Front Matter

action type, enzyme concentration (Han et al., 1990; Shiomori et al., 1996), and

whether the surfactant or water concentration is held constant for a series of reac-

tions (e.g., Figure 3) (Hayes, 1991; Shiomori et al., 1996). Also, theoretically, it is not

clear whether the overall enzyme concentration, or the enzyme concentration in the

dispersed aqueous phase should be held constant (Han et al., 1990). Of interest, when

water was added to a lipase-encapsulated w/o-ME solution at awovalue that is below

optimal, the anticipated increase in activity did not occur (Han et al., 1990). This may

reflect the role of interfacial curvature on the activity retention achieved during

encapsulation, as will be discussed in Section 3.2.5.

For w/o-ME employing AOT, the most commonly employed surfactant, the spe-

cific activity often decreases with AOT concentration (at a constantwo) (Prazeres et

al., 1992; Marangoni, 1993; Miyake et al., 1993a; 1993b; Patel et al., 1996). Since an

increase in AOT (and water, at constantwo) should just increase the concentration of

w/o-MEs without any other change in medium properties, the decrease in activity

reflects that AOT must inactivate lipase (Section 3.2.5). [An exception to this rule is

for hydrolysis under conditions where the additional water substrate is required for

the reaction to proceed (Han et al., 1987b).]

Reactions catalyzed by w/o-ME-encapsulated lipases are quite sensitive to sub-

strate (and product) concentrations due to their roles as inhibitors (discussed above)

and their effect on the properties of the interface and the bulk solvent. For instance,

the degree of hydrolysis decreases with triglyceride concentration for the AOT sur-

factant system due to the presence of increasing levels of product (FFA), acting as

inhibitor (Han and Rhee, 1985a; Chang et al., 1991; Tsai and Chiang, 1991; Hayes

and Kleiman, 1993a; Prazeres et al., 1994; Patel et al., 1995). This makes w/o-MEs

impractical for hosting lipolysis. Regarding fatty acid–fatty alcohol esterification, an

optimal concentration of each substrate exists; and, the optimal levels vary with

enzyme type (Hayes and Gulari, 1990; Rao et al., 1991; Oliveira and Cabral,

1993; Stamatis et al., 1993d). For polyol-fatty acid ester synthesis, the concentra-

tions of both substrates control the success of the reaction by their strong influence

on the phase behavior of the w/o-ME system (Hayes and Gulari, 1991; 1992; 1995).

To form fatty acid ethyl esters, it is best to deliver the substrate ethanol slowly during

the course of reaction, as ethanol/water mixtures are known to inactivate lipases (Rao

et al., 1991).

Several comparisons have been made between w/o-MEs and alternate reaction

systems for hosting lipase-catalyzed reactions. Russell and co-workers screened sev-

eral nonaqueous enzymology systems for lipase-catalyzed hydrolysis (Yang and

Russell, 1995). They report that w/o-ME systems yielded the highest specific lipase

activity, but also the lowest rate of product formation and product concentration, due

in part to the low enzyme and substrate loadings achievable in w/o-ME systems. A

similar finding was also determined when screening reaction systems for lipase-cat-

alyzed esterification (Borzeix et al., 1992; Bornscheuer et al., 1994; Ayyagari and

John, 1995). The most discouraging attributes of w/o-ME reaction systems are the

difficulty in product and enzyme recovery, due to the presence of surfactant, and the

poor suitability for continuous reactor schemes. Approaches to address these flaws

are given in Section 3.4

3.2 Lipases encapsulated in water-in-oil microemulsions 53