product recovery. The bioreactor might be used for sterilization of the substrate. In-situ sterilization can reduce the risks of contamination during loading
of the bioreactor, although the bioreactor then needs to be designed to enable
this.
The issues of substrate loading and unloading have not received much attention in the SSF bioreactor literature. On the large scale, loading is probably
best achieved by conveyor belts or screw augers. Unloading might be facilitated
if the bioreactor can be opened, allowing the substrate bed to drop onto a conveyor belt or similar device. Pneumatic conveying may also be appropriate.
In some cases it might be desirable to use the bioreactor for processing of the
final product. For example, for a product which simply needs to be dried, it is
only necessary to blow warm dry air through the reactor. Alternatively, for a
product which needs to be leached out of the solid, the leaching fluid can be
sprayed onto the top of the substrate bed and withdrawn from the bottom.
5.1.7
Mode of Bioreactor Operation
Batch, fed-batch, repeated fed-batch, and continuous modes of bioreactor operation have been used in SSF processes, although batch processes are by far the
most common. Fed-batch or continuous operations that involve the addition of
fresh, uninoculated substrate particles require interparticle colonization to occur, which is a relatively slow process, and will lead to bioreactor dynamics very
different from those obtained during similar processes in SLF. Despite this,
laboratory studies have demonstrated potential advantages of such operation.
Abdullah et al. [68] compared batch, fed-batch, and repeated fed-batch culture
of Chaetomium cellulolyticum on wheat straw. Under optimal conditions in
batch culture, protein production ceased after three days, with a maximum
protein level of 12 g per gram of solids. In fed-batch culture fresh straw was
added at three-day intervals. Protein productivity was maintained for 12 days at
a slowly declining rate. In the repeated fed-batch culture half of the fermenting
straw was removed and replaced with fresh straw at three-day intervals. Protein
productivity was maintained for 12 days at a steady rate and the protein level
reached a maximum level of 14 g per gram of solids.
Continuous SSF processes are usually operated in plug flow mode. Such processes will require pasteurization or sterilization of the substrate as it enters the
bioreactor, mixing with an inoculum, and at the outlet end of the bioreactor,
continuous removal of spent substrate. Such a process was operated on a pilot
scale for the production of ethanol from fodder beets by Saccharomyces
cerevisiae [81, 82]. The bioreactor had a screw within a 4.7 m long and 15.25 cm
diameter tube. The screw was rotated intermittently to mix the substrate and
move it along the tube. At the front end was a hammer-mill and a pasteurization chamber for substrate preparation and a port for inoculation. New substrate was added, inoculated, and the screw rotated at 12-h intervals, resulting
in a residence time of 72 h.
Successful implementation of such fed-batch or continuous techniques on
commercial scales will require development of effective technologies for solids
100
D.A. Mitchell et al.
of the bioreactor, although the bioreactor then needs to be designed to enable
this.
The issues of substrate loading and unloading have not received much attention in the SSF bioreactor literature. On the large scale, loading is probably
best achieved by conveyor belts or screw augers. Unloading might be facilitated
if the bioreactor can be opened, allowing the substrate bed to drop onto a conveyor belt or similar device. Pneumatic conveying may also be appropriate.
In some cases it might be desirable to use the bioreactor for processing of the
final product. For example, for a product which simply needs to be dried, it is
only necessary to blow warm dry air through the reactor. Alternatively, for a
product which needs to be leached out of the solid, the leaching fluid can be
sprayed onto the top of the substrate bed and withdrawn from the bottom.
5.1.7
Mode of Bioreactor Operation
Batch, fed-batch, repeated fed-batch, and continuous modes of bioreactor operation have been used in SSF processes, although batch processes are by far the
most common. Fed-batch or continuous operations that involve the addition of
fresh, uninoculated substrate particles require interparticle colonization to occur, which is a relatively slow process, and will lead to bioreactor dynamics very
different from those obtained during similar processes in SLF. Despite this,
laboratory studies have demonstrated potential advantages of such operation.
Abdullah et al. [68] compared batch, fed-batch, and repeated fed-batch culture
of Chaetomium cellulolyticum on wheat straw. Under optimal conditions in
batch culture, protein production ceased after three days, with a maximum
protein level of 12 g per gram of solids. In fed-batch culture fresh straw was
added at three-day intervals. Protein productivity was maintained for 12 days at
a slowly declining rate. In the repeated fed-batch culture half of the fermenting
straw was removed and replaced with fresh straw at three-day intervals. Protein
productivity was maintained for 12 days at a steady rate and the protein level
reached a maximum level of 14 g per gram of solids.
Continuous SSF processes are usually operated in plug flow mode. Such processes will require pasteurization or sterilization of the substrate as it enters the
bioreactor, mixing with an inoculum, and at the outlet end of the bioreactor,
continuous removal of spent substrate. Such a process was operated on a pilot
scale for the production of ethanol from fodder beets by Saccharomyces
cerevisiae [81, 82]. The bioreactor had a screw within a 4.7 m long and 15.25 cm
diameter tube. The screw was rotated intermittently to mix the substrate and
move it along the tube. At the front end was a hammer-mill and a pasteurization chamber for substrate preparation and a port for inoculation. New substrate was added, inoculated, and the screw rotated at 12-h intervals, resulting
in a residence time of 72 h.
Successful implementation of such fed-batch or continuous techniques on
commercial scales will require development of effective technologies for solids
100
D.A. Mitchell et al.