Front Matter

al., 1997; 1998a,b; 1999a). Public perception about the advantages of solvent-free

biotransformations might also bring about a shift from early R&D experiments using

organic solvents to industrial implementation of solvent-free bioprocesses. This also

holds true for the production of sTAG containing PUFA (Shimada et al., 1996b;

1999). All experiments about lipase-catalyzed sTAG formation described by the

author’s group have been performed in solvent-free systems (Han and Yamane,

1999; Han et al., 1999a,b; Iwasaki et al., 1999; Rosu et al., 1999a,b).

9.3.3 Increasing the yield in a microaqueous system

A reaction scheme of the lipase-catalyzed synthesis of sTAG containing PUFA can

be generally formulated as shown in Equations (4) and (5):

S-S’ + S“!sTAG + S’

(4)

S+S“! sTAG + H 2 O

(5)

Equation (4) is a transesterification, which is further subdivided into acidolysis,

alcoholysis and interesterification depending on acid, alcohol or ester serving as

S“, respectively (Yamane, 1987). Equation (5) represents an esterification between

an acid and an alcohol, liberating H 2 O. There are several strategies that may be

successfully applied to the reactions shown in Equations (4) and (5) in order to in-

crease the yield of sTAG.

Substrate ratio

In Equations (4) and (5), one of the substrates is a PUFA or a PUFA derivative (e.g.

ethyl ester). Pure PUFAs are relatively expensive, and so should be used as the

‘limiting substrate’ to achieve a total conversion of the substrate, while an excess

molar amount of the other substrate (e.g. a triglyceride) often results in good yields

due to a favored equilibrium.

Thermodynamic shift

Because both reactions in Equations (4) and (5) are reversible, the yield of the tar-

geted sTAG increases as the byproducts S’ or H 2 O are removed from the reaction

mixture, by further shifting of the equilibrium (‘thermodynamic shift’). The principle

is straightforward, and can be always applied to any (bio-)chemical reaction. Meth-

ods to achieve this include winterization (Yamane et al., 1993) when the solubility of

S’ becomes low at reduced temperature, N 2 gas bubbling or vacuum (reduced pres-

sure) when S’ is volatile or has a low boiling point [e.g. ethanol in Equation (4)] or

when H 2 O [Equation (5)] is removed by dehydration using activated molecular

sieves, etc. Tautomerization of vinyl alcohol is another way of eliminating S’

9.3 Increasing the yield 161