For extraction of hydrophilic substrates and/or products, like proteins, a second
aqueous phase may be suitable (Hustedt et al., 1988; Zijlstra et al., 1998). However,
widespread industrial application of aqueous two-phase systems is hampered by the high
costs of the polymers involved, and by the complexity of aqueous two-phase systems
(Andersson and Hahn-Hägerdal, 1990).
It is obvious that in the case of two dissimilar substrates (both hydrophobic and
hydrophilic), selection of a suitable solvent that is not deleterious to the enzyme and that
solubilises high concentrations of both substrates equally well becomes even more
difficult. For some specific bioconversions, solutions are reported that rely on substrate
modification (e.g. by making it more hydrophobic; Adelhorst et al., 1990; Fregapane et
al., 1991; Scheckermann et al., 1995), or on cosolvent addition (Wolf et al., 1999).
However, a general approach for biocatalytic synthesis at high substrate concentrations
and at low costs remains to be formulated.
BIOCATALYSIS IN MULTI-PHASE SYSTEMS WITH SOLID
SUBSTRATE AND/ OR PRODUCT
An attractive alternative for the multi-phase systems described above, is the use of solid
substrate and product phases as water-immiscible reservoir and sink phases. In these
systems, the solid substrate dissolves, is converted in the liquid phase by the biocatalyst,
and if the product concentration exceeds the solubility limit, the product crystallises.
These liquid-solid-solid three-phase systems have the following advantages: 1) extremely
high overall substrate concentrations can be used (Erbeldinger et al., 1998a); note that at
least a part of the reactor volume must be liquid, as López-Fandiño et al. (1994a) and
Kuhl et al. (1995) found that the biocatalytic conversion takes place in the liquid phase;
2) high conversions and rates can be attained (see below); 3) the formation of product
crystals facilitates downstream processing (centrifugation or filtration followed by
drying). The advantages of not needing an organic solvent or expensive polymers are
obvious. All of these advantages give rise to lower overall production costs than in
conventional multi-phase systems.
These findings have increased the interest in multi-phase systems with solid phases of
substrate and/or product; this is reflected by the increased number of applications
(Bornscheuer and Yamane, 1994; Cao et al., 1996; Gill and Vulfson, 1994; Kasche,
1986; Michielsen et al., 1999a; Petkov and Stoineva, 1984; Wolf et al., 1997).
In the present paper, a short review on the latest developments in batch solid-to-solid
bioconversions is given. These developments indicate that, for a specific solid-to-solid
bioconversion, two systems may be commercially attractive: batch systems with high
concentrations of undissolved substrate, and continuous systems. Since continuous
systems for solid-to-solid bioconversions have not been developed yet, proposals for such
systems are given. Finally, a general method is presented for selecting the most attractive
kind of system for a specific solid-to-solid bioconversion.
BATCH SOLID-TO-SOLID BIOCONVERSIONS
Solid-to-solid bioconversions 239