Every fermentation, explained.

A coffee cherry is a fruit with two seeds inside. Between the seed and the skin sits mucilage — a sticky layer of sugar and pectin. The moment the cherry is picked, yeasts and bacteria start eating that sugar, producing acids, alcohols and aromatic compounds. Some of those compounds migrate into the seed. That is fermentation: controlled decay, harnessed for flavour.

Every coffee you have ever drunk was fermented. The only question is how long, at what temperature, with how much oxygen, and with what else in the tank. Everything below is a different answer to those four questions.

Washed. The skin and mucilage are removed mechanically within hours, usually after a short tank ferment of 12 to 36 hours to loosen the last of the pectin. The seed then dries naked. Because the fruit is gone early, washed coffees taste like the variety and the soil: clean, transparent, higher-toned acidity, lighter body. It is the reference process — the control against which everything else is judged.

Natural (dry process). The whole cherry is dried intact, fruit and all, over two to five weeks. Fermentation happens slowly inside the fruit as it shrivels. The result is heavier, sweeter and fruitier — berry, dried fruit, rum. Done badly it turns to compost and alcohol; done well it is the most generous cup on the table.

Honey (pulped natural). The skin is removed but some or all of the mucilage is left on to dry. The names refer to how much stays and how slowly it dries: yellow honey (most mucilage stripped, quick dry), red honey (more left on, shaded), black honey (nearly all of it, dried slowly under cover). More mucilage means more sugar, more fermentation time, more body — honey sits deliberately between washed clarity and natural sweetness.

Anaerobic fermentation. Cherry or pulped coffee is sealed in a tank with a one-way valve. Oxygen is used up in the first hours, after which only bacteria and yeasts that work without it survive. The microbial cast changes, and so does the flavour: less vinegary acetic edge, more lactic and fruit-ester character. Anaerobic can be applied to a washed, honey or natural lot — it is a condition, not a process in itself, which is why 'anaerobic natural' is a complete description and 'anaerobic' alone is not.

Carbonic maceration. Borrowed from Beaujolais winemaking. Whole, unbroken cherries are sealed in a tank flushed with CO₂. Fermentation begins inside each intact fruit rather than in the surrounding juice, which produces a distinctive set of aromatics — candied red fruit, bubblegum, cinnamon. The signature is unmistakable and, at its worst, uniform: badly run carbonic lots all taste like the tank rather than the farm.

Extended and cold fermentation. Simply running the ferment longer, or holding it at 12 to 18 °C so it runs slower. Time buys complexity; temperature buys control. Beyond a point it buys defects — a vegetal, onion-like edge that never leaves the cup.

Lactic fermentation. An anaerobic ferment steered toward lactic-acid bacteria, usually by keeping oxygen out and temperature low. The acidity turns creamy and rounded rather than bright and sharp — yoghurt, cultured butter, stone fruit — with a noticeably thicker body.

Yeast inoculation. A specific commercial or wild-isolated yeast strain is added at the start so it outcompetes whatever arrived on the fruit. This is the most underrated technique in specialty coffee: it is mostly used not to create fireworks but to make results repeatable from tank to tank.

Double fermentation. Two distinct stages, usually a sealed anaerobic phase in cherry followed by a second ferment after pulping, often with the water changed in between. Each stage can have its own temperature and duration, which is where the modern competition lots come from.

Thermal shock. Between stages the coffee is washed with hot then cold water. It halts microbial activity abruptly and, its practitioners argue, locks in the aromatics of the first ferment before a second one blurs them.

Co-fermentation means fermenting coffee together with another substrate, so its sugars and its microbes join the tank. Fruit pulp or juice — orange, passionfruit, mango, cascara from the coffee's own skins. Kombucha or a live SCOBY culture, which brings an acetic-lactic community with it. Wine or beer must, cane molasses, honey. Spices, cacao husk, or barrel staves.

Done seriously, the additive is a food source that shifts which microbes dominate; the flavour that arrives in the cup is created by fermentation, not donated by the ingredient. Done lazily, the coffee is just soaking in juice.

Ageing is the slow cousin: parchment or green coffee rested in whisky, rum or wine barrels, or held in controlled humidity for months. Time and the wood do the work, after fermentation has finished.

An infused coffee has flavour compounds added to it, usually after fermentation is complete: a botanical or synthetic aroma sprayed on parchment, or green coffee soaked in a concentrate. Nothing is fermented; something is applied. That is not cheating, but it is not the same craft, and the two should never share a label.

The honest test is simple: could a blind taster find that flavour in the fruit's own chemistry? Cinnamon in a carbonic Bourbon, yes. Precise, unwavering strawberry candy in every bag of a 40-tonne lot, no. We do not buy infused coffee, and we say so plainly rather than pretending the category does not exist.

Decaf is processing too, applied to green coffee after it leaves the farm. Sugarcane (ethyl acetate) decaffeination, which we use, dissolves caffeine in a solvent derived from fermented cane molasses — a fermentation product doing the work. Swiss Water and mountain-water methods use only water and carbon filtration. CO₂ methods use pressurised carbon dioxide. All of them remove around 97 to 99.9 per cent of the caffeine; what differs is how much flavour survives the trip.

A useful process description names the vessel, the condition, the duration and anything added: 'sealed anaerobic natural, 96 hours at 18 °C' tells you more than 'experimental'. When a bag says only 'special process' or 'funky', the producer's work is being hidden rather than described.

And none of it is a hierarchy. A perfectly executed washed Bourbon can be more interesting than a seventy-two-hour co-ferment. Process is a set of choices, not a ranking — the point is to know which choice you are tasting.