Handbook of Solvents - George Wypych - ChemTech - Ventech!

14.4.1 Organic solvents in microbial production 857
formed readily by microbes and locally high concentrations may occur. In fact, in the beginning
of the 20th century, very large production facilities were in operation for the microbial
production of butanol and acetone. Furthermore, terpenes are natural solvents that are produced
mainly by plants and locally they can reach high concentrations. For instance,
limonene is present in tiny droplets in the peel of oranges. All these solvents are toxic to microbial
cells. Some others as higher hydrocarbons that are present for instance in olive oil,
are not toxic to microbes as will be discussed later.
With the advent of the chemical industry, this picture has changed dramatically. In
polluted locations, microorganisms may be confronted with a large number of solvents at
high concentrations. With a few exceptions only, it has turned out that microbes can be
found that are able to degrade these compounds if their concentration is low. This
degradative potential is not unexpected in view of trace amounts that may be present locally
in the natural biosphere. But the exposure of cells to unnatural high concentrations of these
solvents usually leads to irreversible inactivation and finally to their death.
The chemical industry is largely based on solvent-based processes. But in biotechnological
processes, the microbes usually are exploited in a water-based system. This approach
is quite understandable in view of the preference of microbes for water and the
problems solvents pose to whole cells. Solvents often are used to extract products from the
aqueous phase but only after the production process has been completed. At this stage, damage
to whole cells is obviously no longer relevant. In both, chemical industry and biotechnology,
organic solvents have many advantages over water because of the nature of either
product or substrate. Consequently, during the last decades many possibilities have been investigated
to use solvents in biocatalytic processes. 1,2 The more simple the biocatalytic system,
the less complex it is to use solvents.
Free or immobilized enzymes have been exploited already in a number of systems.
Here, biocatalysis may take place in reversed micelles or in an aqueous phase in contact
with an organic solvent. 3 In a powdered state some enzymes are able to function in pure organic
solvents. 4 Furthermore, modified enzymes such as polymer bound enzymes 5 or
surfactant-coated enzymes 6 have been developed so that they can solubilize in organic solvents
to overcome diffusion limitation. The advantages of enzymatic reactions using organic
solvents can be briefly summarized as follows: 1,3,4
1) hydrophobic substances can be used;
2) synthetic reactions can take place;
3) substrate or production inhibition can be diminished and
4) bioproducts and biocatalysts can easily be recovered from the systems containing
organic solvents.
Although organic solvents have often been used in enzymatic reactions, the application
of organic solvents for biotransformation with whole-cell systems is still limited. Cells
might be continuously in direct contact with the organic phase in a two-phase water-solvent
system during the whole production cycle (Figure 14.4.1.1). 7 Alternatively, cells may remain
separated from the bulk organic phase by using membrane bioreactors
(Figure14.4.1.2). 8 In these instances, cells encounter phase toxicity 9 or molecular toxicity,
respectively.
Because whole bacterial cells are more complex than enzymes, they pose by far
greater problems in operating bioproduction processes when organic solvents are present.
The most critical problem is the inherent toxicity of solvents to living organisms. 1,2,10,11 As