Biotechnology
Results and discussion
Screening of the lipase
Five microbial lipases were tested for their ability to catalyse esterification between stoichiometric amounts of ARA and methanol or in
presence of methanol excess (fig. 2). For both methanol concentrations,
the lipase from C. antarctica was the sole enzyme able to catalyse ARA
methyl ester (ARAME) synthesis. A methanol excess led to an increase
in ARAME synthesis with the lipase from C. antarctica, while no effect
was shown for the other lipases.
The lipase from C. antarctica was hence chosen for further experiments.
Figure 2
Screening of the lipases
for ester synthesis
from stoichiometric
amounts of ARA
and methanol (A)
or in presence
of methanol excess (B).
X Immobilized C. antarctica
□ P. fluorescens
O C rugosa
+ R. arrhizus
Δ Immobilized R. miehei
Screening of the alcohol
Six alcohols were tested in stoichiometric amounts for ARA esterification
catalysed by the lipase from C. antarctica: methanol, ethanol, propanol,
isopropanol, butanol and heptanol (fig. 3). After 50 h of reaction,
equilibrium concentrations were not reached whatever the alcohol. The
best ester concentrations were obtained with the shortest chain alcohols.
Initial ARAME production rates were inversely linked to the alcohol chain
length: the shorter the alcohol chain length, the higher the initial
production rate. However, isopropanol, a secondary alcohol showed a
particular behaviour. The kinetic profile of ARAME production was the
same with isopropanol as with heptanol, in spite of their distinct carbon
chain length.
These results were consistent with those of Pan et al. (1990) and Hills
et al. ( 1990) who have already noted a inverse relationship between the
alcohol chain length and the degree of esterification as well as an
adverse effect of secondary alcohols. These phenomena could be due either
to the increase in alcohol hydrophobicity or in steric hindrance when
increasing carbon chain length. As suggested by Bloomer (1992), the
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