260 Marine Macro- and Microalgae: An Overview
compared to the traditional solvent extraction method with supercritical fluid extraction (Mendes et al.
2005; Andrich et al. 2006; Xu et al. 2008) of oils from microalgal strains, such as A. maxima, A. platensis,
B. braunii, C. vulgaris, Ochronomas danica, Skeletonema costatum, and Isochrysis galbana (Santos et al.
1997; Mendes et al. 1999; Perretti et al. 2003). Supercritical fluid extraction efficiency is affected by four
main factors: pressure, temperature, flow rate, and extraction time (Andrich et al. 2006; Xu et al. 2008).
These factors, along with the use of a co-solvent (i.e., ethanol), can be altered and adjusted to optimize
extractions. When ethanol is used as a co-solvent, the polarity of the extracting solvent is increased and
the viscosity of the fluid is subsequently altered. The resulting effect is an increase of solvating power of
CO 2 , and the extraction requires lower temperature and pressure, making it more efficient (Wiyarno et al.
2011). High moisture content can reduce contact time between the solvent and sample. Hence samples
are dried prior to supercritical fluid extraction (Sahena et al. 2009). Its main disadvantages are associated
with the high capital cost and the high energy requirement for supercritical fluid compression.
Transesterification for biodiesel production
The main disadvantage of vegetable and microalgae oils is that they usually have lower volatility and
higher viscosity than that of petroleum diesel (Fuls et al. 1984; Ma and Hanna 1999; Zuhair 2007; Rawat
et al. 2011). For instance, the fuel properties of biodiesel, such as cetane number, heat of combustion,
melting point and viscosity, increase with increasing carbon number, and saturation degree (Pinto et al.
2005). Therefore, these oils cannot be used directly due to engine coking, carbon depositing, and gelling
of the lubricating oil (Ma and Hanna 1999; Meher et al. 2006; Akoh et al. 2007), requiring conversion
to lower molecular weight constituents with higher volatility and lower viscosity (Rawat et al. 2011).
Hence, the properties of biodiesel can be improved by several methods, namely (i) genetic engineering of
the cells producing the parent oils in order to enrich the fuel with specific fatty acids (Narasimharao et al.
2007), (ii) dilution, (iii) pyrolysis, (iv) cracking, and (v) transesterification (Fukuda et al. 2001; Helwani
et al. 2009). Therefore, these oils need to be modified or transesterified to biodiesel using catalysts.
Transesterification (i.e., alcoholysis) is the process of treating a triacylglycerol (TAG) molecule with
alcohol in the presence of a catalyst to produce alkyl esters and glycerol. These alkyl esters are known
as biodiesel. As the reaction is reversible, an excess of alcohol must be used. Methanol, ethanol, and
propanol are the most commonly used alcohols, but often methanol is used due to its low price and high
availability. Stoichiometrically, for each mole of TAG, three moles of alcohol are required; however, in
industrial processes this is usually upgraded to a 6:1 molar ratio to increase biodiesel yield (Fukuda et al.
2001). Transesterification is often carried out by two methods, namely chemical transesterification and
enzymatic esterification (Fangrui et al. 1999; Shimada et al. 2002).
Chemical-catalysed transesterification: Chemically, the reaction is catalysed by either an acid or an
alkali. Biodiesel production from low quality oils (i.e., waste cooking oils) is challenging due to the
presence of free fatty acids (FFA) and water (Schuchardt et al. 1998). Usage of homogeneous base catalyst
for transesterification of such feedstock suffers from soap formation, which creates serious problems
of product separation and lowers the biodiesel yield substantially (Sharma et al. 2008). In this case,
biodiesel production catalysed by acids (Georgogianni et al. 2008; Zhang et al. 2008) can replace base
catalysts since they do not show measurable susceptibility to FFA and can catalyse esterification (i.e.,
by acid catalyst) and transesterification (i.e., by acid or base catalyst) simultaneously (Fig. 1). In current
commercial processes; however, excess alkali is added to remove all FFA from crude feedstock (Fukuda
et al. 2001). For alkali-catalysed transesterification, as water and FFA do not favour transesterification,
anhydrous triacylglycerol and alcohol are necessary to minimize the production of soap. Soap production
decreases the amount of biodiesel and the separation of glycerol and esters becomes difficult. In spite
of this limitation, as alkaline catalysts are often less corrosive than their acidic counterparts and alkalicatalysed transesterification is typically faster than acid-catalysed reactions (Freedman et al. 1984),
industrial processes usually favour metal hydroxides (Wimmer 1995) and metal alkoxides (Freedman
et al. 1984; Schwab et al. 1987) as well as its carbonates (Filip et al. 1992). Therefore, alkaline catalysts
such as sodium and potassium hydroxides are commonly used as commercial catalysts at a concentration
of about 1% by weight of the oil. Sodium alkoxides are the most active catalysts, since they give excellent
compared to the traditional solvent extraction method with supercritical fluid extraction (Mendes et al.
