**4.2 Supercritical fluid extraction**

Supercritical fluid extraction (SFE) is a separation technique that utilizes supercritical fluids as the extracting solvent. A supercritical fluid is a substance that is above its critical temperature and pressure, which results in unique properties that make it an effective solvent for extraction.

The principle of SFE is based on the fact that the solubility of a substance in a supercritical fluid increases with pressure, while the density of the fluid increases with pressure and temperature. By adjusting the temperature and pressure, the solubility of the substance can be controlled and optimized for extraction.

In SFE, the supercritical fluid is pumped into a vessel containing the sample to be extracted. As the fluid passes through the sample, it dissolves the target compounds, which are then carried out of the vessel and into a collection vessel by depressurization or by lowering the temperature. The extracted compounds can then be separated from the supercritical fluid by condensation or by other means.

SFE has several advantages over traditional extraction methods, including reduced solvent use, shorter extraction times, and higher yields of target compounds.

Supercritical fluid extraction (SFE) can be performed in a variety of ways: batch, semi-batch, or continuous. Plant material is placed in a container and supercritical fluid is added at a specific flow rate until the appropriate extraction conditions are reached. Compared with conventional solvent extraction methods, supercritical fluid extraction offers several advantages, including a lower temperature suitable for thermosensitive compounds and a solvation power that can be controlled by modifying pressure and/or temperature, enabling high selectivity. Supercritical fluids are more effective than liquid solvents in penetrating porous materials and extracting compounds, resulting in faster extraction and a more environmentally friendly process. CO2 and small amounts of organic solvents can be used as nontoxic fluids, and this method can be used on an industrial scale [38].

However, high pressures should be avoided when extracting essential oils to prevent the extraction of undesirable compounds.

To ensure the success of EFS, various factors need to be taken into account, such as sample type, preparation, fluid type, delivery method, and extraction conditions. CO2 is commonly used due to its low critical temperature, cost-effectiveness, nontoxicity, absence of odor and taste, and ease of disposal. Adjusting the process conditions makes it possible to selectively extract the desired components. Compared with steam distillation, EFS has shorter extraction times, lower energy costs, and greater selectivity. The EFS method also makes it easier to manipulate oil composition by modifying extraction parameters [39].
