in cellulose content to 45.39% which is a twofold increase from the native SCT.
Maximum reducing sugar of 0.448 g/g was obtained after pretreatment at optimum
conditions like 2.5% (w/w) Triton X-100, 1.5% H 2 SO 4 concentration, and 30%
(w/w) of biomass loading [6].
5 Alkaline Pretreatment
Alkaline pretreatments are considered to be among the major chemical pretreatment
technology besides the acidic pretreatments. It can either use chemicals like sodium
hydroxide, calcium hydroxide, or ammonia as the reagent. Pretreatments including
sodium hydroxide have enhanced cellulose digestibility. Alkaline pretreatments
require lower temperatures and pressures, but the incubation time is recorded in
terms of hours or days. It results in the removal of lignin in the solid fraction which
can be recovered using appropriate recovery measures, and the solid fraction contains hemicelluloses and celluloses. The residual alkali obtained can be reused by the
chemical recovery process. This pretreatment is based on delignification process
which includes a high amount of hemicellulose being solubilized. The major aim is
to remove the lignin from the biomasses and thus improve the reactivity of the
polysaccharides present in it. Apart from it, this pretreatment also helps to swell the
cell wall, thus improving the cell wall accessibility for subsequent enzymatic
hydrolysis. The reaction mechanism supposed to take place is the solvation and
saponification of the intermolecular ester bonds that cross-link with the hemicellulose and lignin leading to the cleavage of the lignin carbohydrate complex and
therefore expose the cellulose microfibrils present in it [20]. Alkali helps in the
removal of acetyl groups and various uronic acid substitutes, and hence steric
hindrance of the enzymes in the hydrolysis process is reduced, thus increasing
their accessibility towards the carbohydrates. The formation of furfural and HMF
in the hydrolysates is found to be lower when compared to the dilute acid
pretreatment. The degree of polymerization of the cellulose is decreased and hence
causes swelling of cellulose leads to increase in its internal surface area [13].
Alkali pretreatment on SCT with 15% w/w biomass loading and 3% NaOH and
incubation time of 60 min yielded 0.684 g of reducing sugar after the enzymatic
hydrolysis. The compositional analysis proved that the amount of cellulose was
almost intact during the pretreatment, but substantial amount of lignin (89.80%) and
hemicellulose (46%) was found to be removed. SEM images revealed that the
pretreated sample had distorted structure and an increase in the surface area in
SCT thus improving the hydrolysis efficiency when compared to the native SCT
which had a compact rigid structure. X-ray spectrum of native and pretreated SCT
showed that the crystallinity index had increased in the preheated sample being
67.4% when compared to the native sample of 37.4% thereby influencing the
enzymatic hydrolysis process [11].
Another study on SCT showed that alkaline followed by acid pretreatment
yielded the maximum reducing sugar after enzymatic hydrolysis. It was observed
Lignocellulosic Sugarcane Tops for Bioethanol Production: An Overview
95
Maximum reducing sugar of 0.448 g/g was obtained after pretreatment at optimum
conditions like 2.5% (w/w) Triton X-100, 1.5% H 2 SO 4 concentration, and 30%
(w/w) of biomass loading [6].
5 Alkaline Pretreatment
Alkaline pretreatments are considered to be among the major chemical pretreatment
technology besides the acidic pretreatments. It can either use chemicals like sodium
hydroxide, calcium hydroxide, or ammonia as the reagent. Pretreatments including
sodium hydroxide have enhanced cellulose digestibility. Alkaline pretreatments
require lower temperatures and pressures, but the incubation time is recorded in
terms of hours or days. It results in the removal of lignin in the solid fraction which
can be recovered using appropriate recovery measures, and the solid fraction contains hemicelluloses and celluloses. The residual alkali obtained can be reused by the
chemical recovery process. This pretreatment is based on delignification process
which includes a high amount of hemicellulose being solubilized. The major aim is
to remove the lignin from the biomasses and thus improve the reactivity of the
polysaccharides present in it. Apart from it, this pretreatment also helps to swell the
cell wall, thus improving the cell wall accessibility for subsequent enzymatic
hydrolysis. The reaction mechanism supposed to take place is the solvation and
saponification of the intermolecular ester bonds that cross-link with the hemicellulose and lignin leading to the cleavage of the lignin carbohydrate complex and
therefore expose the cellulose microfibrils present in it [20]. Alkali helps in the
removal of acetyl groups and various uronic acid substitutes, and hence steric
hindrance of the enzymes in the hydrolysis process is reduced, thus increasing
their accessibility towards the carbohydrates. The formation of furfural and HMF
in the hydrolysates is found to be lower when compared to the dilute acid
pretreatment. The degree of polymerization of the cellulose is decreased and hence
causes swelling of cellulose leads to increase in its internal surface area [13].
Alkali pretreatment on SCT with 15% w/w biomass loading and 3% NaOH and
incubation time of 60 min yielded 0.684 g of reducing sugar after the enzymatic
hydrolysis. The compositional analysis proved that the amount of cellulose was
almost intact during the pretreatment, but substantial amount of lignin (89.80%) and
hemicellulose (46%) was found to be removed. SEM images revealed that the
pretreated sample had distorted structure and an increase in the surface area in
SCT thus improving the hydrolysis efficiency when compared to the native SCT
which had a compact rigid structure. X-ray spectrum of native and pretreated SCT
showed that the crystallinity index had increased in the preheated sample being
67.4% when compared to the native sample of 37.4% thereby influencing the
enzymatic hydrolysis process [11].
Another study on SCT showed that alkaline followed by acid pretreatment
yielded the maximum reducing sugar after enzymatic hydrolysis. It was observed
Lignocellulosic Sugarcane Tops for Bioethanol Production: An Overview
95