Natural Immobilization and Fluidized Bed Fermentation

Abstract

Immobilization of biomass within a fermenter produces a large increase in cell concentration, which results in a substantial increase in volumetric productivity (r_p, mass of product per unit volume of fermenter per unit time). For primary metabolites, immobilization allows continuous culture operation at dilution rates in excess of μ_max; whilst for secondary metabolites, immobilization allows continuous culture operation at dilution rates which support the maximum rate of product synthesis. If naturally adhesive cells are immobilized on small (1mm) support particles, it is possible to operate the fermenter as a fluidized bed (FBF) and thereby benefit from the efficient mass transfer which characterises this mode of operation. For example, the biomass concentration of Zymomonas mobilis in the FBF was up to 12 times that expected in conventional chemostat culture and r_p for ethanol was up to 10 times higher; with Streptomyces coelicolor, the volumetric productivity of actinorhodin (an antibiotic) was over 20 times higher than that obtained in the best batch culture.