A patent culture of Alcaligenes faecalis var. myxogenes (ATCC 31749) synthesizes "curdlan" during the stationary phase of an aerobic batch fermentation following the depletion of assimilable nitrogen. Because this beta-1,3-homoglucan exopolymer is water insoluble, the fermentation broth is of a relatively low viscosity, and consequently offers little resistance to oxygen transfer from gas to the liquid. However, the layer of insoluble exopolymer surrounding the cell mass offers a resistance to oxygen transfer from the liquid to the cell, thereby necessitating an unexpectedly high dissolved oxygen concentration for maximal productivity. The shear sensitive nature of this fermentation restricts improving oxygen transfer by increasing agitation intensity. 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. The specific respiration rates for growing and curdlan-producing stationary-phase culture were determined to be 7 and 2.7 mmol O2/g cell/h, respectively. At a cell density of 3 g cell/L, the maximal rate of curdlan biosynthesis was about 100 mg/g cell/h, with a requirement for dissolved oxygen (DO) of 6.5 Mg O2/L (86% air saturation at 30-degrees-C and 1 atm.). Whereas, at constant impeller speed, the volumetric oxygen transfer was improved both by increasing the air sparging rate and by using O2-enriched air (30% O2), productivity was not consistently improved by operating at a twofold higher cell density and at a DO > 6.5 mg O2/L. At higher cell densities, it would appear that shear must be increased to minimize culture clumping and assist in ox en transfer to the cell.
