Supercritical fluid extraction (SFE)
One of the alternative methods for extracting essential oils is supercritical fluids. SFE is based on the use of solvents in their supercritical state, meaning that they are exposed to temperatures and pressures above their critical points. SCF have a unique property and are between the liquid and gas phases, which depends on the pressure, temperature and composition of the liquids [4, 6]. The most widely used solvent for this extraction is carbon dioxide, CO2(figure 9). Because it has modest critical conditions (Pc: 72.9 atm, and Tc: 31.2oC), unaggressive for heat sensitive compounds [57], it has no any toxicity and is nonflammable, it is available in high purity at low cost, it can be easily removed from the obtained extract [58], No solvent remains in the final product [59] and etc.
CO2 extracts show some of the properties of essential oils and absolutes. Like essential oils, they have many beneficial therapeutic properties and unlike absolutes, they are not extracted by solvent and instead of chemical solvents, they are extracted using CO2 gas under pressure and at room temperature.
In general, CO2 extracts has excellent quality and better functional with biological activities [60] compared to essential oil produced by hydro-distillation or liquid solvent [61, 62]. The high cost of capital and skills required in a supercritical carbon dioxide extraction plant, as well as the wide range of applications of this process, limit it except for large flavor producers. rosemary, fennel and anise essential oils were obtained by supercritical fluid extraction. The results showed that extraction of supercritical fluids to obtain extracts is economically viable. [63]. Supercritical CO2 extraction of essential oil from lavender was performed by E. Reverchon [64]. This compound showed 34.7% efficiency of the oil.