Processing Methods

A coffee bean is the seed of a fruit. Everything between picking that fruit and drying the seed is processing, and a 2019 review of the post-harvest literature concluded it shapes the aroma of the final coffee as much as origin or variety does¹.

The cherry has, from outside in: skin, sticky fruit pulp (mucilage), a parchment layer, a silverskin, and the two seeds². Processing is about when and how each layer is removed, and how long the seed sits in contact with sugary, fermenting fruit.

Washed (wet process)

The skin and most of the pulp are stripped mechanically (depulped) within hours of picking. The remaining mucilage is removed by fermentation in tanks, typically for 12–72 hours depending on climate and practice — microbes break down the sticky layer — then the parchment coffee is washed and dried on patios or raised beds¹².

• Cup: clean, bright, transparent. Acidity and origin character are front and centre³.
• Brewing: the most predictable process. Extracts evenly and rewards high extraction. This is Cup Engineering practice.
• Where: Colombia, Kenya, most of Central America, much of Ethiopia³.

Natural (dry process)

The whole cherry is dried intact, on beds or patios, for two to four weeks with constant turning. The seed ferments inside the fruit as the sugars concentrate, then the dried husk is milled off². Because the seed stays in contact with the fruit for so long, the natural process produces the largest shift in aroma precursors of the traditional methods¹.

• Cup: fruit-forward — berries, wine, tropical fruit; heavier body, lower perceived acidity, more sweetness. Poorly managed naturals can taste fermented, boozy, or "barnyard"³.
• Brewing: naturals extract easily and tend to have more fines; go a touch coarser or cooler than a washed coffee of the same roast. Cup Engineering practice, consistent with practitioner guidance⁴.
• Where: Ethiopia, Brazil, Yemen; increasingly everywhere³.

Honey / pulped natural

Depulped like a washed coffee, but some or all of the mucilage is left on during drying. Named by how much is left: white/yellow honey (most removed) through red and black honey (nearly all left on, dried slowly in shade)³.

• Cup: in between — the sweetness and syrupy body of a natural with more of the clarity of a washed coffee³.
• Where: Costa Rica, El Salvador, Brazil ("pulped natural")³.

Anaerobic and extended fermentation

Cherries or depulped beans are sealed in tanks with limited oxygen, sometimes with added yeast, fruit, or CO₂ ("carbonic maceration"). Fermentation runs for days under controlled conditions¹.

• Cup: intense and unusual — tropical fruit, cinnamon, rum, bubblegum, "funk". Divisive and expensive.
• Brewing: these are often brewed as espresso or with immersion methods where the intensity works; extract carefully as the fermented notes can turn harsh at high extraction. Cup Engineering practice.

What the chemistry says so far: a 2026 study from the Yunnan Academy of Agricultural Sciences split one batch of Catimor cherries into washed, natural, anaerobic-then-washed and anaerobic-then-natural treatments and ran untargeted metabolomics on the green beans. The anaerobic natural treatment enriched the largest number of flavour-precursor compounds, and cupping scored it above the washed lot on most attributes⁵. That is one study, on one variety, from one farm, so treat it as a mechanism worth watching rather than a rule; the 2019 review is clear that fermentation outcomes depend heavily on the microbial population, temperature and duration¹.

Wet-hulled (giling basah)

Unique to Indonesia. The coffee is depulped, briefly fermented, then hulled from its parchment while still at high moisture (commonly quoted as 30–40 %) and dried as bare green beans³.

• Cup: earthy, herbal, cedar, heavy body, very low acidity. Sumatra Mandheling is the archetype³.
• Brewing: suits darker roasts, immersion, and milk; rarely rewards light-roast pour over treatment. Cup Engineering practice.

Decaffeination

Not a processing method in the same sense, but it happens to green coffee. Swiss Water and mountain water processes use water and carbon filters; sugarcane / EA process uses ethyl acetate derived from sugarcane and preserves sweetness well; CO₂ process uses supercritical CO₂³. Decaf green coffee is browner and more fragile, roasts faster, and is easier to over-roast³ — brew it a little cooler, which is Cup Engineering's practical suggestion.

Matching process to recipe

The recipe library lets authors specify a preferred process. The table is Cup Engineering's rule of thumb, drawn from the cup descriptions above rather than from a study: