Food Lipids: Chemistry, Nutrition, and Biotechnology

and removal of reactants and products is performed except at the initial and final
stages of the reaction (Fig. 16). The equation to characterize the kinetics of a stirred
batch reactor is:
[S ]X � K� ln(1 � X) =
0 m
k E t
cat T
V
where [S 0] is the initial substrate concentration, X is the fraction of substrate converted
to product at any given time (1 � [S]/[S 0]), t is the reaction time, E T is the
total number of moles of enzyme present in the reactor, and V is the volume of the
reactor.
Kurashige (94) found that a batch reactor was useful in reducing the diacylglycerol
content in palm oil by converting existing diacylglycerols and free fatty
acids to triacylglycerols. Using lipase coadsorbed with lechithin on Celite under
vacuum to keep the water content below 150 ppm, the author was able to increase
the triacylglycerol content from 85% to 95% in 6 hours. The rate of conversion in
a stirred batch reactor decreases over time since there is a high initial level of substrate,
which is reduced over time, with conversion to product. In order to maintain
the same rate of conversion throughout the reaction, it would be necessary to add
more immobilized enzyme to the reaction mixture (80). A stirred batch reactor has
the advantage of being relatively easy to build and free enzymes can be used, but it
has the disadvantage that, unless immobilized, the enzyme cannot be reused. As well,
a larger system or longer reaction times are required to achieve equivalent degrees
of conversion in comparison with other systems, and side reactions can be significant
(63). Macrae (9) used a batch reactor to produce cocoa butter equivalents from the
interesterification of palm oil midfraction and stearic acid. While product yields were
high, by-products such as diacylglycerols and free fatty acids were formed. Therefore,
it was necessary to isolate the desired triacylglycerols products using fat fractionation
techniques.
C. Continuous Stirred Tank Reactor
A continuous stirred tank reactor combines components of both fixed bed and batch
reactors. It is an agitated tank in which reactants and products are added and removed
at the same rate, while providing continuous stirring to eliminate mass transfer lim-
Figure 16 Stirred batch reactor for immobilized or free lipase–catalyzed interesterification.
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