In the operation of fermentation bio-reactors, the design of the stirring system is one of the core elements affecting cell growth and product synthesis, playing a crucial role in multiple key stages such as mixing, mass transfer, heat transfer, and shear force control. The core function of the stirring system is to achieve uniform mixing of the gas, liquid, and solid phases in the fermentation broth through mechanical motion, ensuring sufficient contact between the cells and nutrients and dissolved oxygen, while maintaining a suitable flow field environment to support metabolic activities. In this process, design parameters such as the type, rotation speed, installation position, and combination of the stirring paddles directly or indirectly affect the physiological state of the cells and the synthesis efficiency of the target product.
The choice of stirring paddle type is the primary consideration in the design. Different types of stirring paddles (such as radial flow paddles, axial flow paddles, or combined paddles) produce significantly different flow field characteristics. Radial flow paddles generate strong shear force through high-speed rotation, which can quickly break up air bubbles, increase the gas-liquid contact area, and thus improve dissolved oxygen efficiency. However, their high shear characteristics may cause mechanical damage to shear-sensitive cells (such as filamentous fungi or animal cells), leading to cell rupture or altered metabolic pathways. In contrast, axial flow impellers, characterized by gentle circulating flow, are suitable for high-viscosity fermentation systems or shear-sensitive microorganisms, but their oxygen dissolution capacity is relatively weak. Therefore, a balance must be struck between dissolved oxygen efficiency and shear protection to meet the needs of different fermentation systems. For example, a combination of radial and axial flow impellers can be used in high-density aerobic fermentation to balance high dissolved oxygen with uniform mixing.
Controlling the stirring speed is a key method for optimizing the fermentation process. The speed directly affects dissolved oxygen levels, mass transfer rate, and shear intensity. In the early stages of fermentation, the microorganisms are in an adaptation phase with weak metabolic activity; a lower speed at this time can prevent excessive shearing while maintaining basic dissolved oxygen requirements. Once the logarithmic growth phase begins, the microorganisms' metabolism becomes vigorous, and oxygen consumption surges, requiring an increased speed to enhance dissolved oxygen and mass transfer efficiency. During the stationary and decline phases, the microorganisms' growth rate slows down, and excessively high speeds may lead to energy waste or shear damage; in these cases, the speed should be appropriately reduced to optimize cost and microorganism survival rate. Furthermore, the dynamic adjustment of the rotation speed must be considered in conjunction with the rheological characteristics of the fermentation broth. For example, in high-viscosity systems, excessively high rotation speeds may induce localized turbulence, thereby reducing mass transfer efficiency.
The design of the stirring system also needs to consider the physical properties of the fermentation broth. For low-viscosity homogeneous systems (such as amino acid fermentation), the strong shear force of radial flow impellers can achieve rapid mixing and dissolved oxygen; while for high-viscosity or solid-containing systems (such as antibiotic mycelial fermentation), axial flow impellers or large-diameter, low-speed impellers are more effective in disrupting the fluid structure, avoiding localized dead zones, and promoting uniform distribution of substrate and cells. In addition, parameters such as the impeller installation height, interlayer spacing, and impeller size need to be optimized through fluid dynamics simulation (CFD) to ensure a uniform and stable flow field within the fermenter, avoiding differences in cell growth caused by uneven flow field distribution.
The influence of shear force on cell morphology and metabolism is a factor that cannot be ignored in the design of the stirring system. High shear force may damage the cell wall or cell membrane, leading to leakage of cell contents or hindering the synthesis of metabolites. For example, in the fermentation of filamentous fungi, excessive shear force can shorten hyphal length, alter the morphology of fungal pellets, and consequently affect the rheological properties and mass transfer efficiency of the fermentation broth. Therefore, for shear-sensitive fungi, low-shear stirring systems (such as axial flow impellers or magnetic stirring) are required, or the local shear stress can be reduced by optimizing the geometry of the impeller (such as increasing blade width, using oblique blades, or curved blade designs).
The synergistic optimization of the stirring and aeration systems is crucial for fermentation efficiency. The aeration rate and stirring speed together determine the dissolved oxygen level in the fermentation broth, and these two factors need to be dynamically matched according to the specific oxygen consumption rate (OUR) of the fungi. For example, under high aeration conditions, excessively high stirring speeds may lead to excessive bubble breakage, increasing gas-liquid contact time but simultaneously exacerbating shear force; while under low aeration conditions, stirring alone is insufficient to meet the needs of high-oxygen-consuming fungi. Therefore, modern fermentation bioreactors often employ a combined control strategy of "stirring + aeration," optimizing both dissolved oxygen efficiency and shear protection by adjusting the type of gas distributor (such as annular distributors or porous aerators) and the form of the agitator.
The design of the stirring system must also consider the cost and efficiency of industrial production. In large fermenters, stirring power consumption accounts for more than 60% of the total energy consumption. Therefore, energy consumption needs to be reduced by optimizing the blade shape (such as using airfoil or composite blades), variable frequency speed control technology, and waste heat recovery systems. Furthermore, the application of modular design concepts (such as disassembling the stirring system into standardized components) can shorten the equipment manufacturing cycle, improve maintenance convenience, and thus adapt to the production needs of different fermentation products.
