A Model Way to Control Fermentation

Answer: Model-Predictive-coordinated control.

DO₂ content is monitored via a side-entry probe and controlled by adjusting either agitator speed, air flow or air pressure in the bioreactor. The addition of acid, base, nutrient or antifoam is controlled using solenoid (On/Off) valves to inject proper solutions from pressurized addition tanks.

Three Phases

The three basic phases for each fermentation run are heat-up, cool down after sterilization, and fermentation. In Phase 1 the bioreactor is filled with the media (nutrients and water) and sealed. Then steam is injected into the jacket, and temperature is raised to 121° C. For proper sterilization, the temperature must stay at 121° C for a predetermined period. During this time the pressure in the bioreactor increases to about 15-20 psi due to steam generated from the water in the media. The combination of temperature, pressure and time sterilize the media and bioreactor system.

During Phase 2, the bioreactor is cooled down to a specified operating temperature. To achieve this, cooling water is injected into the jacket. During the cooling phase, pressure in the bioreactor drops rapidly due to steam condensation. At this point, sterile air is introduced into the bioreactor to compensate for the pressure drop. This is necessary to avoid creating a negative pressure and to maintain the liquid level in the bioreactor.

In Phase 3, fermentation starts when the bioreactor is inoculated. The culture starts growing, consuming nutrients and oxygen, producing heat, and in some cases, acids and other byproducts that may be harmful to the culture. Maintaining correct growth conditions (temperature, pressure, pH and DO₂ content) is essential for successful completion of the fermentation process. The reaction may last from a few hours to days, but typically it does not last longer than one week. Samples are periodically taken to observe microbial growth and assay for products and various nutrients. When the fermentation is complete, the bioreactor is usually chilled, followed by myriad downstream steps that are required to isolate and purify the fermentation product(s).

DO₂ Concentration

DO₂ concentration is affected by the speed of the agitator, by air supply, and by the pressure of the vessel. The crucial question is: When and how should these factors be used to best achieve the tightest control of DO₂  in a particular application? Waksman needed the ability to change strategy on the fly, at any point of the fermenting process. The new control system gave us that ability by helping us control critical variables including:

Agitator RPM: If a motor with a variable frequency drive is controlling the speed of the agitator, a PID controller can regulate the motor's speed, which can then be used to increase the DO₂. Alternatively, the agitator can be set at a desired constant speed.

Air Flow: Air flow is effective in directly increasing DO₂. The air flow loop is a PID loop whose purpose is to properly maintain the air flow setpoint.