Effects of increased impeller power in a production-scale Aspergillus oryzae fermentation

The goal in this study was to determine how increased impeller power affects enzyme expression in large-scale (80 m(3)), fed-batch Aspergillus oryzae fermentations. An approximate 50% increase in average impeller power was achieved by increasing impeller diameter approximately 10%, while operating at slightly reduced speed. Measured decreases in terminal (95%) mixing time show increased power improved bulk mixing. However, batches operated at increased power had lower recombinant enzyme productivity. Biomass assays and image analysis tests showed no significant difference between high power and control batches, suggesting that slower growth, altered morphology, or increased hyphal fragmentation were not the cause of reduced productivity. Off-line tests on the shear-thinning, highly viscous broth show oxygen limitation occurred after transport through the air-liquid interface and imply the limitation may involve bulk mixing. Specifically, oxygen transfer may be limited to a small zone surrounding each impeller. When this is the case, oxygen mass transfer will be determined by both impeller shear and fluid circulation, which have been characterized with the energy dissipation/circulation function (EDCF). EDCF values during control fermentations were approximately constant at 25 kW m(-3) s(-1), while EDCF values during high power batches fell linearly from 40 to 15 kW m(-3) s(-1). The point at which high power EDCF values drop below those in control fermentations corresponds almost exactly with the point at which product titer stops increasing. Thus, our findings suggest oxygen mass transfer was less efficient during the latter half of high power fermentations because of reductions in impeller speed and subsequent decreases in EDCF values. This observation has clear implications during the scale-up of viscous fungal fermentations, implying that not only is the level of impeller power important, but also relevant is how this power is applied.