and peroxidases (lignin and manganese peroxidases), but it is known that there are
many other enzymes whose role in lignin degradation has not been clarified. In
biological treatment, microorganisms and their enzymes are used for the selective
delignification of lignocellulose materials; this treatment has high yields with low
cost, low energy requirements, and generates nonpolluting by-products; on the
other hand, biodelignification takes much longer than chemical or thermal processes, usually 8–12 weeks (Yu et al. 2010). Even when white, brown, and soft rot
fungi have been used to degrade lignocellulose, white rot fungi remains the most
effective for delignification of lignocellulosic materials, and it has been demonstrated that white rot fungi are able to selectively degrade lignin in wheat straw and
are good choices for delignification of lignocellulosic residues (Fan et al. 1987;
Arora and Chander 2002; Yu et al. 2010; Sánchez 2011). Lignin degrading
enzymes have been extensively expressed in homologous organisms, most in filamentous fungi with variable results; a special case is the expression of laccase
gene from Cyathus bulleri that has been expressed in E. coli with very good results
making it the first laccase successfully expressed in a bacterial host (Salony et al.
2006).
7.5 Solid-State Fermentation
Solid-state fermentation (SSF) is the growing of microorganisms over solid substrate in near absence of liquid free water; the water necessary for microbial
growth is present in the culture absorbed into substrate or associated to other
components of the culture medium. SSF is a good system for fungi cultivation as
culture conditions are similar to the conditions that fungi meet in the natural
habitat. As a result, fungi produce higher enzyme and protein yields, and can even
synthetize some metabolites that are produced in very low yields or are not produced at all in submerged fermentation (Raimbault 1998; Viniegra-González et al.
2003). SSF is a practical treatment for lignocellulose degradation from complex
substrates as agricultural, forestry and food-processing wastes, and SSF may be
used with fast growing fungi capable to degrade lignin in an exclusive way, so it
may be possible to have a lignin-degrading system that may leave cellulose and
hemicellulose ready for further use or engineered microbes can be used to perform
lignocellulose degradation and compound production at the same time. If that is
accomplished, then SSF can be much more efficient than the typical two steps of
lignocellulose degradation and fermentation normally planned for biorefining, as
in SSF enzymes activities produced, are coordinated for the degrading substrate
which results in both less enzyme requirement and shorter process times. There is
an increase in hydrolysis rate by conversion of sugars that can inhibit enzymatic
activity and less water content results in smaller volume reactors and less contamination chances (Sun and Cheng 2002; Malherbe and Cloete 2003). Problems
associated with SSF are due to its nature; heat build up, heterogeneity, possible
contamination, difficult scaling-up, biomass growth estimation, but these have not
7 Integral Management of Lignocellulosic Biomass by Biorefining
241
many other enzymes whose role in lignin degradation has not been clarified. In
biological treatment, microorganisms and their enzymes are used for the selective
delignification of lignocellulose materials; this treatment has high yields with low
cost, low energy requirements, and generates nonpolluting by-products; on the
other hand, biodelignification takes much longer than chemical or thermal processes, usually 8–12 weeks (Yu et al. 2010). Even when white, brown, and soft rot
fungi have been used to degrade lignocellulose, white rot fungi remains the most
effective for delignification of lignocellulosic materials, and it has been demonstrated that white rot fungi are able to selectively degrade lignin in wheat straw and
are good choices for delignification of lignocellulosic residues (Fan et al. 1987;
Arora and Chander 2002; Yu et al. 2010; Sánchez 2011). Lignin degrading
enzymes have been extensively expressed in homologous organisms, most in filamentous fungi with variable results; a special case is the expression of laccase
gene from Cyathus bulleri that has been expressed in E. coli with very good results
making it the first laccase successfully expressed in a bacterial host (Salony et al.
2006).
7.5 Solid-State Fermentation
Solid-state fermentation (SSF) is the growing of microorganisms over solid substrate in near absence of liquid free water; the water necessary for microbial
growth is present in the culture absorbed into substrate or associated to other
components of the culture medium. SSF is a good system for fungi cultivation as
culture conditions are similar to the conditions that fungi meet in the natural
habitat. As a result, fungi produce higher enzyme and protein yields, and can even
synthetize some metabolites that are produced in very low yields or are not produced at all in submerged fermentation (Raimbault 1998; Viniegra-González et al.
2003). SSF is a practical treatment for lignocellulose degradation from complex
substrates as agricultural, forestry and food-processing wastes, and SSF may be
used with fast growing fungi capable to degrade lignin in an exclusive way, so it
may be possible to have a lignin-degrading system that may leave cellulose and
hemicellulose ready for further use or engineered microbes can be used to perform
lignocellulose degradation and compound production at the same time. If that is
accomplished, then SSF can be much more efficient than the typical two steps of
lignocellulose degradation and fermentation normally planned for biorefining, as
in SSF enzymes activities produced, are coordinated for the degrading substrate
which results in both less enzyme requirement and shorter process times. There is
an increase in hydrolysis rate by conversion of sugars that can inhibit enzymatic
activity and less water content results in smaller volume reactors and less contamination chances (Sun and Cheng 2002; Malherbe and Cloete 2003). Problems
associated with SSF are due to its nature; heat build up, heterogeneity, possible
contamination, difficult scaling-up, biomass growth estimation, but these have not
7 Integral Management of Lignocellulosic Biomass by Biorefining
241