Biochemistry of Fatty Acids

Once the crude oils and fats are obtained from the feedstock the key transformation is the transesterification of the triacylglycerides therein. As much of 98% of the oil in these feedstocks is triacylgycerides, that is, a triester made up of glycerol and three fatty acids (Figure 8.40). As noted above, some amount of FFAs also accompanies the TAGs, a reality that complicates biodiesel processing (see below). Fatty acids almost always contain an even number of carbons due to their biosynthesis from the essential building block acetyl CoA, shown in the simplified overview in Figure 8.41. A Claisen condensation with concomitant decarboxylation of the intermediate yields

FIGURE 8.41 The biosynthetic pathway for fatty acids.

the four-carbon P-ketoacyl skeleton (1). Reduction of the ketone functionality (2) followed by dehydration (3) and conjugate reduction (4) yields the four-carbon acyl building block that can again undergo condensation and decarboxylation to lengthen the chain. Of course, all of these steps are catalyzed by enzymes and the fatty acid carboxylate is ultimately freed from its enzyme tether. Subsequent enzyme-cata­lyzed transformations produce the variety of unsaturated and polyunsaturated fatty acids listed in Table 8.8. Also note that almost all naturally occurring unsaturated fatty acids have the cis double bond geometry.

The fatty acids found in TAGs vary depending on the source and climate. In algae, for example, the fatty acids cis-5,8,11,14,17-eicosapentenoic acid (EPA) and cis-4,7,10,13,16,19-docosahexenoic acid (DHA) predominate. In tropical areas, shorter-chain fatty acids are more common (C12-C14), while in cooler climates, longer chains (>C18) dominate. A representative sampling of fatty acid components in various oils and fats is shown in Table 8.9.

The fatty acids that make up the biodiesel feedstock strongly influence the prop­erties of the biodiesel, as one might expect. In general, carbon chains of 18 carbons or fewer are needed to achieve the desired viscosity properties. Given that biodiesel may be used in a wide range of climates, several performance specifications must be met in its manufacture. For example, the cloud point (the temperature at which wax crystals begin to appear) and the pour point (the temperature at which the material can no longer be poured) may limit the use of a specific biodiesel in cold climates. These values are intimately related to the conformational properties of the TAGs

and FFAs in the feedstock. Straight-chain, saturated fatty acids have higher melting points due to increased intermolecular interactions, whereas those TAGs and FFA containing unsaturation are lower-melting—the “kink” from the cis double bond(s) in the fatty acid chain prevents close contact between molecules, with a resultant decrease in the intermolecular forces responsible for melting point (Figure 8.42a and b). Unsaturation also plays an important role with respect to the stability of the biodiesel product and biodiesel from common feedstocks contains a large propor­tion (60-85%) of unsaturated esters (Schumacher et al. 2009). The iodine value (or iodine number) of a fat or oil (including FAE biodiesel) is a measure of the amount of unsaturation present, an important consideration in that double bonds add reactivity. Unsaturation can lead to polymerization and unsaturated fatty acids/esters are espe­cially prone to allylic radical oxidation—problems when it comes to biodiesel deg­radation and engine performance. The typical iodine value for petroleum diesel is negligible, while for biodiesel the typical limit is 120 (Hart Energy Consulting 2008).