11.3 Kinetics of lipase-catalyzed interesterification
Lipase-catalyzed interesterification most likely takes place in the sequential hydro-
lysis – esterification hypothesis via a ping-pong bi-bi mechanism. Hydrolysis is often
treated as a single-substrate reaction system that can be explained by the Michaelis –
Menten equation (Miller et al., 1991). In the microaqueous reaction system, the
content of water is limited and, therefore, must be considered to be one of the sub-
strates in the kinetic description. Consequently, both hydrolysis and esterification
steps can be treated as two-substrate systems, which can be described as a ping-
pong bi-bi mechanism. The initial reaction rate can be expressed in the form as:
V¼
VmaxCACB
KACAþKBCBþCACB
ð 13 Þ
whereCAandCBare concentrations of substrates A and B, respectively,KAandKB
are reaction constants for A and B, respectively, andVmaxis the maximum reaction
rate.
To account for the effects of all chemical species participating in the interester-
ification between triacylglycerols and free fatty acids throughout the entire reaction,
the models were derived from the sequential hydrolysis and esterification hypothesis
and the model therefrom was further simplified by experiments (Reyes and Hill,
1994). The disappearance rate of fatty acid acyl donors, which equals to the incor-
poration rate into the substrate triacylglycerols, was described as:
r¼
ðVRCFADCDAGVMCWCSTÞET
ð 1 þKICDAGÞðCFADþCRFAÞ
ð 14 Þ
whereVR,VM, andKIare the combined reaction rate constants,Cis concentration,
FAD is fatty acid acyl donor, DAG is diacylglycerol, W is water, ST is newly formed
triacylglycerol, RFA is released fatty acid, andETis the total enzyme concentration.
As can be seen from the model, the contents of water and diacylglycerols were in-
cluded in the equation. The simulation work using the equation provided that: (i)
there was a limit beyond which increasing the initial water content produced no
further increase in the initial rate of the reaction; (ii) an increase in the initial
DAG concentration produced a concomitant increase in the rate of the interesteri-
fication; (iii) the free fatty acids inhibited the hydrolysis reaction; and (iv) there was a
limit beyond which increasing the initial content of substrate triacylglycerols pro-
duced no significant increase in the rate of either the hydrolysis or the esterification
reactions.
Balcao and Malcata (1996) derived a single rate equation in a general form for all
type of interesterifications including hydrolysis and esterification. However, the
model was so complicated and too many constants were involved in the equation
that it is difficult to use it in practice. Kinetic models based on ping-pong bi-bi
mechanisms or the educated simplifications thereof have been increasingly uti-
lized. Kyotani et al. (1988a) demonstrated that the models derived from the acyl-
glycerol-enzyme complex hypothesis were best to fit the experimental results in
11.3 Kinetics of lipase-catalyzed interesterification 197
