handling, automated or semi-automated measurement, control of environmental parameters, and prevention of contamination. Continuous supply of
inoculum in an identical state would be a challenge in such a process: A nondormant inoculum would change over time, while a dormant form of inoculum
such as fungal spores would introduce a lag due to the time required for
germination.
Fed-batch operations can also be operated where soluble nutrients are added
to the bed. They can be sprayed onto the bed as a fine mist [89, 90, 129], or
simply mixed in with the substrate, such as has been done with soluble starch
during a fed-batch culture of Gibberella fujikuroi on wheat bran [130]. This
technique of fed-batch operation is appropriate for soluble nutrients which
cause substrate inhibition if added in sufficient amounts for the whole fermentation at the start of the fermentation.
Fed-batch and continuous modes of operation require agitation, either to
mix in fresh substrates or nutrients, or to move the substrate bed along the
bioreactor from inlet to outlet. Therefore these modes of bioreactor operation
can only be used if the process microorganism tolerates mixing.
5.1.8
Key Considerations in Choosing a Bioreactor for a Particular Process
Depending on the particular substrate and organism used in the process, different considerations may become of overriding importance. For example, for a
fast-growing organism the overriding problem is to remove sufficient heat from
the bioreactor, whereas for a slow-growing organism the most important consideration is to protect the process against contaminants. For the slow-growing
organism aseptic procedure is absolutely essential, whereas for the fast-growing
organism, fermentations may not actually be carried out aseptically. The
provision of high densities of an active inoculum leads to rapid growth which
means that the process organism will compete well against any entering contaminants. For the fast-growing organism aeration plays a key role in achieving
the desired heat removal.
The sensitivities of the microorganism and the substrate particles to shear
forces generated by mixing are also crucial. Nothing quantitative is known
about the magnitudes of the shear forces in mixed beds and their effects on the
microorganism, although Stuart et al. [110] noted a steady decrease in the final
protein content of the fermented substrate in a rotating drum bioreactor when
the rotation speed was increased from 10 rpm to 50 rpm. Since the maximum
substrate temperature was similar for all rotational speeds, it is likely that this
decrease was related to shear forces. This is potentially quite an important issue
for the growth of fungi, which are commonly used in SSF processes. In a mixed
bed particles will rub against one another, which might damage hyphae.
Intermittent mixing with long intervening periods of static operation will cause
disruption of those hyphae which extend between particles during the static
periods.
Growth rates and sensitivity to mixing can readily be determined in laboratory scale studies, and this information can be used to guide bioreactor selecBiochemical Engineering Aspects of Solid State Bioprocessing
101
inoculum in an identical state would be a challenge in such a process: A nondormant inoculum would change over time, while a dormant form of inoculum
such as fungal spores would introduce a lag due to the time required for
germination.
Fed-batch operations can also be operated where soluble nutrients are added
to the bed. They can be sprayed onto the bed as a fine mist [89, 90, 129], or
simply mixed in with the substrate, such as has been done with soluble starch
during a fed-batch culture of Gibberella fujikuroi on wheat bran [130]. This
technique of fed-batch operation is appropriate for soluble nutrients which
cause substrate inhibition if added in sufficient amounts for the whole fermentation at the start of the fermentation.
Fed-batch and continuous modes of operation require agitation, either to
mix in fresh substrates or nutrients, or to move the substrate bed along the
bioreactor from inlet to outlet. Therefore these modes of bioreactor operation
can only be used if the process microorganism tolerates mixing.
5.1.8
Key Considerations in Choosing a Bioreactor for a Particular Process
Depending on the particular substrate and organism used in the process, different considerations may become of overriding importance. For example, for a
fast-growing organism the overriding problem is to remove sufficient heat from
the bioreactor, whereas for a slow-growing organism the most important consideration is to protect the process against contaminants. For the slow-growing
organism aseptic procedure is absolutely essential, whereas for the fast-growing
organism, fermentations may not actually be carried out aseptically. The
provision of high densities of an active inoculum leads to rapid growth which
means that the process organism will compete well against any entering contaminants. For the fast-growing organism aeration plays a key role in achieving
the desired heat removal.
The sensitivities of the microorganism and the substrate particles to shear
forces generated by mixing are also crucial. Nothing quantitative is known
about the magnitudes of the shear forces in mixed beds and their effects on the
microorganism, although Stuart et al. [110] noted a steady decrease in the final
protein content of the fermented substrate in a rotating drum bioreactor when
the rotation speed was increased from 10 rpm to 50 rpm. Since the maximum
substrate temperature was similar for all rotational speeds, it is likely that this
decrease was related to shear forces. This is potentially quite an important issue
for the growth of fungi, which are commonly used in SSF processes. In a mixed
bed particles will rub against one another, which might damage hyphae.
Intermittent mixing with long intervening periods of static operation will cause
disruption of those hyphae which extend between particles during the static
periods.
Growth rates and sensitivity to mixing can readily be determined in laboratory scale studies, and this information can be used to guide bioreactor selecBiochemical Engineering Aspects of Solid State Bioprocessing
101