Serine alkaline protease (SAP) production in a complex medium based on physically pretreated molasses by recombinant Bacillus subtilis carrying pHV1431::subC gene is described. The effects of oxygen transfer were investigated in 3.5 dm(3) bioreactor systems with controls for agitation rate, dissolved oxygen, pH, temperature, and foam formation under two different agitation rates, ie N = 500 and 750 min(-1), and four different air flow rates, ie Q/V-R = 0.2, 0.5, 0.7, and 1.0 vvm, at a molasses concentration equivalent to initial sucrose concentration (C-So) of 20 kg m(-3). The yield values (Y-x/s, Y-x/o, Y-s/o) and maintenance coefficient of oxygen (m(o)), were calculated. mo decreased with the increase in the air-inlet rate. Increase in oxygen transfer rate increased the rate of growth and SAP activity, and affected the cultivation time to achieve maximum expression of SAP activity. At Q/V-R = 0.5 vvm and N = 750 min(-1), SAP activity reached 2250 U cm(-3) at t = 36 h. The oxygen transfer coefficient (K(L)a) and oxygen uptake rate (-r(o)) were measured throughout the fermentations and their variation with the oxygen transfer conditions determined. New correlations for the calculation of KLa and -r(o) are proposed. (C) 2004 Society of Chemical Industry.