ethyl acetate or carbon dioxide (CO2). Neither method has consistently produced satisfying flavor in the cup. Ethyl acetate, a synthetic fruit ester, leaves a fruity aftertaste in the coffee--unfortunately nothing like the berry and citrus flavors we find in East African coffees. And we had high hopes for the CO2 process in the early '90s. Carbon dioxide is the carbonation in sparkling water, but it is forced into the coffee at pressures well in excess of 1000 pounds per square inch to extract the caffeine. Perhaps it's the pressure that also forces out the coffee flavor.
The last method to discuss is dihydro-oxide --water. At one time, water process was the most damaging to coffee flavor. That general statement is no longer true, due to improvements in the processing by some companies. Although some water-process decaf has flavor approaching methylene chloride (and a relatively new North American company is making great strides in cup quality), further development will be required before it can be methylene chloride's equal.
In general the process uses water as the solvent, supersaturated with soluble solids from green coffee beans, except caffeine. The idea is that when the warm solvent is circulated through the coffee, it will extract only caffeine, which in turn is removed from the circulating fluid with activated carbon. It's a thesis that in practice has not produced great cups of decaf.
The old standby water decaffeination company, Swiss Water (the only attempt to brand a process), of Vancouver, Canada, is doing all the advertising while others are improving their process. We prefer the cup quality of other companies, and I deplore the marketing tactics of Swiss Water.
In the late '80s, when I first wrote to the previous owners, I decried their deliberately misleading advertising. Here they go again. They are falling back into advertising tactics that assume the ignorance of the audience. Their attempt to associate the chemical names of the other processes with some chemo-hysteria is unethical.
To summarize, of the four major processes for decaf, only methylene chloride and water are widely used in specialty coffee (here, by the way, Wikipedia on decaffeination is less strong than it is on caffeine). Methylene chloride can produce the best cup results when good coffee and careful processing are used. Good coffee and careful processing also produce the best results from dihydro-oxide, but the best is still second in cup quality to MC.