removal, physical factors like pressure were included in
chemical methods such as alkaline pretreatment. Several
physicochemical bioconversion technologies like a steam
explosion, ionic liquid, oxidation, CO 2 explosion, aqueous
ammonia pretreatment, organosolvent, and liquid hot water
pretreatment have been carried out by topical researchers.
The cellulose solvent‐based fractionation and ionic liquids
and of lignocellulose have been recommended recently.
These physiochemical bioconversion technologies depend
upon conditions for processing and usage of solvents, which
affect the physiochemical possessions of lignocellulosic
biomass (Rabemanolontsoa and Saka 2016).
Organosolvent
Organosolvent pretreatment of biomass uses many organic
or aqueous organic solvent combinations for solubilization
of hemicellulose and extraction of lignin. Organic solvents
like acetone, ethanol, ethylene glycol, methanol, tetrahydrofurfuryl alcohol, tri-ethylene glycol are utilized commonly in this method, although organic acids include
acetylsalicylic, oxalic, and salicylic are used as a catalyst in
organosolvent method (Kumar et al. 2009). The organosolvent lignocellulosic pretreatment via alcohol usage leads to
internal bonds hydrolysis of hemicellulose and lignin, as
well as hydrolysis of ester and ether inter-polymer bonds
among them. These outcomes come in the form of lignin
removal and nearly whole hemicellulose solubilization. In
organosolvent method, the existence of organic acids results
in the formation of hydrogen ions that facilitate biomass
delignification and lignin dissolution. The optimum temperature 100 °C–250 °C required for this process but
depends on biomass nature although catalyst usage permits
the procedure to efficiently run at low temperatures. In
reaction mixture when inorganic acid is added, it causes
hemicellulose hydrolysis that significantly enhances cellulose availability (Jedrzejczyk 2019).
Ionic Liquid
The ionic liquid method is a new comparative technology,
which provides the lignocellulosic deconstruction with salts
having low melting points. Ionic liquid usage as waste processing solvents began with the forming of cellulose that
dissolves ionic liquid and then followed by usage of already
known ionic liquid as an alternative solvent for synthetic
cellulose fibers rotation, i.e., ion cell-F method. Two different
strategies for the ionic liquid pretreatment of lignocellulosic
biomass are being developed: The first tactic is a breakdown
of the lignocellulose structure including crystalline cellulose
(dissolution pretreatment), which came directly from the
dissolution of cellulose via ionic liquid. The second, more
current tactic uses ionic liquid to replace lignocellulose by
dissolving hemicellulose and lignin but leaving the cellulose
residue as a filterable solid. This method is analogous to
organosolvent treating but takes place at atmospheric pressure
(Lopes 2017; Elgharbawy et al. 2016).
Deep Eutectic Solvent
Deep eutectic solvent (DES) is a mixture of two or more
than two components in which act as a hydrogen bond
acceptor (HBA) and the other acts as a hydrogen bond donor
(HBD). It comprises those compounds having a low melting
point as compared to distinct components. Deep eutectic
solvent (DES) application for biomass pretreatment is an
alternative to conventional ionic liquids, because of their
lesser charges. As compared to ionic liquids, the deep
eutectic solvent biosynthesis is easy and it can be obtained
from broadly accessible and low-priced components (i.e.,
quaternary salt of ammonium and metallic chloride) (Loow
et al. 2018). Three kinds of deep eutectic solvents differ in
hydrogen bond donors. The choline chloride urea, choline
chloride citric acid, and choline chloride glycerol were
produced and utilized for the sago waste pretreatment from
enzyme hydrolysis into simple sugars (Wan and Mun 2018).
Oxidative Pretreatment
Oxidative pretreatment utilized oxidizing agents like oxygen, ozone, air, or hydrogen peroxide for lignocellulose
biomass treatment. These methods involve lignin removal
from the structure of biomass and enhance cellulose accessibility. Unluckily, oxidation of biomass is not a careful
process. The lignin removal is often done with the loss of
cellulose and hemicellulose. The process of delignification
shows its efficiency by aromatic rings oxidation existing
oxidizing agents into carboxylic acids. Time and oxidizing
agent concentration can be affected by the oxidation method
(Uzuner et al. 2018; Ayeni and Daramola 2017).
Steam Explosion
Steam explosion is a physicochemical method of lignocelluloses biomass pretreatment using high-pressure steam
(saturated) which is quickly dropped causing volatile
decompression. The conditions required while performing
steam explosion method are 160 °C–240 °C temperature
and 0.7–4.8 MPa pressure (Agbor et al. 2011). Steam
explosion aims to improve the cellulose accessibility and
solubilization of hemicellulose by evading the formation of
the inhibitor for enzymatic processing. The steam explosion
method causes hemicellulose partial hydrolysis with the
release of acetic acid. To a small extent, lignin is also
removed but again melting repolymerization and depolymerization cause its reorganization on the exterior of fiber
(Kumar and Sharma 2017; Kumar et al. 2009).
