For each class of biocatalytic process the medium in which the conversion occurs is
dependent on the properties of the reactants and products. In many cases the reaction
components have relatively low water solubilities (less than 50 mM). Increasingly, the
substrate(s), product(s), or both are novel compounds which may be aromatic, multicyclic
or bulky molecules. These compounds bear little relationship in size, shape or
hydrophilicity to the natural substrates of a given enzyme. This leads to biocatalytic
conversions at low rates and, since most are poorly water-soluble, operation at low
reactant concentrations. Consequently the process streams leaving the reactor frequently
contain low product concentrations, making a difficult problem downstream (separation
of structurally similar compounds in water) still more difficult.
The use of a two-liquid phase medium to effect bioconversions at higher overall
concentrations has clear advantages for those reactions where one or more of the reaction
components is poorly water-soluble (Lilly and Woodley, 1985; Lilly et al., 1987). It has
also been found to have particular benefits where these compounds are inhibitory and/or
toxic to the biocatalyst by providing a reservoir for the molecules away from the vicinity
of the biocatalyst. The use of a multiphasic liquid-liquid medium, however, also presents
a number of unique problems for the biochemical engineer designing and scaling-up such
a process. The potential advantages and disadvantages of two-liquid phase biocatalytic
processes are summarised in Table 5.1. In this Chapter we will address the issues of when
such a technology may be applied and the basis for the design of both the reactor and the
downstream process.
FUNDAMENTALS
In two-liquid phase biocatalytic processes a virtually water-immiscible organic solvent is
added to an aqueous phase (at a concentration well above the aqueous phase saturation
limit) containing the biocatalyst to create a biphasic reaction medium. Where the
substrate is hydrophobic the organic phase will be initially rich in substrate which will
then partition into the aqueous phase. Once in the aqueous phase the substrate will be
acted upon by the biocatalyst which can only operate at concentrations up to the aqueous
phase saturation
Table 5.1 Potential advantages and disadvantages
of two-liquid phase biocatalysis
Process feature Advantages and disadvantages
Reactor Operation +Solubilisation of poorly water-soluble molecules
+Reduced inhibition/toxicity
+Excellent substrate/catalyst contact
−Reduced activity per unit volume
−Diffusional limitations
−Liquid-liquid interfacial damage to biocatalyst
Advances in the selection and design of two-liquid phase biocatalytic reactors 123