Front Matter

from the reaction system (Bornscheuer and Yamane, 1995). When an industrial-scale

production is concerned, a combination of a packed-bed reactor, a substrate reser-

voir, and a vacuum apparatus may be more realistic than a large stirred-tank reactor

operated under reduced pressure (Yoshida et al., 1997).

Side reactions (hydrolysis and acyl migration)

Notable side reactions that are concomitant with Equation (4) are hydrolysis and acyl

migration. The former depends naturally on the water content. Excess amounts of

water always decrease the final yield of the desired product due to hydrolysis of the

desired ester. Acyl migration which is confirmed by the formation of 1,3-DAG, may

(more or less) not be inevitable. This depends on a number of factors such as water

content, reaction temperature, enzyme load, reaction time and substrate ratio (Xu et

al., 1998a; 1999). Xu et al. (1998b,c) studied the lipase-catalyzed interesterifications

of fish oil with caprylic acid, and of medium-chain TAG with sunflower oil, in a

solvent-free system in pilot batch and continuous operations. In a pilot batch pro-

duction 0.22–1.37 % h–1acyl migration occurred in the former operation, whereas

in the latter reaction acyl migration was further reduced 4-fold at a similar extent of

incorporation (see also Chapter 11).

Water content

In applying the effect of the thermodynamic shift, however, special precautions must

be taken against trace amounts of H 2 O existing in the reaction mixture (Yamane,

1987, 1988; Yamane et al., 1998). In a microaqueous solvent-free system, water

plays two roles: (i) it is necessary to maintain the catalytic activity of the lipase;

and (ii) it promotes hydrolysis, an unfavorable side reaction [Equation (4)]. It

has been well documented that the water level in the reaction system critically af-

fects the performance of enzymatic reactions in organic media. Water has a profound

influence on both yield and rate of reactions. Essential water or bound-water that is

actually in equilibrium with water in the bulk solution must be retained to keep both

activity and stability of the enzyme molecule. However, an excess amount of water

always reduces the final yield of the targeted sTAG due to hydrolysis that results in

the formation of byproduct(s). When dry N 2 gas bubbling or vacuum is applied for

removing S’ [Equation (4)], it may also remove water that is essential for the en-

zyme’s catalytic activity. As depicted in Equation (5), the liberated water should be

removed, but essential water should be retained. The effect of water on the reaction

performance is usually controlled by adjusting the thermodynamic water activity

(aw) of the reaction components and the enzyme. The crucial role of trace amounts

of water in solvent-free biotransformations makes it a major factor in ‘microaqueous

organic media’ systems (Yamane, 1987; Yamane, 1988; Yamane et al., 1998).

162 9 Lipase-Catalyzed Synthesis of Structured Triacylglycerols