20/03/2026
Scale-up Strategies and Engineering Considerations for Bioreactors in Different Bioprocesses
With technological advancements and a deeper understanding of how biological processes occur, high-purity biological molecules can be produced at a large scale. Today, various bioproducts—including organic acids, biofuels, bioplastics, biopesticides, pharmaceuticals, and aromas—can be manufactured via fermentation. However, such processes are generally more complex than chemical routes, primarily because microbial cells have specific physicochemical requirements. Furthermore, bringing bioproducts to market requires a structured approach to implement appropriate processes and scale-up. Bioreactors are central equipment used in many bioprocesses, and various bioreactor technologies have been developed for different applications. Their scale-up involves developing bioprocesses at laboratory, pilot, and industrial scales.
Typically, bioprocess development proceeds across three scales: laboratory, pilot, and finally industrial. The process begins at the laboratory scale, where optimal conditions are screened and defined to enable transfer to larger scales. This transfer requires the replication of appropriate conditions and performance, which presents a major challenge because critical factors such as aeration and agitation are essential for cell growth. In this context, scale-up strategies must be adopted to maintain bioprocess performance. These strategies are based on similarities in bioreactor geometry, agitation, aeration, and other parameters, and must comply with the requirements of each bioprocess and the microbial strain used. Operating conditions significantly influence cell growth, which in turn affects the biosynthesis of various biomolecules, and these conditions must ultimately be replicated at larger scales. To this end, one or more operating factors can be kept constant during scale-up, enabling the prediction of parameters such as power consumption in large-scale bioreactors or aeration conditions in aerobic cultures.
At the laboratory scale, small-volume systems—either shake flasks (e.g., 50–500 mL) or bioreactors (e.g., 1–15 L)—are used for condition screening and process optimization. It is straightforward to evaluate the effects of different nutrient sources and physicochemical conditions (e.g., temperature, pH, agitation, and aeration rates) on maximizing biomass and product yields while minimizing costs. Furthermore, optimization and simulation can be performed using design of experiments (DoE), and models can be developed and validated at this scale. Once all optimal conditions are determined and tested in laboratory-scale bioreactors, the process can be transferred to the pilot scale (e.g., 50–500 L bioreactors).
This transfer requires maintaining constant key parameters (primarily agitation and aeration) as well as geometric similarity across scales. The pilot scale serves as a demonstration step to verify the feasibility of the developed bioprocess and to establish critical parameters that cannot be optimized at the laboratory scale (e.g., the impact of agitation on shear stress). Once the economic and technical feasibility of the project is validated at the pilot scale, the bioprocess can finally advance to industrial and commercialization stages. Scaling from pilot to industrial scale follows similar criteria to ensure success. Thus, bioprocess scale-up always involves bioreactor design at all scales. This equipment can be considered the core of bioprocessing, as it supports the microbial cells required for bioproduction. Therefore, accurate selection of bioreactors at all scales—based on the operating mode, microbial strain, and available capital investment—is critical.
In this sense, bioprocess and bioreactor scale-up are directly interconnected. During scale-up, it is essential to provide precise conditions for microbial growth and production while ensuring the economic and technical feasibility of the project.
Bioprocess Factors Affecting Scale-up: Operating Mode
Bioprocesses can be conducted in different ways during fermentation, depending on the microbial requirements for substrate consumption, product formation, and potential inhibition. Overall, there are three main operating modes: batch, fed-batch, and continuous, which differ in the feeding of fresh medium and/or the removal of fermentation broth during operation.
The simplest operating mode is batch processing, in which no medium is added or removed throughout fermentation. Consequently, the culture environment evolves continuously due to metabolic dynamics, resulting in a non-steady state. Batch operation is widely used for initial production testing at the laboratory scale and for screening optimal conditions for the production of target biomolecules.
Similar to batch fermentation, fed-batch processing does not involve the removal of fermentation broth. However, fresh medium is added during fermentation. The feed may consist solely of a carbon source, nutrient supplements, or complete medium, depending on the nutritional needs of the strain, and is supplied to the bioreactor either in pulses (intermittently) or continuously. The primary goal of fed-batch is to extend the logarithmic growth phase of the microbial culture, thereby achieving higher biomass and product titers. Determining the optimal time to initiate feeding can be challenging and requires thorough investigation. Typically, feeding starts when the carbon source is partially or fully depleted, or when microbial growth reaches its maximum, with the aim of prolonging this phase. Feeding must follow a well-defined strategy to avoid dilution of the medium and culture, which would compromise the final yield of the bioprocess.
Another critical factor in fed-batch operation is the control of process parameters. The addition of external nutrients affects several aspects of fermentation: biomass, product and substrate concentrations, dissolved oxygen, growth rate, and more. Open-loop and closed-loop control systems are the most commonly used in fermentation; however, new strategies are continuously developed and studied to ensure successful fed-batch operation.
In continuous operation, both fresh medium is fed into the bioreactor and fermentation broth is removed, maintaining an internal steady state (chemostat) with generally constant volume throughout the process.
