Front Matter

11.6 Bioreactors for the lipase-catalyzed

interesterification

Enzyme reactors are often classified by operation (batch or continuous) and mixing

performance (from well mixed to plug flow). The often referred to stirred-tank re-

actor (STR) can be operated in batch or continuous mode, and is often treated as the

well-mixed reactors in which the molecules are maintained in a constant state of

agitation. A packed-bed reactor is a type of reactor for continuous operation which

can be treated as plug flow in the ideal state, meaning that no back-mixing occurs. In

batch well-mixed reactors, the composition of the reactants varies during the course

of the reaction but is constant throughout the reactor; in contrast, in plug-flow re-

actors the composition of the reactants is time-invariant, and varies only along the

length of the reactor. Thus, in plug-flow reactors, the effectiveness factor will be high

near the inlet and low near the outlet, due to the decrease in substrate concentration as

the reactants pass through the reactor, whereas in batch reactors the effectiveness

factor varies with the time of the reaction because the substrate concentration de-

creases with time (Gacesa and Hubble, 1987; Prenosil et al., 1987; Chaplin and

Bucke, 1990). In practice, despite careful design and operation, the ideal states

can be only approached. For example in packed-bed reactors with Lipozyme IM,

more than 2-fold calculated residence time (ideal state) was needed to utilize the

original reactants after a new substrate had been fed in at the inlet, indicating the

wide actual residence time distribution (Xu et al., 1998b).

11.6.1 Batch stirred-tank reactors

Batch reactors are a versatile and traditional form of reactors that are still useful in

certain applications and product development trials, especially for infrequent opera-

tions. Michaelis – Menten kinetics can be used as a basis for a kinetic description of

batch reactors, after integration with regard to time to give total conversion of sub-

strate achieved. For an uninhibited, irreversible reaction employing a single enzyme

in a batch reactor under isothermal conditions, the equation that describes the per-

formance is:

XSKmlnð 1 XÞ¼

kE t

V

ð 19 Þ

whereXis the proportion of substrate converted,Sis the initial substrate concentra-

tion,Kmis the Michaelis constant,kEis the maximum activity of the total enzyme in

the reactor,tis the reaction time, andVis the volume of the substrate mixture. The

characteristic behavior of the lipase-catalyzed acidolysis between oils and free fatty

acids was previously reported (Xu et al., 1998a). The incorporation of acyl donors

(Inc) in the batch reactor can be described in the form of the Michaelis – Menten

equation as given in Equation (15). Incmaxin the equation is likely only in relation to

substrate molar ratios, butKiis mostly influenced by water content and temperature

(Xu et al., 1998a).

11.6 Bioreactors for the lipase-catalyzed interesterification 203