Aspergillus niger β-galactosidase production by yeast in a continuous high cell density reactor

The continuous production of extracellular heterologous β-galactosidase by a recombinant flocculating Saccharomyces cerevisiae, expressing the lacA gene (coding for β-galactosidase) of Aspergillus niger was investigated. A continuous operation was run in a 6.5 l airlift bioreactor with a concentric draft tube using lactose as substrate. Data on the operation with semi-synthetic medium with 50 and 100 g/l initial lactose concentrations are presented. The best result for β-galactosidase productivity-6.2×105 U/1 h-was obtained for a system operating at 0.24 h−1 dilution rate and for a lactose concentration in the feed of 50 g/l. This value represents a 11-fold increase in β-galactosidase productivity when compared to batch culture. Together with extracellular β-galactosidase production an ethanol productivity of 9 g/1 h was obtained for the bioreactor fed with 50 g/l initial lactose concentration at 0.45 h−1 dilution rate. In addition to β-galactosidase and ethanol production, this system allowed for complete lactose metabolism. The feasibility and advantages of using continuous high-cell-density systems operating with flocculent yeast cells for extracellular protein production is clearly shown.