3.1. Supercritical Fluid Extraction

Supercritical fluid extraction is a technology that uses a supercritical fluid as an extractant to separate a component (extract) from its mixture (matrix). Most supercritical solvents are hydrocarbons, aromatics, alcohols, and some gases. CO₂ is the most mainstream supercritical fluid because it is safe, lacks solvent residue after extraction, is cheap and easy to obtain, and can separate heat-sensitive compounds [95]. Co-solvent, pressure, and time effects on the extraction process should be considered in CO₂ supercritical fluid extraction. Abrahamsson et al. [96] extracted carotenoids and chlorophyll A from microalgae at a temperature of 40~60 °C, a pressure of 15~30 MPa, with a liquid CO₂ flow rate of 1–4 g/min, and co-solvent ethanol 0~0.2 mL/min. The extracted compounds were detected using a UV-vis spectrophotometer and modeled using the extractable amount and ethanol mole fraction. The carotenoid yield was 0.25 mg/g, the chlorophyll A yield was 0.96 mg/g, and the extraction amount depended on the co-solvent amount. DaPorto et al. [97] extracted polyphenols from white grape seeds at 40 °C, different pressures, CO₂ fringes, and co-solvent ratios. The optimal extraction conditions were as follows: the highest total polyphenol concentration was obtained at 80 bar pressure, 6 kg/h of the CO₂ flow rate, and 20% (w/w) of the co-solvent. Combined with previous research, increasing pressure can increase the yield, but not significantly.