4. Enable the environmental conditions to be controlled at values which favor
production of the desired product
5. Maintain homogeneity within the substrate bed
6. Facilitate other aspects of processing, including substrate preparation,
sterilization, loading, unloading, and product recovery
7. Operate in the required mode, whether batch, fed-batch, or continuous
Some general points can be made about how these functions can be achieved in
SSF bioreactors.
5.1.1
Containing of the Substrate Bed
The main criterion here is strength. This issue has not received direct attention
in the literature concerning SSF bioreactors. However, some general rules can
be outlined. First, although materials such as plastic, glass, and wood are used
on the small scale, larger bioreactors, especially those containing hundreds of
kilograms of substrate, will need to be constructed of metal. Considerations in
the selection of the metal include cost, resistance to corrosion, and potential
toxicity to the process organism. Stainless steel is a strong material, with good
resistance to corrosion and a lack of toxicity. For this reason it is used extensively in the construction of bioreactors for SLF, and is also appropriate for
large-scale SSF bioreactors. However, its major disadvantage is cost.
5.1.2
Containing of the Process Organism
Release of the process organism into the environment could be potentially
dangerous, especially if pathogens are used in the process. Even if the process
organism is not pathogenic, since the majority of SSF processes involve
filamentous fungi, there is the potential for release of spores into the environment. Such release of fungal spores can cause allergic reactions and respiratory
problems in the process workers. Prevention of release requires enclosed
systems and filters on outlet air streams. Note that there is also potential for
release of spores when the bioreactor is unloaded at the end of the fermentation.
5.1.3
Protecting the Process Organism Against Contamination
The importance of preventing the entry of contaminating organisms depends
on the particular SSF system. If the process organism grows quickly, and if the
low water content, or some other condition, such as a low pH, select against contaminants, then prevention of contamination may not be crucial. However, for
slow growing organisms this will be quite important. As with liquid culture,
entry of contaminants can be prevented by careful design of seals and filtration
of the inlet air stream. However, such design features increase cost and can
potentially negate one of the advantages usually quoted for SSF, namely that SSF
98
D.A. Mitchell et al.
production of the desired product
5. Maintain homogeneity within the substrate bed
6. Facilitate other aspects of processing, including substrate preparation,
sterilization, loading, unloading, and product recovery
7. Operate in the required mode, whether batch, fed-batch, or continuous
Some general points can be made about how these functions can be achieved in
SSF bioreactors.
5.1.1
Containing of the Substrate Bed
The main criterion here is strength. This issue has not received direct attention
in the literature concerning SSF bioreactors. However, some general rules can
be outlined. First, although materials such as plastic, glass, and wood are used
on the small scale, larger bioreactors, especially those containing hundreds of
kilograms of substrate, will need to be constructed of metal. Considerations in
the selection of the metal include cost, resistance to corrosion, and potential
toxicity to the process organism. Stainless steel is a strong material, with good
resistance to corrosion and a lack of toxicity. For this reason it is used extensively in the construction of bioreactors for SLF, and is also appropriate for
large-scale SSF bioreactors. However, its major disadvantage is cost.
5.1.2
Containing of the Process Organism
Release of the process organism into the environment could be potentially
dangerous, especially if pathogens are used in the process. Even if the process
organism is not pathogenic, since the majority of SSF processes involve
filamentous fungi, there is the potential for release of spores into the environment. Such release of fungal spores can cause allergic reactions and respiratory
problems in the process workers. Prevention of release requires enclosed
systems and filters on outlet air streams. Note that there is also potential for
release of spores when the bioreactor is unloaded at the end of the fermentation.
5.1.3
Protecting the Process Organism Against Contamination
The importance of preventing the entry of contaminating organisms depends
on the particular SSF system. If the process organism grows quickly, and if the
low water content, or some other condition, such as a low pH, select against contaminants, then prevention of contamination may not be crucial. However, for
slow growing organisms this will be quite important. As with liquid culture,
entry of contaminants can be prevented by careful design of seals and filtration
of the inlet air stream. However, such design features increase cost and can
potentially negate one of the advantages usually quoted for SSF, namely that SSF
98
D.A. Mitchell et al.