25
enhancing the growth rate or the final fungal biomass achieved. Since cellulases are
inducible enzymes, an inducer like pure cellulose or a small molecule inducer like
lactose/cellobiose may also be added for enhanced production. One of the advantages of SSF for cellulase production is the possibility of using a lignocellulosic
substrate for fermentation, which will serve as a carbon source as well as the inducer
(Nigam and Singh 1996). The usage of the same raw material as the feedstock to be
used in a biorefinery for cellulases production has the added advantage of a more
appropriate enzyme cocktail being elaborated by the organism, and this approach
has been tested successfully by many (Sukumaran et al. 2009; Roslan et al. 2011;
Maeda et al. 2013; Pirota et al. 2014). It should be noted that the choice of carbon
source has a significant impact on cellulase production under SSF since differential
induction of cellulases in response to the type of carbon sources used is a common
phenomenon in cellulase production by fungi.
The major advantages proposed for solid-state fermentation include (1) higher
enzyme production associated with higher biomass, (2) lower product inhibition,
(3) lower protease activity, (4) ability to use water-insoluble substrates as C and N
sources, which allows use of cheap and abundant lignocellulosic biomass, and (5)
concentrated enzyme (Viniegra-González et al. 2003; Hölker and Lenz 2005). It has
also been shown that SSF offers a situation of lowered catabolite repression in comparison to SmF (Díaz-Godínez et al. 2001). The immobility of typical SSF cultures
can be an advantage and disadvantage at the same time. Since this replicates a natural growth environment for filamentous fungi, without any shear (as would be
encountered in agitated SmF cultures), there is better growth and less autolysis due
to hyphal rupture. At the same time, this also results in mass and heat transfer limitations. As the fermentation proceeds on an organic substrate like wheat bran, the
substrate is decomposed leading to collapse of pores and aggregation of particles,
which lead to less oxygen supply to cells and lower heat dissipation, both of which
affect the growth and enzyme production by fungi (Chen 2013). The classical setup
used for solid-state fermentation for large-scale enzyme production is tray reactors,
though other designs like drum reactors and packed bed reactors are also employed.
A typical tray reactor setup includes several trays of optimal size kept in racks
inside a climate-controlled chamber (Fig. 1.7). The trays may be constructed of
stainless steel, aluminum, wood, or even plastic and may or may not have lids. The
trays and lids in some setups are perforated to allow aeration and heat dissipation.
The rooms are maintained at controlled temperature and humidity and often with
forced circulation of HEPA filtered air (though this may not be essential). This configuration is easy to set up and easily scalable with low labor intensity.
The mixing and heat transfer issues associated with stationary tray fermenters
can be addressed through use of rotating drum-type reactors which are either mixed
continuously or intermittently and with or without forced aeration. These types of
reactors were patented by the French company Lyven and the Indian company
Biocon and have been in use for commercial enzyme production. There are also
several other types of reactor designs in use, while tray reactors are the dominant
types used in commercial SSF.
1 Enzymes for Bioenergy
enhancing the growth rate or the final fungal biomass achieved. Since cellulases are
inducible enzymes, an inducer like pure cellulose or a small molecule inducer like
lactose/cellobiose may also be added for enhanced production. One of the advantages of SSF for cellulase production is the possibility of using a lignocellulosic
substrate for fermentation, which will serve as a carbon source as well as the inducer
(Nigam and Singh 1996). The usage of the same raw material as the feedstock to be
used in a biorefinery for cellulases production has the added advantage of a more
appropriate enzyme cocktail being elaborated by the organism, and this approach
has been tested successfully by many (Sukumaran et al. 2009; Roslan et al. 2011;
Maeda et al. 2013; Pirota et al. 2014). It should be noted that the choice of carbon
source has a significant impact on cellulase production under SSF since differential
induction of cellulases in response to the type of carbon sources used is a common
phenomenon in cellulase production by fungi.
The major advantages proposed for solid-state fermentation include (1) higher
enzyme production associated with higher biomass, (2) lower product inhibition,
(3) lower protease activity, (4) ability to use water-insoluble substrates as C and N
sources, which allows use of cheap and abundant lignocellulosic biomass, and (5)
concentrated enzyme (Viniegra-González et al. 2003; Hölker and Lenz 2005). It has
also been shown that SSF offers a situation of lowered catabolite repression in comparison to SmF (Díaz-Godínez et al. 2001). The immobility of typical SSF cultures
can be an advantage and disadvantage at the same time. Since this replicates a natural growth environment for filamentous fungi, without any shear (as would be
encountered in agitated SmF cultures), there is better growth and less autolysis due
to hyphal rupture. At the same time, this also results in mass and heat transfer limitations. As the fermentation proceeds on an organic substrate like wheat bran, the
substrate is decomposed leading to collapse of pores and aggregation of particles,
which lead to less oxygen supply to cells and lower heat dissipation, both of which
affect the growth and enzyme production by fungi (Chen 2013). The classical setup
used for solid-state fermentation for large-scale enzyme production is tray reactors,
though other designs like drum reactors and packed bed reactors are also employed.
A typical tray reactor setup includes several trays of optimal size kept in racks
inside a climate-controlled chamber (Fig. 1.7). The trays may be constructed of
stainless steel, aluminum, wood, or even plastic and may or may not have lids. The
trays and lids in some setups are perforated to allow aeration and heat dissipation.
The rooms are maintained at controlled temperature and humidity and often with
forced circulation of HEPA filtered air (though this may not be essential). This configuration is easy to set up and easily scalable with low labor intensity.
The mixing and heat transfer issues associated with stationary tray fermenters
can be addressed through use of rotating drum-type reactors which are either mixed
continuously or intermittently and with or without forced aeration. These types of
reactors were patented by the French company Lyven and the Indian company
Biocon and have been in use for commercial enzyme production. There are also
several other types of reactor designs in use, while tray reactors are the dominant
types used in commercial SSF.
1 Enzymes for Bioenergy