Xanthan gum is a polyanionic, extracellular polysaccharide produced by the aerobic bacterium Xanthomonas campestris. The commercial value of xanthan gum derives from its ability to impart extremely viscous and shear thinning behavior to fluids. Recently xanthan is used extensively by the petroleum industry in oil-well drilling and secondary recovery procedures and the use of xanthan in drilling mud composition has resulted in a more efficient, lower cost process.
The continuous fermentation of xanthan by X. campestris led to and improved productivity, however, the strain stability was maintained only for a short period. Thus long term continuous production of xanthan has not yet been successful.
In this study, xanthan productivity and strain stability was investigated throughout fermentation by X. campestris. In batch fermentation higher C/N ratio in the same initial glucose concentration yielded higher final xanthan gum concentration and lower cell mass concentration. With increasing glucose concentrations the growth of the microorganism was not inhibited, but the conversion yield to xanthan gum was decreased since high broth viscosity during fermentation probably resulted in the reduction of mass transfer of substrates and oxygen to the cells. X. campestris produced little xanthan gum below pH 5.5. To improve oxygen mass transfer, soy- bean oil was used to reduce the fermentation time by about 10% at an initial glucose concentration of 45 g/L.
Rapidly growing substrain was screened from the continuous culture of X. campestris NRRL B-1459, which exhausted glucose in less than 30 hurs while it took 50 hours for X. campestris NRRL B-1459 to use up 22.5 g/l glucose concentration. The xanthan gum yield of this substrain was almost the same as the original strain. It produced 25.0 g/l xanthan gum in 30 hours of fed-batch operation. Also, although sucrose was used as carbon source sucrose was exhausted for 50 hours and 16 g/l of xanthan was produced at 25.0 g/l of sucrose.
Effect of dissolved oxygen tension(DOT) on xanthan production was investigated by controlling DOT adaptively to a constant level. Fermentation time decreased with increasing DOT, but at above 50% air saturation of DOT the time was not shortened. Xanthan yield was higher when DOT control was started after DOT reached a given set point than at the starting of fermentation.
Strain stability of X. campestris in continuous culture was investigated. Turnover, Q(Q is a value of total feeding volume divided by the fermentor hold-up when the non-mucoid strain occurred) were 6.5 and 10 under nitrogen- and glucose-limited condition, respectively, using complex media. Also, Q value was 12. 0 when the cells were grown at the lower pH(5.0-6.0) and xanthan productivity was the same as that at higher pH(6.5-7.0) in the glucose-limited continuous culture.
In repeated-batch fermentation non-mucoid strain did not appear under any metal ion-deficient conditions. In continuous culture using synthetic media Fe-rich fermentation ran for 700 hours, equivalent to 65 turnovers whereas the Fe-deficient fermentation worked only for 120 hours of 12 turnovers. Cell production was higher and xanthan productivity and conversion yield were lower under Fe-rich fermentation compared to Fe-deficient fermentation.
The current success of long-term continuous production of xanthan gum by X. campestris will shed light on increase of productivity, which will reduce the cost of commercial xanthan production and enlarge the demands of future markets in related industries.
