The modified rotating simplex method has been successfully used to determine the best combination of agitation rate and aeration rate for maximum production of extracellular proteases by Staphylococcus aureus mutant RC128, in a stirred tank bioreactor operated in a discontinuous way. This mutant has shown altered exoprotein production, specially enhanced protease production. Maximum production of proteases (15.28 UP/ml), measured using azocasein as a substrate, was obtained at exponential growth phase when the bioreactor was operated at 300 rpm and at 2 vvm with a volumetric oxygen transfer coefficient (K(L)a) of 175.75 h(-1). These conditions were found to be more suitable for protease production.

-Mixing time is one of the criteria most widely used to characterize mixing intensity in bioprocesses. In bioreactors, mixing mainly depends on amount of energy consumed, reactor and stirrer shapes, airing speed and the rheology of the medium. In this work we experimentally determined the mixing times for a lab-scale bioreactor equipped with a stirrer propelled by two Rushton turbines. From these experiments we could obtain expressions to evaluate the effects of stirring speed, superficial gas velocity, specific power consumption and system geometry on mixing times under various flow regimes. The resulting correlations were employed to analyze the effect of scale-up on mixing times for the production of Staphylococcus aureus Smith.

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