Other Questions to Consider

Once a bioreactor giving a certain agitation and aeration regime has been selected, various considerations will affect the details of its design:

• How important is it to have aseptic operation?

• To what degree is it necessary to contain the process organism?

• Is continuous operation desirable?

• How easy is loading and unloading and how much does labor cost?

• How much substrate is to be fermented?

• Will the bioreactor also be used for one or more of the downstream processing steps?

The degree to which sterile operation is required. Some SSF processes involve fast-growing organisms growing under conditions of low moisture that give the process organism a competitive advantage over contaminants. For example, in many fungal processes, the water activity is below that which is optimal for bacteria, so there are not serious problems with growth of bacterial contaminants, although fungal contaminants might cause problems. It may be possible to operate without strict asepsis: The process organism might be given sufficient advantage over any contaminants through cooking of the substrate, avoidance of gross contaminations, and the provision of a relatively pure and vigorous inoculum. However, in other cases the organism grows slowly and care must be taken to design the bioreactor for sterile operation and to operate it in such a manner as to prevent contamination. In this case it is necessary to sterilize the bioreactor before operation, to properly seal openings, to filter the inlet air and to add solutions to the bio-reactor during the fermentation in an aseptic manner. The various bioreactors that have been used to date differ with respect to their ability to operate aseptically.

The degree to which containment of the process organism is required. In general, transgenic organisms are not used in SSF, and processes rarely involve dangerous pathogens (although some do involve opportunistic pathogens). However, many processes do involve fungi and workers can suffer from allergies or other health problems if spores are allowed to escape freely into the environment. The bioreactor may need to be enclosed, and filters may be required on the outlet air stream. Bioreactors that have been used to date differ with respect to the ease of containing the process organism.

The desirability of continuous operation. Continuous operation in a well-mixed bioreactor is not a useful option for SSF. In SLF the nutrients added to a continuous stirred tank reactor are distributed throughout the bioreactor, becoming available to all the microorganisms. In SSF, any solid particles added to the fermentation would need to be colonized, a process that would take a significant period of time. Even if the particles were inoculated at the time of addition, early growth might be expected to be slow, especially in a mixed bed, and an unduly high fraction of poorly colonized substrate particles would leave in the outflow. However, continuous operation of the "plug-flow type" certainly is an option.

The ease of loading and unloading and the cost of labor. Loading and unloading of the bioreactor are handling operations that are required for all SSF processes. Note that the type of operation can affect how loading and unloading must be done: In continuous bioreactors the loading and unloading operations must be continuous or at least semi-continuous, while in batch operation they are done at distinct times. These operations have received little attention. The general principle is that, depending on labor costs, it may be desirable to avoid bioreactor types that require manual handing in the loading and unloading steps.

The amount of substrate to be fermented. The dimensions of the bioreactor will be determined by the volume of substrate that it must hold at any one time. This will depend on the mass of substrate that it must hold and the bulk packing density of the bed. Note that the allowable height of the bed might be limited by the mechanical strength of the substrate particles.

Involvement of the bioreactor in downstream processing steps. At times, it might be desirable either to dry the substrate bed or to leach a product from it as one of the first downstream processing steps. It may be desirable to undertake such steps within the bioreactor itself. This may influence bioreactor design.

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