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How can you optimize bacterial fermentation in a stirred-tank bioreactor?

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Bacterial fermentation is a process that converts organic substrates into valuable products, such as ethanol, lactic acid, or antibiotics, by using microorganisms as biocatalysts. A stirred-tank bioreactor (STB) is a common type of vessel that provides optimal conditions for the growth and metabolism of bacteria. However, to achieve high productivity and quality of the fermentation products, you need to optimize various parameters that affect the performance of the STB. In this article, you will learn how to optimize bacterial fermentation in a stirred-tank bioreactor by considering four key aspects: substrate, oxygen, pH, and temperature.

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1 Substrate selection and feeding

The substrate is the main source of energy and carbon for the bacteria, and it determines the yield and type of the fermentation products. Therefore, you need to choose a suitable substrate that matches the metabolic pathway of the bacteria, and provide it at a sufficient and constant rate to avoid substrate limitation or inhibition. You can use different feeding strategies, such as batch, fed-batch, or continuous, depending on the kinetics and stoichiometry of the fermentation process. You also need to monitor the concentration of the substrate and the products in the bioreactor, and adjust the feeding rate accordingly.

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2 Oxygen transfer and aeration

Oxygen is essential for aerobic bacteria, as it is involved in the oxidative phosphorylation that generates ATP, the energy currency of the cell. Oxygen is supplied to the bioreactor by aeration, which is the injection of air or pure oxygen into the liquid medium. Aeration also enhances the mixing and homogenization of the medium, and prevents the formation of gradients or dead zones. However, too much aeration can cause shear stress or foam formation, which can damage the cells or reduce the volume of the bioreactor. Therefore, you need to optimize the oxygen transfer rate (OTR), which is the amount of oxygen that dissolves in the medium per unit time, by controlling the aeration rate, the agitation speed, and the geometry of the impeller.

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Per Dedenroth Pedersen

Consultant - microbial food cultures incl. probiotics - strategi - R&D - production - application - value creation
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For some microorganism CO2 can be just as crucial for optimal yields! Hence sparging with CO2 may be needed if not possible to add as carbonate, though for some enough to use in head space!

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3 pH control and regulation

pH is a measure of the acidity or alkalinity of the medium, and it affects the enzyme activity, the cell membrane stability, and the solubility of the products. Most bacteria prefer a neutral or slightly acidic pH, but some can tolerate extreme pH values. The pH of the medium can change during the fermentation due to the production or consumption of acids or bases by the bacteria. Therefore, you need to control and regulate the pH by adding buffers or acid/base solutions to the bioreactor. You also need to measure the pH continuously by using a pH probe or a pH meter, and maintain it within a narrow range that is optimal for the bacteria.

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Per Dedenroth Pedersen

Consultant - microbial food cultures incl. probiotics - strategi - R&D - production - application - value creation
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Adding the base on op can give too high a pH gradient and inactivate bacteria in the top hence this also has to be considered, if in base conncentration, fermenter and stirrer configuration or addition point

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Andy M.

Biomanufacturing Subject Matter Expert Supporting the US Government | Program Management | Investment Strategy | Advanced Manufacturing
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Make sure the ammonia dosing valves and control systems are well maintained and have pressure relief loops in place. When machines malfunction (as they sometimes do) it helps to have a backup. Nothing spoils a batch quicker than flooding the tank with base.

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4 Temperature control and regulation

Temperature is a factor that influences the growth rate, the metabolic rate, and the product formation of the bacteria. Different bacteria have different optimal temperature ranges, depending on their adaptation to different environments. The temperature of the medium can change due to the heat generation or dissipation by the bacteria or the aeration. Therefore, you need to control and regulate the temperature by using a heating or cooling system, such as a jacket, a coil, or a heat exchanger, that surrounds or immerses in the bioreactor. You also need to measure the temperature continuously by using a thermometer or a thermocouple, and keep it within a range that is optimal for the bacteria.

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5 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?

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How can you optimize bacterial fermentation in a stirred-tank bioreactor?

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1 Substrate selection and feeding

2 Oxygen transfer and aeration

3 pH control and regulation

4 Temperature control and regulation

5 Here’s what else to consider

Bioengineering

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