| Summary: | Xanthan is a well-known extracellular polysaccharide, produced by a Gram negative bacterium Xanthomonas campestris (X. campestris) under aerobic conditions. Solutions of xanthan exhibit high viscosities and non-Newtonian behaviour even at low concentrations. This biopolymer has a wide range of valuable commercial and industrial applications, for example; it can be used as a food thickening agent and a stabilizer in some other industries. Traditionally the production of xanthan has predominantly been performed in stirred tank fermenter (STR). This study sought to compare the cultivation of the bacterium, X. campestris for the production of the viscous biopolymer xanthan gum in two different reactor systems, a novel oscillatory baffled reactor (OBR) and the conventional industry workhorse, the stirred tank reactor (STR). Overall biopolymer production occurred at similar rates in the well stirred and aerated STRs, albeit at the cost of higher energy inputs for mixing and aeration. Despite much previous literature promoting the use of the OBR for transporting and reacting very viscous systems, this was the first actual study attempting to investigate the use of the OBR for a highly viscous non-Newtonian fermentation process. The experimental results show that xanthan production was similar in the OBR than in the STR, the OBR is however readily suitable for the cultivation of xanthan. The probable reasons for the inability of the OBR to match the production rates of the STR may well lie in the complex nature of this fermentation process. Unlike a previous study on pullulan production (Gaidhani 2004) where the OBR outperformed the STR, X. campestris initially needs high oxygen transfer rates and the OBR, although it provides good bulk mixing and low energy consumption, seemed unable to equal the STR in this respect, especially in a very viscous system. The result shows that xanthan production in the OBR was similar to the equivalent process in the STR. In order to attempt to improve the OBR a number of technical modifications were made including a novel sparger design to improve gas dispersal. These were not successful in improving xanthan production. Similarly, attempts to achieve improvements via wider amplitude ranges led to damage to the equipment. The conclusion was that significant improvements to the physical robustness of the OBR were necessary before it could be successfully used to process highly viscous bio-fluids.
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