ENHANCING XANTHAN FERMENTATIONS BY DIFFERENT MODES OF GLUCOSE FEEDING

This paper is the fourth in a series aimed at improving the understand ing and operation of conventional agitated fermenters for the producti on of the commercially important gum, Xanthan. In the first, reproduci ble fermentations were established and this protocol was used in studi es of different agitator types and of bulk mixing and dissolved oxygen concentration in the next two. Here, building on the previous work, t he influence of different glucose feeding strategies on Xanthan produc tion in a 20-L agitated fermenter under equivalent conditions of agita tion and dissolved oxygen is reported. The biological performances in three types of fed-batch cultures (a two-step glucose addition, multip le glucose-pulse feeding and continuous feeding of glucose) are compar ed to two batch fermentations with different initial glucose concentra tions, 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 bee n essentially exhausted and under conditions of nonlimiting dissolved oxygen and similar bulk mixing, can result in a greatly enhanced perfo rmance compared to batch fermentations. Using the final Xanthan gum co ncentration, the yield on glucose and the overall productivity as perf ormance 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 valu es for conventional stirred bioreactors of 15-30 g/L, 0.27-0.86 g of X anthan/g of glucose, and 0.12-0.43 g/(L.h).