102 Marine Macro- and Microalgae: An Overview
“Solvent-free” ultrasound-assisted extraction of lipids from fresh microalgae cells has been
investigated and the sonication parameters optimized (Adam et al. 2012). The optimum conditions
identified by this study required an ultrasonic power of 1,000 W, an extraction time of 30 min and a
biomass with a dry weight of 5%. Under these conditions the lipid recovery from N. oculata was 0.48%,
which was 12 times less than the recovery from the Bligh and Dyer conventional method. This study
concluded that while the oil recovery was not affected by the power of sonication, it was significantly
controlled by the percentage of dry weight of biomass, that is, the medium density.
Homogenizers
The main components extracted from microalgae are lipids, carotenoids, and polymers. Most metabolites
of interest produced by microalgae are kept intracellularly. Microalgal lipids are currently very popular
as they have the potential to meet the global demand of biodiesel and therefore, many articles are written
regarding the perspectives. As most microalgal species do not excrete the lipids they synthesize, cell
membrane disruption needs to be performed to free the lipids. Lee et al. (2013) investigated the energy
requirement for microalgal cell disruption using atomic force microscope evaluation and found out that
the average energy for disruption of an individual cell was 17.4 pJ, which is equivalent to a specific
disruption energy of 673 J/kg of dry microalgal biomass, while hydrodynamic cavitation, the most energy
efficient mechanical cell disruption process, had a specific disruption energy of 33 MJ/kg of dry biomass.
These results demonstrate the inefficiency of existing mechanical cell disruption processes.
Hydrodynamic cavitation
This process is a high pressure homogenization pretreatment which forces the algal fluid through small
orifices thus creating a rapid pressure change as well as a high liquid shear that cause cell disruption.
High pressure homogenization has been investigated by Samarasinghe et al. (2012). The alga chosen
for this study was N. oculata. The use of various nozzle sizes did not affect the degree of cell rupture
which implies that the pressure differential alone, and not the shear induced by smaller nozzles, had a
significant effect on cell walls rupture. The authors therefore note that this will have practical implications
as larger nozzles, which will not clog during operation, could be used without compromising the cell
lyses. Cell disruption was close to complete after two passes of the cell culture through the high pressure
homogenizer operated at 276 MPa.
This process has been used to extract lipids from the marine microalga, Scenedesmus sp. (Cho et al.
2012) at 35
o
C and a pressure of 83 bar for 30 min. The algal lipid yield was about 24.9% through this
process while it was only 19.8% when the extraction was carried out with the conventional lipid extraction
procedure using the solvent chloroform:methanol (2:1, v/v) at 65
o
C for 5 h. Hydrodynamic cavitation can
therefore be very interesting as the extraction is performed at a low temperature and in a much shorter
time. In a comparative study of various disruptive techniques (high pressure homogenization, acid,
ultrasonic, and bead beating treatments), Halim et al. (2012) found out that high pressure homogenization
was the most efficient technique with an average disruption of 73.8% of the initial intact cells of the
microalga Chlorococcum sp. stock culture. Increasing operating pressure from 500 to 850 bar was shown
to increase the disruption rate of the cells. Furthermore, the efficiency at a given pressure was higher with
low-density stock culture as this could be expected as lower density cultures absorb more kinetic energy
per individual cell.
Mechanical solid shear with bead milling or mixer
Bead milling is a common process to obtain cell lysis. The algal cells are placed in vessels packed with
glass beads that are agitated at great speed. Cell disruption then occurs through physical grinding of the
cells against the surface of the beads.
Sheng et al. (2012) showed that extraction of lipids from Synechocystis PCC 6803 with bead beating
gave the lowest yield of total FAME as percentage of total biomass.
“Solvent-free” ultrasound-assisted extraction of lipids from fresh microalgae cells has been
investigated and the sonication parameters optimized (Adam et al. 2012). The optimum conditions
identified by this study required an ultrasonic power of 1,000 W, an extraction time of 30 min and a
biomass with a dry weight of 5%. Under these conditions the lipid recovery from N. oculata was 0.48%,
which was 12 times less than the recovery from the Bligh and Dyer conventional method. This study
concluded that while the oil recovery was not affected by the power of sonication, it was significantly
controlled by the percentage of dry weight of biomass, that is, the medium density.
Homogenizers
The main components extracted from microalgae are lipids, carotenoids, and polymers. Most metabolites
of interest produced by microalgae are kept intracellularly. Microalgal lipids are currently very popular
as they have the potential to meet the global demand of biodiesel and therefore, many articles are written
regarding the perspectives. As most microalgal species do not excrete the lipids they synthesize, cell
membrane disruption needs to be performed to free the lipids. Lee et al. (2013) investigated the energy
requirement for microalgal cell disruption using atomic force microscope evaluation and found out that
the average energy for disruption of an individual cell was 17.4 pJ, which is equivalent to a specific
disruption energy of 673 J/kg of dry microalgal biomass, while hydrodynamic cavitation, the most energy
efficient mechanical cell disruption process, had a specific disruption energy of 33 MJ/kg of dry biomass.
These results demonstrate the inefficiency of existing mechanical cell disruption processes.
Hydrodynamic cavitation
This process is a high pressure homogenization pretreatment which forces the algal fluid through small
orifices thus creating a rapid pressure change as well as a high liquid shear that cause cell disruption.
High pressure homogenization has been investigated by Samarasinghe et al. (2012). The alga chosen
for this study was N. oculata. The use of various nozzle sizes did not affect the degree of cell rupture
which implies that the pressure differential alone, and not the shear induced by smaller nozzles, had a
significant effect on cell walls rupture. The authors therefore note that this will have practical implications
as larger nozzles, which will not clog during operation, could be used without compromising the cell
lyses. Cell disruption was close to complete after two passes of the cell culture through the high pressure
homogenizer operated at 276 MPa.
This process has been used to extract lipids from the marine microalga, Scenedesmus sp. (Cho et al.
2012) at 35
o
C and a pressure of 83 bar for 30 min. The algal lipid yield was about 24.9% through this
process while it was only 19.8% when the extraction was carried out with the conventional lipid extraction
procedure using the solvent chloroform:methanol (2:1, v/v) at 65
o
C for 5 h. Hydrodynamic cavitation can
therefore be very interesting as the extraction is performed at a low temperature and in a much shorter
time. In a comparative study of various disruptive techniques (high pressure homogenization, acid,
ultrasonic, and bead beating treatments), Halim et al. (2012) found out that high pressure homogenization
was the most efficient technique with an average disruption of 73.8% of the initial intact cells of the
microalga Chlorococcum sp. stock culture. Increasing operating pressure from 500 to 850 bar was shown
to increase the disruption rate of the cells. Furthermore, the efficiency at a given pressure was higher with
low-density stock culture as this could be expected as lower density cultures absorb more kinetic energy
per individual cell.
Mechanical solid shear with bead milling or mixer
Bead milling is a common process to obtain cell lysis. The algal cells are placed in vessels packed with
glass beads that are agitated at great speed. Cell disruption then occurs through physical grinding of the
cells against the surface of the beads.
Sheng et al. (2012) showed that extraction of lipids from Synechocystis PCC 6803 with bead beating
gave the lowest yield of total FAME as percentage of total biomass.