27
1.5.4 Submerged Fermentation (SmF)
Submerged fermentation is by far the most commonly used technique for production of industrial enzymes, which also includes cellulases. SmF is preferred by the
industry since it allows easy online monitoring of the process parameters and allows
their control (Hansen et al. 2015). Heat and mass transfer are better in SmF, since
the cells are dispersed in a very conductive aqueous environment and also due to the
fact that the system allows for regulation of temperature with external heating/cooling besides having efficient mechanisms for mixing. Biomass-hydrolyzing enzymes
from the common industrially employed cellulase producers (e.g., Trichoderma
reesei) are inducible and the best activities are reported when grown in media containing cellulose. Untreated lignocellulosic substrates are generally found to support lesser enzyme yields compared to pure cellulose forms like Solka Floc or
Avicell (Mathew et al. 2008). A range of different cellulosic materials has been tried
as substrates for cellulase production and this includes pure celluloses (Hendy et al.
1982), paper pulp (Zhuang et al. 2007), corn cob residue (Liming and Xueliang
2004), sugar cane bagasse (Pereira et al. 2013), and even dairy manure (Wen et al.
2005). Most of these natural substrates are capable of inducing the cellulase systems
in fungi often at par with known inducers or sometimes even better (Mathew et al.
2008). The choice of raw materials for cellulase production can influence both the
yields and the final compositions of the cellulase preparation, which in turn will also
affect the cost of enzyme preparations. Agro residues like corn cobs, rice or wheat
straws, bagasse, etc. are cheap raw materials that can induce cellulase production
due to their content of cellulose and hemicellulose polymers. However, they might
need pretreatment for delignification before being used in cellulase production
media. In a biorefinery context, especially for onsite enzyme production, this would
not pose a big problem since the same feedstock that goes for biofuel production
can serve as carbon source/inducer for cellulase production.
Media used for cellulase production have typically followed a composition after
the original T. reesei medium used by Mandels and Weber (1969), but with modifications as appropriate for the strain/organism. Typically for T. reesei the fermentations are carried out in an acidic pH range with the optimal temperature being in the
range of 25–30 °C (Mathew et al. 2008). Besides the carbon source, the choice as
well as the concentration of nitrogen sources are known to affect cellulase production and a high C/N ratio is proposed to be conductive for enhanced cellulase production by T. reesei (Liming and Xueliang 2004). Stirred tank reactors are commonly
employed for SmF production of cellulases, which has the advantages of better
mixing and aeration. However, fungal fermentation can present very unique problems associated with the control of their morphology. The morphology of fungi in
SmF may vary based on various factors including the type of organism, inoculum
density, pH, presence of surface active agents, agitation rates, etc., and careful selection of parameters might be essential to maintain the optimal fungal morphology. In
typical submerged fermentations, fungi may grow in pelleted form or they may be
dispersed as mycelial mats or aggregates. The latter form is often a serious limitation since the aggregates result in mass transfer limitations and cell damage due to
1 Enzymes for Bioenergy
1.5.4 Submerged Fermentation (SmF)
Submerged fermentation is by far the most commonly used technique for production of industrial enzymes, which also includes cellulases. SmF is preferred by the
industry since it allows easy online monitoring of the process parameters and allows
their control (Hansen et al. 2015). Heat and mass transfer are better in SmF, since
the cells are dispersed in a very conductive aqueous environment and also due to the
fact that the system allows for regulation of temperature with external heating/cooling besides having efficient mechanisms for mixing. Biomass-hydrolyzing enzymes
from the common industrially employed cellulase producers (e.g., Trichoderma
reesei) are inducible and the best activities are reported when grown in media containing cellulose. Untreated lignocellulosic substrates are generally found to support lesser enzyme yields compared to pure cellulose forms like Solka Floc or
Avicell (Mathew et al. 2008). A range of different cellulosic materials has been tried
as substrates for cellulase production and this includes pure celluloses (Hendy et al.
1982), paper pulp (Zhuang et al. 2007), corn cob residue (Liming and Xueliang
2004), sugar cane bagasse (Pereira et al. 2013), and even dairy manure (Wen et al.
2005). Most of these natural substrates are capable of inducing the cellulase systems
in fungi often at par with known inducers or sometimes even better (Mathew et al.
2008). The choice of raw materials for cellulase production can influence both the
yields and the final compositions of the cellulase preparation, which in turn will also
affect the cost of enzyme preparations. Agro residues like corn cobs, rice or wheat
straws, bagasse, etc. are cheap raw materials that can induce cellulase production
due to their content of cellulose and hemicellulose polymers. However, they might
need pretreatment for delignification before being used in cellulase production
media. In a biorefinery context, especially for onsite enzyme production, this would
not pose a big problem since the same feedstock that goes for biofuel production
can serve as carbon source/inducer for cellulase production.
Media used for cellulase production have typically followed a composition after
the original T. reesei medium used by Mandels and Weber (1969), but with modifications as appropriate for the strain/organism. Typically for T. reesei the fermentations are carried out in an acidic pH range with the optimal temperature being in the
range of 25–30 °C (Mathew et al. 2008). Besides the carbon source, the choice as
well as the concentration of nitrogen sources are known to affect cellulase production and a high C/N ratio is proposed to be conductive for enhanced cellulase production by T. reesei (Liming and Xueliang 2004). Stirred tank reactors are commonly
employed for SmF production of cellulases, which has the advantages of better
mixing and aeration. However, fungal fermentation can present very unique problems associated with the control of their morphology. The morphology of fungi in
SmF may vary based on various factors including the type of organism, inoculum
density, pH, presence of surface active agents, agitation rates, etc., and careful selection of parameters might be essential to maintain the optimal fungal morphology. In
typical submerged fermentations, fungi may grow in pelleted form or they may be
dispersed as mycelial mats or aggregates. The latter form is often a serious limitation since the aggregates result in mass transfer limitations and cell damage due to
1 Enzymes for Bioenergy