Perhaps the most natural pretreatment of biomass is a purely biological method. Nature commonly employs lignin-degrading microorganisms such as white, brown or soft-rot fungi (Lee, 1997; McMillan et al., 1999). A study that investigated the effect of high-yield concen­trated recombinant MnP (rMnP), produced from the yeast Pichia pastoris on the biobleaching of kraft pulps found that rMnP applied at 30 U/g pulp for 24 h fol­lowed by alkali extraction removed a significant quan­tity of lignin from both hardwood and softwood unbleached kraft pulps (Xu et al., 2010). The rMnP — treated pulp was more susceptible to subsequent peroxide bleaching compared to the control pulp. More than 60% of the kappa number was reduced by sequential rMnP treatments and alkaline extractions. When using white-rot fungi, such as Ceriporiopsis subyer — mispora, to treat sugar maple chips, the amount of extracted hemicellulose can be increased (Barber,

2007) . The biotreatment alters the physical and chemical structures of the LB and removes a portion of the non­carbohydrate mass.

Because biological pretreatment is safe, environmen­tally friendly and energy saving it is gaining more attention (Okano et al., 2005). The downside is that bio­logical pretreatment is too slow for some industrial ap­plications and some material is lost to the microorganism as it is a consumer of hemicellulose, cel­lulose and lignin (Bohlmann, 2006). The microorganisms are also susceptible to poisoning by lignin derivatives (Hamelinck et al., 2003). Biological pretreatment by itself may not be the best solution but it could provide value when employed in conjunction with other pretreatment options.

Acid Hydrolysis

Water is a weak acid by itself; however, adding a salt to water will enhance the activity of the acid. Aqueous acids, especially those with a salt, autoseparate into hydrogen cations and hydroxyl anions, where one side of the cleaved sugar polymer receives the hydrogen cation and the other receives the hydroxyl group. One can apply acid hydrolysis either as a pretreatment or as a main hydrolysis step. A variety of acids act well at ambient temperatures to pretreat LB and prepare the material for anaerobic digestion. LB is eventually hydro­lyzed into monosaccharides, furfural, HMF and other volatile products. The lignin, however, condenses and precipitates out as a result of the pretreatment (Esteghla — lian et al., 1997; Liu and Wyman, 2003; Shevchenko et al.,

1999) .

Concentrated acids are quite powerful, act at mild temperatures and result in rapid reactions. However, H2SO4, H3PO4 and HCl are highly toxic, corrosive, and hazardous. Reactors for acid hydrolysis need to resist corrosion. Furthermore, recovering the concentrated acid from the hydrolysis effluent is important to reduce the negative environmental consequences and to reduce costs.

Hydrolysis using a DA is an effective pretreatment for LB (Hinman et al., 1992). It produces high sugar yields from some hardwoods, like poplar and aspen. In one study, poplar wood was pretreated with a 2% sulfuric acid at 190 °C for 1.1 min and this was followed by an enzymatic hydrolysis (Wyman et al., 2009). In this particular study, the xylose yield was 18.5% and the glucose yield was 64.3% where the raw material contained 25.8% xylose and 74.2% glucose (Wyman et al., 2009). In another study of aspen wood, the wood was pretreated with a 1.1% sulfuric acid at 170 °C for 30 min and followed by enzymatic hydrolysis (Tian et al., 2011). The xylose and mannose yield was 13 wt% (18 wt% theoretical contents) and the glucose yield was 85% following treatment (Tian et al., 2011). See Table 27.5 for a comparison of concentrated and DA pretreatments.

There are essentially two classes of DA pretreatment processes: high-temperature continuous-flow and low

temperature batch processes. High-temperature systems operate at temperatures over 160 °C and are appropriate for solutions with a low concentration of solids, between 5% and 10%. Low-temperature systems operate under 160 °C and are appropriate for solutions with a high concentration of solids, between 10% and 40%.

Even though a simple acid pretreatment significantly improves the rate of a hydrolysis process, it costs higher than other physicochemical pretreatment processes. One such process is steam explosion and was discussed previously in this chapter. Another consideration for an acid hydrolysis pretreatment is that one must neutralize the hydrolysate prior to subsequent enzy­matic hydrolysis or fermentation (Sun and Cheng, 2002).

