Biofuels in Perspective

Triglyceride
Methanol
E 3
E 3
E 3
E 1
E 1 E1
E 2
E 3
E 2
E 3
E 2
E 3
E 3
E 3
Enzymatic Production of Biodiesel 131
Methyl esters
Glycerol
Figure 8.2 Three types of lipase for methanolysis reaction. E1: Extracellular lipase; E2: Intracellular lipase;
E3: Cell-surface-displayed lipase.
Figure 8.1b. 19 The first step is the conversion of triglycerides to diglycerides, which is
followed by the conversion of diglycerides to monoglycerides and of monoglycerides to
glycerol, yielding one fatty acid ester molecule from each glyceride at each step. 20,21
The kinetics of triglyceride transesterification with methanol, i.e. methanolysis,
catalyzed by Rhizopus oryzae lipase appears to follow a sequential reaction mechanism. 22
That is, triglycerides and partial glycerides are first hydrolyzed by lipase to partial glycerides
and free fatty acids, respectively, after which methyl esters (MEs) are synthesized
from free fatty acids and methanol (see Figure 8.1(b)). This suggests that, unlike in the
case of alkali-catalyzed methanolysis, free fatty acids contained in used oils can be easily
converted to MEs.
Three types of lipase, i.e. the extracellular, intracellular, and cell-surface-displayed
lipases shown in Figure 8.2, can be utilized for methanolysis reaction. Since they can be
prepared by different methods and also utilized in different forms, each reveals a range of
characteristics in methanolysis reaction.
8.3 Use of Extracellular Lipases
Methanolysis reaction can be carried out using many kinds of extracellular lipase from
different microorganisms. It is notable that both non-regiospecific and 1(3)-regiospecific
lipases are utilized in methanolysis reaction and also that acyl migration in triglycerides is
significant in the use of 1(3)-regiospecific lipase as it gives a higher conversion rate. The
present section includes description of transesterification reactions using various types of
extracellular lipase and a description of the effect of acyl migration on ME conversion with
1(3)-regiospecific lipase.
8.3.1 Transesterification with Various Types of Alcohol
As shown in Table 8.1 various types of alcohol – primary, secondary, and straight- and
branched-chain – can be employed in transesterification using lipases as catalysts. 19 Linko