No abstract

A bubble column (0.05 m(3)) and an air-lift fermentor (1.2 m(3)) were used for the production of the exocellular microbial polysaccharide xanthan with Xanthomonas campestris in a synthetic medium. Upon oxygen depletion in the liquid, the xanthan production rate dropped sharply and then became a linear function of the oxygen transfer rate. The volumetric mass transfer coefficients for oxygen conformed to the correlation of Suh et al. Using this correlation in combination with the model for xanthan batch fermentation suggested by Peters et al., the xanthan fermentations in the bubble column were well described. The model also correctly predicted the time course of the molecular weight of the polysaccharide even when a complex medium was used. In the air-lift fermentor, however, the xanthan production rate and the xanthan yields with respect to oxygen and glucose were lower than expected at the overall oxygen transfer rate. The poor performance of the air lift was traced back to the lack of any oxygen supply in the downcorner.

The effects of liquid‐phase viscosity, pseudoplasticity, and viscoelasticity on gas‐liquid mass transfer in a bubble column have been investigated using aqueous solutions of sucrose, xanthan, polyacrylamide, and mixtures of xanthan and polyacrylamide. The elastic properties, considered in the form of the Weissenberg number, decreased the volumetric mass transfer coefficient by a factor of up to 5. The effect was strong even at low Weissenberg numbers encountered in many liquids, the elasticity of which is usually neglected.

The degree of pyruvyiation of the xanthan side chains decreased strongly when the microbial oxygen demand was not met. There was no significant dependence on the growth rate.

An empirical model for the production of xanthan polysaccharide by batchwise cultivation of Xanthomonas campestris was developed. The effects of oxygen depletion in the liquid and bacterial growth rate on the xanthan production rate and the time‐course of the xanthan molar mass were taken into account. When the oxygen transfer was rate limiting in the non‐growth phase, the molar mass of the final xanthan product was lower than that observed in the absence of oxygen limitation. A constant feed of medium, containing citric acid, increased the xanthan productivity and allowed a polysaccharide concentration of 50 kg/m3 to be reached within 57 h. Low productivity observed in a 3 m3 reactor with small‐diameter Rushton turbines was attributed to stagnant zones in the wall region.