Biochars contain very stable forms of carbon (fixed carbon) (Blackwell et al., 2010). The proportion of the original biomass carbon as total solid carbon in biochar ranges from around 5% from gasification technologies to about 35% in slow pyrolysis technologies. Well — managed large-scale biomass power plants can produce biochar of a consistent quality, whereas often small — scale "low-tech" technologies tend to produce very variable quality biochars, and are highly dependent on the homogeneity of the heating regime and the duration of heating. (See Tables 26.3 and 26.4.) A low-cost high — volume supply of sustainable biochar with a high carbon fraction will be needed to generate meaningful climate change mitigation benefits. However, currently

low carbon prices will not provide sufficient commercial incentives to simply apply biochar in soils for mitigation alone. The unacceptably high uncertainties of the direct and indirect influences and residence times of biochars and other organic carbon species in soils, their suitability for carbon markets (Intergovernmental Panel on Climate Change, 2000), and even the commercial incentives of high-volume production are fundamental barriers to widespread biochar use. Therefore, there is a growing need for researchers to quantify the net effect of specific biochars and application methods within niche agroeco­logical systems (particularly grains and livestock) and to verify any stable sequestration of carbon fractions (McHenry, 2011). Furthermore, in terms of farm applica­tion risks, some biochars can contain toxic materials that are controlled by "permissible exposure limit" stan­dards. The levels of these toxic materials in the biochar is dependent on both the biomass feedstock and the bio­char manufacture process, thus no simple permissible exposure limit is available for biochar to date (Blackwell et al., 2009). Thus, the development of a secure and responsible biochar industry will require awareness of safe methods of handling agricultural inputs and will need to be justified economically, and be in­tegrated with existing agricultural production systems (McHenry, 2011).

Irradiation is an option for biomass size reduction. Following a gross procedure to reduce field supplies into at least chip size, one can employ high-energy radiation such as gamma radiation and/or microwave radiation to accomplish fine particle reduction (Wasikiewicz et al., 2005). Not only does a high-energy radiation treatment produce fine particles, but it can also favorably alter the physical and chemical properties of the biomass, depending on dosage (Bouchard et al.,

2006) . Irradiation has been shown to decrease the DP (Bouchard et al., 2006) and make microstructural changes to the irradiated cellulose pulp (Dubey et al.,

2004) . These changes include an increase in the carbonyl contents and an overall improvement in the vulnera­bility of the cellulose crystalline regions to reagents (Stepanik et al., 1998). This in turn leads to a higher rate of enzymatic hydrolysis. Furthermore, irradiation leads to a significant increase in sugar yield (Yang et al., 2008).

An electron beam cuts biopolymers such as cellulose, hemicellulose and lignin into smaller chains. Analysis by powder X-ray diffractometer and Fourier transform infrared spectroscopy confirm the electron beam treat­ments reduce the degree of crystallinity and improve the sugar yields from enzymatic hydrolysis from treated samples (Karthika et al., 2012).

Electron beam irradiation is preferred over irradia­tion using a radioisotope. First of all, electron beam is safer. Turn off the power and the electron beam stops. A radioisotope is continuous and thus requires signifi­cant safety precautions to handle and dispose of. Furthermore, dosages delivered by high-energy electron beam can be controlled and they can provide more po­wer per dose. This is a feature that would be useful in the continuous treatment of LB (Auslender et al., 2002).

Compared with microwave and gamma ray treat­ments, treatment by electron beam is more energy effective. The larger particle sizes, that it can treat, signif­icantly offset the negative effect of higher dosage. That said, there still remains the challenge of the limitation of electron beam source and the potential limitation on the scale of operations. Even though all these irradiation options reduce the particle size and reduce the DP, they are too expensive to use in full-scale operations. Currently, the prospects of engaging an irradiation treat­ment, even if in conjunction with other environmentally friendly treatment options, does not look promising due to the excessive energy requirements. Table 27.3 shows a comparison of these irradiation treatment methods on a variety of wood species.

