from microalgae can be feasible on industrial scale when the method applied for
hydrolysis is easy to handle, cost-effective, energy efficient, and maximum yield of
reducing sugars is obtained. The absence of lignin makes the saccharification
process easier and reduces the overall cost. Starch is stored by the microalgae inside
the cells, and these cells can be separated periodically from photobioreactors and
raceway ponds. Biomass harvested can be further disrupted, and starch extraction
can be carried out via water or an organic solvent. Acids (concentrated and diluted)
are mostly used for the disruption of the biomass. Zhou et al. (2011) reported that
addition of 2.5% MgCl 2 in 2% HCl resulted in effective disruption and subsequent
hydrolysis of the algal biomass, and 83% of the total sugars consisting of xylose,
glucose, and arabinose were recovered via this process. Starch can also be saccharified using enzymes such as alpha amylase and gluco-amylase. Large amount
of starch and glycogen have been reported to be present in microalgae like
Chlorella, Chlamydomonas, Dunaliella, Spirulina, and Scenedesmus which can be
processed for bioethanol production. The starch can be converted into ethanol with
the step of anaerobic fermentation and pretreatment can be typically used to
maximize the formation of sugars in first step and then ethanol in the second
step. Study was performed using microalgal strains M. afer and S. abundans for
bioethanol production, and it was reported from the study that dilute acid and
cellulase-treated S. abundans was better feedstock yielding 0.103 g of ethanol per g
of dry weight of microalgae. The process was optimized for sulfuric acid pretreatment, and 52% higher yield of ethanol was obtained with 10 mg/L microalgae
using 3% v/v sulfuric acid treatment at 160 °C for 15 min (Guo et al. 2013).
The utilization of residual lipid extracted algae (LEA) for bioethanol production
is also gaining attention in recent years. Chlorococum sp. was analyzed as a
feedstock in a study to produce bioethanol. The lipid extraction was performed via
supercritical method, and LEA was dried and further subjected to ethanol production giving a yield of 3.83 mg/L from 10 mg/L LEA (Harun et al. 2010). In
another study, C. vulgaris FSP-E was reported to be used as biomass for bioethanol
production with improvement based on pretreatment. Biomass was subjected to
pretreatment using diluted acid and enzymes. It was reported that pretreatment with
enzyme mixture of amylase/cellulase and dilute sulfuric acid were both effective
techniques. The biomass was subjected to fermentation via SSF (simultaneous
hydrolysis and fermentation) and SHF (separate hydrolysis and fermentation)
processes. SHF process gave a higher ethanol yield of 11.66 mg/L as compared to
SSF (Ho et al. 2013). El-Dalatony et al. (2016) performed a study on use of
immobilized yeast and combination of sonication with enzymatic hydrolysis step. It
was reported that sonication combined with hydrolysis gave higher yield of 445
mg/mg of total reducing sugars. Also it was reported that SSF gave higher ethanol
72
S. Joshi and P. Gogate
hydrolysis is easy to handle, cost-effective, energy efficient, and maximum yield of
reducing sugars is obtained. The absence of lignin makes the saccharification
process easier and reduces the overall cost. Starch is stored by the microalgae inside
the cells, and these cells can be separated periodically from photobioreactors and
raceway ponds. Biomass harvested can be further disrupted, and starch extraction
can be carried out via water or an organic solvent. Acids (concentrated and diluted)
are mostly used for the disruption of the biomass. Zhou et al. (2011) reported that
addition of 2.5% MgCl 2 in 2% HCl resulted in effective disruption and subsequent
hydrolysis of the algal biomass, and 83% of the total sugars consisting of xylose,
glucose, and arabinose were recovered via this process. Starch can also be saccharified using enzymes such as alpha amylase and gluco-amylase. Large amount
of starch and glycogen have been reported to be present in microalgae like
Chlorella, Chlamydomonas, Dunaliella, Spirulina, and Scenedesmus which can be
processed for bioethanol production. The starch can be converted into ethanol with
the step of anaerobic fermentation and pretreatment can be typically used to
maximize the formation of sugars in first step and then ethanol in the second
step. Study was performed using microalgal strains M. afer and S. abundans for
bioethanol production, and it was reported from the study that dilute acid and
cellulase-treated S. abundans was better feedstock yielding 0.103 g of ethanol per g
of dry weight of microalgae. The process was optimized for sulfuric acid pretreatment, and 52% higher yield of ethanol was obtained with 10 mg/L microalgae
using 3% v/v sulfuric acid treatment at 160 °C for 15 min (Guo et al. 2013).
The utilization of residual lipid extracted algae (LEA) for bioethanol production
is also gaining attention in recent years. Chlorococum sp. was analyzed as a
feedstock in a study to produce bioethanol. The lipid extraction was performed via
supercritical method, and LEA was dried and further subjected to ethanol production giving a yield of 3.83 mg/L from 10 mg/L LEA (Harun et al. 2010). In
another study, C. vulgaris FSP-E was reported to be used as biomass for bioethanol
production with improvement based on pretreatment. Biomass was subjected to
pretreatment using diluted acid and enzymes. It was reported that pretreatment with
enzyme mixture of amylase/cellulase and dilute sulfuric acid were both effective
techniques. The biomass was subjected to fermentation via SSF (simultaneous
hydrolysis and fermentation) and SHF (separate hydrolysis and fermentation)
processes. SHF process gave a higher ethanol yield of 11.66 mg/L as compared to
SSF (Ho et al. 2013). El-Dalatony et al. (2016) performed a study on use of
immobilized yeast and combination of sonication with enzymatic hydrolysis step. It
was reported that sonication combined with hydrolysis gave higher yield of 445
mg/mg of total reducing sugars. Also it was reported that SSF gave higher ethanol
72
S. Joshi and P. Gogate