Food Lipids: Chemistry, Nutrition, and Biotechnology

supports include high losses of activity due to changes in conformation of the lipase,
steric hindrance, and prevention of access of hydrophobic substrates (7). Common
hydrophobic supports include polyethylene, polypropylene, styrene, and acrylic polymers,
while hydrophilic supports include Duolite, Celite, silica gel, activated carbon,
clay, and Sepharose (7). The effectiveness of the immobilization process is influenced
by the internal structure of the support. If a support with narrow pores is used, most
of the enzyme will be immobilized on the surface of the support, which prevents the
occurrence of internal mass transfer limitations. If a support containing larger pore
sizes is used, such as Spherosil DEA, with an average diameter of 1480 A˚ , some lipase
will be immobilized inside the pores, which can prevent access of the substrate to
some of the lipase. This is due to preferential filling of pores and crevices by the
lipase during immobilization (84,85). The activity of lipases tends to decrease upon
immobilization, with activity being reduced by 20–100% (79,81). The activity of an
immobilized enzyme relative to the free form can be compared by an effectiveness
value, which is defined as the activity of immobilized enzyme divided by the activity
of an equal amount of free enzyme determined under the same operating conditions.
The effectiveness value can be used as a guide to the degree of inactivation of the
enzyme caused by immobilization. For values close to 1.0, very little enzyme activity
has been lost upon immobilization, whereas values much lower than 1 indicate high
degrees of enzyme inactivation (80).
The performance of an immobilized lipase can also be affected by handling
and reaction conditions. Freeze drying of the immobilized enzyme before interesterification
to substantially reduce the moisture content has been reported to dramatically
improve activity. Molecular sieves can also be added to reaction systems to
reduce the amount of water that accumulates during the reaction, which would in
turn reduce the degree of hydrolysis (4). The main disadvantage associated with
adsorption as an immobilization method is that changes in pH, ionic strength, or
temperature can cause desorption of lipase that has been adsorbed by ion exchange.
Lipases adsorbed through hydrophobic or hydrophilic interactions can be desorbed
by changes in temperature or substrate concentration (79).
A. Factors Affecting Immobilized Lipase Activity
Immobilization can have an impact on the activity of lipases through steric, mass
transfer, and electrostatic effects. During immobilization, the enzyme conformation
can be affected and parts of the enzyme can be made inaccessible to the substrate
due to steric hindrance.
1. Mass Transfer Effects
The kinetics of lipase-catalyzed interesterification can be affected by mass transfer
limitations. The substrate must diffuse through the fluid boundary layer at the surface
of the support into the pore structure of the support and react with the lipase. Once
products have been released by the lipase, they must diffuse back out of the pore
structure and away from the surface of the support. Mass transfer limitations fall into
two categories: internal and external mass transfer. Internal mass transfer is the transport
of substrate and product within the porous matrix of the support; it is affected
by the size, depth, and smoothness of the pores. Internal mass transfer is diffusionlimited
only. When the rate of diffusion inward is slower than the rate of conversion
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