the sugars released by enzymes were consumed immediately by
the yeast and converted into ethanol. Thus, the feedback inhibition
on enzymes was prevented, which could be a probable reason for
higher ethanol yield (54.4%) during SSF. Additionally, in this study,
enzyme was added in two steps, which might have also been
responsible for increased hydrolysis of wheat straw resulting in
higher ethanol concentration during SSF in comparison to SHF
process.
A comparison of SSF process for ethanol production from lignocellulosic
biomass from available literature has been presented in
Table 3. SSF results in present study were better as higher ethanol
yield with lower enzyme dosage were obtained in comparison to
the report of Barron et al. (1995) who performed SSF of milled filter
paper using 25 FPU/g cellulose and obtained ethanol yield of 39%.
Presently, in-house produced cellulases was more efficient during
SSF process for ethanol production. SSF has been previously shown
to be a better process configuration for bioethanol production from
lignocellulosic biomass (Soderstrom et al., 2005). In present study,
despite attaining ethanol concentration of 21.6 g/L, the yield of
54.4% was comparatively low, probably due to the incomplete
hydrolysis of cellulose, as temperature of 40 C used during the
process, was below the optimum for cellulases. Varga et al.
(2004) achieved higher ethanol yield (63%) from pretreated corn
stover, however, a higher biomass loading of 12% was used in their
study. Cellulose to glucose conversion by cellulases is a key parameter
to obtain reasonable yields in SSF, as ethanol conversion efficiency
is greatly affected by glucose remaining in the form of
unhydrolyzed cellulose (Castro and Roberto, 2014). Some studies
have reported that higher efficiency of cellulose to ethanol process
via SSF may be attributed to several factors such as different pretreatment
method, higher enzyme and higher biomass loadings,
different nature or composition of biomass etc. (Linde et al.,
2007). For instance, Maeda et al. (2013) used acid and alkali pretreated
sugarcane bagasse at high solids loading in a fed-batch
mode and obtained higher yields. Presently, SSF was carried out
in batch mode using acid soaked hydrothermally pre-treated
wheat straw and lignin remaining in biomass might have hindered
the enzyme hydrolysis. The SSF process in present study was conducted
using crude enzyme without addition of antibiotics to prevent
contamination, which makes overall process more realistic
from the point of view of scalability.
the sugars released by enzymes were consumed immediately by
the yeast and converted into ethanol. Thus, the feedback inhibition
on enzymes was prevented, which could be a probable reason for
higher ethanol yield (54.4%) during SSF. Additionally, in this study,
enzyme was added in two steps, which might have also been
responsible for increased hydrolysis of wheat straw resulting in
higher ethanol concentration during SSF in comparison to SHF
process.
A comparison of SSF process for ethanol production from lignocellulosic
biomass from available literature has been presented in
Table 3. SSF results in present study were better as higher ethanol
yield with lower enzyme dosage were obtained in comparison to
the report of Barron et al. (1995) who performed SSF of milled filter
paper using 25 FPU/g cellulose and obtained ethanol yield of 39%.
Presently, in-house produced cellulases was more efficient during
SSF process for ethanol production. SSF has been previously shown
to be a better process configuration for bioethanol production from
lignocellulosic biomass (Soderstrom et al., 2005). In present study,
despite attaining ethanol concentration of 21.6 g/L, the yield of
54.4% was comparatively low, probably due to the incomplete
hydrolysis of cellulose, as temperature of 40 C used during the
process, was below the optimum for cellulases. Varga et al.
(2004) achieved higher ethanol yield (63%) from pretreated corn
stover, however, a higher biomass loading of 12% was used in their
study. Cellulose to glucose conversion by cellulases is a key parameter
to obtain reasonable yields in SSF, as ethanol conversion efficiency
is greatly affected by glucose remaining in the form of
unhydrolyzed cellulose (Castro and Roberto, 2014). Some studies
have reported that higher efficiency of cellulose to ethanol process
via SSF may be attributed to several factors such as different pretreatment
method, higher enzyme and higher biomass loadings,
different nature or composition of biomass etc. (Linde et al.,
2007). For instance, Maeda et al. (2013) used acid and alkali pretreated
sugarcane bagasse at high solids loading in a fed-batch
mode and obtained higher yields. Presently, SSF was carried out
in batch mode using acid soaked hydrothermally pre-treated
wheat straw and lignin remaining in biomass might have hindered
the enzyme hydrolysis. The SSF process in present study was conducted
using crude enzyme without addition of antibiotics to prevent
contamination, which makes overall process more realistic
from the point of view of scalability.
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