4 Bioprocessing of Lignocellulosic Materials
Various methods are presently accessible for the conversion
of lignocellulosic biomass to ethanol and various chemical
value-added products. Usually, bioprocessing of raw biomass
into their products involves three steps: system optimization,
process design, and model development (Pothiraj et al. 2006).
4.1 Solid-State Fermentation (SSF)
Solid-state fermentation (SSF) is one of the key options that
help to recover microbial enzymes with low operating cost
and capital investment from lignocellulosic wastes materials
(Chahal et al. 1996). Thus, it could an ideal process for
developing countries. The process occurs without the presence of free liquid. The most necessary medium for microbial activity (for growth) is the presence of water in an
absorbed or complex type (Cannel 1980). Microorganisms in
SSF can also cultivate beneath environment comparable to
their usual habitats (Jecu 2000). In SSF, the forestry, agricultural and food waste, and other wastes are considered as
resources of carbon to produce enzymes (Haltrich et al.
1996). Sun and Cheng (2002) reported few advantages of
SSF process: (1) lower enzyme requirement; (2) less reactor
volume; (3) increase the rate of hydrolysis by sugar conversion to slow down the action of enzyme; (4) less sterile
conditions as glucose has been removed instantly and ethanol is formed; (5) higher product yield; and (6) shorter
process time. Malherbe and Cloete (2002) reported that
combining SSF technology with a suitable fungus results in
a selective degradation of lignin that will be achievable in an
industrial scale. Mudgett (1986) reported the disadvantages
usually related to SSF are scale-up, buildup of heat, biomass
growth assessment, bacterial contamination, and control of
substrate content. Based on reactor operation and design
part, the SSF can manufacture various microbial products
(Lonsane et al. 1992).
4.2 Microorganisms and Their Lignocellulytic
Enzymes
Fungi (Baldrian and Gabriel 2003) and bacteria (McCarthy
1987) have been isolated as a varied spectrum of lignocellulolytic microorganisms over the years. Among all the lignocellulolytic microorganisms, Trichoderma reesei and its
mutants are broadly engaged in profitable manufacture of
cellulases and hemicellulases (Jørgensen et al. 2003). T. reesei
was discovered in 1950s and is the first cellulolytic organisms,
which able to degrade hemi- and cellulolytic enzymes but not
lignin. White-rot fungi belong to basidiomycetes, which is
mainly capable as well as widespread lignin degraders (Akin
et al. 1995) with P. chrysosporium. P. chrysosporium is one of
the characteristic set of lignocellulytic enzymes. It has drained
significant consideration as a suitable host for the producing
enzymes that degrades lignin (Ruggeri and Sassi 2003).
White-rot fungi viz. Phlebia fascicularia, Daedalea flavida,
P. floridensis and P. radiate are used to considerably degrade
wheat straw lignin (Arora et al. 2002). Pal et al. (1995)
reported the hemicellulose and lignin degradation for the
period of cultivation of white-rot fungus Trametes versicolor
on sugarcane bagasse and mushroom Flammulina velutipes
for about 40 days. Manganese-peroxidase and laccase are
produced by Trametes versicolor. A bacterial strain recovered
from plant decompose, Pseudomonas putida was also
able to degrade lignin-associated compounds (Pothiraj et al.
2006).
(a) Lignases
Lignases falls in a family of extracellular enzyme that has
been used by fungi in efficient breakdown of lignin aerobically. They are usually low in molecular weight and used to
break the lignin structure. In this regard, two families of
lignolytic enzymes viz. oxidase (laccase) and phenol peroxidases (manganese peroxidase and lignin peroxidase) play
an important role during enzymatic degradation (Krause
et al. 2003). Some enzymes role, that are not yet discovered
consist of glyoxal oxidase (Kersten and Kirk 1987), glucose
oxidase (Kelley and Reddy 1986), veratryl alcohol oxidases
(Bourbonnais and Paice 1988), methanol oxidase (Nishida
and Eriksson 1987), oxido-reductase (Bao and Renganathan
1991; Call and Mücke 1997) and H 2 O 2.
(b) Cellulases
Cellulases, an intricate combination of proteins with diverse
specificities to hydrolyze glycosidic bonds, are accountable
for hydrolysis of cellulose. It is separated into three main
classes of enzyme activity (Goyal et al. 1991) viz.
endoglucanases or endo-1, 4-b-glucanase, cellobiohydrolase, and b-glucosidase. Endoglucanases, frequently known
as carboxy methylcellulose (CM)-cellulases, are expected to
degrade the cellulose fiber which is amorphous in nature.
Thus, helps for successive attack by cellobiohydrolases
(Wood 1992). Among the fungal cellulase, cellobiohydrolase is a most important constituent which accounts for 40–
70% of the total cellulase proteins and use to hydrolyse the
cellulose with high crystallinty. Saul et al. (1990) recognized
a cellulase with exo- and endo-activities from Caldocellum
saccharolyticum.
