Enhancing xanthan fermentations by different modes of glucose feeding

This paper is the fourth in a series aimed at improving the understanding and operation of conventional agitated fermenters for the production of the commercially important gum, Xanthan. In the first, reproducible fermentations were established and this protocol was used in studies of different agitator types and of bulk mixing and dissolved oxygen concentration in the next two. Here, building on the previous work, the influence of different glucose feeding strategies on Xanthan production in a 20-L agitated fermenter under equivalent conditions of agitation and dissolved oxygen is reported. The biological performances in three types of fed-batch cultures (a two-step glucose addition, multiple glucose-pulse feeding and continuous feeding of glucose) are compared to two batch fermentations with different initial glucose concentrations. The work confirmed that improved performance cannot be achieved by increasing the initial glucose concentration above 50 g/L nor by a single 10 g/L pulse addition (initial glucose concentration of 40 g/L) while significant nitrogen is still present. On the other hand, the simple pulse and continuous feeding strategies, after nitrogen has been essentially exhausted and under conditions of nonlimiting dissolved oxygen and similar bulk mixing, can result in a greatly enhanced performance compared to batch fermentations. Using the final Xanthan gum concentration, the yield on glucose and the overall productivity as performance indices, values of 62 g/L, 0.82 g of Xanthan/g of glucose, and 0.72 g/(L.h), respectively, were obtained compared to literature values for conventional stirred bioreactors of 15-30 g/L, 0.27-0.86 g of Xanthan/g of glucose, and 0. 12-0.43 g/(L.h).