pretreated with microwave showed higher hydrolysis percentages and release of total organic carbons into solution.
Solubilization increases significantly with the increase in
temperature. During microwave pretreatment lignocellulosic, biomass is solubilized using acid and alkali treatments
in conjugation with enzymatic hydrolysis. Hence, it is the
most effective method to alter cellulose structure (Xiong
et al. 2000), coupled with lignin and hemicelluloses degradation and thereby increases enzymatic vulnerability (Lu
et al. 2011). Sugar yield after microwave pretreatment can be
enhanced along with some chemicals (Segneanu et al. 2011).
(c) Extrusion
It is one of the most extensively utilized physical pretreatment processes where the materials are passed through a die
of the desired cross-section having huge potential for lignocellulosic materials for biogas production. In a study
conducted by Perez-Rodriguez et al. (Pérez-Rodríguez et al.
2018), a twin-screw extruder was applied for pretreatment of
vine-trimming shoots for the production of methane via
anaerobic digestion. An increment of 15–21% biogas, 50%
reduction in hemicellulose content and increase in soluble
chemicals portion (lipids, carbohydrates, minerals, proteins,
and vitamins) was observed for pretreated samples in comparison to untreated feedstocks. It was also reported that
extrusion induced 50% reduction in hemicellulose content
while increased soluble such as. The rapid conversion ability
of these soluble components by methanogenic microorganisms causes higher methane yield and also establishes that
the process efficiency of ball milling is minute (Jędrzejczyk
et al. 2019).
(d) Ultrasonication
Cellulose’s chemical reactivity and accessibility are
increased by many folds due to ultrasound pretreatment as it
can penetrate the crystalline regions of cellulose and
decompose lignin molecules but gets limited for fine structure of cellulose. The negative impact of fiber-to-surface area
ratio on enzymatic hydrolysis is reduced by ultrasonic
decomposition of hemicellulose. According to some studies
cellulose, saccharification can be improved via ultrasonic
pretreatment of biomass (Bosma et al. 2003; Yachmenev
et al. 2009; Sun and Tomkinson 2002). Variability in the
structure of raw material and its influence on the rate of
saccharification both pre- and post-ultrasonic pretreatment
has been well documented by Zhang et al. (2008). In their
investigation, they suggested that the vibration energy of
ultrasound is very low to induce any conformational change
at surface. Nevertheless, the hydrogen bond among the
molecules of lignocellulosic materials can be broken and
reduces its crystallinity by employing ultrasound-assisted
alkali pretreatment. Thus, it subsequently increases the rate
of lignin degradation and enzymatic saccharification. Furthermore, the sagging of cavitational bubbles generates a
mechanical impact, which creates an environment for
enzymatic action on substrates (Jędrzejczyk et al. 2019).
3.2 Physicochemical Pretreatments
(a) Steam Explosion/Hydrothermal
It is basically a thermochemical pretreatment method where
steam is employed to disintegrate lignocellulosic material
with zero to minimum utilization of chemical (Chornet and
Overend 1988; Kaar et al. 1998). To promote hemicelluloses
hydrolysis high temperature (between 160 and 240 °C) and
pressure (0.7 and 4.8 MPa) is maintained within the reactor
containing mixture of biomass and steam, which is followed
by decompression (Agbor et al. 2011). This pretreatment
method is responsible for causing elevated solubility of the
hemicellulose (chiefly oligosaccharides). Fermentable sugar
quantity can be enhanced by using steam explosion process
coupled with enzymatic saccharification. The major drawbacks of this pretreatment process are incomplete degradation of hemicellulose and creation of toxic compounds,
which can diminish the efficiency of fermentation process.
Phenolic and aromatics compounds along with aldehydes,
aliphatic acids, bioaclohols, ions and other fermentation
products may act as inhibitors.
(b) Ammonia Fiber Explosion (AFEX)
Biomass is mixed with liquid ammonia under modest temperature (70–200 °C) and pressure (0.7–2.8 MPa) followed
by a rapid release in pressure during ammonia fiber explosion method. Biomass fibers are ruptured due to sudden
release of pressure and cause disintegration of lignin,
hemicellulose, and cellulose polymers into finer constituents
which in turn enlarge the pore number and size within the
cell wall. The increased water holding capacity and accessibility toward enzymatic degradation due to structural
change in the material leads to higher sugar recovery during
AFEX pretreatment (Kumar et al. 2009). Some of the major
advantages are: efficient lignin elimination, creation of fewer
inhibitors, and high carbohydrates recovery.
