Shear conditions in clavulanic acid production by Streptomyces clavuligerus in stirred tank and airlift bioreactors

Abstract

In biochemical processes involving filamentous microorganisms, the high shear rate may damage suspended cells leading to viability loss and cell disruption. In this work, the influence of the shear conditions in clavulanic acid (CA) production by Streptomyces clavuligerus was evaluated in a 4-dm³ conventional stirred tank (STB) and in 6-dm³ concentric-tube airlift (ALB) bioreactors. Batch cultivations were performed in a STB at 600 and 800 rpm and 0.5 vvm (cultivations B1 and B2) and in ALB at 3.0 and 4.1 vvm (cultivations A1 and A2) to define two initial oxygen transfer conditions in both bioreactors. The average shear rate ($\dot{\gamma }_{\rm {av}}$) of the cultivations was estimated using correlations of recent literature based on experimental data of rheological properties of the broth (consistency index, K, and flow index, n) and operating conditions, impeller speed (N) for STB and superficial gas velocity in the riser (UGR) for ALB. In the same oxygen transfer condition, the $\dot{\gamma }_{\rm {av}}$ values for ALB were higher than those obtained in STB. The maximum $\dot{\gamma }_{\rm {av}}$ presented a strong correlation with a maximum consistency index (K _max) of the broth. Close values of maximum CA production were obtained in cultivations A1 and A2 (454 and 442 mg L⁻¹) with similar maximum $\dot{\gamma }_{\rm {av}}$ values of 4,247 and 4,225 s⁻¹. In cultivations B1 and B2, the maximum CA production of 269 and 402 mg L⁻¹ were reached with a maximum $\dot{\gamma }_{\rm {av}}$ of 904 and 1,786 s⁻¹. The results show that high values of average shear rate increase the CA production regardless of the oxygen transfer condition and bioreactor model.