cellobiose, which is then repeated numerous times along its
chain to form a linear polymer. This structural configuration
contributes to its highly crystalline, tightly packed and
resistance toward depolymerization. Conversion of cellulose
into glucose and further into ethanol can be achieved either
through chemical (via H 2 SO 4 ) or enzymatic hydrolysis (via
cellulases) (Singh and Mishra 1995; Canilha et al. 2011).
Hemicellulose with molecular weight lesser than cellulose is
the second major fraction of lignocellulosic material. It is a
heteropolysaccharide consisting acetic acid, 4-O-methyl-Dglucuronic acid, D-glucuronic acid, pentoses (Dxylose,
L-arabinose), and hexoses (D-glucose, D-galactose, and
D-mannose). Based on type of sugar in the main chain within
the polymeric, hemicellulose is classified as xylan, glucomannan, and galactan (Kuhad et al. 1997). It can be easily
hydrolyzed in comparison to cellulose owing to its amorphous nature (Taherzadeh and Karimi 2007). Pretreatment
methods such as hydrothermal and acid hydrolysis are
employed to remove hemicelluloses from lignocellulosic
materials, which release sugars (xylose) that is consequently
converted into ethanol (Canilha et al. 2012; Sun and Cheng
2002).
Lignin is amorphous heteropolymer comprised of three
phenylpropane units (coniferyl, p-coumaryl, and sinapyl
alcohol), and phenylpropanoid monomeric units (p-hydroxyphenyl, guaiacyl, and syringyl). The presence of cellulose
elementary fibrils along with hemicelluloses and lignin protects cell wall against chemical and/or biological degradation
(da Silva et al. 2010). The resistance towards enzymatic
hydrolysis of lignocellulosic materials by restricting enzyme
accessibility is caused by lignin content and its distribution.
Hence, to improve the rate of enzymatic hydrolysis, delignification plays a crucial role (Hideno et al. 2009).
These three components are unevenly distributed in the
cell walls, which depend on the type of tissue, plant species,
and its maturity (Canilha et al. 2012). Apart from this, some
of the major characteristics investigated for mixed food
waste by various authors have been tabulated below
(Table 2). From Table 2, it is evident that food waste is
primarily acidic in nature and has moderate percentage of
carbon.
3 Pretreatment of Biomass
According to the literatures, there are a number of available
pretreatment technologies that can efficiently breakdown the
biomass components into smaller fractions (e.g., oligosaccharides and monosaccharides) to obtain an array of products. The primary objective of a pretreatment process is to
promote cellulose hydrolysis for its conversion into fuels or
value-added products. The accelerated hydrolysis rates is
achieved due to physical and chemical changes within the
structure of lignocellulosic biomass caused by the various
pretreatment employed. These changes include the reduction
of cellulose crystallinity and lignin removal which in turn
increases porosity. The pretreatment and deconstruction
method employed in conjugation with physicochemical
properties of biomass directly influence the success of the
fabrication of biofuels and other bioproducts. Apart from the
efficient pretreatment technology, usage of toxic and hazardous material free aqueous media for solubilization is also
important factor for the production of carbohydrates with
reduced molecular weight. The broad classification and
hierarchy of pretreatment methods are shown (Fig. 3) and
discussed below.
Fig. 2 Schematic representation
of the chapter
Bioconversion of Food Waste into Biogas
83
chain to form a linear polymer. This structural configuration
contributes to its highly crystalline, tightly packed and
resistance toward depolymerization. Conversion of cellulose
into glucose and further into ethanol can be achieved either
through chemical (via H 2 SO 4 ) or enzymatic hydrolysis (via
cellulases) (Singh and Mishra 1995; Canilha et al. 2011).
Hemicellulose with molecular weight lesser than cellulose is
the second major fraction of lignocellulosic material. It is a
heteropolysaccharide consisting acetic acid, 4-O-methyl-Dglucuronic acid, D-glucuronic acid, pentoses (Dxylose,
L-arabinose), and hexoses (D-glucose, D-galactose, and
D-mannose). Based on type of sugar in the main chain within
the polymeric, hemicellulose is classified as xylan, glucomannan, and galactan (Kuhad et al. 1997). It can be easily
hydrolyzed in comparison to cellulose owing to its amorphous nature (Taherzadeh and Karimi 2007). Pretreatment
methods such as hydrothermal and acid hydrolysis are
employed to remove hemicelluloses from lignocellulosic
materials, which release sugars (xylose) that is consequently
converted into ethanol (Canilha et al. 2012; Sun and Cheng
2002).
Lignin is amorphous heteropolymer comprised of three
phenylpropane units (coniferyl, p-coumaryl, and sinapyl
alcohol), and phenylpropanoid monomeric units (p-hydroxyphenyl, guaiacyl, and syringyl). The presence of cellulose
elementary fibrils along with hemicelluloses and lignin protects cell wall against chemical and/or biological degradation
(da Silva et al. 2010). The resistance towards enzymatic
hydrolysis of lignocellulosic materials by restricting enzyme
accessibility is caused by lignin content and its distribution.
Hence, to improve the rate of enzymatic hydrolysis, delignification plays a crucial role (Hideno et al. 2009).
These three components are unevenly distributed in the
cell walls, which depend on the type of tissue, plant species,
and its maturity (Canilha et al. 2012). Apart from this, some
of the major characteristics investigated for mixed food
waste by various authors have been tabulated below
(Table 2). From Table 2, it is evident that food waste is
primarily acidic in nature and has moderate percentage of
carbon.
3 Pretreatment of Biomass
According to the literatures, there are a number of available
pretreatment technologies that can efficiently breakdown the
biomass components into smaller fractions (e.g., oligosaccharides and monosaccharides) to obtain an array of products. The primary objective of a pretreatment process is to
promote cellulose hydrolysis for its conversion into fuels or
value-added products. The accelerated hydrolysis rates is
achieved due to physical and chemical changes within the
structure of lignocellulosic biomass caused by the various
pretreatment employed. These changes include the reduction
of cellulose crystallinity and lignin removal which in turn
increases porosity. The pretreatment and deconstruction
method employed in conjugation with physicochemical
properties of biomass directly influence the success of the
fabrication of biofuels and other bioproducts. Apart from the
efficient pretreatment technology, usage of toxic and hazardous material free aqueous media for solubilization is also
important factor for the production of carbohydrates with
reduced molecular weight. The broad classification and
hierarchy of pretreatment methods are shown (Fig. 3) and
discussed below.
Fig. 2 Schematic representation
of the chapter
Bioconversion of Food Waste into Biogas
83