Principles of extraction
The purpose of extraction is to separate the active compounds of plant tissues from the inactive components by using selective solvents. During extraction, solvents diffuse into the solid plant material and solubilise compounds with similar polarity. Solid-liquid extraction techniques are still vastly used while novel technologies such as supercritical fluid extraction are gaining popularity.
Solid liquid extraction methods
Solid-liquid extraction methods used for the extraction of phytochemicals from plants include maceration, infusion and Soxhlet extraction. The choice among them depends of the physicochemical properties and stability of the phytoconstituents to be extracted.
Maceration is produced by steeping the plant material in a an organic solvent at room temperature.
Infusion is similar to maceration but the extraction is carried out at higher temperature.
It is a continuous process of extraction with a hot solvent. One of the best extraction method for the recovery of big yields of extract.
Choice of Solvent
Following the principle of “like dissolves like”, the solvent choice depends of the polarity of the targetted substance. Nonpolar solvents are used to solubilize lipophilic compounds (e.g., alkanes, fatty acids, pigments, waxes, sterols, some terpenoids, alkaloids, and coumarins). Medium polarity solvents are used to extract compounds of moderate polarity (e.g., some alkaloids, flavonoids). High polarity ones are used for more polar compounds (e.g., flavonoids glycosides, tannins, some alkaloids). On the other hand when we want to extract a large spectrum of phytochemicals, the best options are alcoholic solvents because they increase the cell wall permeability and allow a nonselective extraction.
Solvents used for active compounds extraction
- Fatty acids
Supercritical fluid extraction
Supercritical fluid extraction (SFE) is the process of separating one compound from the herb using supercritical fluids as the extracting solvent. Supercritical fluid combine both the advantages of liquid and gas phases : high density and low viscosity. The typical set-up use supercritical carbon dioxide with sometimes the addition of a polar co-solvent such as methanol or ethanol to improve the extraction of polar compounds. Unlike other processes, this extraction method leaves no solvent residue behind. In addition the CO2 is nonflammable, odorless, tasteless, environmentally safe and food-grade: when used to process foods it does not need to be declared on the food label.