Since moving to decaf recently (many people were moaning about my incessant wittering so I knew the time was right) I started to wonder just how coffee is decaffeinated. Well, here is the answer…
General Chemistry Online
Full info above but this is my favourite bit…
Supercritical fluid CO2 extraction
When a sealed vial containing both gaseous and liquid carbon dioxide under high pressure is heated, the liquid density drops while the gas density rises. If the pressure is above 72.8 atm, and the temperature rises above 304.2 K, the density of the liquid and the density of the gas become identical. The meniscus between the liquid and gas phases vanishes. The carbon dioxide becomes a supercritical fluid which has both gaslike and liquidlike properties. The fluid fills the container like a gas, but can dissolve substances like a liquid. Supercritical fluid carbon dioxide is an excellent nonpolar solvent for many organic compounds, including caffeine.
The extraction process is simple. Supercritical carbon dioxide is forced through green coffee beans. Its gaslike behavior allows it to penetrate deep into the beans, and it dissolves 97-99% of the caffeine present.
Coffee manufacturers recover the caffeine and resell it for use in soft drinks and medicines. The caffeine-laden CO2 is sprayed with high pressure water and caffeine is then isolated by a variety of methods, including charcoal adsorption, distillation, recrystallization, or reverse osmosis.