ClassificationThe classification of batch solid-to-solid bioconversions shown in Figure 8.1 is based on
the preparation of each of these reaction mixtures. Types 1 and 2 in Figure 8.1 are
systems in which no solvent is present, apart from the reactant(s). In systems of type 1,
one of the substrates is added as a liquid phase. In systems of type 2, so-called semi-
liquid eutectics, a liquid phase is formed on mixing of two solid substrates. Formation of
the latter systems is based on the principle that the melting temperature of a mixture of
two compounds can display a minimum as a function of mixture composition, the so-
called eutectic temperature (TE, see Figure 8.2); such a mixture is called a eutectic.
Below the eutectic temperature, an entirely solid mixture of the constituent components is
present (Figure 8.2). Above the melting line (above the solid line in Figure 8.2), an
entirely liquid phase exists at every composition of the mixture. At the remaining
combinations of temperature and mixture composition, semi-liquid eutectic mixtures are
formed. For a binary eutectic mixture, this means that a solid phase of only one of the
components and a liquid phase consisting of both components are present (Figure 8.2).
The third type in Figure 8.1 are those systems in which all substrates are in part solid,
but also dissolved in a separate, non-reacting solvent phase.
The last type of systems (type 4 in Figure 8.1) comprises bioconversions of a solid salt
into another solid salt. The systems of this type are formed by salt addition to an aqueous
solution. In contrast to the systems of types 1, 2, and 3, the substrate and product
concentrations in the aqueous phase of these systems can be regulated by the amount of
salt added. Note that, in addition to the bioconversion, dissociation and complexation
reactions occur in the aqueous phase of these systems.
Figure 8.2 Phase diagram for a model
binary eutectic system; TA is the
melting point of component A, TB is
the melting point of component B, and
Solid-to-solid bioconversions 241