Microbial fermentation is a cornerstone of biotechnological research and industrial production. Among the most commonly used methods are shake flask culture and tank (bioreactor) culture, each offering distinct advantages and challenges. Understanding the differences between these methods is essential to ensure successful scale-up from laboratory research to industrial production.
While shake flask cultivation is widely favored in basic research and teaching due to its simplicity and low cost, researchers often encounter inconsistencies when applying parameters optimized in shake flasks directly to large-scale tank fermentation. These discrepancies stem from several critical factors, including variations in dissolved oxygen levels, CO₂ concentration, and mechanical forces. Below, we explore the key differences between shake flask and tank cultures, as well as strategies to bridge these gaps.
Key differences between shake flask culture and tank culture
1. Dissolved oxygen levels
- Shake Flask Culture: Oxygen is supplied through breathable membranes or loose bottle caps, making oxygen transfer highly dependent on environmental conditions.
- Tank Culture: Aeration is precisely controlled via integrated air systems. The dissolved oxygen transfer rate (Kd value) is significantly higher and more consistent than in shake flasks.
2. CO₂ concentration
- Shake Flask Culture: Operates under normal atmospheric pressure, resulting in relatively low CO₂ accumulation.
- Tank Culture: Often conducted under positive pressure, leading to higher CO₂ concentrations that can directly affect microbial growth and product synthesis.
3. Mechanical stress
- Shake Flask Culture: Microorganisms experience minimal mechanical damage, primarily from liquid movement and surface interaction.
- Tank Culture: Mechanical forces generated by impellers and prolonged stirring can impose considerable shear stress on microbial cells, potentially impacting viability and productivity.
4. Additional influencing factors
| Parameter | Shake Flask Culture | Tank Culture |
| pH Control | Generally uncontrolled | Precisely maintained |
| Pressure | Atmospheric pressure | Variable (often positive pressure) |
| Feeding Strategies | Limited or absent | Controlled and continuous feeding possible |
Strategies to Minimize Differences Between Shake Flask and Tank Fermentation
To enhance the correlation between small-scale and large-scale fermentation results, several adjustments can be applied during shake flask cultivation:
Optimizing Dissolved Oxygen
- Increase shaker speed to boost oxygen transfer.
- Reduce culture volume to improve aeration (while monitoring for evaporation loss).
- Introduce sterile air or oxygen directly into the flask.
- Add glass beads to simulate mixing effects similar to tank agitation.
Balancing Mechanical Effects
- Attach baffles inside the flask to create turbulence and mimic shear forces.
- Choose tank impellers that minimize shear, such as propeller or disc-type mixers.
- Optimize stirring speed and time in tanks to limit mechanical damage.
- Increase the viscosity of the fermentation broth, where applicable, to buffer cells from shear forces without compromising oxygen transfer.
Conclusion
Successful microbial fermentation depends on understanding the operational differences between shake flask and tank cultures. By carefully adjusting experimental conditions and applying balance methods, researchers can reduce variability, enhance reproducibility, and ensure a smoother transition from laboratory-scale experiments to industrial-scale production.
For optimal results, it is essential to consider these factors early in the research and development process, ensuring that small-scale studies effectively inform large-scale operations.
