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Phytotechnology with Biomass Production
proportion of hemicellulose and some of cellulose are hydrolyzed into slurries during pretreatment, and sugar degradation compounds such as furfural
and hydroxymethylfurfural (HMF) as well as aromatic lignin degradation
compounds can be generated (Mosier et al., 2005; Zhao et al., 2020a). Dilute
acid pretreatment is commonly performed with high temperature (>150°C)
and short time (<1 hour). Moreover, ethanol yield depends on pretreatment
conditions applied (acid dose, reaction time, and temperature) (Ji et al., 2015).
Alkali pretreatment can efficiently cleave and decompose the chemical crosslinks (ether and ester bonds) between carbohydrates and lignin, resulting
in a structural alteration of lignin and elimination of hemicellulose (Zhao
et al., 2020b). The solubilization of disrupted lignin and hemicellulose renders Miscanthus biomass more amenable to enzymes due to the increment of
cellulosic accessibility. Furthermore, alkali pretreatment is conducted under
relatively low temperatures but long residence times, followed by multiplewashing for removing small lignin units and other inhibitors.
LHW and steam explosion as the category of hydrothermal pretreatments
have attracted considerable attention for pretreatment of lignocellulosic biomass and solubilization of amorphous hemicellulose. During pretreatment,
water can be autoionized into acidic hydronium ions that cleave the glycosidic bonds of hemicellulose, resulting in the formation of acetic acid, which
in turn catalyzes breaking cellulose and hemicellulose into oligosaccharides
and monomeric sugars (glucose and xylose) (Mosier et al., 2005). Thus, harsh
pretreatment temperatures (180°C–230°C) induced further degradation and
decomposition of monosaccharides into inhibitors (e.g., furfural and HMF)
(Li et al., 2013). The formation of inhibitors in hydrolysates can cause sugar
loss and inhibit subsequent enzymatic hydrolysis and fermentation. Thus, the
detoxification process is commonly needed. Also, since the cellulose and lignin
are more robust than hemicellulose, they are amenable for recovery. Therefore,
hydrophobic interaction between residual lignin and cellulose during enzymatic saccharification is inevitable if there is no surfactant addition.
Organosolv and ionic liquids as green solvents offer the advantage of
clean fractionation of lignocellulosic biomass into individual components
with high purity (Brosse et al., 2009; Dash & Mohanty, 2019; Kim et al., 2018).
Organosolv allows for the efficient fractionation of starting biomass into a
solid residue rich in cellulose and a liquid fraction containing organosolv
and water-soluble lignin and hemicellulose (Brosse et al., 2009). Ionic liquids
owing hydrogen bond acceptor with high polarity can dissolve Miscanthus
biomass, and ionic liquids having acetate, chloride, and phosphate anions
show desirable solubility properties (Padmanabhan et al., 2011). However,
excessive reagents are consumed for washing pretreated biomass to avoid
lignin recondensation. Besides, the sealed condition required for organosolv
and ionic liquid recoveries increases production costs, limiting their feasibility in commercialization.
To compensate for the drawbacks of a single pretreatment, the physical
and chemical combinations such as microwave-assisted with acid and alkali