FERMENTATION

Related Terms

Fermentation is a widely used process in industry as well as in the production of many foods, beverages, and pharmaceuticals. Fermentation uses microorganisms to convert raw materials to product. Strictly speaking, fermentation is microorganism metabolism on a carbon. Fermentation processes include chemical reactions such as oxidations, reductions, polymerizations, and hydrolysis, as well as biosynthesis and the formation of cells. Some processes may require the presence of air (aerobic), others the absence of air (anaerobic). The rate of fermentation depends on the concentration of microorganisms, cells, cellular components, and enzymes as well as temperature and pH. Product recovery always involves the concentration of the dilute solution. Products may be excreted into the broth whether or not products are excreted by the micoorganisms.

In the conventional batch process, the reactor is filled with a sterile nutrient substrate and inoculated with the microorganism. In the course of the entire fermentation, nothing is added except oxygen (in the form of air), an antifoam agent, and acid or base to control the pH. The culture is allowed to grow until no more of the product is being made, at which point the reactor is "harvested" and cleaned out for another run. Four typical phases of growth are observed: lag phase (when the organisms adapt to their surroundings), exponential growth (when they grow in numbers), stationary phase (when they stop growing), and death phase.

An enhancement of the closed batch process is the fed-batch fermentation, which is used in the production of substances such as penicillin. In the fed-batch process, substrate is added in increments as the fermentation progresses. This is employed to avoid the inhibition of substrate consumption at high substrate concentrations due to catabolite regulation. In this process, fermentation is started batchwise with a small substrate concentration. When all the initial substrate is consumed, a new addition of fermentation medium is made in an amount such that the substrate concentration remains just below the point where it produces inhibitory effects. At the same time, some of the fermentation product is removed and taken off for processing.

Since it is usually not possible to measure the substrate concentration directly and continuously during the fermentation, indirect parameters which are correlated with the metabolism of the critical substrate must be measured in order to control the feeding process. For instance, in the production of organic acids, the pH value may be used to determine the rate of glucose feeding. In the production of ethanol, the rate of CO2 production can be measured and used to continuously vary the glucose feed rate based on the following formula:

C6H12O6 --> 2 C2H5OH + 2 CO2

In fermentations with critical osmotic valves, feeding can be regulated by monitoring the pO2-value or the CO2 content in the exhaust air.

An extension of fed-batch fermentation is continuous fermentation.

There are also industrial considerations related to the fermentation process. For instance, to avoid biological process contamination, the fermentation medium, air, and equipment are sterilized. Foam control can be achieved by either mechanical foam destruction or chemical anti-foaming agents. Several other factors must be measured and controlled such as pressure, temperature, agitator shaft power, and viscosity. An important element for industrial fermentations is scale up. This is the conversion of a laboratory procedure to an industrial process. It is well established in the field of industrial microbiology that what works well at the laboratory scale may work poorly or not at all when first attempted at large scale. It is generally not possible to take fermentation conditions that have worked in the laboratory and blindly apply them to industrial-scale equipment. Although many parameters have been tested for use as scale up criteria, there is no general formula because of the variation in fermentation processes. The most important methods are the maintenance of constant power consumption per unit of broth and the maintenance of constant volumetric transfer rate.

banasc@rpi.edu
Fri Dec 8 3:02 AM EST 1995