and chemical composition as it plays an important role in product yield (Ibrahim
et al. 2017). Therefore, high lignin content is not preferred as it will take a long time
in pretreatment process.
In the first step, the substrate that has a high lignin content should undergo the
pretreatment. The pretreatment methods vary with the biomass used for biobutanol
production. The often used pretreatments include sulphuric acid pretreatment, alkaline peroxide pretreatment, steam explosion pretreatment, hydrothermal
pretreatment, and organic acid pretreatment. These pretreatments are needed to
degrade the structure of lignin that holds the cellulose and hemicellulose
components. After the pretreatment, the substrate can be easily hydrolysed into
fermentable sugar and subsequently into biobutanol. Pretreatment can be categorized
into three types which are physical, chemical, and biological.
Physical pretreatment commonly is conducted for the woody biomass in order to
reduce the woody size before chemical or biological processing. Woody biomass
typically is very energy-intensive as high energy consumptions are needed for
milling the wood chips into fibres and approximately 500 to 800 Wh/kg are used
(Schell and Harwood 1994; Zhu et al. 2009, 2010a). Therefore, in order to reduce
energy consumption and to ensure viable commercial cellulosic bioenergy production from woody biomass, post-chemical pretreatment size reduction approach has
been proposed (Zhu et al. 2010b).
Chemical pretreatment includes alkali, dilute acid, oxidizing agents, and organic
solvents. Chemical pretreatment has become the most promising method to remove
the lignin and/or hemicelluloses of biomass and subsequently decrease the degree of
polymerization and crystallinity of the cellulosic components (Behera et al. 2014).
Furthermore, some chemicals used do not produce toxic residues for the downstream
processes although those chemicals have a significant effect on the genuine structure
of lignocellulosic biomass when the process has been carried out at room temperature and pressure (Mtui 2009). However, concentrated acid is not favourable as it is
corrosive and not feasible for the economic pretreatment.
Biological pretreatment has provided more eco-friendly and economically viable
strategy for enhancement of enzymatic saccharification rate as this pretreatment uses
metabolite of a microorganism in nature for the biofuel production (Sindhu et al.
2016). Currently, pretreatment using rot fungi seems promising as it consumes less
energy and contributes less damage to the environment (Chen et al. 2010). In
addition, as the pretreatment is conducted at mild condition, the by-product produced during the pretreatment do not inhibit the subsequent hydrolysis.
4.4.2 Medium Formulation
An optimum medium composition is essential for a high biobutanol production
especially carbon and nitrogen. It was reported that the amount of carbon source
particularly sugar should not exceed more than 160 g/L. The cells growth is inhibited
if more than 80 g/L of sugar is supplied and the cells cannot grow in medium
containing more than 160 g/L of sugar (Monot et al. 1982). However, low sugars
76
N. H. Alias et al.
et al. 2017). Therefore, high lignin content is not preferred as it will take a long time
in pretreatment process.
In the first step, the substrate that has a high lignin content should undergo the
pretreatment. The pretreatment methods vary with the biomass used for biobutanol
production. The often used pretreatments include sulphuric acid pretreatment, alkaline peroxide pretreatment, steam explosion pretreatment, hydrothermal
pretreatment, and organic acid pretreatment. These pretreatments are needed to
degrade the structure of lignin that holds the cellulose and hemicellulose
components. After the pretreatment, the substrate can be easily hydrolysed into
fermentable sugar and subsequently into biobutanol. Pretreatment can be categorized
into three types which are physical, chemical, and biological.
Physical pretreatment commonly is conducted for the woody biomass in order to
reduce the woody size before chemical or biological processing. Woody biomass
typically is very energy-intensive as high energy consumptions are needed for
milling the wood chips into fibres and approximately 500 to 800 Wh/kg are used
(Schell and Harwood 1994; Zhu et al. 2009, 2010a). Therefore, in order to reduce
energy consumption and to ensure viable commercial cellulosic bioenergy production from woody biomass, post-chemical pretreatment size reduction approach has
been proposed (Zhu et al. 2010b).
Chemical pretreatment includes alkali, dilute acid, oxidizing agents, and organic
solvents. Chemical pretreatment has become the most promising method to remove
the lignin and/or hemicelluloses of biomass and subsequently decrease the degree of
polymerization and crystallinity of the cellulosic components (Behera et al. 2014).
Furthermore, some chemicals used do not produce toxic residues for the downstream
processes although those chemicals have a significant effect on the genuine structure
of lignocellulosic biomass when the process has been carried out at room temperature and pressure (Mtui 2009). However, concentrated acid is not favourable as it is
corrosive and not feasible for the economic pretreatment.
Biological pretreatment has provided more eco-friendly and economically viable
strategy for enhancement of enzymatic saccharification rate as this pretreatment uses
metabolite of a microorganism in nature for the biofuel production (Sindhu et al.
2016). Currently, pretreatment using rot fungi seems promising as it consumes less
energy and contributes less damage to the environment (Chen et al. 2010). In
addition, as the pretreatment is conducted at mild condition, the by-product produced during the pretreatment do not inhibit the subsequent hydrolysis.
4.4.2 Medium Formulation
An optimum medium composition is essential for a high biobutanol production
especially carbon and nitrogen. It was reported that the amount of carbon source
particularly sugar should not exceed more than 160 g/L. The cells growth is inhibited
if more than 80 g/L of sugar is supplied and the cells cannot grow in medium
containing more than 160 g/L of sugar (Monot et al. 1982). However, low sugars
76
N. H. Alias et al.