7.5 Physico-chemical Methods
201
to prevent the evaporation of water during the process [18]. The pH of the reaction
plays an important role in LHW process and should always be controlled between
4 and 7. More acidic or basic conditions would result in the degradation of sugars
and formation of inhibitors [18, 20]. The advantages of this method include low
temperature requirements, minimum formation of inhibitory compounds, low cost
of the solvent and no need for chemicals; however, due to the large amounts of water,
the process is energy intensive. It also leads to the formation of products at low
concentrations due to dilution in water [18, 20, 27].
7.6 Biological Methods
Compared to the chemical and physical pre-treatment methods, biological treatments
are environmentally friendly, efficient, low cost and low-energy processes. These
methods are used to treat lignocellulosic biomass prior to the enzymatic saccharification, in which cellulose is converted into oligomers and sugar monomers by
cellulase enzymes [17, 18, 27]. Biological pre-treatment involves the use of lignindegrading bacteria or fungi, as whole-cell or enzymes for the deconstruction of the
lignin structure in cell wall. The common lignin-degrading enzymes include laccases,
lignin peroxidase, manganese peroxidase, and versatile peroxidase [18, 21].
Fungi are known as microorganisms that produce plant cell-wall-degrading
enzymes such as cellulose-, hemicellulose, and lignin-degrading hydrolyzing
enzymes. Other types of microorganisms such as Actinomycetes have been reported
to be effective for grasses pre-treatment [21]. Different types of rot-fungi such as
brown-, white- and soft-rot fungi were used to degrade lignin and hemicellulose.
Brown rots mainly disrupt the cellulose component, while white and soft rots can
disrupt both cellulose and lignin components of biomass [17]. Since white-rot fungi
are capable of producing high sugar yield during enzymatic saccharification, they
are more commonly used for biological treatments [18]. Degradation of lignin by
white-rot fungi occurs owing to the generation of lignin-degrading enzymes such as
peroxidases and laccases [17, 27].
The advantages of biological pre-treatment are low energy requirements and mild
operating conditions. However, they have the drawbacks of very low hydrolysis rate,
low efficiency, extremely precise growth conditions, large space requirement and
long residence time of 10–14 days, which hamper their industrial-scale application
[17, 18, 21, 27].
201
to prevent the evaporation of water during the process [18]. The pH of the reaction
plays an important role in LHW process and should always be controlled between
4 and 7. More acidic or basic conditions would result in the degradation of sugars
and formation of inhibitors [18, 20]. The advantages of this method include low
temperature requirements, minimum formation of inhibitory compounds, low cost
of the solvent and no need for chemicals; however, due to the large amounts of water,
the process is energy intensive. It also leads to the formation of products at low
concentrations due to dilution in water [18, 20, 27].
7.6 Biological Methods
Compared to the chemical and physical pre-treatment methods, biological treatments
are environmentally friendly, efficient, low cost and low-energy processes. These
methods are used to treat lignocellulosic biomass prior to the enzymatic saccharification, in which cellulose is converted into oligomers and sugar monomers by
cellulase enzymes [17, 18, 27]. Biological pre-treatment involves the use of lignindegrading bacteria or fungi, as whole-cell or enzymes for the deconstruction of the
lignin structure in cell wall. The common lignin-degrading enzymes include laccases,
lignin peroxidase, manganese peroxidase, and versatile peroxidase [18, 21].
Fungi are known as microorganisms that produce plant cell-wall-degrading
enzymes such as cellulose-, hemicellulose, and lignin-degrading hydrolyzing
enzymes. Other types of microorganisms such as Actinomycetes have been reported
to be effective for grasses pre-treatment [21]. Different types of rot-fungi such as
brown-, white- and soft-rot fungi were used to degrade lignin and hemicellulose.
Brown rots mainly disrupt the cellulose component, while white and soft rots can
disrupt both cellulose and lignin components of biomass [17]. Since white-rot fungi
are capable of producing high sugar yield during enzymatic saccharification, they
are more commonly used for biological treatments [18]. Degradation of lignin by
white-rot fungi occurs owing to the generation of lignin-degrading enzymes such as
peroxidases and laccases [17, 27].
The advantages of biological pre-treatment are low energy requirements and mild
operating conditions. However, they have the drawbacks of very low hydrolysis rate,
low efficiency, extremely precise growth conditions, large space requirement and
long residence time of 10–14 days, which hamper their industrial-scale application
[17, 18, 21, 27].