Bioreactor: Parameter, Parts, Types, and Application

Bioreactors are containers created and designed to offer an efficient environment for whole cells or enzymes to convert bio-chemicals into products. In other circumstances, such as water treatment, the bioreactor is used to inactivate or sterilize cells.

Numerous distinct bioreactors include stirred tank bioreactor, packed bed bioreactor, fluidized bed bioreactor, airlift bioreactor, and bubble column bioreactor. Depending upon the type of bioreactor, it holds various applications, such as those for cell growth, enzyme production, catalysis, biosensors, food production, milk processing, extrusion, tissue engineering, algae production, protein synthesis, anaerobic digestion, algae, and chitinolytic culture.

Parameters to be Considered for Bioreactor

Bioreactor only works well when it meets specific parameters like temperature, mixing rate of microorganisms, uniform nutrient transfer, and gassing.

• Culture mixing: The culture for the bioreactor must be mixed constantly and carefully to ensure the proper mixing of nutrients and maintain the suitable pH and temperature among cells. The mixing rate varies according to the organisms used for culturing. For example, the rate for mixing fungi and yeast is 500-1500 per min, and for mammalian, insect, and plant cells is 30-300 per min
• Temperature and pH: Correct temperature and pH play crucial roles in the processes occurring in the bioreactor because culture cells and enzymes work efficiently in certain temperatures and pH. The specific temperature range for bacteria, fungi, and yeast cells is 20 °C to 60 °C, and the pH range for these organisms is 4.5-7.
• Nutrient required:    Depending on the bioprocess strategy, nutrients such as water, energy sources like glucose, carbon, salt, and other trace elements (e.g., vitamins) are either added gradually over time (in a fed-batch or continuous process) or made available at the start of a bioprocess (batch bioprocess).
• Construction material: The materials (glass or stainless steel) used during construction should withstand the pressure and have to be anti-corrosive.
• Ideal foam collector: The ideal bioreactor must have a foam controller to prevent various side effects caused by it. The detector must sense when the foam touches it so that it adds the anti-foaming agent to a fermenter.
• Supplying air:   The continuous stirring causes disturbance in nutrients and reduces bubbles’ size. So, it is essential to release the air molecules in the nutrient broth as air molecules are taken by the cell only when it dissolves in the nutrient solution. The demands of air molecules depend on the types of organisms or cells used (aerobic or anaerobic) and the fermentation phase. At the very beginning, only a few amounts of oxygen are needed, but the supply of oxygen is increased as there is faster growth of cells.

Parts of Bioreactor

The various parts of the bioreactor work together to meet the parameters like temperature, nutrients, aeration, and pH for the growth and development of the cells, enzymes, and many more. Some essential parts of a bioreactor are discussed below:

1. Fermenter vessel: Most fermented containers are made of glass and stainless steel to reduce pressure and corrosion. It provides a workable environment for production.
2. Heating and cooling apparatus: The cooling jacket and silicon in a reactor help to remove excess heat, while internal coils provide heat during fermentation.
3. Feed ports: The silicon tubes are available for adding nutrients and acid/alkali for fermentation.
4. Foam control: The foam produced during the fermentation process has many side effects like it decreases efficiency, and productivity, degrading product quality, and many more. So, the foam detector is placed in a reactor, adding some anti-foam is used to deform the fermenter.
5.  Valves: In the fermenter, valves regulate the liquid flow. Most of the reactor contains at least three valves in it.
6. Sparger: It is used in introducing sterile air to a fermentation vessel. It also aids in providing the vessel with the correct aeration.
7. Impeller: The role of an impeller in a fermenter is to distribute microbial cells in a nutrient media evenly and also to reduce the bubbles produced with the help of impeller blades.
8. Computer: Modern automated and semi-automated software programs are used for collecting data, monitoring, and controlling the process, such as Ambr® Clone Selection software application used for cell line screening and Software called Biostat® T CHO introduces users to bioprocess engineering and controls a bioreactor as well.
9. Baffles: Baffles are the metal stripes attached to the wall of the container to prevent vertex and improve aeration in the fermenter.
10.  Regulator: This device is used to control and maintain the temperature, pH, nutrients, oxygen concentration, and product concentration.

Types of Bioreactors

Different bioreactors have been constructed depending on their use. Some of them are listed below :  (4)

1. Continuous stirred tank bioreactor

• A Continuous stirred tank bioreactor is a cylindrical vessel with a motor-driven central shaft holding one or even more agitators (impellers).
• It consists of different features for proper agitation and baffles for aeration.
• It creates a homogeneous environment, is easy to operate, easy to clean, cheap, and which is why used to a great extent in industries. 