88
N. Bordoloi et al.
Various methods are presently accessible for the conversion
of lignocellulosic biomass to ethanol and various chemical
value-added products. Usually, bioprocessing of raw biomass
into their products involves three steps: system optimization,
process design, and model development (Pothiraj et al. 2006).
4.1 Solid-State Fermentation (SSF)
Solid-state fermentation (SSF) is one of the key options that
help to recover microbial enzymes with low operating cost
and capital investment from lignocellulosic wastes materials
(Chahal et al. 1996). Thus, it could an ideal process for
developing countries. The process occurs without the presence of free liquid. The most necessary medium for microbial activity (for growth) is the presence of water in an
absorbed or complex type (Cannel 1980). Microorganisms in
SSF can also cultivate beneath environment comparable to
their usual habitats (Jecu 2000). In SSF, the forestry, agricultural and food waste, and other wastes are considered as
resources of carbon to produce enzymes (Haltrich et al.
1996). Sun and Cheng (2002) reported few advantages of
SSF process: (1) lower enzyme requirement; (2) less reactor
volume; (3) increase the rate of hydrolysis by sugar conversion to slow down the action of enzyme; (4) less sterile
conditions as glucose has been removed instantly and ethanol is formed; (5) higher product yield; and (6) shorter
process time. Malherbe and Cloete (2002) reported that
combining SSF technology with a suitable fungus results in
a selective degradation of lignin that will be achievable in an
industrial scale. Mudgett (1986) reported the disadvantages
usually related to SSF are scale-up, buildup of heat, biomass
growth assessment, bacterial contamination, and control of
substrate content. Based on reactor operation and design
part, the SSF can manufacture various microbial products
(Lonsane et al. 1992).
4.2 Microorganisms and Their Lignocellulytic
Enzymes
Fungi (Baldrian and Gabriel 2003) and bacteria (McCarthy
1987) have been isolated as a varied spectrum of lignocellulolytic microorganisms over the years. Among all the lignocellulolytic microorganisms, Trichoderma reesei and its
mutants are broadly engaged in profitable manufacture of
cellulases and hemicellulases (Jørgensen et al. 2003). T. reesei
was discovered in 1950s and is the first cellulolytic organisms,
which able to degrade hemi- and cellulolytic enzymes but not
lignin. White-rot fungi belong to basidiomycetes, which is
mainly capable as well as widespread lignin degraders (Akin
et al. 1995) with P. chrysosporium. P. chrysosporium is one of
the characteristic set of lignocellulytic enzymes. It has drained
significant consideration as a suitable host for the producing
enzymes that degrades lignin (Ruggeri and Sassi 2003).
White-rot fungi viz. Phlebia fascicularia, Daedalea flavida,
P. floridensis and P. radiate are used to considerably degrade
wheat straw lignin (Arora et al. 2002). Pal et al. (1995)
reported the hemicellulose and lignin degradation for the
period of cultivation of white-rot fungus Trametes versicolor
on sugarcane bagasse and mushroom Flammulina velutipes
for about 40 days. Manganese-peroxidase and laccase are
produced by Trametes versicolor. A bacterial strain recovered
from plant decompose, Pseudomonas putida was also
able to degrade lignin-associated compounds (Pothiraj et al.
2006).
(a) Lignases
Lignases falls in a family of extracellular enzyme that has
been used by fungi in efficient breakdown of lignin aerobically. They are usually low in molecular weight and used to
break the lignin structure. In this regard, two families of
lignolytic enzymes viz. oxidase (laccase) and phenol peroxidases (manganese peroxidase and lignin peroxidase) play
an important role during enzymatic degradation (Krause
et al. 2003). Some enzymes role, that are not yet discovered
consist of glyoxal oxidase (Kersten and Kirk 1987), glucose
oxidase (Kelley and Reddy 1986), veratryl alcohol oxidases
(Bourbonnais and Paice 1988), methanol oxidase (Nishida
and Eriksson 1987), oxido-reductase (Bao and Renganathan
1991; Call and Mücke 1997) and H 2 O 2.
(b) Cellulases
Cellulases, an intricate combination of proteins with diverse
specificities to hydrolyze glycosidic bonds, are accountable
for hydrolysis of cellulose. It is separated into three main
classes of enzyme activity (Goyal et al. 1991) viz.
endoglucanases or endo-1, 4-b-glucanase, cellobiohydrolase, and b-glucosidase. Endoglucanases, frequently known
as carboxy methylcellulose (CM)-cellulases, are expected to
degrade the cellulose fiber which is amorphous in nature.
Thus, helps for successive attack by cellobiohydrolases
(Wood 1992). Among the fungal cellulase, cellobiohydrolase is a most important constituent which accounts for 40–
70% of the total cellulase proteins and use to hydrolyse the
cellulose with high crystallinty. Saul et al. (1990) recognized
a cellulase with exo- and endo-activities from Caldocellum
saccharolyticum.
88
N. Bordoloi et al.