Food Lipids: Chemistry, Nutrition, and Biotechnology

esters as surface active agents during the immobilization process can increase activity
10-fold when the preparation is used under microaqueous conditions (19). In contrast,
using surface-active agents to form an emulsion can dramatically decrease the rate
if interesterification because they prevent contact between the lipase and substrate
(111). Adsorption at the interface can be inhibited by the presence of other nonsubstrate
molecules, such as proteins. The presence of proteins other than lipase at the
interface reduces the ability of the lipase to bind to the interface. Addition of protein
in the presence of lipase can cause desorption of lipase from the interface.
Phospholipids, such as phosphatidylcholine, phosphatidylethanolamine, and
phosphatidylinositol, can be found as minor components in oil, in quantities of 0.1–
3.2%. The presence of phospholipids can have a negative effect on lipase activity.
The initial rate of reaction can be decreased due to initial competition between
phosphatidylcholine and the triacylglycerols for the active site of the lipase. Phosphatidylethanolamine
seems to have the most inhibitory effect on lipase action possibly
due to the presence of the amine group. Due to their effects, the phospholipid
content of oils must be less than 500 ppm in order to prolong the half-life of immobilized
lipases during interesterification (112).
G. Product Accumulation
During interesterification of two triacylglycerols, the production of monoacylglycerols
and diacylglycerols can lead to an increase in the rate of reaction, whereas the
presence of high levels of free fatty acids can inhibit the initial hydrolysis of triacylglycerols
(51). In lipase-catalyzed interesterification, where hydrolysis is extensive,
or in acidolysis reactions, the level of free fatty acids can have an impact on
the rate of the reaction. During acidolysis of butter oil with undecanoic acid, Elliott
and Parkin (65) reported that concentrations of undecanoic acid greater than 250
mM decreased the activity of porcine pancreatic lipase. Inhibition of lipase activity
by free fatty acids agrees with the Michaelis–Menten model for uncompetitive inhibition
by a substrate (65):
� =
V max[S 0]
[S 0]
[S ] �1 � � � K
Ki 0 m
where S 0 is the initial free fatty acid concentration, K i is the inhibition constant, and
K m is the Michaelis constant.
The loss of activity by lipase in the presence of high concentrations of free
fatty acids has been attributed to several factors. High levels of free fatty acids would
produce high levels of free or ionized carboxylic acid groups, which would acidify
the microaqueous phase surrounding the lipase or cause desorption of water from
the interface. Also, with short and medium chain fatty acids, there could be partitioning
of fatty acids away from the interface into the surrounding water shell due
to their increased solubility in water. This would limit access by the substrate to the
interface (113). Kuo and Parkin (113) found that there was less inhibition when
longer chain fatty acids, such as C13:0 and C17:0, were used during acidolysis,
compared with C5:0 and C9:0. The decrease in lipase activity was attributed to both
increased solubility of the short chain fatty acids in and acidification of the aqueous
phase.
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