tion medium. Using finallyn-hexane, supercritical carbon dioxide, or even the fatty
acid to be esterified, the sugar substrates should be mainly in a solid state. Investiga-
tions carried out since 1986 (Therisod and Klibanov) have shown many approaches
to have been performed in this manner. The logPvalues of solvents ranged from –
1.30 to + 6.60. Alternatively, free acids, alkyl-, trichloroethyl-, vinyl- or glyceryl-
esters were used for lipid reactands.
Water generated during the esterification was removed by the addition of activated
molecular sieves. In the case of vinyl esters of fatty acids, the resulting byproduct
vinyl alcohol tautomerizes to the low-boiling-point acetaldehyde. However, one
must be cautious since it has been recently reported that several lipases (e.g.,
from Candida rugosa and Geotrichum candidum) lost most of their activity
when exposed to acetaldehyde (Weber et al., 1995). In comparative studies, Coulon
et al. (1995) showed that, in the presence of solvent, the transesterification gave
better results than direct esterification. In solvent-free conditions, only direct ester-
ification was available.
In most cases the immobilized lipases ofCandida antarcticaand ofRhizomucor
miehei(formerlyMucor miehei) served as catalysts. Among the carbohydrates stu-
died, fructose was used very frequently. Unfortunately, due to their specific behavior
in solution, four isomers (furanose and pyranose forms,a- andb-anomers) of mono-
acylated fructose were found (e.g., Scheckermann et al., 1995; Jung et al., 1998). In
17.2 Lipase- and protease-catalyzed esterification of lipids 365
Figure 1. Scheme of the lipase-catalyzed synthesis of fructose-monooleate (main isomer) (e.g., R¼H,
CH 3 ,CH¼CH2,diacylglyceryl).