2. Airlift bioreactor

• An airlift bioreactor, also known as a tower reactor, contains a draft tube that boosts circulation and air transfer. The draft tube even increases the shear forces in a bioreactor.
• It consists of two zones, one is called riser, which is sparged with gas, and another one is known as downcomer, which does not contain sparged gas.
• Some products are temperature-dependent, and growing different types of cells in a wide range of temperatures (30-40℃) in the same instrument is complex. A two-stage reactor is necessary for producing such products where the cell growth occurs in one reactor, and the remaining process occurs in another reactor.
• It does not require agitators because of the simple design. This property makes airlift bioreactor affordable, requires low energy, and is easy to clean. 

3. Bubble column reactor

• It is a cylindrical vessel with an aspect ratio of 4:10 and contains a gas distributor at the bottom section of the vessel where gas is sparged in the form of bubbles.
• Like an airlift reactor, it doesn’t have a draft tube.
• It contains perforated pipes or porous metal spargers to sparge gas.
• Gas flow rate and fluid rheological characteristics significantly impact O2 transfer, mixing, and other operational aspects.
• The advantages of the bubble column reactor over other reactors are its high productivity in terms of volume and superior heat management, and self-regulation.
• It is specially used to produce aerobic products. (2)

4. Fluidized-bed bioreactor

• This type of reactor consists of packed beds (solid-liquid material) where solid particles are retained, and liquid particles flow out.
• Except for the top position expanding to lower fluid velocity, a fluidized bed bioreactor is similar to a bubble column bioreactor.
• The merits of using this reactor are the uniform mixing of particles, uniform temperature gradients, and can operate in a continuous state.
• Inadequate fluidization occurs due to the accumulation of relatively massive or densely packed particles on the distributor plate, which is a significant issue with the steady functioning of fluidized beds.
• The major issue of fluidized bed bioreactors is inadequate fluidization due to the accumulation of relatively massive or densely packed particles on the distributor plates. (3)

5. Packed bed bioreactor

• A packed bed bioreactor consists of a column of solid particles with biocatalysts on or within the solids matrix. It is also called a fixed-bed reactor.
• A nutrition broth is continuously applied on top of the immobilized biocatalyst.
• It is challenging to adjust the pH of the packed bed bioreactors because of improper adding of acid or alkali combined with poor mixing. The issues like undesired temperature gradient, difficulty in cleaning, hard-to-replace catalyst, and undesirable side effects are frequently observed.
• This type of reactor offers many advantages like a high catalyst conversion rate, simplicity to use, low construction time, affordable operating costs, better reactant-catalyst contact, and can operate at high temperatures and pressures.
• In addition, a packed bed bioreactor is used to create bio-artificial liver support systems and expanded bone marrow cells. (1)

6.  Photobioreactor

• A photobioreactor is a fermenter that uses either direct sunlight or artificial illumination.   
• As the light must pass through a container, this reactor is constructed using glass, plastics, and flat panels.
• It operates efficiently at a temperature ranging from 25°C-40°C.
• This fermenter has limitations since it depends on light (requires maximum light); it needs frequent cleaning for light to pass through, and controlling temperature is also challenging.

                        

Application of Bioreactor

A bioreactor’s application highly depends on the reactor type used.

• A continuous stirred tank reactor is suitable for the production of alcohol, antibiotics (like penicillin), certain enzymes (like a ligninolytic enzyme), tissue mass culture, citric acid, exopolysaccharides (like dextran), and amino acids (like glutamate).
• Bubble column fermenters are used for culturing algal and Chitinolytic enzyme cultures. It is also used for the bioconversion of glucose to gluconic acid using an enzyme called glucose oxidase. 
• An airlift bioreactor is usually used to produce single-cell protein, methanol, cellulose, lactic acid, gibberellic acid, wastewater treatment, and pectinolytic enzymes like polygalacturonases.
• Fluidized bioreactors are suitable for producing enzymes like laccase, fluid-suspended biocatalysts (immobilized enzymes, cells, and microbial flocks), and also in bulk drying of materials.
• A packed bed reactor is mainly constructed for wastewater and sewage treatment and for cultivating anaerobic ammonium-oxidizing bacteria (ANAMMOX).
• Photobioreactors are usually preferred for producing photosynthetic cultures such as cyanobacteria, and microalgae, and for β-carotene.

References

1. Chaudhuri, J., & Al-Rubeai, M. (2005). Bioreactors for tissue engineering: Principles, design, and operation. Bioreactors for Tissue Engineering: Principles, Design and Operation, (July), 1–372. https://doi.org/10.1007/1-4020-3741-4
2. Kantarci, N., Borak, F., & Ulgen, K. O. (2005). Bubble column reactors. Process Biochemistry, 40(7), 2263–2283. https://doi.org/10.1016/j.procbio.2004.10.004
3. Seong, Y. S., Dong, H. L., & Sang, D. K. (2006). Effect of uniformity of gas distribution on fluidization characteristics in conical gas fluidized beds. Studies in Surface Science and Catalysis, 159, 557–559. https://doi.org/10.1016/s0167-2991(06)81657-5
4. พวงผกา มะเสนา และประณ. (2557). Bioreactors and their types. 4(1), 88–100.
5. Chisti, Y., & Moo-Young, M. (2003). Bioreactor.