may be limited to the aqueous phase saturation concentration of the components. This
may also be used to limit exposure of the biocatalyst to poorly water-soluble inhibitory or
toxic substrates/products. The high overall concentration of product produced, however,
facilitates downstream processing. As stated previously the product may or may not be
preferentially soluble in the same phase as the substrate and, where the latter situation is
the case, separation of the phases enables a primary isolation of product from substrate.
CLASSIFICATION
A number of different two-liquid phase systems can be identified and these have been
discussed in detail elsewhere (Lilly and Woodley, 1985; Lilly et al., 1987). Some are
listed in Table 5.2. Initially a distinction can be drawn between those reactions where
catalysis occurs at the interface between the liquid phases and those where catalysis
occurs in the bulk of the aqueous phase. With the exception of lipases (which have a
tertiary structure to enable them to work in an amphipathic environment) all other
enzyme and microbial biocatalysts examined to date have been found to catalyse
conversions in the bulk of the aqueous phase (e.g. Woodley et al., 1991a; 1991b).
Nevertheless the interface may still contribute to the reaction rate, or loss of catalytic
activity over time, via direct contact between the cells or enzyme and the interface. It is
well known that immobilisation to prevent damage to biocatalysts as a result of this direct
interfacial contact has implications even for those reactions where conversion occurs in
the bulk of the aqueous phase.
From the perspective of reactor and downstream process design two features are
particularly critical:
(a) Organic phase. Reactions may operate with an organic phase which is either the
substrate itself or which has the substrate disssolved in it. The former case is of benefit
where the substrate is not inhibitory or toxic. In this way direct transfer of substrate to
catalyst via interfacial contact can help contribute to the reaction rate. In the latter case
the organic solvent can be selected to act as a diluant for an inhibitory
Table 5.2 Possible classes of two-liquid phase
biocatalytic reactors
Feature Option
Organic phase Substrate or carrier solvent
Light or dense phase
Continuous or dispersed phase
Reaction Substrate/product distribution
Bulk phase or at interface
Catalyst Whole-cell or immobilised enzyme
or toxic substrate, reducing the interfacial concentration and also that which
partitions into the aqueous phase.Multiphase bioreactor design 126