Static mixers were introduced in the draught tube of a three-phase, concentric tube type airlift bioreactor, resulting in a 30% increase in ethanol productivity during glucose fermentation with a highly flocculent strain of Saccharomyces cerevisiae. Increased productivity was obtained as a consequence of the flee size reduction provoked by the new design of the draught tube and of the smaller applied aeration rates, allowing a higher dilution rate to be used. Steady state data at different dilution rates were measured for both systems and the results were compared in terms of specific consumption / production rates and ethanol productivity. An approach based on heterogeneous catalysis principles was used in order to consider the presence of porous yeast flocs. Total glucose conversion was achieved at a dilution rate of 0.34 h(-1). The smaller flee size contributed to the higher observed reaction rates by reducing diffusional limitations inside the flocs. The respiratory quotient had a constant value (around 23) at all dilution rates, meaning that the metabolic state of the cells in the flocs remained constant, having a strong fermentative metabolism.