A Model Way to Control Fermentation

Whether you're brewing beer, creating clean water from wastewater, or producing pharmaceuticals, flexibility is what you need when you're controlling fermentation. Along with effective coordination, flexibilty is crucial when you want to minimize waste of material and maximize product quality throughout all phases of a typical fermentation process (that is, sterilization followed by cool down and then fermentation).

This article focuses on the factors that impact dissolved oxygen (DO₂) and how to simplify and maintain tight, responsive control.

Dissolved oxygen is an important factor because it is a regulator of metabolic activity. Nutrient issues aside, the amount of dissolved oxygen available to the growing culture can easily become a growth-limiting factor, especially when conducting high cell density fermentations.

When the Waksman Institute of Microbiology at Rutgers (Piscataway, N.J.) needed to improve process control in its fermentation applications, its staff conferred with the process control engineers at ControlSoft, Inc. (Highland Heights, Ohio), and outlined a specific goal: faster and tighter control of DO₂ to process requirements. To accomplish this and to achieve the highest possible product quality, we knew that we needed a flexible control strategy. Of course, we also wanted to save money wherever possible. ControlSoft’s engineers recommended its MANTRA Advanced Process Control system.

Change is Good

The ability to change control strategy is critical for slow processes such as fermentation because of the mechanical problems that can occur during the lengthy phases of the process. At Waksman, the typical key parameters are temperature, pressure, pH and DO₂, enabling the growth of both anaerobes and organisms requiring high oxygen transfer. Waksman needed flexibility to provide its process engineers with the opportunity to save a batch in spite of faulty conditions that may develop over during the process. A few months after our first meeting the new control software was ready to be added to our PC network.

The Application

A typical fermenter is shown in the Figure. The circulation pump runs hot or cold water through the jacket to maintain temperature in the vessel (in this case, 121° C for sterilization or 37° C for fermentation). Sterilized air is run through a sparger into the media in the fermenter (typically proteins, sugar, water and the culture), and then is released through the exhaust piping. The air exhaust valve maintains pressure in the fermenter. The sparger control valve maintains the desired flow of air. Agitator speed is maintained via a motor and variable-frequency drive (VFD).

Fermenter Control Points

Dissolved oxygen concentration is affected by a number

of factors: agitator speed, air supply and vessel pressure. The

question is: How best to achieve tight control of the variables?