How is coffee decaffeinated, and does it remove all the caffeine?

By washing the caffeine out of green, unroasted beans with water, a solvent or carbon dioxide — and no, it does not remove all of it.

How much remains. Regulations differ by market, but decaffeinated coffee typically must have had at least 97–99.9% of its caffeine removed depending on the standard. A cup of decaf generally contains a small but real amount — commonly cited in the range of a few milligrams, against around 70–140mg in a regular cup. Enough to matter to someone extremely sensitive, and negligible for most people.

The processes:

Solvent-based (direct and indirect). Beans are steamed or soaked, then treated with a solvent — usually ethyl acetate or methylene chloride — which bonds selectively to caffeine. The solvent is then removed and the beans are steamed and dried. Residual solvent limits are tightly regulated and the roasting temperature far exceeds the solvents' boiling points.

Ethyl acetate occurs naturally in fruit, which is why coffee processed with it is sometimes marketed as "naturally decaffeinated" — a description that is technically defensible and somewhat misleading, since the solvent used is typically synthesised.

Swiss Water Process. Beans are soaked in water, which removes caffeine and flavour compounds. The water is passed through activated carbon filters that capture caffeine but not the larger flavour molecules, producing a caffeine-free, flavour-saturated solution. Fresh beans are then soaked in that solution — so caffeine leaves but flavour has nowhere to go, because the water is already saturated with it. Chemical-free and more expensive.

CO₂ process. Carbon dioxide under high pressure becomes supercritical — behaving between a liquid and a gas — and selectively dissolves caffeine while leaving flavour compounds. Efficient, used at scale, and expensive in equipment.

Why decaf has a poor reputation. Historically justified: early processes damaged flavour badly. Modern methods are far better, and the bigger factor now is that decaf is frequently made from lower-quality beans, stored longer and roasted less carefully — which is a commercial choice rather than a limitation of decaffeination.

Decaf is not caffeine-free, and the two terms are not interchangeable on labels.