Bioreactor design considerations in the production of high-quality microbial exopolysaccharide

An examination into the effect of bioreactor design on the production of β,1,3-glucan exopolysaccharide (“curdlan”) by selected patent cultures ofAlcaligenes faecalis andAgrobacterium radiobacter revealed that low shear mixing achieved through the replacement of the radial-flow flat-blade impellers that are commonly supplied in “standard” commercial bioreactors, by low shear (high-pumping) axialflow impellers, leads to an increase in thequality of the exopolymer recovered during the stationary-phase of batch fermentations. Whereas “Rushton turbine” impellers were effective in providing high rates of oxygen transfer necessary for high cell density fermentations, the high shear-to-flow ratio characteristic of this design produced a product of inferior quality, but with characteristics very similar to that of the commercially available “curdlan standard.” Curdlan is water insoluble, and consequently, the fermentation broth is of a relative low viscosity compared to other soluble microbial polysaccharides. Whereas curdlan does not constrain mass transfer from gas to liquid, it nevertheless offers a resistance to oxygen transfer from the liquid to the cell by virtue of the layer of insoluble exopolymer surrounding the cell mass, thereby necessitating an unexpectedly high dissolved oxygen concentration for maximal productivity. The requirement for high volumetric oxygen transfer can be met by low shear designs with axial-flow impellers, providing gas dispersion is assisted by the use of sparging devices consisting of microporous materials.