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In aerobic fermentations, oxygen is vital for microbial growth and metabolite production. Since air comprises only about 20% oxygen and the gas is poorly soluble in water—just 9 ppm at 20°C—supplying sufficient oxygen becomes a critical challenge, especially in high-demand processes like yeast growth or citric acid production. Even a fully saturated broth may offer only a few seconds of oxygen availability.
To address this, sterile or scrubbed air is introduced into the fermentor via a sparger placed below the lowest impeller. The sparger creates fine bubbles, which increase the gas-liquid interface, enhancing oxygen transfer. However, finer holes in the sparger require greater pressure, raising energy costs. Optimal hole sizes typically range from 0.25 to 3.0 cm to balance efficiency and clogging risks.
Agitation, usually by impellers, is essential—especially in viscous media or when filamentous organisms are used. Impellers distribute air uniformly, create turbulence, and maintain even temperature and microbial dispersion. Baffles prevent vortex formation, improve mixing, and further aid bubble dispersion. Effective design depends on medium viscosity and requires pilot-scale testing.
Oxygen must pass several barriers before reaching microbial cells: gas and liquid film resistances, interfacial and bulk liquid resistances, cell cluster resistances, and finally, cellular uptake. The most significant barrier is often the liquid film, which agitation helps disrupt.
Agitation supports four primary functions: distributing air, minimizing liquid film resistance, ensuring homogenous mixing, and controlling microbial clump size. Finer bubbles present a larger surface area for gas exchange, making agitation and aeration key components in stirred tank fermentors. These processes are energy-intensive at scale and thus heavily optimized through engineering calculations.
Exhaust air is filtered to prevent contamination, especially with pathogenic organisms, and incoming sterile air can be produced via filtration or other sterilization techniques.
