Supercritical Fluids as Green Solvents for Biotransformations

Supercritical fluids (SCFs) are compounds that exist at a temperature and pressure that are above their corresponding critical values [70,71]. They exhibit the properties of both gases and Hquids. With gases, they share the properties of low surface tension, low viscosity, and high diffusivity. Their main Hquid-like feature is the density, which results in enhanced solubility of solutes compared with the solubility of gases. Furthermore, the solubility of solutes can be manipulated by changes in pressure and temperature near the critical point [72]. 

Among the SCFs, supercritical carbon dioxide (SCCO2) provides additional benefits [73], since it is environmentally benign, inexpensive, available in large quantities, nonflammable, and exhibits low toxicity. Its critical pressure is relatively low (73.4 bar) and it has an ambient critical temperature (31.3 °C). CO2 can be easily removed from reaction mixtures by depressurization [74]. 

In several studies, SCCO2 has shown to have adverse effects on enzymatic activity [79]. This fact results in the demand for novel methods of enzyme stabilization. 

The most effective of these include immobilization [80], lipid coating [81], surfactant coating [82], use of cross-linked enzyme crystals [83], cross-linked enzyme aggregates [84], and membrane reactors [85]. 

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