Carbon Sequestration

The potential of biochar to sequester atmospheric C for centuries is certainly one of its most attractive qualities. As global anthropogenic C emissions continue to increase, C sequestration using biochar employs photosynthesis to draw C from the atmosphere, and pyrolysis to convert that photosynthetically sequestered C into forms that are mostly not biologically degradable. Fossil fuel-derived energy and most biofuels are regarded as carbon-positive, due to the positive net emis­sions of C from their production and use. Carbon-neutral biofuels are those that result in no net emissions of C resulting from their production and use. Pyrolysis energy production combined with biochar incorporation into soil has been described as what may be the only carbon-negative energy production system known (Mathews, 2008). Carbon-negative energy (Figure 25.2) sequesters more C from the atmosphere than is released through its production and use. If biochar is pro­duced from waste feedstocks that would otherwise be microbially degraded and contribute to anthropogenic C emissions, then it is possible to sequester a significant portion of anthropogenic C emissions through pyrolysis (Lehmann et al., 2006). While estimates of biochar C sequestration potential vary between studies depend­ing on methodological details, recent studies reported 1.65 GtC/y or 19% of anthropogenic carbon dioxide emis­sions could be offset (Lee and Day, 2013; Lee et al., 2013). When combined with projected adoptions of renewable energy systems by the year 2100, it has been estimated that through the pyrolysis of agricultural residues, silvi­cultural residues, organic waste from industry, and urban waste, 5.5—9.5 GtC/y is achievable (Lehmann et al., 2006). This would exceed current fossil fuel emis­sions and thereby represent a potential remediation, as opposed to a conservation tool. Others have recently projected the possibility of up to 15.6 GtC/y (Smith et al., 2013). While the C sequestration potential of biochar is currently appealing, it must be deployed carefully in order to minimize the risk of damaging the soil, due to the irreversibility of biochar application to soil (Sohi et al., 2008).

Alkaline pretreatment is viewed as a viable treatment method because of its low energy requirement and low capital equipment and operational costs (Zhao et al.,

2008) . This process operates at lower temperatures and pressures than other pretreatment methods. However, at these conditions, the process is measured in hours or days vs. minutes or seconds for high-temperature, high-pressure methods (Karr and Holtzapple, 2000). Additionally, one may recover or regenerate many of the caustic salts.

Alkaline pretreatment may follow an SEP and may be followed by an enzymatic hydrolysis pretreatment (Montane et al., 1994; Pan et al., 2006). The initial reac­tions of alkaline pretreatment involve solvation and saponification. Solvation, similarly associated with
dissolution or diffusion, is where the solvent surrounds an ion, typically sodium dissolved in water. Traditional NaOH treatment requires high temperatures to be effec­tive (Zhao et al., 2008). It may be supplemented with urea to lower operational temperatures and improve dissolution (Zhao et al., 2008).

The alkaline solvent then saponifies the intermolec­ular ester bonds that cross-link xylan hemicelluloses and other components including lignin and other hemi — celluloses. Removing these cross-links increases the porosity of the lignocellulosic materials. This improves the penetrability of the material to the solvent and swelling of the biomass follows. The swollen biomass is thus more vulnerable to enzymatic and bacterial activity.

Compared with acid hydrolysis, alkaline hydrolysis generally causes less sugar degradation. That said, dissolution or solubilization of LB increases with alkali concentrations. At strong alkali concentrations, peeling of end-groups may occur. This leads to alkaline hydroly­sis and degradation of the dissolved polysaccharides. Furthermore, this may also produce unwanted byprod­ucts. However, there may be a downstream advantage in subsequent conversion treatments. It increases the inter­nal surface area, decreases the DP, decreases crystallinity and separates linkages between lignin and carbohy­drates causing an overall disruption of the lignin struc­ture (Fengel, 1984). This provides opportunity for increased enzymatic and bacterial activity in down­stream processes.

An alternative process to improve sugar content is to use aqueous potassium hydroxide, which selectively removes xylan. Keeping the temperature low, at or
below room temperature, prevents peeling (Hon and Shiraishi, 2001).

It appears that monomeric forms of hemicelluloses are easily degradable to other volatile compounds. Glu — comannans and xylans are particularly vulnerable to peeling. However, by pretreating with a 3% NaOH and 12% urea at —15 °C one can achieve a 60% glucose conversion (Zhao et al., 2008).

Calcium hydroxide, or slake lime, is yet another effective alkaline pretreatment agent. It is one of the least expensive and it is highly recyclable (Karr and Holtzapple, 2000). Using common lime kiln technology, one can recover calcium hydroxide by regenerating it from insoluble calcium carbonate. Lime pretreatment removes lignin and hemicellulose and increases the CrI.

Pretreatment with dilute NaOH decreases the lignin content within a range of 24—55% to 20% and increases the digestibility of NaOH-treated hardwood from 14% to 55%. No effect was observed for softwoods with lignin content greater than 26% (Bjerre et al., 1996). Dilute NaOH pretreatment causes swelling, which, as stated previously, has downstream benefits.

