Microalgae biomass at 100 g/L was hydrolyzed according to the optimized conditions to yield soluble carbohydrates preparations. These preparations were inoculated with S. cerevisiae cells and accumulated about 23 g ethanol per liter,
representing approximately 81% of the maximum theoretical. These results indicated that microalgae biomass could be converted into ethanol by baker’s yeast
efficiently as commercial grade dextrose and that other nutrients, usually used to
improve fermentation, such as the N-source, were already present in the hydrolyzed
microalgal biomass. Both almost complete exhaustion of carbohydrates from the
fermentation broth and high conversion efficiency of carbohydrates into ethanol
indicated very high enrichment of fermentable sugars in the biomass of the strains
selected in this study and in their corresponding hydrolysates. It also indicated that
sugar loss and/or generation of fermentation inhibitors from microalgal biomass
remained at negligible levels after the optimized saccharification treatment. These
results contribute to support the potential of microalgae biomass as an alternative
feedstock for bioethanol and the value of bioprospecting programs to identified
candidate strains among natural biodiversity.
Yuan et al. (2016) evaluated liquid hot water pretreatment prior to enzymatic
hydrolysis of Scenedesmus sp. The concentration and recovery of total sugars and
glucose at 100 °C were 0.85 and 0.26 g/L, respectively, while 13.4 and 0.16 g/L at
200 °C. These results indicated that the increase of temperature could accelerate the
motions of solvent molecules (sulfuric acid) and improve the liberation of sugars.
Thus, according to these authors, liquid hot water pretreatment could greatly
enhance the enzymatic efficiency and could be regarded as an ideal method for
glucose recovery from microalgae.
Mixed microalgae cultures could be considered as an attractive research area
compared to traditional pure culture to dominate cultivation contamination risk and
enhance economic feasibility of large-scale biofuel production. In this sense,
Shokrkar et al. (2017) evaluate the effect of different pretreatment strategies
including acidic, alkaline, and enzymatic hydrolysis on the sugar extraction from
mixed microalgae. According to these authors, total carbohydrates content of
microalgal biomass increased about 20.1% in the absence of nitrogen (about 36% in
terms of volatile suspended solids amount). Dilute acids decompose cellulose, and
starch in the biomass to release simple sugars. Hydrolysis kinetic depends on the
type of substrate, temperature, acid concentration, and reaction time. Results
showed that the mixture of dilute sulfuric acid and MgSO 4 exhibited a higher sugar
yield than dilute acid. Among all pretreatments used, the enzymatic treatment with
thermostable enzymes showed the highest recovery of 0.951 g of extracted glucose
per gram of total sugar. Moreover, the enzymatic pretreatment of wet microalgae
was compared with dried ones at identical operational conditions and dried biomass
concentration of 50 g/L, and similar sugar yields were achieved which would be
advantageous to reduce the need for drying of the microalgae biomass.
Fermentation of the acidic and enzymatic treated samples to ethanol using
Saccharomyces cerevisiae showed yield of 0.38 and 0.46 g/g glucose, corresponding to 76 and 92% of the theoretical values, respectively. These authors
reported that bioethanol yield after enzymatic hydrolysis of mixed microalgae
11 Biofuels from Microalgae: Bioethanol
239
representing approximately 81% of the maximum theoretical. These results indicated that microalgae biomass could be converted into ethanol by baker’s yeast
efficiently as commercial grade dextrose and that other nutrients, usually used to
improve fermentation, such as the N-source, were already present in the hydrolyzed
microalgal biomass. Both almost complete exhaustion of carbohydrates from the
fermentation broth and high conversion efficiency of carbohydrates into ethanol
indicated very high enrichment of fermentable sugars in the biomass of the strains
selected in this study and in their corresponding hydrolysates. It also indicated that
sugar loss and/or generation of fermentation inhibitors from microalgal biomass
remained at negligible levels after the optimized saccharification treatment. These
results contribute to support the potential of microalgae biomass as an alternative
feedstock for bioethanol and the value of bioprospecting programs to identified
candidate strains among natural biodiversity.
Yuan et al. (2016) evaluated liquid hot water pretreatment prior to enzymatic
hydrolysis of Scenedesmus sp. The concentration and recovery of total sugars and
glucose at 100 °C were 0.85 and 0.26 g/L, respectively, while 13.4 and 0.16 g/L at
200 °C. These results indicated that the increase of temperature could accelerate the
motions of solvent molecules (sulfuric acid) and improve the liberation of sugars.
Thus, according to these authors, liquid hot water pretreatment could greatly
enhance the enzymatic efficiency and could be regarded as an ideal method for
glucose recovery from microalgae.
Mixed microalgae cultures could be considered as an attractive research area
compared to traditional pure culture to dominate cultivation contamination risk and
enhance economic feasibility of large-scale biofuel production. In this sense,
Shokrkar et al. (2017) evaluate the effect of different pretreatment strategies
including acidic, alkaline, and enzymatic hydrolysis on the sugar extraction from
mixed microalgae. According to these authors, total carbohydrates content of
microalgal biomass increased about 20.1% in the absence of nitrogen (about 36% in
terms of volatile suspended solids amount). Dilute acids decompose cellulose, and
starch in the biomass to release simple sugars. Hydrolysis kinetic depends on the
type of substrate, temperature, acid concentration, and reaction time. Results
showed that the mixture of dilute sulfuric acid and MgSO 4 exhibited a higher sugar
yield than dilute acid. Among all pretreatments used, the enzymatic treatment with
thermostable enzymes showed the highest recovery of 0.951 g of extracted glucose
per gram of total sugar. Moreover, the enzymatic pretreatment of wet microalgae
was compared with dried ones at identical operational conditions and dried biomass
concentration of 50 g/L, and similar sugar yields were achieved which would be
advantageous to reduce the need for drying of the microalgae biomass.
Fermentation of the acidic and enzymatic treated samples to ethanol using
Saccharomyces cerevisiae showed yield of 0.38 and 0.46 g/g glucose, corresponding to 76 and 92% of the theoretical values, respectively. These authors
reported that bioethanol yield after enzymatic hydrolysis of mixed microalgae
11 Biofuels from Microalgae: Bioethanol
239