This time, the washed process: crisp, refreshing, and liked by almost everyone. What does "washed" mean, what exactly is being washed, and why does it taste so clean? This article stays with what is specific to washed coffee and how we do it on our farm. The mechanics of fermentation itself (the four stages, how pH falls, how acids form) are gathered in The four stages of fermentation and Reading the fermentation pH curve.
As the name says, washed coffee is washed before it is dried. Our working definition is this.
1. Pulp the cherries (remove the skin),
2. ferment the parchment with the mucilage (the slippery fruit layer) still attached,
3. wash until no skin or pulp remains,
4. dry.
After harvest the cherries are washed once and pulped into mucilage-covered parchment. A fermentation step follows. Dry it without washing and you have honey; wash it and you have washed.
There are two main ways to ferment mucilage-covered parchment.
1. Put only the parchment in the tank (dry fermentation). It is done when the sticky mucilage no longer clings to your hand.
2. Put the parchment and water in the tank (wet, or underwater, fermentation). It is done when the mucilage lifts off the parchment.
The difference is whether you add water. Either way, fermentation finishes in about 20 to 36 hours. The longer and slower you can ferment, the better the flavour, but beyond 36 hours the risk of mould rises sharply. Too short, and it ends before the microbes have developed properly, leaving a grassy flavour. That is why we set 20 to 36 hours. The more water you use, the more sugar leaches out, so you also have to make sure the microbes do not run out of food. What rises and what falls in underwater fermentation is in Underwater fermentation in practice.
Our 20 to 36 hours is a rule of thumb, but in recent years the optimum has been measured head-on. In an experiment that fermented Yunnan arabica in open tanks and varied only the time, the 36-hour lot scored highest in the cup (77.25 points) and both shorter and longer lots scored lower (Yuan et al., Fermentation 2024). Fermentation has a summit. Get off before it, or walk past it, and the cup drops.
That optimum is not a fixed number, though. At higher temperatures you reach the same point sooner, and sealed versus open tanks move it too. So we decide when to stop by measuring the progress itself, with pH, rather than by time. Go by the clock alone and on a hot day you walk into over-fermentation.
In a washed tank, lactic acid bacteria take the lead. A study tracking the microbes of washed fermentation found lactic acid bacteria such as Weissella and Leuconostoc increasing throughout and driving the process (Shen et al., Molecules 2023). Lactic acid bacteria turn sugar into lactic acid. That is one source of the "clean acidity" of washed coffee. Which acids appear when, and where over-fermentation begins, is in Fermentation time and organic acids.
The same study tells us something to watch. Among the fungi in a fermentation tank can be Aspergillus, a source of the mycotoxin OTA (Shen et al., 2023). So the design of washed coffee comes down to this: run a fermentation led by lactic acid bacteria, stop it at the right time, and dry it cleanly and quickly. It needs no flair. It needs discipline.
There is a reason washed coffee tastes clean. Because the mucilage is removed early, there are fewer fruit-derived fermentation aromas (esters and the like), and the bright acidity of the bean itself comes forward. Fermentation is the process of turning sugar into acid, and deciding how far to take it, by choosing when to stop, is the maker's job.
Washing and soaking are not just clean-up. Through osmosis they rinse simple sugars and microbial metabolites away from around the bean, lowering acidity, softening the cup, and evening out the lot (De Bruyn et al., AEM 2017). If you want to keep individuality, soak lightly; if you want cleanliness and uniformity, let the soak work. It is a lever you can choose in the design. Such differences of method remain in the bean as an aromatic "fingerprint" that instruments can tell apart (Suleria et al., Fermentation 2023).
Washed coffee uses water, so we pay attention to its quality and quantity. Ferment or wash in dirty water and you invite unwanted microbes and off-flavours. A clean cup starts with clean water. Water that has been used becomes an environmental load if simply discharged, so we think about recirculation and treatment where we can. Pulp and fermentation liquid that would be thrown away can sometimes be used as a starter for the next fermentation. Caring for water protects both the taste and the environment.
What we put real effort into is washed coffee made with mossto. Mossto is the cherry juice produced during anaerobic fermentation, packed with the sugar of ripe cherries and the microbes that grew in it. Soak the parchment in it, then wash the mucilage away at the end, and you get a washed coffee with the complexity of mossto. More in the article on mossto.
Washed coffee has a crisp, light mouthfeel and a fruitiness that lifts through the nose after you swallow. It suits the first cup of the morning and a refreshing cup after lunch. Even within washed coffees, farm and variety make a large difference, so please also look at the aroma, flavour and body notes on each product page.
It moves with the environment, but in open-tank research 36 hours was the peak of cup quality and both shorter and longer lots scored lower. We work within 20 to 36 hours and stop by pH, not by time.
Because the mucilage is removed early there are fewer fruit-derived fermentation aromas; a fermentation led by lactic acid bacteria makes clean acids; and washing and soaking rinse away surplus compounds and even out the cup.
Whether water is added to the tank. Underwater fermentation holds temperature more steadily and research has identified characteristic aroma compounds. But the more water you use, the more sugar leaches out, so you must watch that the microbes do not run short of food.
For the whole mechanism of fermentation see The four stages of fermentation; for management by pH, Managing fermentation by numbers. For methods that dry without washing, see natural and honey.
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