Factors Affecting Scale-up: Agitation, Aeration, and Viscosity
Most industrial microbial fermentation processes are aerobic and carried out in aqueous media rich in macro- and micronutrients. Generally, these fermentation broths are viscous and exhibit non-Newtonian fluid behavior. In these processes, oxygen is essential for microbial growth, maintenance, and the production of biological metabolites. Therefore, sufficient oxygen supply to the system must be ensured, and oxygen transfer in the fermentation broth must be characterized.
In industrial aerobic processes, aeration is challenging due to the low solubility of oxygen in water, which must overcome several diffusion barriers to reach the microbial cells. These factors must therefore be considered during scale-up. Different agitation and aeration systems, as well as viscosity control over the course of fermentation, can be implemented in bioreactors to provide the required oxygen supply, efficient mass and heat transfer, and homogeneity within the vessel. Agitation is strictly dependent on the type of bioreactor used. The microbial strain must also be considered, as some microorganisms are more sensitive to high agitation rates than others.
Aeration is typically provided by a system consisting of an air compressor (which pumps air into the bioreactor), an air cooler (to cool the air temperature if necessary), and a sparger (which helps distribute air within the bioreactor in conjunction with the agitation system). The aeration rate should be determined by the Oxygen Uptake Rate (OUR), i.e., the amount of oxygen consumed by the microorganisms over time. On the other hand, the Oxygen Transfer Rate (OTR) is related to the oxygen concentration transferred through the medium over time and is directly linked to the volumetric mass transfer coefficient (kLa). Thus, OTR and kLa can be used as scale-up criteria to ensure adequate aeration of the bioreactor system.
Agitation and aeration may induce foaming during fermentation. Moreover, certain bioprocesses—such as the production of biosurfactants—generate more foam due to the nature of the bioproducts. Since foam impairs air dispersion, adequate headspace volume must be considered in bioreactor design, and a foam control system (including antifoam agents if necessary) should be installed.
Viscosity is another parameter influencing aeration. Increased medium viscosity affects OTR, bubble coalescence, and distribution. Viscosity can arise from the presence of solids, elevated microbial biomass, and product formation—especially when the product itself is viscous. In such cases, the fermentation broth becomes increasingly viscous over time, making agitation and aeration critical factors that must be optimized at laboratory and pilot scales before industrial implementation. Investigating different impeller configurations and aeration rates is essential for successful bioprocessing.
Similarity Criteria
Beyond all the relationships established for bioprocess scale-up, scale-up ultimately relies on similarity. The developed bioprocess and/or bioreactor will have certain conditions, parameters, relationships, and/or ratios that must be kept constant across all scales to ensure process reproducibility and feasibility, with the same goal: high-yield production of a specific biomolecule.
It is therefore necessary not only to establish a robust and optimized process at the laboratory scale but also to maintain these conditions during scale-up. Obviously, it is impossible to preserve all characteristics across scales, but several are indispensable. These similarities can be chemical and biochemical, mechanical, thermal, and geometric.
The main geometric ratio to be maintained across scales is the ratio of tank height (H) to diameter (D), which varies with bioreactor type. Generally, a higher H:D ratio implies greater heterogeneity within the bioreactor, as agitation may differ between the top and bottom regions. This ratio must therefore be considered in large-scale bioreactor design (for stirred tanks, by adjusting the number and/or spacing of impellers to ensure coverage of all zones in the system).
Other defined relationships include the ratio of liquid height (HL) to reactor height (HR), which should be between 0.7 and 0.8 to ensure adequate headspace and accommodate foaming. Additionally, relationships between impeller diameter (DI), baffle width (Db), and bioreactor diameter (Dt) in stirred tanks are important. These geometric relationships are highly useful for scale-up, and establishing such criteria is critical. However, other biochemical parameters (e.g., physicochemical properties, microbial growth rate) must also be considered, alongside multivariate analysis, to achieve proper process replication.
A bioprocess begins at the laboratory scale, where process conditions are defined and optimized. These defined conditions must then be transferred to larger scales (pilot and industrial). The success of process transfer depends on the correct selection of scale-up strategies based on key parameters—such as agitation and/or aeration—that must be maintained at the new scale.
Each process is unique, with specific conditions where the ideal combination of aeration and agitation promotes optimal microbial growth and efficient production of the desired bioproduct. It is also important to select the appropriate bioreactor model and operating mode, and to define a combination of scale-up criteria for improved process performance. Furthermore, even while the basic bioreactor design remains consistent, new studies on modifications and scale-up are continuously conducted to adapt to the evolving biotechnology industry.
Duoing Biotechnology fully supports microbial fermentation-based bioproduction, including the manufacturing of recombinant proteins, nucleic acids, and synthetic biology-related products. Our bench-top glass bioreactors and stainless-steel bioreactors support the full spectrum from laboratory process development to pilot and commercial production, helping to enable faster, more efficient, and more sustainable biomanufacturing.
