Airlift-driven fibrous-bed bioreactor for continuous production of glucoamylase using immobilized recombinant yeast cells

Continuous production of a fungal glucoamylase by immobilized recombinant Saccharomyces cerevisiae strain C468 containing plasmid pGAC9. Yeast cells were immobilized on hydrophilic cotton cloth in an inverse internal loop airlift-driven bioreactor. Free-cell culture in the airlift and stirred tank bioreactors confirmed the plasmid instability of the recombinant yeast. Enhanced glucoamylase productivity and plasmid stability were observed both in the free and immobilized cell cultures in the airlift bioreactor system. The glucoamylase level of the free-cell culture in the airlift bioreactor was ∼20% higher than that in the in stirred tank bioreactor due to high cell density (cell dry weight/volume of bioreactor) and fraction of the plasmid-carrying cells. A potentially high glucoamylase activity of 161 U/L and a corresponding volumetric productivity of 3.5 U/L h were achieved when a cell density of ∼85 g/L (or 12.3 g/g fiber) was attained in the fibrous-bed immobilized cell bioreactor system. The stable glucoamylase production was achieved after five generations, at which time a fraction of ∼62% of the plasmid-carrying cells was realized in the immobilized cell system. Plasmid stability was increased for the immobilized cells during continuous culture at the operating dilution rate. The volumetric and specific productivities and fraction of plasmid-carrying cells in the immobilized cell system were higher than in the free-cell counterpart, however. This was in part due to the high viability (∼80%) in the immobilized cell system and the selective immobilization of the plasmid-carrying cells in the fibrous bed, and perhaps increased plasmid copy number.