The overall effectiveness of alkaline pretreatment depends on the lignin content of the biomass. Further­more, it changes the cellulose structure such that it is less dense and more thermodynamically stable than native cellulose (Hendriks and Zeeman, 2009; Liu and Wyman, 2003).

Available research to date has shown that biochar al­ters various soil properties in a number of ways. (See Table 26.5.) In the context of the siliceous sandy soils of the West Midlands, the most sought effects are improved microbial habitats and improved nutrient supplies from relatively low (~1 t/ha) rates of biochar use. (See Table 26.6 for crop and pasture research responses in the West Midlands.) It is clear that more research is needed on how various biochars influence the flows of nutrients through the soil profile (Lehmann
et al., 2006; Laird et al., 2008), particularly under Austra­lian conditions (McHenry, 2011). To date, the major claims have been related to biological immobilization of inorganic N, adsorption of dissolved ammonium, ni­trates, P, and hydrophobic organic pollutants (Beaton et al., 1960; Gustafsson et al., 1997; Accardi-Dey and Gschwend, 2002; Lehmann et al., 2003; Bridle, 2004; Mizuta et al., 2004). However, the available research scope does not include an assessment of whether this adsorption could reduce some transport of agricultural fertilizers or other pollutants into ground and surface waters in agricultural catchments (Lehmann et al., 2006; Lehmann, 2007). Early work by Bridle (2004) sug­gested that biochar applications reduce nitrate leaching, as his research found levels of nitrate and ammonium did not change in soils for 56 days after application. The soil incubation study further revealed that in contrast, soil bicarbonate availability and plant available P levels would increase slowly (Bridle, 2004). The labo­ratory results suggested that biochar would provide a source of P for plant growth and could have applications on soils as a slow release form of P, yet some research suggest a reduced uptake of N. This may be more useful in deep sandy soils where P leaches from the surface into groundwater. Biochars are also hypothesized to slow the N cycle by increasing the carbon to N soil ratio, possibly due to increased soil aeration reducing anaerobic conditions (Lehmann et al., 2006). Rondon et al. (2005) found a significant reduction of nitrous oxide emissions, and a near-complete suppression of methane emissions in glasshouse environments at biochar additions of 30 g/kg of soil for some crops (Rondon et al., 2005; Lehmann et al., 2006). However, in some circumstances a high carbon to N ratio and abiotic buffering of mineral N may lead to low N availability (Lehmann and Rondon, 2006). Therefore, medium-scale crop biochar trials are required with regionally common soil biota and mineralogy, and also crop, pasture, and animals for greater understanding of commercial agricultural applicability in a particular region.

ECONOMIC ANALYSIS

A recent analysis by Blackwell et al. (2010) on biochar effects on profitability of dryland wheat production in WA provided a perspective of the breakeven investment costs per hectare of different responses over the medium term. Table 26.7 shows that for the West Midlands area (high rainfall north) a 10% yield increase from 1 t/ha application of banded biochar with a declining response over 12 years would break even at $130/ha, based on the previous 12-year data. This breakeven cost included estimated biochar application costs of between $20 and
$50/ha; thus a production/purchase and transport cost would need to be no higher than about $50—$100/t to enable some income from the biochar use, which encourages further work toward low-cost biochar pro­duction technology development (Blackwell et al., 2010).

When aqueous ammonia, 10—15 wt%, is percolated through biomass at temperatures between 150 and 170 °C with a fluid velocity of 1 cm/min and a residence time of 14 min, lignin depolymerizes and the lignin — carbohydrate linkages break. This process is known as ammonia recycle percolation (ARP) (Iyer et al., 1996). This process is advantageous in that it does not inhibit downstream biological processes. A water wash is there­fore not necessary (Kumar et al., 2009a). Additionally, it is possible to recover and recycle the ammonia. On the downside, ARP is inefficient when used to pretreat soft­wood pulp (Mosier et al., 2005) where lignin had already been removed.

Ozonolysis Pretreatment

A pretreatment option that is appropriate for grassy biomass and some softwood is ozonolysis.

