Application of high cell density airlift bioreactors to bio-ethanol production – study on optimal bioreactor operation

Abstract

In this paper a hydrodynamic and rheological analysis of a continuous airlift bioreactor with high cell density system is presented. A highly flocculating recombinant strain of Sacharomyces cerevisae containing genes for lactose transport (lactose permease) and hydrolysis (β-galactosidase) was exploited to ferment lactose from cheese whey to ethanol. The magnetic particle-tracer method was used to assess the effect of operational conditions (air flow rate, biomass concentration) on hydrodynamic behaviour of an the airlift bioreactor during the fermentation process. Measurements of liquid circulation velocity showed the existence of a critical value of biomass concentration at which a dramatic deceleration of net liquid flow appeared with increasing biomass quantity. Rheological analysis revealed dramatic changes in viscosity of the yeast floc suspension at the same biomass concentration of about 73g.dmˉ³ corresponding to 42.8 % v/v of solid fraction. These facts have a particular importance for the successful processing of high cell density airlift bioreactor as only a circulated flow regime will be favourable to keep the solid particles in suspension state and evenly distributed throughout the bioreactor.