Now biodiesel is commonly produced by using basic catalysts,
such as sodium or potassium hydroxides, since these catalysts give
high yield in short reaction times. However, in the case of oils with
high contents of free fatty acids (FFAs), such as microalgal oils,
alkaline catalysis presents some problems due to the formation of
soaps, which decreases the biodiesel yield and increases the purification
costs. Acid catalysts transform both FFAs and acylglycerols
into methyl esters (5). However, the reaction velocity of transesterification
with an acid-catalyst is much slower than with an
alkaline one and more methanol is required (6). In addition, acid
catalysts are highly corrosive and so are seldom used on an industrial
scale.
Lipases also catalyse the transformation of both FFAs and acylglycerols
to methyl esters, but at lower temperatures than in
alkaline and acid catalysis (3040C). Moreover, the purification of
biodiesel is easier, cheaper and entails a lower consumption of
water, because neither alkalis nor acids need to be separated, only
the excess of alcohol and the glycerol. This glycerol is also easily
recuperated at a high degree of purity (without mixing with alkalis
or acids), and it can be used in nobler applications. On the other
hand, if the lipase is immobilized, the separation of the reaction
mixture and the catalyst is straightforward. Many works on the
enzymatic production of biodiesel have studied the effect of
methanol on the stability of the lipase, because when methanol
exceeds the solubility limit in the reaction mixture the lipase is
irreversibly deactivated. To address this problem several alternatives
have been put forward, such as the addition of methanol by
steps (7), the immobilization of lipase (8) and the utilization of
solvents, such as tert-butanol (9).