Liquid Hot Water
Liquid hot water is also called compressed hot water and shows
similarity with the steam explosion method although its name
Bioconversion of Agro-Industrial Waste into Value-Added Compounds
355
chemical methods such as alkaline pretreatment. Several
physicochemical bioconversion technologies like a steam
explosion, ionic liquid, oxidation, CO 2 explosion, aqueous
ammonia pretreatment, organosolvent, and liquid hot water
pretreatment have been carried out by topical researchers.
The cellulose solvent‐based fractionation and ionic liquids
and of lignocellulose have been recommended recently.
These physiochemical bioconversion technologies depend
upon conditions for processing and usage of solvents, which
affect the physiochemical possessions of lignocellulosic
biomass (Rabemanolontsoa and Saka 2016).
Organosolvent
Organosolvent pretreatment of biomass uses many organic
or aqueous organic solvent combinations for solubilization
of hemicellulose and extraction of lignin. Organic solvents
like acetone, ethanol, ethylene glycol, methanol, tetrahydrofurfuryl alcohol, tri-ethylene glycol are utilized commonly in this method, although organic acids include
acetylsalicylic, oxalic, and salicylic are used as a catalyst in
organosolvent method (Kumar et al. 2009). The organosolvent lignocellulosic pretreatment via alcohol usage leads to
internal bonds hydrolysis of hemicellulose and lignin, as
well as hydrolysis of ester and ether inter-polymer bonds
among them. These outcomes come in the form of lignin
removal and nearly whole hemicellulose solubilization. In
organosolvent method, the existence of organic acids results
in the formation of hydrogen ions that facilitate biomass
delignification and lignin dissolution. The optimum temperature 100 °C–250 °C required for this process but
depends on biomass nature although catalyst usage permits
the procedure to efficiently run at low temperatures. In
reaction mixture when inorganic acid is added, it causes
hemicellulose hydrolysis that significantly enhances cellulose availability (Jedrzejczyk 2019).
Ionic Liquid
The ionic liquid method is a new comparative technology,
which provides the lignocellulosic deconstruction with salts
having low melting points. Ionic liquid usage as waste processing solvents began with the forming of cellulose that
dissolves ionic liquid and then followed by usage of already
known ionic liquid as an alternative solvent for synthetic
cellulose fibers rotation, i.e., ion cell-F method. Two different
strategies for the ionic liquid pretreatment of lignocellulosic
biomass are being developed: The first tactic is a breakdown
of the lignocellulose structure including crystalline cellulose
(dissolution pretreatment), which came directly from the
dissolution of cellulose via ionic liquid. The second, more
current tactic uses ionic liquid to replace lignocellulose by
dissolving hemicellulose and lignin but leaving the cellulose
residue as a filterable solid. This method is analogous to
organosolvent treating but takes place at atmospheric pressure
(Lopes 2017; Elgharbawy et al. 2016).
Deep Eutectic Solvent
Deep eutectic solvent (DES) is a mixture of two or more
than two components in which act as a hydrogen bond
acceptor (HBA) and the other acts as a hydrogen bond donor
(HBD). It comprises those compounds having a low melting
point as compared to distinct components. Deep eutectic
solvent (DES) application for biomass pretreatment is an
alternative to conventional ionic liquids, because of their
lesser charges. As compared to ionic liquids, the deep
eutectic solvent biosynthesis is easy and it can be obtained
from broadly accessible and low-priced components (i.e.,
quaternary salt of ammonium and metallic chloride) (Loow
et al. 2018). Three kinds of deep eutectic solvents differ in
hydrogen bond donors. The choline chloride urea, choline
chloride citric acid, and choline chloride glycerol were
produced and utilized for the sago waste pretreatment from
enzyme hydrolysis into simple sugars (Wan and Mun 2018).
Oxidative Pretreatment
Oxidative pretreatment utilized oxidizing agents like oxygen, ozone, air, or hydrogen peroxide for lignocellulose
biomass treatment. These methods involve lignin removal
from the structure of biomass and enhance cellulose accessibility. Unluckily, oxidation of biomass is not a careful
process. The lignin removal is often done with the loss of
cellulose and hemicellulose. The process of delignification
shows its efficiency by aromatic rings oxidation existing
oxidizing agents into carboxylic acids. Time and oxidizing
agent concentration can be affected by the oxidation method
(Uzuner et al. 2018; Ayeni and Daramola 2017).
Steam Explosion
Steam explosion is a physicochemical method of lignocelluloses biomass pretreatment using high-pressure steam
(saturated) which is quickly dropped causing volatile
decompression. The conditions required while performing
steam explosion method are 160 °C–240 °C temperature
and 0.7–4.8 MPa pressure (Agbor et al. 2011). Steam
explosion aims to improve the cellulose accessibility and
solubilization of hemicellulose by evading the formation of
the inhibitor for enzymatic processing. The steam explosion
method causes hemicellulose partial hydrolysis with the
release of acetic acid. To a small extent, lignin is also
removed but again melting repolymerization and depolymerization cause its reorganization on the exterior of fiber
(Kumar and Sharma 2017; Kumar et al. 2009).
Liquid Hot Water
Liquid hot water is also called compressed hot water and shows
similarity with the steam explosion method although its name
Bioconversion of Agro-Industrial Waste into Value-Added Compounds
355