7.5 Role of Enzymes in Biorefinery
7.5.1 In Biological Pretreatment
As discussed above, the biorefinery process involves three main steps, of which
pretreatment of biomass is one of the important steps to enhance the production of
fermentable sugar. Although pretreatment could be of three types, physical, chemical or biological, the biological pretreatment is more preferred as it is eco-friendly,
easy, safe to use and involves the use of microbial enzymes and several microorganisms itself, e.g. white rot (Myrothecium verrucaria) and brown rot fungi
(Trametes versicolor, Pleurotus ostreatus). It can be efficiently used in the
delignification process without much requirement of energy (Kumar et al. 2009).
Various enzymes, such as laccases, lignin peroxidases, manganese-dependent peroxidases, etc., have been employed for the delignification process (Agrawal et al.
2019). This process makes inner hemicellulose and cellulose part more accessible for
the other hydrolytic enzymes such as endo-xylanases and cellulases, respectively, for
the hydrolysis process (Bhardwaj et al. 2019). After this step, the accessibility of
cellulose (carbon source) increases for efficient fermentation by microorganisms
leading to the cost-effective enzyme production followed by hydrolysis of the same
pretreated biomass. Therefore, it can be inferred that the rate of hydrolysis can be
increased up to 90% after the pretreatment (Saini et al. 2015).
The pretreatment process via enzymes utilizes crude or purified enzymes or
partially purified ligninolytic or hydrolytic enzymes. This may help to remove lignin
via fungal pretreatment within less time period (Plácido and Capareda 2015).
Although the complete efficiency of enzymatic pretreatment process is not yet
studied properly as compared to thermal and chemical pretreatment process, treatment of sugarcane using alkaline (NaOH) and crude Anthracophyllum discolor
enzyme extracts for the production of bioethanol resulted in 48.7% and 33.6% lignin
removal by NaOH enzymatic methods, i.e. 31% lower than the enzymatic process
alone (Asgher et al. 2013). However, in the study by Asgher et al. (2013) when
sugarcane bagasse was treated enzymatically with the increased cellulose load,
hydrolysis yeild of about 79% was obtained suggesting effectve treatmnet of the
lignocellulosic biomass (Asgher et al. 2013). Hence, these results can be the
examples of continuing new researches on the use of both ligninolytic and cellulolytic enzymes to disrupt the structure of lignocellulosic plant biomass for a better
saccharification and hydrolysis process (Asgher et al. 2013). There are various
reports in the enzymatic hydrolysis process such as a microalgal pretreatment for
the biomethane gas production (Vanegas et al. 2015), production of biohydrogen
(Mahdy et al. 2014), extraction of lipids for biodiesel generation (Fu et al. 2010) and
production of bioethanol (Kim et al. 2014). Similarly, manganese peroxidase in the
crude extract of Anthracophyllum discolor was used for the pretreatment of
Botryococcus braunii for the production of biogas (Ciudad et al. 2014). Enzymatic
pretreatment can be performed by using individual or cocktails of enzymes. Cocktails of enzymes are made by using either crude or partially purified enzymes.
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7.5.1 In Biological Pretreatment
As discussed above, the biorefinery process involves three main steps, of which
pretreatment of biomass is one of the important steps to enhance the production of
fermentable sugar. Although pretreatment could be of three types, physical, chemical or biological, the biological pretreatment is more preferred as it is eco-friendly,
easy, safe to use and involves the use of microbial enzymes and several microorganisms itself, e.g. white rot (Myrothecium verrucaria) and brown rot fungi
(Trametes versicolor, Pleurotus ostreatus). It can be efficiently used in the
delignification process without much requirement of energy (Kumar et al. 2009).
Various enzymes, such as laccases, lignin peroxidases, manganese-dependent peroxidases, etc., have been employed for the delignification process (Agrawal et al.
2019). This process makes inner hemicellulose and cellulose part more accessible for
the other hydrolytic enzymes such as endo-xylanases and cellulases, respectively, for
the hydrolysis process (Bhardwaj et al. 2019). After this step, the accessibility of
cellulose (carbon source) increases for efficient fermentation by microorganisms
leading to the cost-effective enzyme production followed by hydrolysis of the same
pretreated biomass. Therefore, it can be inferred that the rate of hydrolysis can be
increased up to 90% after the pretreatment (Saini et al. 2015).
The pretreatment process via enzymes utilizes crude or purified enzymes or
partially purified ligninolytic or hydrolytic enzymes. This may help to remove lignin
via fungal pretreatment within less time period (Plácido and Capareda 2015).
Although the complete efficiency of enzymatic pretreatment process is not yet
studied properly as compared to thermal and chemical pretreatment process, treatment of sugarcane using alkaline (NaOH) and crude Anthracophyllum discolor
enzyme extracts for the production of bioethanol resulted in 48.7% and 33.6% lignin
removal by NaOH enzymatic methods, i.e. 31% lower than the enzymatic process
alone (Asgher et al. 2013). However, in the study by Asgher et al. (2013) when
sugarcane bagasse was treated enzymatically with the increased cellulose load,
hydrolysis yeild of about 79% was obtained suggesting effectve treatmnet of the
lignocellulosic biomass (Asgher et al. 2013). Hence, these results can be the
examples of continuing new researches on the use of both ligninolytic and cellulolytic enzymes to disrupt the structure of lignocellulosic plant biomass for a better
saccharification and hydrolysis process (Asgher et al. 2013). There are various
reports in the enzymatic hydrolysis process such as a microalgal pretreatment for
the biomethane gas production (Vanegas et al. 2015), production of biohydrogen
(Mahdy et al. 2014), extraction of lipids for biodiesel generation (Fu et al. 2010) and
production of bioethanol (Kim et al. 2014). Similarly, manganese peroxidase in the
crude extract of Anthracophyllum discolor was used for the pretreatment of
Botryococcus braunii for the production of biogas (Ciudad et al. 2014). Enzymatic
pretreatment can be performed by using individual or cocktails of enzymes. Cocktails of enzymes are made by using either crude or partially purified enzymes.
166
N. Bhardwaj and P. Verma