Taking one molecule out

Make a cup of tea and you have done an extraction: hot water pulls caffeine, colour and flavour molecules out of the leaves, and the leaves stay in the bag.

In solvent extraction, a solvent dissolves the molecules you want and leaves the rest behind. Like dissolves like: water takes up sugars and other polar molecules, with their δ+ and δ− ends, and oily solvents take up non-polar ones. Chemists often shake a mixture with two liquids that stay in separate layers, such as water and an oil, and each kind of molecule moves into the layer it dissolves in best. A separating funnel has a tap at the bottom, so the lower layer can be run off and the two layers kept apart.

Decaf coffee

Caffeine makes up about 1 to 2% of a coffee bean. A cup of ordinary coffee holds about 100 mg of it, and a cup of decaf just a few milligrams. Decaf makers take the caffeine out of green beans, before roasting, because roasting makes the molecules that give coffee its taste and smell. First they soak or steam the beans: the water swells them and gives the caffeine a way out. Then a solvent carries the caffeine away. Caffeine, C₈H₁₀N₄O₂, has two rings that share one side, with four nitrogen atoms and two C=O groups. These make it partly polar, so it dissolves in water and in several other solvents too.

Ethyl ethanoate, the sweet-smelling ester in some nail polish removers, is found in fruit too, and decaf packs call it ethyl acetate. The beans are rinsed with it about ten times, each rinse carrying off more caffeine, then steamed to drive out the last of the solvent, which boils at 77 °C.

The Swiss Water process uses only water. The beans soak in water that already holds every molecule that dissolves out of green coffee, apart from caffeine, at the same strength as the beans. Molecules spread from where they are crowded to where they are less crowded, so the flavour molecules stay in the beans and only the caffeine moves out. Carbon filters catch it, and the water goes back for the next batch.

Above 31 °C and about 73 times the pressure of the air, carbon dioxide becomes supercritical: a fluid that flows into the beans like a gas and dissolves caffeine like a liquid. Decaf plants squeeze it to around 300 times air pressure. It takes up the caffeine and leaves most of the flavour molecules in the beans. When the pressure is let go, it turns back into an ordinary gas and leaves nothing behind in the beans.