EFFECT OF THE DISSOLVED-OXYGEN TENSION DURING CULTIVATION OF XANTHOMONAS-CAMPESTRIS ON THE PRODUCTION AND QUALITY OF XANTHAN GUM

The effect of the dissolved oxygen tension (DOT) on the production and quality of xanthan gum was studied in a range of 10% to 100% of saturation. Oxygen tension was controlled by gas blending in a well-mixed bioreactor. The evolution of the power law theological indexes, as well as the apparent yield stress of the medium, was determined. Pyruvic acid residues were quantified in the xanthan produced. The mean molecular weight (MMW) was estimated by a theological technique and the molecular weights distribution was estimated by gel permeation chromatography. Pyruvic acid content was higher as cultivation evolved. Pyruvate was very similar when oxygen tension was high (> 40% DOT), but showed a higher value if compared with 10% DOT. DOT strongly determines the theological behaviour of the culture medium. The quality of xanthan was influenced by the DOT level. A saturation-type behaviour exists between the oxygen tension and the mean molecular weight. Micromixing could be playing a role in determining this behaviour. MMWs of 10 x 10(6) kg kgmol(-1) were found above 40% DOT. In general, the proportion of high molecular weight polymers was higher as DOT was higher.