Aspergillus terreus Broth Rheology, Oxygen Transfer, and Lovastatin Production in a Gas-Agitated Slurry Reactor

A 20 L gas-agitated slurry bubble column bioreactor was used to investigate the effects of nonmechanical low-intensity agitation on development of broth rheology and fungal pellet morphology during production of lovastatin by the filamentous fungus Aspergillus terreus. Fermentations were carried out under elevated dissolved oxygen levels (400% of air saturation) at gassing rates that ranged from 0.5 to 1.5 vvm. Various initial concentrations of the growth limiting nitrogen source were used to attain different total biomass concentrations, to observe the effect of this variable on development of pellets and the rheology of the fermentation broth. The non-Newtonian rheology of the fermentation broth was influenced both by the biomass concentration and the size of the fungal pellets. The stable pellet diameter ranged from ∼2300 to ∼2900 μm. Too low turbulence (gassing rate of 0.5 vvm) and low dissolved oxygen levels adversely affected lovastatin production. The best biomass specific production of lovastatin was attained at high biomass concentrations under oxygen-rich conditions that were not excessively turbulent. In fermentation broths with various rheologies, the oxygen-transfer coefficient in the bubble column correlated with the aeration velocity, biomass concentration, and effective viscosity of the broth. The correlations obtained were significantly different for broths with pelleted growth and those with filamentous growth.