Subcritical Water as a Green Solvent for Plant Extraction

Abstract

Subcritical water extraction (SWE), as a method, is non-toxic, non-flammable, cheap, readily available, safe, environmental friendly and uses a green solvent. Chemicals with different functional groups such as flavonoids, vitamins, antioxidants and antimicrobials, can be extracted selectively using SWE. SWE has become a popular green extraction method for different classes of compounds present in numerous kinds of matrices and samples, such as those from environmental, food or botanical sources. Plant oils normally contain a complex mixture of organic compounds. They are largely composed of a range of saturated or partially saturated cyclic and linear molecules of relatively low molecular mass and within this range a variety of hydrocarbons and oxygenated compounds occur. SWE is a technique based on the use of water as an extractant, at temperatures between 100 and 374 °C and at a pressure high enough to maintain the liquid state. SWE of plant materials is a powerful alternative to traditional methods because it enables a rapid extraction, and the use of moderate temperatures. This avoids the loss and degradation of volatile and thermo labile compounds. Additional positive aspects of the use of SWE are its simplicity, low cost, and a more favourable environmental impact than traditional solvents. The extraction of phenolic compounds, essential oil, carotenoids, flavonoids, flavour and fragrance compounds has been carried out using SWE. SWE is also selective in that the operator is able to extract various polar and non-polar organic compounds by choice, through varying the temperature. When doing this, the water must be kept in a liquid state using minor adjustments in pressure. In the extraction of essential oils from herbs, SWE has been seen to give recoveries comparable to those of steam distillation and solvent extraction.