2005; Andrich et al. 2006; Xu et al. 2008) of oils from microalgal strains, such as A. maxima, A. platensis,
B. braunii, C. vulgaris, Ochronomas danica, Skeletonema costatum, and Isochrysis galbana (Santos et al.
1997; Mendes et al. 1999; Perretti et al. 2003). Supercritical fluid extraction efficiency is affected by four
main factors: pressure, temperature, flow rate, and extraction time (Andrich et al. 2006; Xu et al. 2008).
These factors, along with the use of a co-solvent (i.e., ethanol), can be altered and adjusted to optimize
extractions. When ethanol is used as a co-solvent, the polarity of the extracting solvent is increased and
the viscosity of the fluid is subsequently altered. The resulting effect is an increase of solvating power of
CO 2 , and the extraction requires lower temperature and pressure, making it more efficient (Wiyarno et al.
2011). High moisture content can reduce contact time between the solvent and sample. Hence samples
are dried prior to supercritical fluid extraction (Sahena et al. 2009). Its main disadvantages are associated
with the high capital cost and the high energy requirement for supercritical fluid compression.
Transesterification for biodiesel production
The main disadvantage of vegetable and microalgae oils is that they usually have lower volatility and
higher viscosity than that of petroleum diesel (Fuls et al. 1984; Ma and Hanna 1999; Zuhair 2007; Rawat
et al. 2011). For instance, the fuel properties of biodiesel, such as cetane number, heat of combustion,
melting point and viscosity, increase with increasing carbon number, and saturation degree (Pinto et al.
2005). Therefore, these oils cannot be used directly due to engine coking, carbon depositing, and gelling
of the lubricating oil (Ma and Hanna 1999; Meher et al. 2006; Akoh et al. 2007), requiring conversion
to lower molecular weight constituents with higher volatility and lower viscosity (Rawat et al. 2011).
Hence, the properties of biodiesel can be improved by several methods, namely (i) genetic engineering of
the cells producing the parent oils in order to enrich the fuel with specific fatty acids (Narasimharao et al.
2007), (ii) dilution, (iii) pyrolysis, (iv) cracking, and (v) transesterification (Fukuda et al. 2001; Helwani
et al. 2009). Therefore, these oils need to be modified or transesterified to biodiesel using catalysts.
Transesterification (i.e., alcoholysis) is the process of treating a triacylglycerol (TAG) molecule with
alcohol in the presence of a catalyst to produce alkyl esters and glycerol. These alkyl esters are known
as biodiesel. As the reaction is reversible, an excess of alcohol must be used. Methanol, ethanol, and
propanol are the most commonly used alcohols, but often methanol is used due to its low price and high
availability. Stoichiometrically, for each mole of TAG, three moles of alcohol are required; however, in
industrial processes this is usually upgraded to a 6:1 molar ratio to increase biodiesel yield (Fukuda et al.
2001). Transesterification is often carried out by two methods, namely chemical transesterification and
enzymatic esterification (Fangrui et al. 1999; Shimada et al. 2002).
Chemical-catalysed transesterification: Chemically, the reaction is catalysed by either an acid or an
alkali. Biodiesel production from low quality oils (i.e., waste cooking oils) is challenging due to the
presence of free fatty acids (FFA) and water (Schuchardt et al. 1998). Usage of homogeneous base catalyst
for transesterification of such feedstock suffers from soap formation, which creates serious problems
of product separation and lowers the biodiesel yield substantially (Sharma et al. 2008). In this case,
biodiesel production catalysed by acids (Georgogianni et al. 2008; Zhang et al. 2008) can replace base
catalysts since they do not show measurable susceptibility to FFA and can catalyse esterification (i.e.,
by acid catalyst) and transesterification (i.e., by acid or base catalyst) simultaneously (Fig. 1). In current
commercial processes; however, excess alkali is added to remove all FFA from crude feedstock (Fukuda
et al. 2001). For alkali-catalysed transesterification, as water and FFA do not favour transesterification,
anhydrous triacylglycerol and alcohol are necessary to minimize the production of soap. Soap production
decreases the amount of biodiesel and the separation of glycerol and esters becomes difficult. In spite
of this limitation, as alkaline catalysts are often less corrosive than their acidic counterparts and alkalicatalysed transesterification is typically faster than acid-catalysed reactions (Freedman et al. 1984),
industrial processes usually favour metal hydroxides (Wimmer 1995) and metal alkoxides (Freedman
et al. 1984; Schwab et al. 1987) as well as its carbonates (Filip et al. 1992). Therefore, alkaline catalysts
such as sodium and potassium hydroxides are commonly used as commercial catalysts at a concentration
of about 1% by weight of the oil. Sodium alkoxides are the most active catalysts, since they give excellent