(c) Carbon dioxide (CO 2 ) Explosion
The basic principle employed in CO 2 explosion method is
that the hydrolysis rate of the material will be accelerated
due to the formation of carbonic acid induced by CO 2 (Sun
Bioconversion of Food Waste into Biogas
85
Solubilization increases significantly with the increase in
temperature. During microwave pretreatment lignocellulosic, biomass is solubilized using acid and alkali treatments
in conjugation with enzymatic hydrolysis. Hence, it is the
most effective method to alter cellulose structure (Xiong
et al. 2000), coupled with lignin and hemicelluloses degradation and thereby increases enzymatic vulnerability (Lu
et al. 2011). Sugar yield after microwave pretreatment can be
enhanced along with some chemicals (Segneanu et al. 2011).
(c) Extrusion
It is one of the most extensively utilized physical pretreatment processes where the materials are passed through a die
of the desired cross-section having huge potential for lignocellulosic materials for biogas production. In a study
conducted by Perez-Rodriguez et al. (Pérez-Rodríguez et al.
2018), a twin-screw extruder was applied for pretreatment of
vine-trimming shoots for the production of methane via
anaerobic digestion. An increment of 15–21% biogas, 50%
reduction in hemicellulose content and increase in soluble
chemicals portion (lipids, carbohydrates, minerals, proteins,
and vitamins) was observed for pretreated samples in comparison to untreated feedstocks. It was also reported that
extrusion induced 50% reduction in hemicellulose content
while increased soluble such as. The rapid conversion ability
of these soluble components by methanogenic microorganisms causes higher methane yield and also establishes that
the process efficiency of ball milling is minute (Jędrzejczyk
et al. 2019).
(d) Ultrasonication
Cellulose’s chemical reactivity and accessibility are
increased by many folds due to ultrasound pretreatment as it
can penetrate the crystalline regions of cellulose and
decompose lignin molecules but gets limited for fine structure of cellulose. The negative impact of fiber-to-surface area
ratio on enzymatic hydrolysis is reduced by ultrasonic
decomposition of hemicellulose. According to some studies
cellulose, saccharification can be improved via ultrasonic
pretreatment of biomass (Bosma et al. 2003; Yachmenev
et al. 2009; Sun and Tomkinson 2002). Variability in the
structure of raw material and its influence on the rate of
saccharification both pre- and post-ultrasonic pretreatment
has been well documented by Zhang et al. (2008). In their
investigation, they suggested that the vibration energy of
ultrasound is very low to induce any conformational change
at surface. Nevertheless, the hydrogen bond among the
molecules of lignocellulosic materials can be broken and
reduces its crystallinity by employing ultrasound-assisted
alkali pretreatment. Thus, it subsequently increases the rate
of lignin degradation and enzymatic saccharification. Furthermore, the sagging of cavitational bubbles generates a
mechanical impact, which creates an environment for
enzymatic action on substrates (Jędrzejczyk et al. 2019).
3.2 Physicochemical Pretreatments
(a) Steam Explosion/Hydrothermal
It is basically a thermochemical pretreatment method where
steam is employed to disintegrate lignocellulosic material
with zero to minimum utilization of chemical (Chornet and
Overend 1988; Kaar et al. 1998). To promote hemicelluloses
hydrolysis high temperature (between 160 and 240 °C) and
pressure (0.7 and 4.8 MPa) is maintained within the reactor
containing mixture of biomass and steam, which is followed
by decompression (Agbor et al. 2011). This pretreatment
method is responsible for causing elevated solubility of the
hemicellulose (chiefly oligosaccharides). Fermentable sugar
quantity can be enhanced by using steam explosion process
coupled with enzymatic saccharification. The major drawbacks of this pretreatment process are incomplete degradation of hemicellulose and creation of toxic compounds,
which can diminish the efficiency of fermentation process.
Phenolic and aromatics compounds along with aldehydes,
aliphatic acids, bioaclohols, ions and other fermentation
products may act as inhibitors.
(b) Ammonia Fiber Explosion (AFEX)
Biomass is mixed with liquid ammonia under modest temperature (70–200 °C) and pressure (0.7–2.8 MPa) followed
by a rapid release in pressure during ammonia fiber explosion method. Biomass fibers are ruptured due to sudden
release of pressure and cause disintegration of lignin,
hemicellulose, and cellulose polymers into finer constituents
which in turn enlarge the pore number and size within the
cell wall. The increased water holding capacity and accessibility toward enzymatic degradation due to structural
change in the material leads to higher sugar recovery during
AFEX pretreatment (Kumar et al. 2009). Some of the major
advantages are: efficient lignin elimination, creation of fewer
inhibitors, and high carbohydrates recovery.
(c) Carbon dioxide (CO 2 ) Explosion
The basic principle employed in CO 2 explosion method is
that the hydrolysis rate of the material will be accelerated
due to the formation of carbonic acid induced by CO 2 (Sun
Bioconversion of Food Waste into Biogas
85