Industrial Fermentation: Bioreactors and Processing

Posted on Jun 20, 2026 in Biotechnology

Core Structural Features

• Vessel Material: Constructed from 316L Stainless Steel for internal components to prevent corrosion and toxic heavy metal leaching, while ensuring the vessel can withstand intense steam sterilization pressures (Durand & Chereau, 1987; Iagati, 2014).
• Aspect Ratio: Typically cylindrical with a hemispherical top and bottom, with a height-to-diameter ratio (H:D) usually ranging between 2:1 and 6:1 to optimize gas bubble ascent times (Iagati, 2014).
• Baffles: Flat vertical strips attached to the inner wall of the tank. They break up wild, vortexing liquid currents and convert them into top-to-bottom turbulent mixing patterns.
• Sparger: A ring or nozzle located at the base of the tank that introduces sterile air or gas under pressure into the liquid.
• Cooling Jacket/Coils: Microbial metabolism produces considerable heat. Cool water is circulated through outer jackets or internal coils to keep the temperature tightly regulated.

Types of Fermenters

Bioreactors are classified by how they handle fluid dynamics, mixing, and aeration.

A. Stirred Tank Reactor (STR)

The Stirred Tank Reactor is the standard workhorse of the biotechnology industry, relying on mechanical energy for mixing.

• Mechanism: Uses an internal motor-driven shaft equipped with multiple impellers (such as flat-blade Rushton turbines) to physically chop gas bubbles into tiny sizes and blend the media.
• Advantages: Exceptional mixing and gas dispersion capabilities; highly flexible and adaptable to varying media viscosities.
• Disadvantages: High power consumption and high shear stress, which can tear apart delicate animal, plant, or filamentous fungal cells.

B. Airlift Reactor

A pneumatic design that eliminates the mechanical agitator shaft, relying instead on density differences for fluid movement.

• Mechanism: Divided into an inner tube (Riser) and an outer annulus (Downcomer). Gas is injected exclusively at the base of the riser. The gas-liquid mixture expands and lowers in density, forcing it upward. At the top, gas escapes, increasing the liquid’s density and causing it to sink back down through the downcomer.
• Advantages: Very low shear stress, highly efficient oxygen transfer, and low power requirements due to the absence of a motor shaft. Excellent for plant, animal, or shear-sensitive cell cultures.
• Disadvantages: Less effective mixing when working with high-viscosity media.

C. Bubble Column Reactor

A simplified pneumatic reactor consisting of a tall vertical column without internal draft tubes.

• Mechanism: Gas is pumped into the very bottom through high-pressure sparger plates (Durand & Chereau, 1987). The upward movement of the rising gas bubbles provides the sole source of mixing and agitation.
• Advantages: Low capital cost, simple design with no moving internal components, minimal maintenance, and high liquid holdup.
• Disadvantages: Poorer mixing performance along the horizontal axis, making it prone to forming stagnant nutrient pockets if the vessel is scaled too wide.

D. Tower Fermenter

A specialized variation of the bubble column designed with a very high aspect ratio (H:D ratio often exceeding 10:1).

• Mechanism: Broth enters from the top while gas bubbles flow from the bottom. It often features internal perforated trays or plates to slow bubble ascent and maximize gas-liquid contact times.
• Advantages: Excellent for continuous processes involving flocculating yeasts (like traditional beer brewing), where yeast clumps sink down against the upward flow of gas, allowing automatic cell separation without external centrifuges.

Upstream Processing (USP)

Upstream processing encompasses all steps prior to the actual harvest of the product. Its main objective is to create the optimum environment for microbial growth and product synthesis.

Key Components of USP

• Producer Organism Selection: Isolation, screening, and genetic modification of a high-yielding, stable microbial strain.
• Media Formulation: Designing a cost-effective substrate containing essential carbon sources (e.g., molasses, glucose), nitrogen sources, vitamins, and minerals.
• Sterilization: Destroying contaminating microorganisms in the media, fermenter vessel, and ancillary equipment using pressurized steam (121°C).
• Inoculum Development: Scaling up the microbial culture from a master cell bank vial to a volume large enough to seed the production fermenter.
• Fermentation / Bioreactor Operation: Culturing the microorganism under strictly controlled conditions (pH, temperature, dissolved oxygen, and agitation) to maximize product yield.

Downstream Processing (DSP)

Once fermentation is complete, the target product must be isolated from a complex broth. DSP usually accounts for the major share of total production costs.

Step A: Separation of Microbial Cells

The first step in DSP is solid-liquid separation to remove the cell mass from the liquid medium.

• Filtration: Passing the broth through a porous membrane or filter cloth. Rotary Vacuum Drum Filtration (RVDF) is commonly used for large-scale filamentous fungi or yeast separation. Cross-Flow Filtration (Microfiltration) avoids rapid membrane fouling by running the broth parallel to the membrane surface.
• Centrifugation: Utilizing centrifugal force to separate particles based on density differences. It is highly effective for smaller bacterial cells (e.g., E. coli) that clog standard filters.

Step B: Cell Disruption

If the target product is intracellular, the cell wall and membrane must be broken.

• Mechanical Methods: High-pressure homogenization or bead milling.
• Non-Mechanical Methods: Enzymatic lysis, chemical treatment, or physical methods (osmotic shock, freeze-thawing).

Step C: Phase Extraction

The volume of the liquid stream is reduced, or the product is partitioned away from contaminants.

• Liquid-Liquid Extraction: Mixing the broth with an immiscible organic solvent.
• Aqueous Two-Phase Systems (ATPS): Mixing two structural polymers (e.g., PEG and dextran) to create two benign, water-based phases.

Step D: High-Resolution Purification

Chromatography separates the target molecule from impurities based on specific chemical attributes.

Step E: Final Formulation

The final steps convert the purified liquid product into a stable, concentrated solid form.

• Crystallization: Altering temperature, pH, or solvent composition to exceed the product’s solubility limit.
• Drying: Removing remaining moisture from the product.