Washed (wet) process

From cherry to dry parchment

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The order below follows the FAO guidance and the description in a 2019 trial of wet processing. Not every mill uses every step.

What the fermentation is for

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The FAO guidance is blunt about it: fermentation may contribute to coffee quality, but its primary purpose is to enable the mucilage to be removed, and the shortest fermentation that loosens the mucilage enough for washing is the best one. It adds that how fast this happens varies with the microbes that arrive on the cherries and with the temperature. The international code of practice for the same subject gives twelve to thirty-six hours as its range and warns that the step has to be watched so the coffee does not turn sour.

The same guidance records a disagreement. Fermentation and sun-drying were both once considered essential for good coffee, it says, and that is now disputed, as machines that remove the mucilage and mechanical dryers have become widespread. It also says that many claims connecting one processing step or another to quality are not supported by any objective comparison of the alternatives.

Fermentation itself is covered at length in the guide to coffee fermentation.

What one large trial found

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A 2019 study is unusual in having tested the choices a wet mill actually makes, at something like mill scale. At a farm in Yunnan, China, two batches of two tonnes of Catimor cherries were each split in two. Half was pulped and fermented under water until the mucilage came away, which took 36 hours in one batch and 48 in the other; half of that was then left in the tank to 84 hours. The other half had its mucilage removed by machine and was then held under water for 12 hours, the local practice, or for 72. Some of each was soaked for a further day. The coffees were roasted and scored on 27 attributes by twelve trained panellists at a coffee company's research centre.

The authors report first that the scores were close to each other across all the samples. Within that, of the three choices, the length of the fermentation changed the green coffee and the cup the most: a long fermentation raised the scores for fruity odour and flavour and for acidity, and lowered those for floral and cereal odours. Removing the mucilage by machine gave a cup the authors call comparable to the fermented one, though it was significantly less fruity in odour and scored higher for an earthy flavour. Soaking made the green coffee from the different lots more alike in composition; in the cup it raised overall flavour intensity and lowered a spicy odour.

The authors add the limit themselves: how strong each effect is would depend on the variety and on local conditions. This was one variety, at one farm, in one cold season. It shows that fermentation time can change the cup, by a margin a trained panel could detect in scores the authors themselves call close. It does not show that longer is better, or that the same hours would do the same thing somewhere warmer.

What it costs in water

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Wet processing uses water for pulping, for fermenting where that is done under water, and for washing, and the water that comes out carries the sugars and pectin of the fruit. A 2024 study of wet mills along one river system in southern Ethiopia found them using about 22 litres to wash a kilogram of coffee where the mill's recycling system worked, and 63 litres on average in practice, because many recycling systems did not. It tabulates other authors' figures from several countries that run from about 22 to 90 litres. The paper does not say whether its kilogram is of cherry or of finished coffee, so the figures are useful for their spread, not as a rate.

The same study describes what the untreated water does downstream: its organic load and acidity damage the rivers that receive it. The 2019 trial notes that a machine which removes the mucilage cuts both the water used and the time taken, which is the main argument for it. The FAO guidance adds one caution about such machines: those that use little water tend to leave more beans stripped of their parchment, and bare beans go mouldy quickly.

Washed coffee and the cup

Tendency, not a guarantee

The trade describes washed coffees as clean and bright, and uses the method as shorthand for a cup in which the fruit has left little trace. That is a description of what buyers expect, and the FAO guidance goes no further than saying that parchment coffee has different sensory qualities from coffee dried in the cherry. None of the documents read for this page compared washed and natural coffee from the same fruit with a tasting panel.

What the documents do show is that washed is not one thing. The same cherries, in the trial above, gave detectably different cups depending on how long they were fermented and how the mucilage was removed. A label that says washed tells you the fruit came off before drying. It does not tell you which of those coffees you have.

The neighbouring methods

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• Natural — The whole cherry is dried and the fruit is hulled off afterwards. No pulping water, but close to twice as much water to dry out of the crop.
• Pulped natural and honey — Pulped as here, then dried at once with the mucilage still on the parchment. No fermentation tank and no washing.
• Wet-hulled — Pulped, fermented and washed as here, then hulled while still wet, so the bare bean is what gets dried.

What this page leaves out about washed coffee

CoffeeHQ explanation

A fermentation time that is correct. The code of practice gives a range and the guidance says shorter is safer; the one trial read found longer changed the cup, at one site.

How washed coffee tastes compared with natural coffee from the same fruit. No controlled comparison was read.

How much of the world's coffee is washed, how the soak used in Kenya and elsewhere affects the cup beyond the one trial described, or how much water a given mill uses.

What to do next

• Coffee fermentation — Which microbes do the work in the tank, what they change in the seed, and what inoculating with a chosen yeast did in one trial.
• Drying coffee — The stage after washing, and the one where the guidance places most of the risk.
• Washed versus natural — The two main methods side by side, with a tasting you can set up yourself.
• Colombia — A producing country of washed coffee, with what producing it costs the grower.
• Become a coffee expert — A structured programme from the plant and its processing to roasting, water and tasting.