AGITATOR SPEED AND DISSOLVED-OXYGEN EFFECTS IN XANTHAN FERMENTATIONS

Agitation speed affects both the extent of motion in Xanthan fermentat ion broths because of their rheological complexity and the rate of oxy gen transfer. The combination of these two effects causes the dissolve d oxygen concentration and its spatial uniformity also to change with agitator speed. Separating these complex interactions has been achieve d in this study in the following way. First, the influence of agitatio n speeds of 500 and 1000 rpm has been investigated at a constant nonli miting dissolved oxygen concentration of 20% of air saturation using g as blending. Under these controlled dissolved oxygen conditions, the r esults demonstrate that the biological performance of the culture was independent of agitation speed as long as broth homogeneity could be e nsured. With the development of increasing rheological complexity lend ing to stagnant regions at Xanthan concentrations >20 g/L, it is shown that the superior bulk mixing achieved at 1000 rpm, compared with 500 rpm, leading to an increased proportion of the cells in the fermenter to be metabolically active and hence higher microbial oxygen uptake r ates, was responsible for the enhanced performance. Second, the effect s of varying dissolved oxygen are compared with a control in each case with an agitator speed of 1000 rpm to ensure full motion, but with a fixed, nonlimiting dissolved oxygen of 20% air saturation. The specifi c oxygen uptake rate of the culture in the exponential phase, determin ed using steady-stare gas analysis data, was found to be independent o f dissolved oxygen above 6% air saturation, whereas the specific growt h rate of the culture was not influenced by dissolved oxygen, even at levels as low as 3%, although a decrease in Xanthan production rare co uld be measured. In the production phase, the critical oxygen level wa s determined to be 6% to 10%, so that, below this value, both specific Xanthan production rate as well as specific oxygen uptake rate decrea sed significantly. In addition, it is shown that the dynamic method of oxygen uptake determination is unsuitable even for moderately viscous Xanthan broths. (C) 1998 John Wiley & Sons, Inc.