After the extraction process, the algal lipids are ready to be converted into biodiesel through transesterification, whereby the lipids react with a short-chain alcohol (e. g., metha­nol) in the presence of a catalyst (Sharma and Singh, 2009). Since the reaction is reversible, an alcohol-to-oil molar ratio of more than 3 is usually used to push the reaction toward the product side at reflux temperature (60-70 °C) using a homogeneous base catalyst (e. g., KOH and NaOH) to accelerate the reaction. At the end of the reaction, two obvious layers will be observed due to gravity separation: The top layer is biodiesel (the main product), whereas the bottom layer is glycerol (byproduct). Subsequently, biodiesel is subjected to sev­eral purification steps, such as water washing and evaporation, to produce pure biodiesel of high quality. In a recent optimization study, more than 90% of algal biodiesel (Chlorella vulgaris) yield was attained at a reaction temperature of 43 °C, with a methanol-to-oil molar ratio of 14, 0.42 wt% of NaOH, and a reaction time of 90 minutes (Plata et al., 2010).

Nevertheless, it is important to note that the presence of a high free fatty acid (FFA) content in algal lipids (more than 0.5% w/w) may prevent the use of a homogeneous base catalyst for the transesterification reaction (Ehimen et al., 2010; Zhu et al., 2008). This is because FFA will react with the base catalyst to form soap, resulting in a low biodiesel yield and causing sig­nificant difficulty in product separation and purification. An alternative acid catalyst (e. g., sulfuric acid, H2SO4) will be a better choice because it is not sensitive to the FFA content in the oil and thus esterification (FFA is converted to alkyl esters) and transesterification can occur simultaneously. Nevertheless, the acid catalyst has a significant drawback from a commercialization aspect for the following reasons: (1) the reaction is extremely slow and a high concentration of catalyst is required to accelerate the reaction, (2) reuse and recycling of the catalyst are not possible, and (3) strong acidic properties of the catalyst will cause serious corrosion on valves, pipelines, and reactor walls (Lam et al., 2010).