The impeller mixes broth and disperses gas bubbles from the sparger,
driving oxygen transfer to the growing cell population (k_La). Cells consume
oxygen and nutrients to grow and secrete product; too little agitation or
aeration starves the culture, while too much shear can damage cells at larger
scale — the core challenge of scaling a bioreactor up.
k_La ∝ (P/V)^0.5 · v_s^0.5 (agitation power & superficial gas velocity)
dX/dt = μ_max · X · (DO / (DO + K_DO)) (Monod-type oxygen limitation)
titer(t) = ∫ q_p · X dt
- Impeller agitation — RPM of the stirrer; raises mixing and oxygen transfer, but excess shear can hurt cells at high RPM.
- Aeration rate — gas flow through the sparger (vvm); more bubbles raise dissolved oxygen, up to a saturation limit.
- Temperature — shifts growth rate; the culture grows fastest near its optimum (≈37 °C) and slows away from it.
- Scale preset — bench, pilot and production vessels keep the same physics but change volume, so the same RPM delivers very different mixing energy per litre — the essence of scale-up.