Sterilization of solid substrates could be done in specialized vessels, in which
case transfer into the bioreactor must be done aseptically. This is more
problematic to achieve than with liquid media, and therefore it is preferable to
sterilize in situ in the bioreactor, meaning that the bioreactor must be designed
to enable this. In most processes batch sterilization will be used [80]. For
processes operated in continuous mode a continuous sterilizer will be required
at the inlet end of the bioreactor, with provision for sterile transfer into the
inoculation chamber [81, 82].
3
Biochemical Engineering Approach to SSF Bioreactors
Both microscale and macroscale phenomena have the potential to control bioreactor performance. These are illustrated in Fig. 3 for an aerobic process. These
processes occur within a spatially heterogeneous physical system, as was
demonstrated in Fig. 1, a substrate bed consisting of moist solid particles between which are gas-filled voids. During the fermentation the bulk of the
growth occurs at the particle surfaces.
The microscale phenomena include:
1. Microbial growth and death rates in response to the environmental conditions.
2. The microbial growth form, especially whether growth occurs as a mycelium
or a biofilm of unicellular organisms.
3. The effect of microbial growth on the environment through the release of
enzymes and end products and the uptake of nutrients.
4. Intraparticle diffusion of compounds such as O 2 , CO 2 , protons, enzymes,
soluble nutrients, hydrolysis products, and products of metabolism.
5. Transfer between the interparticle regions and either the substrate particle
or biomass of compounds such as O 2 , CO 2 , water, and volatile end products
of metabolism.
6. Destruction of the particle due to growth if the carbon source contributes to
the physical structure of the solid particle.
The macroscale phenomena include:
1. Bulk flow of air into and out of the bioreactor, carrying sensible energy and
compounds such as O 2 , CO 2 , and water.
2. If the bioreactor is operated with forced aeration or mixing, bulk flow of air
in the interparticle spaces, carrying sensible energy and compounds such as
O 2 , CO 2 and H 2 O.
3. Natural convection, diffusion, and conduction, which are usually unimportant in the direction of airflow but can be important normal to the direction of airflow or in the absence of forced aeration.
4. Conduction across the bioreactor wall and convective cooling to the surroundings, which could be surrounding air, or could be water in a water jacket.
5. Shear effects caused by mixing within the bioreactor, including damage to
either the microorganism itself, or to the integrity of the substrate particles.
Biochemical Engineering Aspects of Solid State Bioprocessing
79
case transfer into the bioreactor must be done aseptically. This is more
problematic to achieve than with liquid media, and therefore it is preferable to
sterilize in situ in the bioreactor, meaning that the bioreactor must be designed
to enable this. In most processes batch sterilization will be used [80]. For
processes operated in continuous mode a continuous sterilizer will be required
at the inlet end of the bioreactor, with provision for sterile transfer into the
inoculation chamber [81, 82].
3
Biochemical Engineering Approach to SSF Bioreactors
Both microscale and macroscale phenomena have the potential to control bioreactor performance. These are illustrated in Fig. 3 for an aerobic process. These
processes occur within a spatially heterogeneous physical system, as was
demonstrated in Fig. 1, a substrate bed consisting of moist solid particles between which are gas-filled voids. During the fermentation the bulk of the
growth occurs at the particle surfaces.
The microscale phenomena include:
1. Microbial growth and death rates in response to the environmental conditions.
2. The microbial growth form, especially whether growth occurs as a mycelium
or a biofilm of unicellular organisms.
3. The effect of microbial growth on the environment through the release of
enzymes and end products and the uptake of nutrients.
4. Intraparticle diffusion of compounds such as O 2 , CO 2 , protons, enzymes,
soluble nutrients, hydrolysis products, and products of metabolism.
5. Transfer between the interparticle regions and either the substrate particle
or biomass of compounds such as O 2 , CO 2 , water, and volatile end products
of metabolism.
6. Destruction of the particle due to growth if the carbon source contributes to
the physical structure of the solid particle.
The macroscale phenomena include:
1. Bulk flow of air into and out of the bioreactor, carrying sensible energy and
compounds such as O 2 , CO 2 , and water.
2. If the bioreactor is operated with forced aeration or mixing, bulk flow of air
in the interparticle spaces, carrying sensible energy and compounds such as
O 2 , CO 2 and H 2 O.
3. Natural convection, diffusion, and conduction, which are usually unimportant in the direction of airflow but can be important normal to the direction of airflow or in the absence of forced aeration.
4. Conduction across the bioreactor wall and convective cooling to the surroundings, which could be surrounding air, or could be water in a water jacket.
5. Shear effects caused by mixing within the bioreactor, including damage to
either the microorganism itself, or to the integrity of the substrate particles.
Biochemical Engineering Aspects of Solid State Bioprocessing
79