woody parts of plant body. The high tensile strength and solvent insoluble properties
are provided by this structure (Sjöström and Westermark 1999). The build-up of
plant cell wall is determined by the regular arrangement of microfibrils of cellulose
(Chen et al. 2007). Hemicellulose that linked together with hydrogen bonds covers
the adjacent spaces between the elementary fibrils (Ali et al. 2015; Saini et al. 2015).
Hemicellulose consists of different monosaccharide units compared to cellulose.
Hemicellulose commonly possesses arabinose (five carbon sugar), xylans (five
carbon sugar), and uronic acid, for instance, sugar acid (Jenol et al. 2014). They
are heteropolymers consisting of linear and branched structure arrangement of
pentoses, hexoses, and sugar acids (Peng et al. 2012). Amorphous morphology of
hemicellulose made it partly soluble in water (Demirbas 2008). In terms of molecular
weight, hemicellulose is lighter compared to cellulose and since it is composed of
short lateral chain, it is easy to hydrolyse. Several components that act as inhibitors
like furfurals and hydroxymethyl furfurals in the fermentation process might be
produced during the degradation of hemicellulose. Hence, hemicellulose should be
removed at least 50% in order to increase the degree of digestibility of cellulose.
Lignin is the most complex compound in lignocelluloses, which provides a
structural support to the plant. Lignin tied the long molecular chains of sugars of
cellulose and hemicellulose together to make sturdy and strong cell walls
(Hüttermann et al. 2001). The tight association between lignin and different
polysaccharides has conferred the mechanical strength to the cell wall. It also
protects the cellulose and prevents the fibre to swell when reacting with water and
acts as protective shield against microbial and enzymatic attacks. Therefore, the
conversion of the cellulose into sugar is extremely slow. Lignin is bound to both
cellulose and hemicellulose by the α-ether linkages, acetal bonds, phenyl glycosidic
bonds, and ester bonds in the matrices (Ali et al. 2015).
There are many types of natural enzymes that can be utilized to depolymerize
starch into glucose and maltose such as α-amylase and glucoamylase. Endoenzyme
or α-amylase is used to hydrolyse α-1,4 glycosidic bond while exoenzyme or
glucoamylase is used for hydrolysis of both α-1,4 glycosidic bond and α-1,6
glycosidic bonds. Each type of enzymes may give a different rate of hydrolysis.
For example, rate of hydrolysis of α-1,4 glycosidic bond using glucoamylase is
20 times faster compared to hydrolysis of α-1,6 glycosidic bond. However, the rate
of reaction are also affected by other factors like pH of solution, starch granules
structure, temperature, and chemical composition of the starch itself (Murthy et al.
2011).
Commonly, the starch conversion is conducted at a first stage called liquefaction,
where starch is hydrolysed by α-amylase to form a shorter chain of dextrin. The
viscosity of starch will be reduced during this step. Then, in the second step, the
maltodextrins are further hydrolysed by glucoamylase in order to release glucose
together with a small amount of disaccharides or trisaccharides. In most practices,
glucose produced then can be used as a feedstock in fermentation for biofuel
productions and other applications (Husin et al. 2018).
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N. H. Alias et al.
are provided by this structure (Sjöström and Westermark 1999). The build-up of
plant cell wall is determined by the regular arrangement of microfibrils of cellulose
(Chen et al. 2007). Hemicellulose that linked together with hydrogen bonds covers
the adjacent spaces between the elementary fibrils (Ali et al. 2015; Saini et al. 2015).
Hemicellulose consists of different monosaccharide units compared to cellulose.
Hemicellulose commonly possesses arabinose (five carbon sugar), xylans (five
carbon sugar), and uronic acid, for instance, sugar acid (Jenol et al. 2014). They
are heteropolymers consisting of linear and branched structure arrangement of
pentoses, hexoses, and sugar acids (Peng et al. 2012). Amorphous morphology of
hemicellulose made it partly soluble in water (Demirbas 2008). In terms of molecular
weight, hemicellulose is lighter compared to cellulose and since it is composed of
short lateral chain, it is easy to hydrolyse. Several components that act as inhibitors
like furfurals and hydroxymethyl furfurals in the fermentation process might be
produced during the degradation of hemicellulose. Hence, hemicellulose should be
removed at least 50% in order to increase the degree of digestibility of cellulose.
Lignin is the most complex compound in lignocelluloses, which provides a
structural support to the plant. Lignin tied the long molecular chains of sugars of
cellulose and hemicellulose together to make sturdy and strong cell walls
(Hüttermann et al. 2001). The tight association between lignin and different
polysaccharides has conferred the mechanical strength to the cell wall. It also
protects the cellulose and prevents the fibre to swell when reacting with water and
acts as protective shield against microbial and enzymatic attacks. Therefore, the
conversion of the cellulose into sugar is extremely slow. Lignin is bound to both
cellulose and hemicellulose by the α-ether linkages, acetal bonds, phenyl glycosidic
bonds, and ester bonds in the matrices (Ali et al. 2015).
There are many types of natural enzymes that can be utilized to depolymerize
starch into glucose and maltose such as α-amylase and glucoamylase. Endoenzyme
or α-amylase is used to hydrolyse α-1,4 glycosidic bond while exoenzyme or
glucoamylase is used for hydrolysis of both α-1,4 glycosidic bond and α-1,6
glycosidic bonds. Each type of enzymes may give a different rate of hydrolysis.
For example, rate of hydrolysis of α-1,4 glycosidic bond using glucoamylase is
20 times faster compared to hydrolysis of α-1,6 glycosidic bond. However, the rate
of reaction are also affected by other factors like pH of solution, starch granules
structure, temperature, and chemical composition of the starch itself (Murthy et al.
2011).
Commonly, the starch conversion is conducted at a first stage called liquefaction,
where starch is hydrolysed by α-amylase to form a shorter chain of dextrin. The
viscosity of starch will be reduced during this step. Then, in the second step, the
maltodextrins are further hydrolysed by glucoamylase in order to release glucose
together with a small amount of disaccharides or trisaccharides. In most practices,
glucose produced then can be used as a feedstock in fermentation for biofuel
productions and other applications (Husin et al. 2018).
74
N. H. Alias et al.