PROCESSING THE GREEN COFFEE @museum.de

The coffee fruits need to be processed straight after picking. It’s the only way to ensure that the desired aromas develop later on. Processing requires fermentation, so two factors are crucial: how ripe the fruit is, and how long it’s allowed to ferment. If it’s left for too long, the coffee pips start to rot, and the result is a “stinker”. And yes, that’s the technical term.

There are various processing methods, each of which affects the quality of the coffee. In wet processing, also known as "fully washed", the pulp is squeezed off the seeds straight after picking.

Then the seeds are transferred into containers full of water. Depending on the climate, it takes up to three days for any remaining fruit pulp to decay. That leaves the clean pips in their yellow, parchment-like husks. Those wet pips then need to be regularly turned as they’re left to dry in the sun.

On the table, you’ll see a simple, wooden machine. It’s a mechanical depulper, also sometimes called a peeler. Inside the machine, there’s a studded roller that’s turned with a crank. The coffee fruit is decanted into the hopper, and then the pulp is stripped with the roller. Machines like this one are still in use on small farms. This particular exhibit is from Tanzania.

Next to it is a long, wooden trough to hold water: a fermentation tank. Wet processing uses a lot of water. In Kenya, it might be as much as 300 litres (or 60 gallons) for a single kilogramme (35 ounces) of coffee. And the water can’t be reused because of toxic fermentation residues.

Dry processing, also called "natural" processing, is the simplest method of preparing the coffee fruit. The whole fruits, skin and pulp included, are dried in the sun right after being picked. That can take up to 30 days. Later, the dry, brown fruit residues are removed by machine.

That process saves water, but the fermentation is uncontrolled and often, all the fruits, whether unripe, ripe or overripe, are dried at the same time. The resulting coffees have more flaws and are less aromatic. Dry-processed coffees are less profitable than those processed by the wet method described earlier. But recently, other methods known as semi-dry or natural-pulped processes have been developed and achieve very good coffee qualities while using only a small amount of water.