In this case, ozone is used to degrade the lignin and hemicellulose in bagasse, green hay, peanut, pine, cotton straw, wheat straw and poplar sawdust (Ben — Ghedalia and Miron, 1981; Vidal and Molinier, 1988). The process primarily acts on the lignin component and only mildly affects the hemicellulose component, while cellulose is negligibly affected. Ozonolysis is notable in that it removes lignin effectively and the reactions take place at room temperature and stan­dard pressure (Ben-Ghedalia and Miron, 1981). The most significant advantage is that following an ozone pretreatment where 60% of the lignin is removed from wheat straw, the rate of enzymatic hydrolysis increases by 500% (Ben-Ghedalia and Miron, 1981). The most notable drawback is that the process is expensive due to the large volume of ozone required (Sun and Cheng, 2002).

Biofuels account for the major proportion of bioenergy production worldwide, with most of the fuels being derived through biochemical processes. For this reason, this review will focus in the main on current prac­tices used in the production of the main biofuels. The major producers of bioethanol are Brazil and the United States, both of which account for about 89% of world pro­duction (World Development Report, 2008; Lichts, 2010), while the European Union is the world’s largest producer of biodiesel (OECD-FAO, 2009). The United States has been the world’s largest producer of ethanol fuel since 2005 and the world’s largest exporter since 2010. In 2011, the United States produced 52.6 billion liters (13.9 billion US liquid gallons) of ethanol, while Brazil pro­duced 21.1 billion liters (5.57 billion US liquid gallons), representing 24.9% of the world’s total ethanol used as fuel (Renewable Fuels Association, 2012).

Fuel ethanol production is considerably more modest in the European Union, where France, Germany and Spain are the largest producers of bioethanol producing 950, 581 and 346 million liters, respectively, in 2008 (European Bioethanol Fuel Association, 2009). Countries such as Poland, Hungary and Slovakia have also increased their bioethanol output producing 200, 150 and 94 million liters of bioethanol, respectively (European Bioethanol Fuel Association, 2009). Sweden is the leading country in Europe in terms of the use of ethanol as fuel, the impetus for which is driven by government policy. Although most of the ethanol is imported, Swedish gas stations are required by an act of parliament to offer at least one alternative fuel. Furthermore, reductions in biofuel prices to the consumer have also encouraged biofuel consumption. Government incentives for biofuel replacement of gasoline are now being implemented in other countries worldwide, motivated by ever-increasing oil costs, depleting fossil fuel resources, GHG emission targets and the need for greater diversification to support agri­cultural and rural development (Mussato et al., 2010).

The major feedstock for bioethanol in Brazil is sugar­cane including bagasse, while corn grain/maize is the main feedstock used for bioethanol production in the United States. As mentioned earlier, bioethanol can be produced from any sugar or starch crop in first — generation processes, but other potential resources for bioethanol include sugar beet, cassava, maize, oil palm, rapeseed, soybean, corn stover, grass, leaves, agri­crop residues and various locally available nonfood plant biomass like Jatropha, Miscanthus, willow, hemp and switchgrass. Table 2.1 summarizes the major crops/biomass currently (ranked in order of impor­tance) in use for biofuel and bioenergy production in different countries. Shapouri (1995), Shapouri et al. (2002) concluded that the energy content of bioethanol was higher than the energy required to produce it, although other researchers would argue as to the economic viability of bioethanol in the absence of an accompanying high-value biorefinery process.

Production of ethanol from lignocellulosic biomass is a complex process where the biomass often requires pre­treatment to render the holocellulose more accessible to a mixture of enzymes, which are utilized to saccharify or hyrolyze the complex polysaccharides to fermentable sugars. Pretreatment processes can be expensive, toxic and corrosive and may require a subsequent costly detox­ification step (Agbor et al., 2011; Zhang and Lynd, 2004; Sun and Cheng, 2002). In addition, preparation of fermentable sugars and the inhibitory effect of lignin and carbohydrate-derived compounds, formed during pretreatment of the lignocelluloses, are the major bottle­necks in bioconversion processes (Viikari et al., 2007). However, since biomass energy is derived from renew­able resources, its production can still be advantageous if proper management technologies are utilized in biomass harvesting, pretreatment and processing, and if biomass feedstocks are produced sustainably. Plant

2. BIOENERGY RESEARCH: AN OVERVIEW ON TECHNOLOGICAL DEVELOPMENTS AND BIORESOURCES

biomass to energy or chemicals can be economical only if all of the components in the biomass are converted into fuel, chemicals or other value-added components in a true biorefinery approach (FitzPatrick et al., 2010; Cherubini, 2010; Percival Zhang, 2008; Kamm and Kamm, 2004).

