processes are cheaper and can be operated by people with low levels of technical
skills. For such processes the extra cost of these anti-contamination measures
must be covered by the value of the product.
5.1.4
Controlling Environmental Conditions to Optimize Growth and Product Formation
Important environmental conditions include pH, water activity, supply of oxygen, and temperature. Maintaining optimum temperatures and water levels is
usually the main challenge in bioreactor design and operation. Aeration can be
achieved by blowing air forcefully through the substrate, or passing air around
a static or mixed bed. This air plays a key role in heat removal through convection and evaporation. Heat removal can also be promoted by having closely
spaced cooling surfaces. Water transfer, air supply, and heat removal are tied
together: aeration rates are typically determined by heat removal needs rather
than oxygen supply needs, and the evaporation of water from the substrate bed
into the process air can dry the bed to undesirably low levels if water is not
replenished during the fermentation.
Although there is nothing that can be done in bioreactor operation to
prevent intraparticle gradients from occurring, it is possible to prevent the
interparticle transport phenomena from becoming limiting. Mixing and high
aeration rates are the best strategies to achieve this. However, mixing is not
always appropriate, and forced aeration in the absence of mixing leads to its
own problems with temperature gradients in the bed. Evaporative heat removal
can be promoted by aeration with dry air, but this tends to dry out the substrate
in a system which already has a low water availability, and introduces problems
of replenishing water evenly across the bed.
5.1.5
Maintenance of Homogeneity Within the Substrate Bed
This issue is linked to the previous issue. It is desirable for optimal conditions
to be maintained over the whole bioreactor volume, since this will lead to
optimal process productivity. The easiest way to achieve homogeneity is by
mixing, which can be either intermittent or continuous. However, if the
organism cannot tolerate mixing, meaning that the bed must remain static, then
homogeneity is impossible to achieve. Thermal gradients across a bioreactor
are more important than gas concentration gradients. Growth and product
formation are quickly affected by deviations from the optimal temperature,
whereas quite large changes in the oxygen and carbon dioxide levels can often
occur with relatively little effect on process performance.
5.1.6
Facilitating Other Aspects of Processing
SSF processes involve not only the fermentation itself but also substrate preparation, inoculum preparation, loading and unloading of the bioreactor, and
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skills. For such processes the extra cost of these anti-contamination measures
must be covered by the value of the product.
5.1.4
Controlling Environmental Conditions to Optimize Growth and Product Formation
Important environmental conditions include pH, water activity, supply of oxygen, and temperature. Maintaining optimum temperatures and water levels is
usually the main challenge in bioreactor design and operation. Aeration can be
achieved by blowing air forcefully through the substrate, or passing air around
a static or mixed bed. This air plays a key role in heat removal through convection and evaporation. Heat removal can also be promoted by having closely
spaced cooling surfaces. Water transfer, air supply, and heat removal are tied
together: aeration rates are typically determined by heat removal needs rather
than oxygen supply needs, and the evaporation of water from the substrate bed
into the process air can dry the bed to undesirably low levels if water is not
replenished during the fermentation.
Although there is nothing that can be done in bioreactor operation to
prevent intraparticle gradients from occurring, it is possible to prevent the
interparticle transport phenomena from becoming limiting. Mixing and high
aeration rates are the best strategies to achieve this. However, mixing is not
always appropriate, and forced aeration in the absence of mixing leads to its
own problems with temperature gradients in the bed. Evaporative heat removal
can be promoted by aeration with dry air, but this tends to dry out the substrate
in a system which already has a low water availability, and introduces problems
of replenishing water evenly across the bed.
5.1.5
Maintenance of Homogeneity Within the Substrate Bed
This issue is linked to the previous issue. It is desirable for optimal conditions
to be maintained over the whole bioreactor volume, since this will lead to
optimal process productivity. The easiest way to achieve homogeneity is by
mixing, which can be either intermittent or continuous. However, if the
organism cannot tolerate mixing, meaning that the bed must remain static, then
homogeneity is impossible to achieve. Thermal gradients across a bioreactor
are more important than gas concentration gradients. Growth and product
formation are quickly affected by deviations from the optimal temperature,
whereas quite large changes in the oxygen and carbon dioxide levels can often
occur with relatively little effect on process performance.
5.1.6
Facilitating Other Aspects of Processing
SSF processes involve not only the fermentation itself but also substrate preparation, inoculum preparation, loading and unloading of the bioreactor, and
Biochemical Engineering Aspects of Solid State Bioprocessing
99