Importance of Stirring
Uniform Mixing: Ensures even distribution of nutrients, microorganisms, and temperature.
Enhanced Mass Transfer: Facilitates oxygen transfer from air bubbles to cells.
Heat Transfer: Aids in distributing heat uniformly, preventing temperature gradients.
Types of Stirring Systems
1) Impellers (Mechanical Agitators)
Rushton Turbine: Flat-blade disk impeller, common for gas-liquid dispersion.
Marine Propeller: Efficient for low-viscosity liquids; large blade area.
Paddle Impeller: Simpler design, often used in low-shear conditions.
2) Airlift Fermenters
Rely on air sparging instead of mechanical stirring.
Circulation is driven by the difference in density between the aerated and non-aerated zones.
Key Parameters in Stirring
Agitation Speed (rpm): Must be optimized to ensure proper mixing without damaging shear-sensitive cells.
Power Consumption: Stirring can be a major energy cost; large-scale setups must balance oxygen transfer needs with efficiency.
Shear Sensitivity: Certain cells (e.g., animal cells, filamentous fungi) can be damaged by high shear forces.
Common Stirring Instruments
Impellers – Rotating blades that generate turbulence.
Propellers – Smaller, simpler versions of impellers.
Magnetic Stirrers – Use a rotating magnetic field to move a stirrer bar.
Air-Lift Fermenters – Rely on air bubbles for mixing.
Bioreactors – Large vessels with mechanical stirring mechanisms like impellers or propellers.
