Table 3 shows
the ED attained at increasing methanol/FFA molar ratios. It can be
seen that the higher the molar ratio the higher the ED, since the
esterification equilibrium is displaced toward the formation of
products when the molar ratio increases. The highest ED was
obtained with a methanol/FFA molar ratio of 2. However, the ED
only improved by 1% with respect to the 1.5 molar ratio, although
the amount of methanol increased 33%. Therefore a methanol/FFA
molar ratio of 1.5 was chosen for future experiments. It is
important to operate with a low methanol/FFA ratio, since the
higher the methanol concentration is, the higher the possibility
that the lipase is deactivated, especially when no solvent is used
(7). However, some authors use high molar ratios to increase the
conversion. For instance, Da Rós et al. (15) use an ethanol/lipids
molar ratio of 12:1 (equivalent to 4:1 alcohol/FFA), in the
transesterification of the lipids from the cyanobaterium
Microcystis aeruginosa, catalysed by Novozym 435, although these
authors used tert-butanol or iso-octane to preserve the stability
of the lipase. On the other hand, the low methanol/FFA ratio used
in this work contrasts with the high ratios that are required for
the production of biodiesel by esterification of FFAs by acid
catalysis. Su (20) established an optimal methanol/FFA molar
ratio of 10:1 in the esterification of FFAs from soybean oil
catalysed by hydrochloric acid, and Hayyan et al. (21) used a
methanol/oil molar ratio of 8:1 (about 2.7:1 as equivalent FFAs) in
the transesterification-esterification of sludge palm oil with 23 wt
% FFAs.