Oxygen transfer effects on recombinant benzaldehyde lyase production

Oxygen transfer (OT) effects on benzaldehyde lyase (BAL) production by recombinant Escherichia coli carrying pUC18:: bal gene were investigated on a defined medium with glucose and (NH 4 ) 2 HPO 4 as the sole carbon and nitrogen sources respectively, in 3.0 dm 3 bioreactor systems at six different conditions with the parameters, air inlet rate of Q O / V R = 0.5 vvm, and agitation rates of N = 250 , 375, 500, 625, 750 min - 1 and at Q O / V R = 0.7 vvm , N = 750 min - 1 . Cell concentration ( C X ), BAL volumetric and specific activities, organic acid concentrations, yield values, maintenance coefficients, oxygen transfer coefficients ( K L a ) and oxygen uptake rates (OUR) were determined throughout the bioprocess. Increase in the oxygen transfer rate increased the cell formation up to 0.5 vvm, 500 min - 1 ; however, further increase inhibited cell formation. The highest BAL volumetric activity was obtained at 0.5 vvm, 500 min - 1 as 860 U cm - 3 . K L a , and enhancement factor E ( K L a / K L a o ) increased with the cultivation time and agitation rate, their values varied between 0.008–0.046 s - 1 and 1.00–3.07, respectively. OUR first increased, and then decreased with the cultivation time at all OT conditions, the highest OUR was observed at 0.5 vvm, 500 min - 1 as 3.6 mmol m - 3 s - 1 at t = 8 h ; while the highest specific OUR was obtained at 0.5 vvm, 250 min - 1 as 0.74 kg kg - 1 h - 1 at t = 1 h . With the increase in Q O / V R and/or N, Damköhler number ( Da ) , that is the OT limitation, decreased. The effectiveness factor ( η ) values were close to 1.0 at medium and high OT conditions at the beginning of the processes as cells consumed oxygen with such a high rate that maximum possible oxygen utilization rate (oxygen demand) value was approached; thereafter, η decreased with the increase in cultivation time and Da, and the decrease rate was higher at lower agitation rates due to the lower K L a . Oxygen consumption for maintenance ( m O ) was the highest at 0.5 vvm, 500 min - 1 and the value was 0.20 kg kg - 1 h - 1 .