e. where xanthan is midway between an ordered and a disordered structure, was determined by NMR at constant temperatures. I m was found to be in the range of 24 m m at 80°C to 60 m m NaCl at 95°C, thus lying in the range of the critical ionic strength of the thermal stability. This suggests a close relationship between thermal stability and secondary structure of xanthan, indicated by the enhanced thermal stability in the ordered state. We believe this enhanced thermostability arises from a double-stranded conformation in the ordered state, as in DNA. The presence of double-stranded xanthan is also indicated by electron micrographs taken at both high and low ionic strengths. The transition temperature (T m ) of xanthan was determined by NMR and optical rotation measurements. At the ionic strength of 7·5 m m the two methods resulted in T m values of 67 and 52°C respectively. This difference in T m can possibly be due to the fact that the observed NMR and optical rotation (OR) effects are caused by different molecular phenomena.