Strategies for penicillin fermentation in tower-loop reactors

Since it has not been possible to produce penicillin in tower-loop reactors with highly viscous filamentous molds of Penicillium chrysogenum which are employed in stirred-tank reactors, a new strategy has been developed to avoid the formation of this morphology and to use the pellet form of the fungi. When employing definite impeller speeds in the subculture in connection with definite inoculum amounts and substrate concentrations in the main culture (bubble column), it is possible to generate a suspension of isolated small pellets, which shows a low broth viscosity up to a sediment content of 45% over the entire fermentation time. Volumetric mass-transfer coefficients k(L)as are by a factor of 4 to 5 higher in these pellet suspensions than in filamentous broths. It was easy to maintain the necessary oxygen supply for penicillin production in these pellet suspensions. Under these conditions the specific penicillin productivities were higher with regard to power input (up to 90%), biomass, and consumed substrate than in the stirred-tank reactors with highly viscous filamentous morphology of the fungi. Under nonoptimized operating conditions the absolute penicillin production in the tower loop was 35% lower than in the stirred-tank reactor due to lower possible biomass concentrations. The separation of the biomass, and therefore the penicillin recovery, is much simpler when employing pellets. It is shown how the particular mass transfer resistances at the gas/liquid and liquid/pellet interfaces and within the pellets change with the pellet diameter. There should be a particular pellet diameter at which penicillin productivity has its maximum. These investigations indicate that the use of tower-loop reactors can, in the future, be an alternative for more economical penicillin production methods.