A variety of nonbiological pretreatment methods have been extensively reviewed. These include physical, chemical, physicochemical and other combinations of procedures (Alvira et al., 2010; Chandra et al., 2007; da Costa Sousa et al., 2009; Hendriks and Zeeman, 2009; Sun and Cheng, 2002; Taherzadeh and Karimi, 2008). Based on their effects on biomass structure, pretreat­ments can be divided into different categories: those that increase enzyme accessibility to crystalline cellulose by decreasing the fiber’s degree of polymerization or by facilitatinghemicellulose and/or lignin removal to create pores in the cellulose fibrils. Since hemicellulose and lignin are the two main protective coats surrounding cellulose, they have to be removed or altered in order to achieve fast enzymatic hydrolysis of the biomass. How­ever, to obtain high sugar yield for both hexoses and pen­toses, an ideal pretreatment procedure should efficiently remove or modify lignin and also hydrolyze hemicellu — lose, but not degrade these hemicellulose sugars (Ohgren et al., 2007). Some of the most widely investigated proce­dures are briefly described.

Physical Pretreatments

These include mechanical methods to chip, grind and mill the biomass to reduce particle size and, potentially, the crystallinity and degree of polymerization of ligno — cellulose in order to maximize the downstream enzyme hydrolysis process (Tassinari et al., 1980). Recently, a novel extrusion method was developed where the biomass materials are subjected to heating, mixing and shearing to cause both physical and chemical modifica­tions to the material in order to increase cellulose acces­sibility (Karunanithy and Muthukumarappan, 2010a, b; Karunanithy et al., 2012).

Chemical Pretreatments

These are mainly alkali and acid pretreatments. Alkali pretreatments increase cellulose digestibility by enhancing lignin solubilization and decreasing cellulose crystallinity. This method is more effective on agricul­tural biomass than on wood material (Kumar et al., 2009; Playne, 1984). Acid pretreatment, mostly diluted acid pretreatments, increase cellulose accessibility mainly by solubilizing hemicellulose. It can be used as either a pretreatment or a direct hydrolysis process but leads to toxic degradation products that inhibit down­stream fermentation (Alvira et al., 2010). On the con­trary, ozonolysis uses the powerful oxidant ozone to delignify lignocellulosic materials at room temperature and does not form inhibitory compounds, yet it is economically unviable due to large amounts of ozone consumed (Sun and Cheng, 2002). On the other hand, organosolv process can efficiently remove lignin and result in minimal cellulose loss. This is a promising pro­cess if economic solvents are available at commercial scales (Wood and Saddler, 1988; Zhao et al., 2009).

Used edible oils generally recycled as animal feed or used as a raw material for lubricant and paints and the rest discharged into the environment (Watanabe et al., 2001). To eliminate environment and human health risk caused by waste oils/fats (Chen et al., 2006) and to lower biodiesel production cost, usage of waste oils/fats for biodiesel production is recommended (Watanabe et al., 2001). Waste cooking oil, animal fats, yellow grease, brown grease and waste from vegetable oil refining industries are major sources of waste oil for biodiesel production (Huynh et al., 2011). Waste oils are rich in high percentage of FFA and high water content, so lipase — mediated transesterification is a promising method for production of biodiesel with high yields (Huynh et al., 2011). Novozym 435 is capable of converting used olive oils (Sanchez and Vasudevan, 2006). Novozym 435 is capable of converting used olive oils in to biodiesel (Sanchez and Vasudevan 2006).

Algae Oils

Usage of algae oil for biodiesel production has considerable interest because of their availability costs compared to edible oils and animal fats. High photosyn­thetic rate, rapid growth rate, and high productivity make algae a good renewable source for oil/fats. High lipid content (20—40%), tolerance to water, and smaller land usage up to 132 times less compared to terrestrial oil crops make them more prominent choice for oil source (Karatay and Donmez, 2011). Usage of algae oil could reduce the food scarcity problem caused by bio­energy crops (Mata et al., 2010). However, technological development is needed to improve the microalgae oil extraction processes.

CHOICE OF ENZYME

Lipases from bacteria and fungi are the most commonly used for transesterification. In general at re­action temperatures 30—50 °C, the best enzymes will show conversions above 90%. Catalytic reaction time, alcohol type and enzyme condition (free enzyme or immobilized) are also the crucial parameters for select­ing enzyme (Table 8.3). Immobilized Pseudomonas cepacia lipases converts the jatropha oil into FAME in 8 h with ethanol; for the same free enzyme, it took 90 h for trans- esterifying soybean oil with methanol.