The main components of plant oils are the fatty acids and their derivatives the mono-, di — and triacylglycerides. Tri-acyl glycerides make up 95% of plant oils. Glycerides are esters formed by fatty acid condensation with tri-alcohol glycerol (propanetriol). Depending on the number of fatty acids fixed on the glycerol molecule, one can have mono-, di — or triacylglycerides. Of course, the fatty acids can be the same or different. As stated in the introduction, biodiesel can be obtained by esterification or transesterification. Esterification is the process by which a fatty acid reacts with a mono-alcohol to form an ester. The esterification reaction is catalyzed by acids. Esterification is commonly used as a step in the process of biodiesel fabrication to eliminate FFAs from low-quality oil with high acid content. Transesterification (or alcoholysis) is the displacement of alcohol from an ester by another alcohol in a process similar to hydrolysis. This process has been widely used to reduce triglyceride viscosity. The transesterification reaction is represented by the general equation (5).

RCOOR’ + R"OH ^ RCOOR" + R’OH (5)

This stepwise reaction occurs through the successive formation of di — and monoglycerides as intermediate products (Canakci et al., 2006). Theoretically, transesterification requires three alcohol molecules for one triglyceride molecule; however, an excess of alcohol is necessary because the three intermediate reactions are reversible (Marchetti et al., 2007; Om

Tapanes et al., 2008). After the reaction period, the glycerol-rich phase is separated from the ester layer by decantation or centrifugation. The resulting ester phase (crude biodiesel) contains contaminants such as methanol, glycerides, soaps, catalysts, or glycerol that must be purified to comply with the European Standard EN 14214.

Different technologies can be used for biodiesel production; these include chemical or enzyme catalysis and supercritical alcohol treatment (Demirbas, 2008b). EN 14214 establishes 25 parameters that must be assessed to certify the biodiesel quality.

In conventional transesterification and esterification processes for the production of biodiesel, strong alkalis or acids are used as chemical catalysts. These processes are highly energy consumptive and the poor reaction selectivity that often results from the physicochemical synthesis justifies the ongoing research on enzymatic catalysis. In addition, an extra purification step is required to remove glycerol, water, and other contaminants from alkyl-esters.

The base catalysis is much faster than the acid catalysis. Low cost and favorable kinetics have turned NaOH into the most-used catalyst in the industry. However, soap and emulsion can be formed during the reaction and complicate the purification process.