Front Matter

3.2.4 Effect of surfactant and solvent type

Most of the early studies of w/o-ME-encapsulated lipases employed AOT [sodium

bis(2-ethylhexyl sulfosuccinate], a two-tailed anionic surfactant, because of its de-

monstrated ability to readily form monodisperse w/o-MEs in a one-phase medium

for a large variety of components without the requirement of a co-surfactant. More-

over, a great body of literature exists on the physico-chemical properties of AOTw/o-

ME systems (Eicke, 1987). More recent efforts have sought substitutes for AOT due

to the frequent lipase activity loss occurring with AOT, the surfactant’s non-biocom-

patibility, and complications it induces for downstream product recovery. Examples

of alternate surfactant systems are given in Table 2. Many of the alternative systems

consist of AOT mixed with less-harsh, more biocompatible surfactants such as

Tweens or Spans (polyoxyethylene sorbitan fatty alcohol esters) (Yamada et al.,

1993; 1994; Hossain et al., 1999), lecithin (Nagayama et al., 1998), taurocholate

and bile salts (Kuboi et al., 1992), or polar cosurfactants (Hayes and Gulari,

1994). It is believed that the presence of the second surfactant or co-surfactant re-

duces the strong interactions between AOT and proteins (Schomaecker et al., 1988)

which probably reduce activity and stability upon encapsulation.

Regarding cationic surfactants, the single-tail amphiphile CTAB (cetyl trimethyl-

ammonium bromide) yields much less complication for downstream separations and

improved lipase activity retention; however, it promotes a much slower specific

catalytic rate (Rees and Robinson, 1995). [Double-tail cationic surfactants are re-

ported to form non-specific aggregates with lipase (Skagerlind and Holmberg,

1994).]

Several groups have employed lecithin, which contains mostly phosphatidylcho-

line, as a natural biocompatible zwitterionic surfactant (Hochkoeppler and Palmieri,

1990; Chen and Chang, 1993; Marangoni et al., 1993; Oliveira and Cabral, 1993;

Kermasha et al., 1995; Avramoiotis et al., 1996; Svensson et al., 1996; Nagayama et

al., 1998). Lipase reactions have been reported to yield biocatalytic rates superior to

AOTwith good activity retention (Oliveira and Cabral, 1993; Svensson et al., 1996).

The performance of the lecithin is sensitive to its head group and fatty acyl composi-

tion, which can vary significantly between commercial sources (Marangoni et al.,

1993). The structure of lecithin w/o-MEs consists of a series of interconnected tubes

or rods rather than typical discrete, spherically shaped w/o-MEs (Walde et al., 1990).

Lecithin w/o-MEs can also form gels, as will discussed in Section 3.5. A minor

problem with lecithin is that lipases are reported to hydrolyze it to a small extent

(Morita et al., 1984). A second natural surfactant is the use of fatty acid soaps

(see Table 2).

Nonionic surfactants, particularly those based on commercially available poly-

ethylene glycol (PEG) fatty alcohol ethers (e.g., Brij) or PEG sorbitan fatty acid

esters, are attractive biocompatible surffactants. They can readily form w/o-MEs

in the absence or presence of co-surfactants given that the surfactant system’s hy-

drophilic–lipophilic balance, or HLB value, is between 8 and 15. Early work by

Holmberg’s group and others demonstrated PEG ether & w/o-ME systems were

effective in hosting lipase-catalyzed acidolysis of triglycerides (Holmberg and

O ̈sterberg, 1987) and esterification of glycerol and fatty alcohols (Stamatis et al.,

1995), but less successful for hydrolysis (Holmberg and O ̈sterberg, 1988; Stark

54 3 Lipid Modification in Water-in-Oil Microemulsions