Efficiency of Free Lipases

Using free lipases in organic media, one problem to overcome is the low activity of crude enzyme powders. High lipase input obviously reduces the economic viability of the system and is hence not feasible. Both physical and chemical modification of free lipases have been developed, describing increased lipase solubility and activity.

In most cases chemical modification involves the covalent coupling of a wide variety of hydrophobic molecules onto lysine amino acid residues on the lipase (45, 46). This generally results in an enhanced activity and stability of the catalyst. It was shown that in certain cases these effects are due to an increased porosity of the lipase-aggregate structure obtained after lyophi-lization of the lipase (47). A similar phenomenon may explain the enhanced activities reported after physical modification of lipases. This technique often involves drying of a lipase in the presence of hydrophobic compounds, often lipids (48, 49).

An alternative "modification" method, referred to as in situ immobilization, is based on the conversion of an emulsion of a lipase solution in oil into a dispersion of solid particles. The latter is achieved by drying the system under reduced pressure. The intensity of mixing during the drying stage determined the average particle size of the biocatalyst thus obtained (50). Despite these developments, dispersion of the dried modified lipase preparations in hydrophobic media remains essential but difficult, especially when highly viscous oils are involved. Moreover, recovery of the suspended lipase from the reaction medium for reuse may require significant catalyst handling and operational cost.

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