Improved performance in viscous mycelial fermentations by agitator retrofitting

Abstract

For viscous mycelial fermentations it was demonstrated at the pilot-plant scale that the replacement of standard radial flow Rushton turbines with larger diameter axial-flow Prochem hydrofoil impellers significantly improved oxygen transfer efficiency. It was also determined that the Streptomyces broth under evaluation is highly shear thinning. Separate experiments using a Norcardia broth with similar Theological properties demonstrated that the oxygen transfer coefficient, K(L)a, can be greatly increased by use of water additions to reduce broth viscosity. These observations are consistent with the hypothesis that the improvement in oxygen transfer by changing agitator types is primarily due to an improvement in bulk mixing. A model is presented, based on the concepts of Bajpai and Reuss, which explains this improvement in performance in terms of enlargement of the well mixed micromixer region for viscous mycelial broths.

Mixing in the Fermentation and Cell Culture Industries

“…Prochem hydrofoil impellers, which produce axial flow, have been tested for use in high viscosity mycelial fermentations. Buckland et al (1988a) demonstrated, in viscous mycelial fermentations at the pilot scale, that the replacement of standard radial flow Rushton impellers with larger diameter axial flow Prochem impellers, significantly improved the oxygen transfer efficiency. This improvement was attributed to the increase in size of the well-mixed, low viscosity cavern in the impeller region.…”

“…Bulk blending is considered to be poor, using such turbines due to their tendency to compartmentalize (Nienow and Ulbrecht, 1985) and can lead to broth inhomogeneities of either pH or oxygen. This may be expected to be amplified in viscous, non-Newtonian broths (Buckland et al, 1988a;Nienow et al, 1995;Amanullah et al, 1998b). Bryant (1977) and Bajpai and Reuss (1982) have suggested that the critical factor that determines the overall effectiveness of oxygen uptake by microorganisms is the frequency at which cells are circulated through the highly oxygenated impeller region.…”

“…Considering the importance of these complex morphologies on fermentation performance, and reports that changes in the morphology of P. chrysogenum can be caused by mechanical forces (Dion et al, 1954;Metz et al, 1981;van Suijdam and Metz, 1981;Smith et al, 1990;Nielsen, 1992;Makagiansar et al, 1993;Ayazi Shamlou et al, 1994;Jüsten et al, 1996Jüsten et al, , 1998a, the direct effect of agitation on morphology in submerged fermentations requires attention. As well as the total power input, the choice of impeller geometry determines the hydrodynamic forces that might affect the morphology (Jüsten et al, 1996;Amanullah et al, 1999) and differentiation (Jüsten et al, 1998a) of filamentous species, thereby influencing growth or production (König et al, 1981;Buckland et al, 1988a;Jüsten et al, 1998a;Amanullah et al, 1999). Other environmental factors, such as pH and spore concentration (in cases where spores are used as inoculum), also significantly influence mycelial morphology (Metz and Kossen, 1977).…”

“…Such results have also been demonstrated in highly viscous xanthan fermentations (see Section 18.3.4). Buckland et al (1988a) recommended that standard Rushton impellers could be replaced by low-powernumber agitators (preferably axial flow) such that the retrofitting would result in similar operating speeds, torque, and power. In this manner, the same shaft, motor, and gearbox could be employed.…”

The Scale‐Up of Microbial Batch and Fed‐Batch Fermentation Processes

“…Sometime later in the early 1990s, Enfors and co-workers came to similar conclusions for fedbatch fermentations with respect to the nutrient feed (see for example, ). At around this time, it was shown that replacing Rushton turbines by high solidity ratio hydrofoil impellers which enhanced bulk mixing (spatial homogeneity) improved fermentation performance (Buckland et al, 1988), Only fairly recently, however has it been established experimentally that spatial and temporal chemical gradients exist in large-scale fed batch bio-reactors where additions of a concentrated, often viscous, carbon source at a single point onto the top surface of the growth medium means that mixing times are high (>~50s even at the 20m 3 scale (Vrabel et al, 2000)). Studies using computational fluid dynamics (CFD) based on Large Eddy Simulation (LES) also showed that considerable glucose gradients could be expected even when a standard 500 g/L glucose solution was fed to the liquid surface in a 22 m 3 bioreactor fitted with four Rushton turbines (Enfors et al, 2001).…”