Supercritical fluid technology is a new extraction and separation technology developed in the past twenty years. Supercritical fluid is a fluid that is below the critical temperature and above the critical pressure. It has unique physical and chemical properties, and has the advantages of both gas and liquid: The viscosity is small (approximate gas), the density is high (close to liquid), and the diffusion coefficient is several tens or even hundreds of times higher than that of liquid. Therefore, it has strong dissolving power and good fluidity and transferability.
In general, the commonly used supercritical fluid is CO2. CO2 is non-toxic, odorless, non-flammable, and inexpensive. The critical temperature of CO2 is 31.4°C (near normal temperature) and the critical pressure is 7.28 MPa. The polarity in the supercritical state is between n-hexane and chloroform. By controlling the pressure and temperature parameters, the target component can be effectively selected. These properties make the supercritical CO2 extraction method particularly suitable for the extraction of fat-soluble volatile oils, pastes, resins, and heat-sensitive products from natural plants.
Vanillin was extracted from vanilla pods using supercritical CO2 extraction technology, and the effects of temperature and pressure on the extraction rate of vanillin were studied. They extracted vanillin from flavonoids produced in Hainan Province and determined the optimal conditions for extraction pressure 35 MPa, extraction temperature 45 °C, separation (I) pressure 16 MPa, separation (I) temperature 40 °C, separation (II) Pressure 6 MPa, separation temperature (II) 40°C, extraction time 150 min. Under this condition, the vanillin extraction rate was 88.3%.
Supercritical CO2 was used to extract cumin and compared with the components obtained by steam distillation and Soxhlet extraction. The yield of supercritical CO2 extraction was significantly higher than that of steam distillation and Soxhlet extraction. The odor and color of the supercritical extraction products were superior to those of the latter two methods. The components of Soxhlet extraction are relatively single, almost all of which are anethole. The main difference between the products obtained by supercritical extraction and steam distillation is the content of fatty acids, and the fatty acids in the products of supercritical extraction. The content is much higher than the steam distilled essential oil.
GCMS was used to compare the components of peppermint oil extracted by steam distillation and supercritical CO2 extraction. The oil yield of the supercritical CO2 extraction method was 2.43%, and the oil recovery rate of the steam distillation method was 1.15%. The main components of the oil extracted by the two methods were menthol, in which the menthol content of the supercritical CO2 extraction product was 61.8%, while the menthol content of steam distilled essential oil was 69.4%. It can be seen that the oil yield of the supercritical CO2 extraction method is more than double that of the steam distillation method, and the difference between the menthol content and the steam distillation method is not large, and thus the yield of menthol is also higher than the latter.
The supercritical CO2 extraction method can effectively avoid the disadvantages of high-temperature destruction of heat-sensitive components and loss of active components during steam distillation, and can also avoid the problems of dissolving pigments, waxes, and solvents in organic solvent extraction. The supercritical CO2 extraction method has the advantages of simple process, low cost, no solvent residue, no toxicity, no pollution, mild temperature conditions, and the like. In the future, the research work of the supercritical CO2 extraction will focus on the selective extraction and product post-processing. At the same time, the combined application of supercritical CO2 extraction and other extraction technologies of plant-based natural flavors needs further in-depth research.