Biodiesel is derived from plant and animal lipids. Lipids are subdivided in two main classes based on their chemical characteristics: polar and nonpolar (neutral) lipids. Neutral lipids include the tri — and diglycerides, waxes, and isoprenoid-type lipids. Monoglycerides divide neutral lipids from polar lipids. Polar lipids include phospholipids (e. g. phosphatidylinositol and phosphatidylethanolamine), free fatty acids, and glyc­erol. Desirable feedstocks for biodiesel production are composed of a higher proportion of saturated fatty
acyl neutral, rather than polar lipids. Compared to ani­mal fats and other seed-based oils, many microalgal spe­cies have been reported to contain a relatively greater proportion of polar lipids to neutral lipids (triglycerides) and the predominance of long-chain polyunsaturated fatty acids (greater than C18). However, several species of microalgae have been shown to produce various lipids, hydrocarbons, and other complex oils suitable for biodiesel production (Banerjee et al., 2002; Guschina and Harwood, 2006). To accurately predict yields from microalgae, it is critical to understand the lipid composi­tion of the feedstock. The fluorescence probe Nile Red is often used to monitor neutral lipid composition within microalgae. However, Nile Red cannot provide informa­tion regarding carbon chain length or saturation of fatty acids. Gas chromatography is often utilized for the iden­tification of specific fatty acids and the separation, identi­fication and quantification of specific lipid classes by High-performance liquid chromatography—evaporative light scattering detection (HPLC-ELSD) has recently been described (Jones et al., 2012). An informed real­time understanding of the lipid composition of the cul­ture may lead to better cultivation practices, which can drive the accumulation of desirable lipids and ultimately higher biodiesel yields.

The oil to biodiesel conversion process is termed transesterification (Figure 10.5). During transesterifica­tion, an alcohol (e. g. methanol and ethanol) is reacted with vegetable oil (fatty acid) in the presence of catalyst. Catalysts include alkalis (e. g. KOH and NaOH) or acids (e. g. H2SO4) to produce fatty acid methyl esters (FAME) or fatty acid ethyl esters and glycerol. Generally, meth­anol is preferred for transesterification because it is less expensive than ethanol. Transesterification requires 3 mol of alcohol for every 1 mol of triglyceride to pro­duce 1 mol of glycerol and 3 mol of methyl esters. This

reaction is reversible in nature and eventually arrives at equilibrium (Fukuda et al., 2001). The produced bio­diesel is immiscible and thus easily separated from glyc­erol by phase partitioning the biodiesel in a nonpolar solvent such as hexane or heptane. The solvent is later recovered by distillation. Transesterification is an inexpensive way of transforming the large, branched molecular structure of the vegetable oils into smaller, straight-chain molecules of the type required in regular diesel combustion engines.

Using microalgae as a feedstock, biodiesel can be pro­duced from extracted algal oils or by direct conversion of the biomass. The production of biodiesel from extracted microalgal oil proceeds as described above. For direct conversion of the biomass to biodiesel, the microalgae are first concentrated to a paste-like consis­tency. The cells are then incubated in methanol or ethanol in the presence of a strong acid or base at an elevated temperature. In this process, fatty acids derived from not only triglycerides but also diglycerides and free fatty acids are transesterified to biodiesel. The remaining residue contains starch and proteins, which can further be processed into ethanol, animal feed, or used as a feedstock in an anaerobic fermenter.