7 Enzymatic Pretreatment
Lignocellulosic biomass being the key building block of plant cell wall comprises of
cellulose, hemicellulose, lignin, extractives, pectin, and ash. These fractions of
components may vary with the plant type, species, their age, and the particular
climatic conditions. The plant cell wall is a complex structure comprising of several
β-glucan chains that are bound by hydrogen bonds to form elementary fibrils which
are further bundled together to form microfibrillar structures. Studies done on biomasses that are pretreated using enzymes have shown that it does not require
additional washing due to the absence of inhibitors and pH neutralization steps as
needed in other chemical or physicochemical pretreatment methods since it is
processed at pH conditions of 11–12. Further it helps to enhance the cellulases and
xylanases accessibility to the polysaccharides present in the delignified biomass.
Laccase is the enzyme usually used for the enzymatic pretreatment since it is said to
fit well with the feedstocks containing higher hemicellulosic content because of the
maximum recovery being achieved when compared to other pretreatment techniques. Laccase pretreatment of the biomasses is non-hazardous to the environment
and also non-corrosive to the bioreactors with no issues regarding the waste disposal
and the recycling of the toxic chemicals, therefore reducing the operational costs to a
great extent.
Laccase-mediated pretreatment studies done on SCT using central composite
design (CCD) based on response surface methodology (RSM) showed that maximum delignification of 79.1% was obtained when the operational process parameters
were processed at temperature of 40
C, pH at 7, biomass loading of 21% (w/v),
enzyme titer of 430.3 IU/mL, and incubation time of 6 h [4].
Other studies done on a combination of biomasses including Ricinus communis
(RC), Saccharum officinarum (SCT) top, Saccharum spontaneum (KG), Lantana
camara (LC), Ananas comosus (PA) leaf wastes, and Bambusa bambos (BB) where
laccase enzyme of 500 IU/ml, with substrate concentration of 30% (w/v), 1 g of
mixed substrate at 35
C, and incubation time of 6 h followed by simultaneous
saccharification and fermentation proved to be efficient in the production of
bioethanol of 1.396 g/L/h [17].
8 Alkaline Hydrogen Peroxide (AHP) Pretreatment
AHP pretreatment is one of the most effective pretreatment for delignification and a
variety of lignocellulosic agricultural residues such as rice husks or hulls [23, 24],
rapeseed straw [25], wheat straw [26, 27], sugarcane bagasse [28–30], sweet sorghum bagasse [31], cashew apple bagasse [32], bamboo [33], olive tree biomass
[34], poplar [35], Jerusalem artichoke stalk [36], and seaweed Ulva prolifera
[37]. AHP has oxidative action on the cell wall of biomass and breaks ester linkages
with less sugar degradation and increased digestibility. No or very less secondary
Lignocellulosic Sugarcane Tops for Bioethanol Production: An Overview
97
Lignocellulosic biomass being the key building block of plant cell wall comprises of
cellulose, hemicellulose, lignin, extractives, pectin, and ash. These fractions of
components may vary with the plant type, species, their age, and the particular
climatic conditions. The plant cell wall is a complex structure comprising of several
β-glucan chains that are bound by hydrogen bonds to form elementary fibrils which
are further bundled together to form microfibrillar structures. Studies done on biomasses that are pretreated using enzymes have shown that it does not require
additional washing due to the absence of inhibitors and pH neutralization steps as
needed in other chemical or physicochemical pretreatment methods since it is
processed at pH conditions of 11–12. Further it helps to enhance the cellulases and
xylanases accessibility to the polysaccharides present in the delignified biomass.
Laccase is the enzyme usually used for the enzymatic pretreatment since it is said to
fit well with the feedstocks containing higher hemicellulosic content because of the
maximum recovery being achieved when compared to other pretreatment techniques. Laccase pretreatment of the biomasses is non-hazardous to the environment
and also non-corrosive to the bioreactors with no issues regarding the waste disposal
and the recycling of the toxic chemicals, therefore reducing the operational costs to a
great extent.
Laccase-mediated pretreatment studies done on SCT using central composite
design (CCD) based on response surface methodology (RSM) showed that maximum delignification of 79.1% was obtained when the operational process parameters
were processed at temperature of 40
C, pH at 7, biomass loading of 21% (w/v),
enzyme titer of 430.3 IU/mL, and incubation time of 6 h [4].
Other studies done on a combination of biomasses including Ricinus communis
(RC), Saccharum officinarum (SCT) top, Saccharum spontaneum (KG), Lantana
camara (LC), Ananas comosus (PA) leaf wastes, and Bambusa bambos (BB) where
laccase enzyme of 500 IU/ml, with substrate concentration of 30% (w/v), 1 g of
mixed substrate at 35
C, and incubation time of 6 h followed by simultaneous
saccharification and fermentation proved to be efficient in the production of
bioethanol of 1.396 g/L/h [17].
8 Alkaline Hydrogen Peroxide (AHP) Pretreatment
AHP pretreatment is one of the most effective pretreatment for delignification and a
variety of lignocellulosic agricultural residues such as rice husks or hulls [23, 24],
rapeseed straw [25], wheat straw [26, 27], sugarcane bagasse [28–30], sweet sorghum bagasse [31], cashew apple bagasse [32], bamboo [33], olive tree biomass
[34], poplar [35], Jerusalem artichoke stalk [36], and seaweed Ulva prolifera
[37]. AHP has oxidative action on the cell wall of biomass and breaks ester linkages
with less sugar degradation and increased digestibility. No or very less secondary
Lignocellulosic Sugarcane Tops for Bioethanol Production: An Overview
97