| Xanthan gum, a microbial polysaccharide secreted from Xanthomonas campestris NRRL B-1459, has been widely used as a suspending and thickening agent in the food, chemical, and pharmaceutical industries. The unique characteristics, such as high viscosity at low concentration and pseudoplasticity extends its market value, yet the distinctive rheology causes difficulties in aeration and agitation during fermentation. The centrifugal, packed-bed bioreactor (CPBR) has been developed with improved xanthan production. However, the cellular metabolism of X. campestris and the oxygen transfer characteristic in CPBR have not been characterized. In this study, metabolic flux analysis (MFA) was employed to characterize the cause-and-effect relationship between glucose utilization and the fermentation strategies during xanthan gum fermentation in CPBR. Based on the information obtained from MFA, the environment impact to cellular metabolism can be further identified. Furthermore, the oxygen transfer properties of CPBR during viscous xanthan gum fermentation were determined in terms of the effect the arrangement of the centrifugal, packed-bed (CPB) and the recirculation loop (RL). Characterized by the maximum volumetric transfer coefficient (kLa), the aeration efficiency of CPBR was compared to those in Stirred-tank reactors (STR) equipped with disc turbine (DT) or marine propeller (MP), and to that in a water-in-ail emulsion. In addition, the effects of polyethylenimine (PEI), a branched cationic polymer, on the immobilization of Xanthomonas campestris onto two different types of fibrous matrices (cotton towel, CT, and viscose towel, VT) were investigated. The surface characteristics of both CT and VT were significantly altered after treatment with 0.25% (w/v) PEI. The amount of cells immobilized on the fibrous matrices was dependent on the contact time and the holding capacity of the fibers. Although the anticipated enhancement in crosslinking exopolysaccharide-free cells with fibrous matrices treated by PEI was greatly hindered in this study mainly due to the presence of xanthan gum polymers, once the cells were immobilized, they were still capable of producing xanthan gum, with final xanthan concentrations higher than those produced solely from suspending cells. Ultimately, the scale-up strategy for xanthan gum fermentation will be developed by integrating information obtained from metabolic flux analysis and oxygen transfer characteristics of CPBR. |
