Front Matter

To improve the performance of the continuous membrane bioreactors, two ap-

proaches were adopted. The first was to introduce an electric field across the mem-

brane surface. This induced electrophoretic and electroosmotic motion of the w/o-

MEs away from the membrane surface. Such treatment improved the permeation

flux rate, leading to a slight improvement in reactor performance (Hakoda et al.,

1996a). However, the presence of a voltage drop led to further lipase inactiva-

tion, (Hakoda et al., 1996a, b). The second approach was to immobilize the lipase

in liposomes, then dissolve the complexes in w/o-ME media. Immobilization re-

duced the permeability and adsorption of lipases on the membrane surface (Chang

et al., 1991). This approach led to a similar rate of glycerolysis as achieved in a batch

system, indicating that immobilization did not promote mass transfer limitations

(Chang et al., 1991). Importantly, the immobilized lipases were quite stable, with

a half-life of 45 days reported (Chang et al., 1991). Similarly, Cabral and co-workers

reported the enhancement of bioreactor operation by the addition of lecithin (Pra-

zeres et al., 1993c). However, the reactor productivity was plagued with the same

problem that occurs in batch systems, namely, decreased yields at higher substrate

concentrations or molar flow rates (Chang et al., 1991).

3.5 Microemulsion gels

It was discovered during the late 1980s that the solid-phase microemulsion gels can

be formed by adding the protein gelatin to AOT-based w/o-ME solutions, adding

water at certain levels to lecithin w/o-ME systems, or adding phenolic compounds

to AOT (Luisi et al., 1990; Yu et al., 1993). Microemulsion gels have since received

much attention, particularly as transdermal drug delivery agents (Dreher et al.,

1997). Generally, the gels are believed to contain a series of networked aqueous

rods stabilized by a surfactant layer, and perhaps co-existing with normal w/o-MEs.

3.5.1 AOT/gelatin gels

Robinson and co-workers demonstrated that AOT-gelatin gels were a particularly

useful form of immobilized lipase (de Jesus et al., 1995; Nascimento et al.,

1992; Rees et al., 1991; 1993). The gels are prepared by combining a w/o-ME solu-

tion containing lipase with an aqueous gelatin solution (or equivalently, mixing

AOT/solvent with aqueous lipase and gelatin solutions) at 55 8 C, and then allowing

the mixture to slowly cool below the gelation temperature (30–35 8 C) (Rees et al.,

1991). The gel can then be carved into small particulates using a scalpel, or mechani-

cally ground using a mortar and pestle in liquid nitrogen, the later yielding a more

reactive, less diffusion-limited product (Jenta et al., 1997a). The gel particulates can

then be suspended in stirred tank reactors, or employed in a packed bed. A typical gel

composition would consist of 24 % water, 9 % AOT, and 14 % gelatin (Rees et al.,

1993). ‘Soft’, optically-transparent gels, containing a minimal amount of gelatin (5–

8 %), exhibit the highest rate of biocatalysis, but are not stable and are susceptible to

60 3 Lipid Modification in Water-in-Oil Microemulsions