of lignin or hemicelluloses and also these catalysts help in optimizing the simple
carbohydrates from the complex ones (celluloses and lignin) (Sun et al. 2014b). If
diluted alkali is used as a catalyst in this mode of pretreatment, then it is observed
that in sugarcane bagasse the contents of lignin were reduced, and the yield of
glucose plus the hydrolysis rates were increased (Miura et al. 2012). Sulfuric acid
was also used as a catalyst for hydrothermal pretreatment in LG biomass to make
their enzymatic hydrolysis easy (Lu et al. 2009).
• Steam explosion: For the pretreatment of lignocellulosic biomass, researchers
commonly use the strategy of steam explosion. This pretreatment is operated by
exposing the chipped LC material to higher pressure with steam at elevated heat
(for multiple seconds to minute), after that pressure is dropped to 760 torr. Due to
the sudden decrease of high pressure, most of the LC materials are converted into
fibers. Also, lignin and hemicellulose get disrupted and can be eliminated from
the mixture (Pan et al. 2005). This process has many benefits as environmentally
friendly, energetically efficient, and lesser hazardous chemicals production
(Alvira et al. 2010).
Steam explosion is applied to LC materials without adding any other
chemicals. When wheat straw was treated with this method, at 170–220
C, the
results were observed to check the hydrolysis behaviors (Horn et al. 2011). The
observation was that when the temperature was kept at 210
C then the hydrolysis
products obtained were at the maximum. Similarly, glucose yields were also
produced at the same percentages when the conditions were harsh, but the
inhibitors of aromatic nature were produced which affected the fermentation
process. So the removal of inhibitor is necessary. Simple wash with water is
also effective for steam pretreated LC biomasses having inhibitors in the solution.
As many inhibitors are produced in this pretreatment process for the next stages
when the conditions of the method are severe, so some impregnating reagents are
used for improved separation of hemicelluloses from the mixture at milder
conditions. Those reagents may include sulfuric acid and sulfur dioxide. Boussaid
et al. (2000) studied the changes in the chemical composition and the behavior of
enzymatic hydrolysis when sulfur dioxide was impregnated on LC material,
under different conditions. With severe conditions of steam explosion method,
the digestibility of material to enzymes was enhanced but the negative side was
that severe conditions resulted in lesser production of simple sugar of hemicelluloses. However, the mild conditions for steam explosion pretreatment
enhanced both glucose levels and enzymatic digestibility for the later stages
(Boussaid et al. 2000). In a study by Martín et al. (2002), the fermentation
abilities and effects of enzymes on hydrolysis were compared at 25
C temperature for a time duration of 10 min, with sulfuric acid and sulfur dioxide as
impregnated agents in sugarcane bagasse LC material. With sulfuric acid as, the
yield of glucose was higher but that of total sugars was lower, on the other hand,
with sulfur dioxide the yields of xylose and total sugar were higher. Sulfuric acid
had played its role in the inhibition of inhibitors, enzymatic hydrolysis catalysis,
and polysaccharides hydrolysis (De Bari et al. 2007; Varga et al. 2004).
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FaizaKausar et al.
carbohydrates from the complex ones (celluloses and lignin) (Sun et al. 2014b). If
diluted alkali is used as a catalyst in this mode of pretreatment, then it is observed
that in sugarcane bagasse the contents of lignin were reduced, and the yield of
glucose plus the hydrolysis rates were increased (Miura et al. 2012). Sulfuric acid
was also used as a catalyst for hydrothermal pretreatment in LG biomass to make
their enzymatic hydrolysis easy (Lu et al. 2009).
• Steam explosion: For the pretreatment of lignocellulosic biomass, researchers
commonly use the strategy of steam explosion. This pretreatment is operated by
exposing the chipped LC material to higher pressure with steam at elevated heat
(for multiple seconds to minute), after that pressure is dropped to 760 torr. Due to
the sudden decrease of high pressure, most of the LC materials are converted into
fibers. Also, lignin and hemicellulose get disrupted and can be eliminated from
the mixture (Pan et al. 2005). This process has many benefits as environmentally
friendly, energetically efficient, and lesser hazardous chemicals production
(Alvira et al. 2010).
Steam explosion is applied to LC materials without adding any other
chemicals. When wheat straw was treated with this method, at 170–220
C, the
results were observed to check the hydrolysis behaviors (Horn et al. 2011). The
observation was that when the temperature was kept at 210
C then the hydrolysis
products obtained were at the maximum. Similarly, glucose yields were also
produced at the same percentages when the conditions were harsh, but the
inhibitors of aromatic nature were produced which affected the fermentation
process. So the removal of inhibitor is necessary. Simple wash with water is
also effective for steam pretreated LC biomasses having inhibitors in the solution.
As many inhibitors are produced in this pretreatment process for the next stages
when the conditions of the method are severe, so some impregnating reagents are
used for improved separation of hemicelluloses from the mixture at milder
conditions. Those reagents may include sulfuric acid and sulfur dioxide. Boussaid
et al. (2000) studied the changes in the chemical composition and the behavior of
enzymatic hydrolysis when sulfur dioxide was impregnated on LC material,
under different conditions. With severe conditions of steam explosion method,
the digestibility of material to enzymes was enhanced but the negative side was
that severe conditions resulted in lesser production of simple sugar of hemicelluloses. However, the mild conditions for steam explosion pretreatment
enhanced both glucose levels and enzymatic digestibility for the later stages
(Boussaid et al. 2000). In a study by Martín et al. (2002), the fermentation
abilities and effects of enzymes on hydrolysis were compared at 25
C temperature for a time duration of 10 min, with sulfuric acid and sulfur dioxide as
impregnated agents in sugarcane bagasse LC material. With sulfuric acid as, the
yield of glucose was higher but that of total sugars was lower, on the other hand,
with sulfur dioxide the yields of xylose and total sugar were higher. Sulfuric acid
had played its role in the inhibition of inhibitors, enzymatic hydrolysis catalysis,
and polysaccharides hydrolysis (De Bari et al. 2007; Varga et al. 2004).
130
FaizaKausar et al.