Xanthan gum fermentation by Xanthomonas campestris immobilized in a novel centrifugal fibrous-bed bioreactor

Current industrial fermentations in conventional stirred-tank fermenters for production of xanthan gum and other polysaccharides are energy-intensive and costly, mainly because the high broth viscosity causes agitation and aeration to be difficult and limits the final product concentration and productivity. In this work, a novel, centrifugal, packed-bed reactor (CPBR) was developed for viscous xanthan gum fermentation. Xanthomonas campestris cells were immobilized in a rotating fibrous matrix by natural attachment to the fiber surfaces. The mixing and aeration problems were overcome by continuously pumping and circulating the medium broth through the rotating fibrous matrix to ensure intimate contact of gas and liquid with the cells and to separate the xanthan polymer from the cells. Almost all of the cells were immobilized on the fiber surfaces as was indicated by the absence of suspended cells in the fermentation broth. Thus, cell-free xanthan broth was obtained, with similar to 85% xanthan yield from glucose. The bioreactor was operated in repeated batch mode to study the feasibility and performance of long-term xanthan gum production using the immobilized cells in the fibrous bed. Consistent xanthan production rate and gum quality were obtained for eight consecutive batches studied during a total operation period of over 3 weeks. The volumetric xanthan productivity achieved in the reactor was similar to 1 g/(L . h) based on the total liquid volume and similar to 3 g/(L . h) based on the fibrous-bed volume. In contrast, conventional batch fermentation in a stirred-tank reactor (STR) with free cells usually has a productivity of only 0.5 g/(L . h) or lower. The high productivity in CPBR was attributed to the relatively high cell density, similar to 7 g/L, in the reactor. The specific xanthan productivity was lower in CPBR than in STR, however. This was because of the relatively low cell viability (similar to 60%) and limitation in oxygen transfer in CPBR, which can be improved by increasing the medium recirculation rate and the rotational speed of the fibrous matrix.