The fungal morphology during submerged cultivations has a profound influence on the overall performance of bioreactors. In this research, glucoamylase production by Aspergillus niger has been taken as a model to improve more insights. The morphology engineering could be conducted effectively by changing the seed morphology, as well as specific power input. During the fed-batch cultivations, pellet formation under milder shear stress field helped to reduce the broth viscosity, thus relieving oxygen limitation and promoting the enzyme production. Furthermore, we found that the relation between the shear stress field, which was characterized by energy dissipation rate/circulation function (EDCF), and enzyme activity was consistent with quadratic parabola, which threw light on the process optimization and scale-up for industrial enzyme production.

Global and local gas–liquid characteristics (gas hold-up, volumetric mass transfer coefficient), and flow field, mixing time of the liquid phase are investigated for various triple-impeller configurations. Four types of impellers (Rushton turbine (RT), hollow blade turbine (HBT), wide-blade hydrofoil impeller pumping down (WHd), and pumping up (WHu)) were used to form four combinations (3RT, HBT+2WHd, HBT+2WHu, and 3WHu). The results show that the axial impellers combination (3WHu) provides more effective homogenization performance than the impellers with combined radial and axial flow (HBT+2WHu, HBT+2WHd), while the radial impellers combination (3RT) is the worst. When the gas superficial velocity is 1.625 mm s–1, 3WHu produces a 53% higher mass transfer coefficient than HBT+2WHd, HBT+2WHu, and 3RT lie between them. When the gas superficial velocity reaches up to 8.124 mm s–1, however, all of the tested configurations give almost similar mass transfer coefficients under equivalent power input. For 3RT, the highest hold-up is in the bottom impeller discharge stream and near the wall for the middle and top impellers. For the HBT+2WHd combination, there was no large variations of gas hold up in the bulk except the region around the bottom impeller. For HBT+2WHu and 3WHu, high gas hold-up was observed between the two up pumping impellers, and moderately low gas hold-up above the top impeller. There are three zones of higher interfacial area for the HBT +2WHu and 3RT combination, two zones of higher interfacial area for 3WHu, and only one zone of higher interfacial area for HBT+2WHd combination. At low gas velocity, the flow pattern generated by each impeller combination results in different gas bubble trajectory and different bubble breakup and coalescence kinetics, which in turn influences both local and average gas holdup directly, and also affects the local specific interfacial area, that is, influences the mass transfer coefficient indirectly. At higher gas velocity, the power drop also contributes to change of the gas hold-up and mass transfer at the same specific power consumption.

Effects of impeller configuration on fungal physiology and cephalosporin C production were investigated by an industrial strain Acremonium chrysogenum in a 12 m3 bioreactor equipped with conventional and novel impeller configuration, respectively. The cell growth and oxygen uptake rate (OUR) profiles were little affected by the impeller configurations. However, differing impeller combinations significantly affected the morphology, which in turn influenced cephalosporin C production. Under the novel impeller configuration, the production of cephalosporin C was 10% higher and an excessive amount of dispersed arthrospores was also observed. Computational fluid dynamics (CFD) simulation further revealed that poor mass and energy exchange as well as inhomogeneous environment existed in the bioreactor equipped with conventional impeller configuration. For equivalent power dissipation, the volume oxygen transfer coefficient (KLa) could be enhanced by 15% compared with that of conventional impeller configuration. Power consumption was dramatically decreased by 25% by using novel impeller configuration.Highlights► The efficiency of cell factory rely on complex co-operation with dynamic variation of extracellular environment. ► Little reasonable knowledge with regard to when and how to improve surrounding environment in the bioprocess. ► We design a novel impeller configuration and CFD modeling, discovering a series of physiology response. ► This was also a case integrating fluid hydrodynamics and physiology which had been relatively few reported.

•Enzyme yields increased significantly (20–60%) with proper agitation system.•Exceed power input of RT impeller enhanced the shear effect rather than mixing.•Pellet morphology is determined mostly by local high shear zone.•The kLa are similar in average but widely divergent in distribution in the fermentor.Significantly influenced by complex cell morphology, glucoamylase fermentation using Aspergillus niger is characterized by high apparent viscosity and shear-thinning rheology. In this study, the influence of liquid flow field patterns on morphology, broth rheology, mass transfer and glucoamylase production was investigated by applying two different configurations with radial (Ruston Turbine, RT) and axial (Wide-blade hydrofoil upward-pumping, WHu) flow impellers. It was found that empirical correlations for averaged quantities, such as the mass transfer coefficient and viscosity, cannot reasonably explain the observations. Therefore, numerical simulation was carried out to study the detailed characteristics of local field in lab-scale bioreactors. The results showed, under similar glucose and oxygen uptake rates, that the WHu configuration formed relatively homogeneous viscosity and mass transfer fields, while the RT configuration was accompanied with significant heterogeneities. Under these conditions, the fraction of active mycelia in pellets could be highly correlated with enzyme production, and a novel parameter (Active Part Percentage, APP) was defined to introduce the effects of flow field on pelletized morphology. The WHu impellers facilitated the formation of pellets and hairy structures, with a higher APP of the pellets. As a result, the culture with the axial flow impeller configuration exhibited a larger glucoamylase production rate (+25%) and product yield on sugar (+23%) and yield on energy (+60%) in comparison to the radial flow impeller. Computational fluid models were proposed to in-depth understand such results based on local mass transfer and viscosity values, since the average values are similar over the entire fermentation processes.