fuels have increased the researcher’s interest in developing sustainable methodologies for the production of biofuel (Yang et al. 2015). The biomass obtained from
lignocellulosic plant is abundantly available in the environment and can be considered as a vital alternative of fossil fuels. The biomass can be found in the environment throughout the year in the bulk amount without being used in the form of
agricultural and forestry waste/residues (Thomas et al. 2016). Most of the residues,
e.g. rice and sugarcane cultivation, are burnt in the open fields mostly in Asian
countries causing environmental pollution (Thomas et al. 2016).
The composition of lignocellulosic plant biomass consists of three main components, i.e. lignin, hemicellulose and cellulose, which together make the recalcitrant
structure of plant biomass (Singh et al. 2017). Due to this, the biorefinery process
involves three major steps such as pretreatment, saccharification and hydrolysis for
the complete bioconversion (Bhardwaj et al. 2020). Other important aspects such as
the type of biomass to be used in the biorefinery process and biomass transportation
are also a matter of concern along with the structure recalcitrance of the biomass to
expose valuable sugars to be utilized in the biorefinery process to fulfil the bioenergy
requirement of the world (Hassan et al. 2019).
The microbial hydrolytic enzymes play an important role in the bioconversion of
biomass by converting it into fermentable sugar (Wei et al. 2012). Therefore, various
strategies have been carried out till date such as isolation of new microbes and
various optimization studies to improve the production of enzymes (Attri and Garg
2014; Haitjema et al. 2014; Nigam 2013). Enzymes are required in all the major
steps of biorefinery processes, e.g. in the biological pretreatment method, using
laccase for the removal of lignin which can help to reduce the recalcitrant nature
of plant cell wall and making inner cellular parts, i.e. hemicellulose and cellulose,
more accessible (Agrawal et al. 2019). Hemicellulases, e.g. xylanases and cellulases,
are required in the hydrolysis and saccharification of plant residues which enhance
the release of sugar molecules (Bala and Singh 2019a). These enzymes can be used
either individually or as a cocktail (Bhardwaj et al. 2019). Although the commercially available enzyme cocktails are costly and affect the economy of the process,
microbial enzyme can be considered as the best alternatives (Vaishnav et al. 2018).
Along with the cost of the enzymes, another important factor to be considered is the
amount/load of enzyme required for the process and futher study has to be done to
identify suitable enzyme preparations to achieve enhanced saccharification rate
(Cunha et al. 2017). Also getting microorganisms which can produce an enzyme
cocktail that can act on multiple agricultural residues is another option to improve
the economic viability of the process (Thomas et al. 2016). With the availability of a
huge range of cellulases, lignocellulases can be utilized to allow the adaptation of
such cocktails (Ang et al. 2015). This can be achieved by xylanase supplementation
as endo-xylanase is known as one of the most suitable enzymes used in the
hydrolysis process by breaking the internal glycosidic linkages present in the
backbone of the complex structure of heteroxylan, resulting in the xylooligosaccharide formation (Thomas et al. 2014a, b). Later these xylooligosaccharides are converted into other fermentable sugars such as trimers
(xylotriose), dimers (xylobiose) and monomers (xylose) (Brienzo et al. 2012).
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lignocellulosic plant is abundantly available in the environment and can be considered as a vital alternative of fossil fuels. The biomass can be found in the environment throughout the year in the bulk amount without being used in the form of
agricultural and forestry waste/residues (Thomas et al. 2016). Most of the residues,
e.g. rice and sugarcane cultivation, are burnt in the open fields mostly in Asian
countries causing environmental pollution (Thomas et al. 2016).
The composition of lignocellulosic plant biomass consists of three main components, i.e. lignin, hemicellulose and cellulose, which together make the recalcitrant
structure of plant biomass (Singh et al. 2017). Due to this, the biorefinery process
involves three major steps such as pretreatment, saccharification and hydrolysis for
the complete bioconversion (Bhardwaj et al. 2020). Other important aspects such as
the type of biomass to be used in the biorefinery process and biomass transportation
are also a matter of concern along with the structure recalcitrance of the biomass to
expose valuable sugars to be utilized in the biorefinery process to fulfil the bioenergy
requirement of the world (Hassan et al. 2019).
The microbial hydrolytic enzymes play an important role in the bioconversion of
biomass by converting it into fermentable sugar (Wei et al. 2012). Therefore, various
strategies have been carried out till date such as isolation of new microbes and
various optimization studies to improve the production of enzymes (Attri and Garg
2014; Haitjema et al. 2014; Nigam 2013). Enzymes are required in all the major
steps of biorefinery processes, e.g. in the biological pretreatment method, using
laccase for the removal of lignin which can help to reduce the recalcitrant nature
of plant cell wall and making inner cellular parts, i.e. hemicellulose and cellulose,
more accessible (Agrawal et al. 2019). Hemicellulases, e.g. xylanases and cellulases,
are required in the hydrolysis and saccharification of plant residues which enhance
the release of sugar molecules (Bala and Singh 2019a). These enzymes can be used
either individually or as a cocktail (Bhardwaj et al. 2019). Although the commercially available enzyme cocktails are costly and affect the economy of the process,
microbial enzyme can be considered as the best alternatives (Vaishnav et al. 2018).
Along with the cost of the enzymes, another important factor to be considered is the
amount/load of enzyme required for the process and futher study has to be done to
identify suitable enzyme preparations to achieve enhanced saccharification rate
(Cunha et al. 2017). Also getting microorganisms which can produce an enzyme
cocktail that can act on multiple agricultural residues is another option to improve
the economic viability of the process (Thomas et al. 2016). With the availability of a
huge range of cellulases, lignocellulases can be utilized to allow the adaptation of
such cocktails (Ang et al. 2015). This can be achieved by xylanase supplementation
as endo-xylanase is known as one of the most suitable enzymes used in the
hydrolysis process by breaking the internal glycosidic linkages present in the
backbone of the complex structure of heteroxylan, resulting in the xylooligosaccharide formation (Thomas et al. 2014a, b). Later these xylooligosaccharides are converted into other fermentable sugars such as trimers
(xylotriose), dimers (xylobiose) and monomers (xylose) (Brienzo et al. 2012).
162
N. Bhardwaj and P. Verma