24
cellulase preparations, BGL from Aspergillus niger is blended, which makes up for
T. reesei’s lower BGL activity and product inhibition. Apparently, the production of
biomass-hydrolyzing enzymes is a complex process which has to take care of fungal
growth parameters, ideal conditions for the optimal enzyme production, and balancing the costs involved. The following discussions will concentrate on the technologies used for production of cellulases.
1.5.2 Fermentation Strategies
Commercial production of cellulases typically employs the submerged fermentation (SmF) strategy while solid-state fermentation (SSF) processes are also used by
companies (Sukumaran et al. 2005). Each method has its own advantages and disadvantages and the question of a best method is rather irrelevant, since the method
adopted would depend on the organism employed, substrate used, growth conditions, existing infrastructure if any, need for purification, etc. While SSF is claimed
to be the cheaper method for microbial enzyme production by many (e.g., Tengerdy
1996; Castilho et al. 2000, Viniegra-González et al. 2003; Krishna 2005), there are
also reports that state the contrary (Nakkeeran et al. 2012). Also it should be noted
that the enzymes elaborated by the same organism can be different in SmF and SSF
(Hansen et al. 2015), and the cost of production also needs to be read along with the
efficiency of the enzyme in catalyzing the desired reaction. In this context the production costs are tightly connected to the productivity of the enzyme-producing
strain and the final activity yield. The hydrolytic efficiency of the multienzyme
complex depends on both the properties of individual enzymes and synergies
between them and their ratio in the cocktail (Gusakov et al. 2014).
1.5.3 Solid-State Fermentation (SSF)
Solid-state fermentation is defined as fermentation with near absence of free water
but substrate moistened to support growth of microorganisms (Pandey 2003). The
substrate can either be a natural organic material like wheat bran or an inert support
material impregnated or coated with the liquid growth medium (Ooijkaas et al.
2000). SSF processes are closer to the natural conditions of growth of most of the
filamentous fungi and are hence expected to be ideal for production of enzymes that
are required for plant cell wall degradation. There have been a large number of studies on solid-state fermentation for cellulase production reported in the literature.
Even earlier studies like those by Chahal (1985) have demonstrated the higher
yields of enzyme in SSF compared to SmF, and there have been reports which indicated significant cost reductions for SSF compared to SmF (Tengerdy 1996). Solidstate fermentation is typically performed by inoculating a natural substrate (e.g.,
wheat bran) moistened/impregnated with the suitable medium that carries essential
nutrients – especially nitrogen source since in the majority of cases the substrate
itself acts as the carbon source. Additional carbon source may be added for
R.K. Sukumaran et al.
cellulase preparations, BGL from Aspergillus niger is blended, which makes up for
T. reesei’s lower BGL activity and product inhibition. Apparently, the production of
biomass-hydrolyzing enzymes is a complex process which has to take care of fungal
growth parameters, ideal conditions for the optimal enzyme production, and balancing the costs involved. The following discussions will concentrate on the technologies used for production of cellulases.
1.5.2 Fermentation Strategies
Commercial production of cellulases typically employs the submerged fermentation (SmF) strategy while solid-state fermentation (SSF) processes are also used by
companies (Sukumaran et al. 2005). Each method has its own advantages and disadvantages and the question of a best method is rather irrelevant, since the method
adopted would depend on the organism employed, substrate used, growth conditions, existing infrastructure if any, need for purification, etc. While SSF is claimed
to be the cheaper method for microbial enzyme production by many (e.g., Tengerdy
1996; Castilho et al. 2000, Viniegra-González et al. 2003; Krishna 2005), there are
also reports that state the contrary (Nakkeeran et al. 2012). Also it should be noted
that the enzymes elaborated by the same organism can be different in SmF and SSF
(Hansen et al. 2015), and the cost of production also needs to be read along with the
efficiency of the enzyme in catalyzing the desired reaction. In this context the production costs are tightly connected to the productivity of the enzyme-producing
strain and the final activity yield. The hydrolytic efficiency of the multienzyme
complex depends on both the properties of individual enzymes and synergies
between them and their ratio in the cocktail (Gusakov et al. 2014).
1.5.3 Solid-State Fermentation (SSF)
Solid-state fermentation is defined as fermentation with near absence of free water
but substrate moistened to support growth of microorganisms (Pandey 2003). The
substrate can either be a natural organic material like wheat bran or an inert support
material impregnated or coated with the liquid growth medium (Ooijkaas et al.
2000). SSF processes are closer to the natural conditions of growth of most of the
filamentous fungi and are hence expected to be ideal for production of enzymes that
are required for plant cell wall degradation. There have been a large number of studies on solid-state fermentation for cellulase production reported in the literature.
Even earlier studies like those by Chahal (1985) have demonstrated the higher
yields of enzyme in SSF compared to SmF, and there have been reports which indicated significant cost reductions for SSF compared to SmF (Tengerdy 1996). Solidstate fermentation is typically performed by inoculating a natural substrate (e.g.,
wheat bran) moistened/impregnated with the suitable medium that carries essential
nutrients – especially nitrogen source since in the majority of cases the substrate
itself acts as the carbon source. Additional carbon source may be added for
R.K. Sukumaran et al.