Chemistry Reference
In-Depth Information
isocyanate; these are hazardous chemicals, and therefore means the process cannot
be considered green. Additives such as bovine serum albumin and diamino com-
pounds can also be added to enhance the procedure [
10
,
41
,
62
].
Cross-linking has the advantage of not requiring a support, and the aggregates
formed also demonstrate high stability, making them suitable for use in more ex-
treme processing conditions. They have also demonstrated good reuse potential
[
10
]. However, cross-linking can result in conformational changes, therefore reduc-
ing enzyme performance. The nature of the aggregates can also result in reduced
mass transfer, thus lessening the effectiveness of the system. The diversity and ro-
bustness of its applications are also limited due to the variable nature of enzymes,
since some enzymes may resist cross-linking [
2
]. Although involving relatively
simple procedures [
48
], the two-step method of precipitation and cross-linking may
not be considered efficient. Cross-linking can be combined with methods involv-
ing supports, therefore allowing the advantages of supports to be implemented, and
resulting in more effective systems [
10
].
Immobilisation through cross-linking of lipase has recently been carried out us-
ing glutaraldehyde as the cross-linking agent, and bovine serum albumin as an addi-
tive to improve stability. The immobilized lipase showed very good stability at high
temperatures, and also in organic solvents. It also showed very good reuse potential,
with only a small drop in activity after ten cycles [
63
].
Another study that used glutaraldehyde as the cross-linking agent found that the
precipitating agent had a major impact on the lipase activity retained, as well as
the yield of enzyme immobilised. For example, polyethylene glycol 200 resulted
in the immobilized material having a higher specific activity than when acetone,
butanone, propane, polyethylene glycol 600 and ammonium sulphate were used
as precipitating agents, while ammonium sulphate and polyethylene glycol 600 re-
sulted in the highest yields of immobilised lipase [
20
].
Lipase immobilized through CLEAs has also been shown to display better stabil-
ity than Novozyme 435, particularly in organic solvents; this includes less leach-
ing, therefore increasing reuse potential [
64
]. There have also been claims of lipase
'hyperactivation' for particular preparations-these made use of additives such as
surfactants or crown ether, and ammonium sulphate, acetone or dimethoxyethane
as precipitating agents [
65
].
4.3.6 Entrapment/Encapsulation
Although involving a support, entrapment and encapsulation methods do not re-
quire direct interactions between the enzyme and support, and instead make use of
the support to confine and trap the enzyme. With entrapment, the enzyme becomes
entrapped in a surrounding lattice/matrix structure, while with encapsulation, the
enzyme becomes encapsulated in small capsule like structures.
Within these approaches of immobilisation, the sol-gel method is by far the
most common approach and this involves immobilising the enzyme within a gel,
