In order to overcome disadvantages of classical extraction techniques several novel extraction technologies has been developed. Among them subcritical water extraction (SWE), or pressurized hot water extraction, is one of the most promising. Water is considered as extremely polar solvent at ambient conditions; therefore, it could not be used for extraction of moderately polar and non-polar compounds. This drawback of water can be overcome by increase of pressure and temperature, which causes decrease of dielectric constant and makes water suitable for extraction of target compounds. This property of water is used in SWE technique.
Table 1 Properties of water at different condition state of state (Brunner, 2014)
| Property | Water at normal conditions | Near-critical Water | Supercritical Water |
|---|---|---|---|
| T (°C) | 25 | 350 | 400 |
| P (bar) | 1 | 250 | 500 |
| ρ (kg m-3) | 997.45 | 625.45 | 577.79 |
| ε (-) | 78.5 | 14.86 | 12.16 |
| pKw (-) | 14.0 | 11.5 | 11.5 |
Water is considered to be in subcritical state above boiling temperature, but below critical temperature (374.15 °C), and at the pressure high enough to keep it in the liquid state. In the SWE selectivity of water (dielectric constant of water) can be changed manily by the increase or decrease of extraction temperature, which is the most influential parameter in SWE.
Baside, pressurized hat water extraction exhibits lower viscosity but higher diffusivity than water at room temperature, which favors the diffusion into the plant matrix and the release of bioactive compounds (Teo et al., 2010). One of important aspects of this extraction technology is that it is accepted as green and environmentally friendly since it uses water as extraction solvent and water is non-toxic for human health and environment. Subcritical water has been applied in wide variety of extraction processes. One of the main is extraction of antioxidant compounds from different plant materials. Beside, SWE has been applied for extraction of anthraquinones from Noni root. (Shotipurk et al., 2004) isolation of gingseng saponins from a gingseng powdered root (Engelberth et al., 2010), extraction of hypericine and pseudohypericine from Hypericum perforatum (Mannila et al., 2002), kava lactones from kava roots (Kubatova et al., 2001), iridoid glycosides from Veronica lonifolia (Suomi, 2000) etc.
However, SWE also has its limitations such as:
- Degradation of sensitive bioactive compounds caused by elevated temperatures.
- Formation of certain undesirable compounds on high process temperatures.