Sterilization aerobic fermentations

The magnitude of the air sterilization problem is seen from the usual needs of a highly aerobic fermentation where roughly 1 volume of air per volume of medium per minute may be used. For a factoiy with 20 fermenters of 100,000 1 (3500 fH) each, 2 million Em (70,000 ftVm) of air is handled. Very large compressors are used, and at least two are required so that one can be down for maintenance. 

The pH is adjusted to 6.9 to 7.0 before sterilization and aerobic fermentation is effected for 24 hours (until the packed cell volume is about 10 to 15%) under the following conditions temperature, 37°C sterile air input, 54 ftVmin pressure, 7 psi and agitation,... 

Prior to sterilizing the abovedescribed medium, adjust the pH to 8. Aerobically ferment for 66 to 90 hours while stirring at 250 rpm with air input at 4.5 C/)2/min and 25 psi. The potency of the antibiotic produced at the end of this period reaches a peak of 150 to 225 jug/ml and remains relatively constant. The pH of the fermentation medium changes slightly during the antibiotic production, varying in the range of 6.8 to 7.3. 

Microorganisms in liquids and gases can be removed by microfiltration hence, air supplied to aerobic fermenters can be sterilized in this way. Membrane filters are often used for the sterihzation of liquids, such as culture media for fermentation (especially for tissue culture), and also for the removal of microorganisms from various fermentation products, the heating of which should be avoided. 

For aerobic fermentations, air needs to be supplied continuously. Typical aeration rates for aerobic fermentation are 0.5 - 1.0 vvm (air volume per liquid volume per minute). This requires an enormous amount of air. Therefore, not only the medium but also the air must be free of microbial contaminants. All of the sterilization techniques discussed for medium can also be employed for air. However, sterilization of air by means of heat is economically impractical and is also ineffective due to the low heat-transfer efficiency of air compared with those of liquids. The most effective technique for air sterilization is filtration using fibrous or membrane filters. 

Fermentation processes, except for sterilization, have in common many of the familiar chemical engineering unit operations. For example, aerobic fermentations involve the mixing of three heterogeneous phases microorganisms, medium, and air. Other unit operations include mass transfer of oxygen from the air to the organisms and heat transfer from the fermentation medium. 

Aerobic fermentation processes also require a continuous supply of large quantities of air, typically on the order of one volume of air per volume of liquid per minute, VVM. Sterilization of this air is mandatory in almost all fermentations. Absolute filter cartridges of polymeric membranes are now used almost exclusively in the fermentation industry. Relatively small units have replaced the large depth filters used in the past. Still, water and particulates pose a major problem for filters thus requiring the use of prefilters and traps to remove these contaminants before they reach the absolute filter. Parallel installation of the filters prevents a total shutdown of the fermentation process in the event of filter clogging. 

All biochemical reactions are enzyme-mediated. The rate of an enzyme reaction depends on the substrate concentration at the location of the enzyme and thereby on the diffusion rate of a substrate to the enzyme. It is therefore important to permanently obtain an intimate contact between a cell or enzyme and substrate molecules. Additionally, the product generated in the bioreactor has to be extracted because it may under certain conditions inhibit its own production. In some processes there may also be even a prepurification in the bioreactor itself. If living micro-organisms have to be applied, it is necessary to provide sufficient nutrition and respiration gases in case of aerobic fermentation. All other reaction parameters such as temperature, pH-value and reaction time have to be controlled precisely. In many cases (generally with modem processes) the maintenance of microbiological integrity (sterile process) is absolutely mandatory for a successful fermentation. 

The most common procedure for large-scale aerobic fermentations is to use deep tanks of a stirred liquid medium. Efficient air exchange is obtained by continuous pumping of sterile air into crossed or coiled perforated pipe placed at the bottom of the tank at rates of up to one volume of air per unit of medium volume per minute [62]. This method promotes aerobic growth throughout the medium, which accomplishes large volume production in a limited space. But it also has problems, which relate to the maintenance of sterility and occasionally from foam formation. Overall, however, submerged... 

For most aerobic fermentation processes, maintenance of absolute sterility is critical to cell growth and biomass production. Culture growth rate determines susceptibility to culture contamination. Mammalian cells divide in a day, while microbials divide in an hour or less. The difference in growth rate makes slower-growing mammalian cell cultures more susceptible... 

Tlnis problem mainly affects petrochemical effluents. A large proportion of refinery WW is relatively sterile dnd takes a long time to seed because of its high pH (spent caustic) or its origin (condensates). In some particular units (lubricating oils) or in petrochemical plants (urea, acetic acid, aldehydes), anaerobic or aerobic fermentation can develop. As a result, strict conditions must be enforced for preserving samples between sample-taking and analysis. 

In aerobic fermentations, it is necessary to sterilize air since the volume of air required in aerobic fermentations is usually large, conventional techniques of heat sterilization are uneconomical. Effective and viable alternatives include the use of membrane or fibrous filters. An important consideration of the filter medium is that it should not be wetted, since this can lead to contamination. Materials such as glass fibers can be used to avoid this problem. As discussed in a previous subsection, the mechanisms by which particles suspended in a flowing stream of air are removed include impaction, diffusion and interception Impaction occurs when particles in the air collide with the fibrous filter due to their higher momentum as compared to air. Smaller particles, on the other hand, travel towards the fiber as a result of diffusion caused by Brownian motion. Particles less than 1 fi,m are collected by this mechanism. Particles that are not small or heavy, but are large in size, are intercepted by the fiber. The efficiency of air filtration is therefore a combination of the three mechanisms. The mathematical... 

Generally, the fermentation process involves the addition of a specific culture of microorganisms to a sterilized liquid substrate or broth in a tank (submerged fermentation), addition of air if aerobic, in a well-designed gas-liquid contactor. The fermentation process is then carried out to grow microorganisms and to produce the required chemicals. Table 11-1 lists examples of the processes used by fermentation. 

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