Subcritical fluids are certain compounds that exist as fluids under conditions where the temperature is above their boiling point but below their critical temperature and the pressure is below their critical pressure. When the temperature does not exceed a certain value, the gas can be pressurized to make the gas liquefied, while above that temperature, no matter how much pressure is added can not make the gas liquefied, this temperature is called the critical temperature of the gas. At the critical temperature, the pressure necessary to liquefy the gas is called the critical pressure. When propane, butane, high-purity isobutane (R600a), 1,1,1,2-tetrafluoroethane (R134a), dimethyl ether (DME), liquefied petroleum gas (LPG), and sulphur hexafluoride exist in a subcritical fluid state, the diffusion of molecules is enhanced, the mass transfer rate is accelerated, and the penetration and dissolution capacity of weakly polarized as well as nonpolarized substances in the natural products are significantly increased.

Subcritical fluid extraction technology is the use of the special properties of the above subcritical fluid, the material in the extraction tank into the subcritical fluid immersion, in a certain material solubility ratio, extraction temperature, extraction time, extraction pressure, extractant and entrainment agent and stirring, ultrasonic extraction process under the assistance. After solid-liquid separation, the extraction mixture enters the evaporation system, and under the action of compressor and vacuum pump, the extractant is converted from liquid to gas according to the principle of decompression evaporation to get the target extract.