3.2.1 Applications
A wide variety of reactions have been investigated in w/o-MEs (Table 1). Microe-
mulsions have hosted both hydrolysis and esterification reactions, demonstrating
their versatility. Many of the applications focus on the ability of w/o-MEs to co-
solubilize polyhydric alcohols (e.g., glucosides, glycerol, cholesterol, and gly-
cols) fatty acyl substrates, lipase, and water in an optically transparent medium,
with sufficient interfacial area between nanophases. Most alternate reaction systems
for polyol ester syntheses involve biphasic/heterogeneous media. With care, lipases
can exhibit quite high turnover rates and remain reasonably stable, as will be dis-
cussed in Sections 3.2.3 and 3.2.5. In addition, the enantioselectivity reported for w/
o-ME-encapsulated lipases is quite high, and can surpass that occurring in alternate
nonaqueous lipase media (Hedstro ̈m et al., 1993). A unique application of w/o-MEs
is for hosting the formation of lactones from long-chainx-hydroxy fatty acids (Rees
et al., 1995a). Chemical means to form lactones from these molecules is quite slow.
The w/o-ME interface attracts both the carbonyl and hydroxyl termini of the sub-
strate, bringing both in close contact to the active site of lipase. Although w/o-ME-
encapsulated lipases have formed structured triglycerides via acidolysis or esterifi-
cation, yields have been less than desired due to a significant extent of hydrolysis,
which also occurred (Holmberg and O ̈sterberg, 1987; Hayes and Gulari, 1992).
Structured lipids consist of triglycerides containing a long-chain acyl group in
the 2-glycerol position and medium chain groups in the outer positions. Applica-
tions include infant formulae and nutritional agents (Soumanou et al., 1998).
The optical clarity and high reaction rates make w/o-MEs well-suited for assaying
lipase activity in situ. For instance,in situlipase assays based on FTIR (Walde and
48 3 Lipid Modification in Water-in-Oil Microemulsions
Table 1.Examples of lipase-catalyzed reactions performed in microemulsions
Reaction type Reference
Hydrolysis of triglycerides Han and Rhee, 1985b
Esterification of glycerol Fletcher et al., 1998
Hayes and Gulari, 1991
Esterification of ethylene glycol Hayes and Gulari, 1992
Esterification of diols Yang and Gulari, 1994
Stamatis et al., 1996
Esterification of glucoside Skagerlind et al., 1997
Structured triglyceride synthesis Hayes and Gulari, 1992
Lactonization ofx-hydroxy-hexadecanoic acid Rees et al., 1995a
Glycerolysis of triglycerides Chang et al., 1991
Acidolysis and Triglycerides Holmberg and O ̈sterberg, 1987
Interesterification between triolein and tributyrin Bello et al., 1987
Cholesterol oleate ester synthesis and hydrolysis Hedstro ̈m et al., 1992
Acidolysis of phosphatidylcholine Holmberg and Eriksson, 1992
Enantioselective esterification of 2-octanol Rees and Robinson, 1995
Enantioselective esterification of ibuprofen Hedstro ̈m et al., 1993
Enantiomeric esterification of menthol Stamatis et al., 1993b
Transesterification of butter fat Kermasha et al., 1995
Lipase hydrolysis coupled with other enzymatic reactions Hochkoeppler and Palmieri, 1990
Yang and Russell, 1994