What CO2 extraction is
Carbon dioxide at or above its critical point becomes a supercritical fluid with the permeation properties of a gas and the solvation properties of a liquid. The critical properties of carbon dioxide are 31.1 °C and 73.8 bar. Supercritical carbon dioxide is a nonpolar solvent with gas-like and liquid-like properties and a low critical temperature and pressure.
Advantages and limits
The advantage of CO2 over steam distillation is that it operates at a lower temperature, which can separate the plant waxes from the oils. Because of its low critical temperature, CO2 minimizes thermal damage to bioactive compounds. CO2 is non-flammable and, unlike solvents such as hexane and acetone, does not leave toxic residue. Because CO2 is a gas at normal atmospheric pressure, it leaves no trace of itself in the final product.
Varying the pressure and temperature changes the properties of the supercritical fluid, which allows selective extraction. Volatile oils can be extracted from a plant at low pressures (100 bar), whereas liquid extraction would also remove lipids. Diffusion is much faster in supercritical fluids than in liquids, so extraction occurs faster: an extraction with an organic liquid may take several hours, whereas supercritical fluid extraction can be completed in 10 to 60 minutes. Carbon dioxide is non-polar and has somewhat limited dissolving power, so it cannot always be used as a solvent on its own. The equipment for CO2 extraction is expensive, which is reflected in the price of the essential oils obtained from the process.
Rose
Supercritical CO2 extraction of rose yields a concrete that may be marketed as a concrete, absolute or CO2 extract. Rose oil is also made by steam distillation, which gives rose otto, and by solvent extraction, which gives rose absolute. The heat of distillation makes some of the compounds of the rose break down, so rose attar does not smell very similar to fresh roses. CO2 extraction of rose takes place at a low temperature, extracts a wide range of compounds and leaves the aromatics unaltered by heat, which gives an essence more faithful to the original.
History
Baron Charles Cagniard de la Tour discovered the supercritical phase in 1822. Thomas Andrews coined the term "critical point" in 1869. In 1967 Kurt Zosel noticed that caffeine dissolved in supercritical carbon dioxide, and three years later he registered his coffee decaffeination process for a patent.
A lot of work was done in the 1980s in the food industry on decaffeination of tea and coffee, extraction of aromas from spices and herbs, and extraction of hops for the beer industry. Liquid CO2 hop extracts were introduced in 1975, and a supercritical CO2 hop extraction process was developed in 1978.
Explore all seven extraits in the Discovery Set.
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