A supercritical fluid extraction system is often referred to as a supercritical CO2 extraction system. Every substance has a critical temperature and pressure point above which it enters the supercritical phase, making many solvents theoretically suitable for SCE. However, CO2 has long been the solvent of choice for SCE due to its low cost, non-toxicity, non-flammability, food compatibility, and relatively low critical temperature and pressure values (around 31.0 °C and 73.8 bar).
CO2 alone is usually more than sufficient to extract most chemical components from dried and ground raw materials. CO2 is particularly effective at solubilizing non-polar compounds of varying molecular weight (such as monoterpenes and sesquiterpenes, fat-soluble vitamins, fatty acids, and even waxes), but more polar components (such as pigments and polyphenols) may require the addition of a co-solvent/modifier, usually ethanol, to improve extraction efficiency. Regardless of the co-solvent, CO2 is always used in a higher proportion. One example of the use of a co-solvent is the extraction of curcumin from turmeric. Sequential extraction of the same batch of raw materials under different pressure and temperature parameters proved extremely useful in this case: the non-polar molecules were first isolated by extraction with pure CO2, and subsequent re-extraction with the addition of ethanol yielded a curcumin-rich fraction.
The density of the extracts is entirely dependent on the extraction pressure and temperature. When working in the lower part of the supercritical pressure range (e.g. 100-150 bar), volatile molecules with lower molecular weights are preferentially extracted. Increasing the pressure (200-300 bar) also increases the density of the CO2 and its solvating power, which leads to the extraction of heavier and (albeit moderately) more polar molecules. Such constituents are often solid or highly viscous at room pressure and temperature. The choice of extraction conditions therefore depends on the target molecules to be extracted and/or fractionated. For example, extracting caffeine from tea leaves produces a solid powdery extract, whereas extracting rice bran takes on a more pasty consistency.
Supercritical CO2 is used for low-temperature drying of expensive materials such as aerogels. CO2 diffuses into the porous structure of the material and replaces organic solvents and/or moisture at low temperatures of 35–40 °C.
For liquid extract, we do not need to add any substance. As we mentioned in the previous question, the extract can be liquid or solid depending on the product. The liquid extract you are talking about can be obtained by maceration. In maceration, the plant is placed in oil, such as olive oil, so that the active substances are transferred to the oil. There is another type of extract in the form of powder, which is usually obtained as follows: the plant is extracted with alcohol, the alcohol is then evaporated, and this extract is mixed with the powder of the extracted plant and encapsulated. In this method, there is a risk of high temperature and alcohol residues.
Maceration is usually time-consuming and is suitable when a liquid solvent (e.g. olive oil) needs to be enriched with solid raw material components (e.g. those contained in rose petals). It is usually time-consuming, while the extract is highly diluted and difficult to process. Liquid-liquid extraction, on the other hand, often uses harmful organic solvents and higher temperatures – often high enough to boil the solvent. Thus, extracts require additional post-processing and quality control to remove harmful solvent residues and assess the degradation of heat-sensitive chemical compounds. Regardless of the extraction method chosen, fractionation of the extract by changing the solvating capacity of the solvent through temperature and pressure remains a unique advantage of SCE. This feature also distinguishes SCE from other modern methods such as microwave and ultrasound extraction.
All CO2 used during extraction returns to the gaseous state after pressure release, ensuring its complete separation from the extract with zero probability of (even harmless) residue.
The complete absence of oxygen and light in the extraction vessel, combined with significantly lower extraction temperatures (32–70 °C, 40 °C on average) minimize oxidation and photodegradation of labile components.
The unique ability to adjust the solubility of CO2 by changing the operating parameters and adding co-solvents allows for the collection of several fractions of the extract. An example is the fractionation of hops. Volatile essential oil can be extracted initially with pure CO2 at lower pressure (around 150 bar, 40 °C), then increased to higher pressure (250 bar) to extract bitter compounds such as lupulones and humulones. Our laboratory and production systems typically recover 90-95% of the extracted material in 2-3 hours.
The SCE equipment is not designed for chemical analysis of extracts, but we can help you develop an extraction process and study the chemical composition of the extracts in our chromatography laboratory.
The loading volume of the product depends on its density. Dry, crushed plants can take up from a third to two thirds of the volume. For example, in an extractor with a capacity of 500 ml, you can load 200 grams of lavender, 150 grams of thyme and 400 grams of hemp seeds. Since the seeds have a high specific gravity, the loading volume increases. The amount of extract also varies depending on the product. In some varieties of lavender and thyme, up to 5% yield can be obtained. Almost all of the extract in the plant can be extracted.
Is the plant subjected to any treatment before extraction? (does it need to be dried or can I use a fresh plant?) The most suitable method is to use dry, crushed plants. Undried plants can act as a barrier to carbon dioxide due to the water they contain. However, extraction can also be performed with fresh plants.The maximum sample mass per batch is determined by the density of the material, which in turn varies with particle size and moisture content. If the raw material is dry and finely ground, the vessel can be loaded with up to 2/3 of the extractor volume (in ml). For example, our 500 ml F-500 system can be loaded with 200 g of lavender, 150 g of thyme, or up to 400 g of the denser black cumin seeds. Likewise, the extract yield varies depending on the natural content of the extractable material present in the sample. For example, yields of up to 3% of the raw material weight can be expected from lavender, thyme, and other essential oil-rich materials. Yields of 20-40% of the mass can be expected from seeds and other raw materials rich in fatty oils.
For SFE of solid raw materials, dried and ground material is preferred. The presence of excess moisture can form a protective coating of hydrogen-bonded water molecules on the surface of the material and prevent diffusion of supercritical CO2 into the tissue, while larger particle size greatly reduces the surface area available for mass transfer.
Any plant material can be extracted using SFE, but the yield and quality of the extract can almost always be significantly improved by process optimization.
SFE is effective for most plant tissues and can recover valuable compounds from materials with low content. Examples include the extraction and deodorization of pigments (e.g. from paprika and algae), collagen (from fish skin), and alkaloids.