28
shear. Also the mycelia may get entangled in the baffles of the reactor and can get
accumulated on the fermenter wall wherever there is limited turbulence. The aggregates or mats can become really large, disallowing any penetration of substrates or
oxygen into the core, leading to death of cells in the center of the aggregates. The
pellets are highly entangled dense masses of hyphae which can assume sizes
between a few hundred micrometers and several millimeters (Domingues et al.
2000). Pelleted growth is generally preferred in fungal fermentations since it allows
better mass and oxygen transfer due to an even distribution and enhanced surface
area. Optimal morphologies in fermentations are dependent on operational conditions and knowledge of these can aid in enhancing the productivities in SmF (Cui
et al. 1998). It has been observed that higher inoculum densities can lead to smaller
pellet size, translating to higher protein secretion and higher filter paper activity
(Domingues et al. 2000). Similarly, in the same study it was observed that the presence of surfactant inhibited pellet formation by T. reesei RUT-C30. In another study
which related cellulase productivity to fungal morphology, it was found that the
buffers and pH conditions that promoted compact pellet formation resulted in
enhanced enzyme yield (Ferreira et al. 2009). A pH of around 4.8 and 100 mM succinate buffer supported maximum cellulase yield by T. reesei RUT-C30 in this study
(Ferreira et al. 2009).
High productivities and yields of cellulases at industrial scale require that the
fermentation is conducted under carbon flux limitation under either fed batch or
continuous mode (Jourdier et al. 2012). Pure forms of cellulose, lignocellulosic
substrates like pretreated plant biomass materials, or soluble carbon sources that can
induce cellulases like lactose are the common carbon sources used in commercialscale production of cellulases. The typical processes can be batch, fed batch, or
continuous. Lactose is used as the carbon source/inducer in commercial production
of cellulases employing Trichoderma, the disaccharide being the most affordable
among highly potent soluble inducers. While cultivation of the fungus in cellulose/
lignocellulose is cheaper, control of glucose concentration becomes a major limitation. At low concentrations of (ligno)cellulose, glucose production might be too
slow to meet the metabolic needs of active growth and enzyme production, while at
high concentrations, a higher rate of glucose generation compared to its consumption can result in catabolite repression (England et al. 2010). Apparently to maintain
the conditions of carbon flux limitation, fed batch or continuous mode becomes
helpful (Jourdier et al. 2013). In fact, most of the commercial production of cellulases employs the fed batch strategy where a soluble inducer like lactose or a cellulosic substrate is carefully dosed into the fermentation medium at appropriate
intervals. The use of insoluble inducers/carbon sources like pure cellulose, paper
pulp, or any lignocellulosic substrate poses the additional challenge of mixing the
medium in the production reactors. This is a very serious limitation when batch
process has to be employed, and often the production strategy has to ensure a size
reduction treatment before the insoluble substrate is fed to the reactor (Shin et al.
2000; Liming and Xueliang 2004). However, this can increase the production cost
due to the need for pulverization of substrate. Here again, a fed batch process may
help to maintain sufficient mixing, since the concentration of insoluble substrate
R.K. Sukumaran et al.
shear. Also the mycelia may get entangled in the baffles of the reactor and can get
accumulated on the fermenter wall wherever there is limited turbulence. The aggregates or mats can become really large, disallowing any penetration of substrates or
oxygen into the core, leading to death of cells in the center of the aggregates. The
pellets are highly entangled dense masses of hyphae which can assume sizes
between a few hundred micrometers and several millimeters (Domingues et al.
2000). Pelleted growth is generally preferred in fungal fermentations since it allows
better mass and oxygen transfer due to an even distribution and enhanced surface
area. Optimal morphologies in fermentations are dependent on operational conditions and knowledge of these can aid in enhancing the productivities in SmF (Cui
et al. 1998). It has been observed that higher inoculum densities can lead to smaller
pellet size, translating to higher protein secretion and higher filter paper activity
(Domingues et al. 2000). Similarly, in the same study it was observed that the presence of surfactant inhibited pellet formation by T. reesei RUT-C30. In another study
which related cellulase productivity to fungal morphology, it was found that the
buffers and pH conditions that promoted compact pellet formation resulted in
enhanced enzyme yield (Ferreira et al. 2009). A pH of around 4.8 and 100 mM succinate buffer supported maximum cellulase yield by T. reesei RUT-C30 in this study
(Ferreira et al. 2009).
High productivities and yields of cellulases at industrial scale require that the
fermentation is conducted under carbon flux limitation under either fed batch or
continuous mode (Jourdier et al. 2012). Pure forms of cellulose, lignocellulosic
substrates like pretreated plant biomass materials, or soluble carbon sources that can
induce cellulases like lactose are the common carbon sources used in commercialscale production of cellulases. The typical processes can be batch, fed batch, or
continuous. Lactose is used as the carbon source/inducer in commercial production
of cellulases employing Trichoderma, the disaccharide being the most affordable
among highly potent soluble inducers. While cultivation of the fungus in cellulose/
lignocellulose is cheaper, control of glucose concentration becomes a major limitation. At low concentrations of (ligno)cellulose, glucose production might be too
slow to meet the metabolic needs of active growth and enzyme production, while at
high concentrations, a higher rate of glucose generation compared to its consumption can result in catabolite repression (England et al. 2010). Apparently to maintain
the conditions of carbon flux limitation, fed batch or continuous mode becomes
helpful (Jourdier et al. 2013). In fact, most of the commercial production of cellulases employs the fed batch strategy where a soluble inducer like lactose or a cellulosic substrate is carefully dosed into the fermentation medium at appropriate
intervals. The use of insoluble inducers/carbon sources like pure cellulose, paper
pulp, or any lignocellulosic substrate poses the additional challenge of mixing the
medium in the production reactors. This is a very serious limitation when batch
process has to be employed, and often the production strategy has to ensure a size
reduction treatment before the insoluble substrate is fed to the reactor (Shin et al.
2000; Liming and Xueliang 2004). However, this can increase the production cost
due to the need for pulverization of substrate. Here again, a fed batch process may
help to maintain sufficient mixing, since the concentration of insoluble substrate
R.K. Sukumaran et al.