How Do They Do It: Coffee Processing and Fermentation - and what our stance on that is at Drip
From Cherry to Cup: How Processing and Fermentation Shape Your Coffee
Roast, brew method, variety, and altitude all shape a coffee's flavour, but one of the biggest influences happens before any of them: what the producer does with the cherry in the days after harvest. Two lots from the same farm can taste like completely different coffees depending on how the fruit is removed, how the seed is dried, and which microbes get involved.
Processing and fermentation aren't the same thing
The two words get used interchangeably, but they mean different things.
Processing is the series of agricultural and mechanical steps that turns a fresh cherry into a stable, exportable product: dry parchment or dry cherry.
Fermentation is a natural event, carried out by microbes feeding on the fruit's sugars. Fermentation happens inside nearly every processing method, but not all processing is fermentation. Even the washing step exists mostly to rinse away what fermentation has broken down.
The three classic processes
The main methods differ mainly in when the fruit comes off the seed.
Natural. The oldest method, and the one with the least intervention. Whole cherries are dried on raised beds or patios, raked regularly, for up to a few weeks. Fermentation happens spontaneously inside the drying fruit, driven by whatever microbes live in that environment. Nobody has studied exactly which species are involved, but it's likely a mixed community of yeasts and bacteria that varies by region. Naturals are known for heady, fruity cups, from delicate peach to jammy blueberry and wine-like notes. They need dry, warm weather and lots of labor, and rain or uneven drying can turn a fruity cup musty, boozy, or vinegary.
Washed. The most common method worldwide, and likely what's in your bag if the label doesn't say otherwise. Within about a day of harvest, a depulper removes the skin and most of the flesh. The seeds, still coated in mucilage, ferment in a tank for anywhere from around 8 hours to a couple of days (sometimes longer if submerged in water). Then they're washed and dried. Washing is faster, takes less labor, and carries less spoilage risk than drying whole fruit, and floating bad cherries are naturally sorted out along the way. The tradeoff is water use, which is why it's rare in water-scarce regions like Yemen. The result is usually clean, bright, and terroir-driven, with citrus and stone fruit more likely than berry.
Honey (pulped natural). Developed in Brazil, where it's known as pulped natural, and later refined by Costa Rican producers. The skin and pulp come off but some mucilage stays on the seed while it dries. Despite the name, there's no honey involved. Producers use colors for how much mucilage remains, usually yellow, red, and black, and sometimes white and purple. White honey can be nearly indistinguishable from washed coffee, while black honey can echo a natural, with raisin and grape notes. The ones in between tend to be sweet and creamy, with caramel, plum, and ripe-cherry character.
What's happening at the microscopic level
Microbes are nearly everywhere, and fermentation can begin before the cherry is even picked. The more fruit exposed to the environment, the faster it goes. As yeasts and bacteria consume sugars, they leave behind alcohols, acids, and other compounds that end up in the seed. In the roaster, those compounds transform into the flavors we taste.
- Yeasts turn sugars into alcohol and CO₂ and are often linked to fruity, floral, and winey notes.
- Lactic acid bacteria produce lactic acid, which can add rounded, creamy acidity. They are thought to dominate in traditional washed tanks.
- Acetic acid bacteria produce the sharp, vinegary notes that producers try to avoid.
When fermentation goes wrong: the "ferment" defect
Cuppers are trained to catch a defect called ferment, which shows up as overripe-fruit flavours that tip into vinegar, rot, or compost. It can affect any process but is most associated with washed coffees left too long in the tank, or fermented in poorly cleaned facilities. Some researchers describe it as a microbial attack on the bean itself rather than just the surrounding fruit, and have suggested defining it chemically (for example, by thresholds of certain ethyl esters) rather than by taste alone.
Modulations: steering the process
Newer techniques are best understood as modulations, adjustments layered onto a basic process. They can be added to natural, honey, or washed coffees, depending on when the skin comes off.
Anaerobic fermentation and carbonic maceration. Coffee is sealed in a tank, usually stainless steel with a one-way valve for gas. As the fruit ferments, CO₂ builds up and pushes out oxygen, favoring different microbes than an open tank would. Carbonic maceration, borrowed from winemaking (think Beaujolais), usually means whole cherries fermented in a sealed tank. "Anaerobic" is looser: it can describe depulped coffee or whole cherries, and tells you little about the rest of the process. The cups are often intensely fruity and sweet, with notes of candy or dessert wine.
Extended fermentation. The producer deliberately holds the cherries, in bags or a reception tank, for hours to days before depulping, or lengthens the tank time. This runs against conventional wisdom: much of the coffee research has aimed at removing mucilage quickly (a rapid fermentation of around 8 hours is often considered optimal). Done carefully, though, longer fermentation can build more elaborate flavors without major equipment investment.
Multi-stage fermentation. Several steps in sequence. The most common is the post-fermentation soak in clean water, often used in East African washed coffees ("double washed"). Others include a brief whole-cherry maceration before depulping, or a dry fermentation followed by a submerged one ("double fermented").
Thermal shock. Another modulation rather than a type of fermentation. Protocols vary, but one example heats ripe cherries to around 70°C briefly, then pulps and ferments them in a sealed, temperature-controlled tank, with pH monitored as fermentation proceeds. The aim is to stop when the mucilage sugars are used up and before alcohol builds. Producers describe an especially sweet, fruity, rounded cup. Thermal shock can also be applied before pulping or during drying, depending on the protocol.
Yeast and culture inoculation. Instead of leaving things to native microbes, producers add selected yeast or bacterial cultures, called starters, to steer flavor and make results repeatable. This is standard in large-scale winemaking, where yeasts are chosen to highlight a grape variety. In coffee it's still fairly rare, though suppliers are developing coffee-specific strains. Since she switched from wine production to coffee in 2014, Lucia Solis - specialised in microbiology and fermentation sciences - has been bringing her knowledge to producers so that they can provide added value in processing and limit risks during fermentation.
Koji. Koji is the fungus behind soy sauce, miso, and sake. Applied to cherries or parchment, it breaks down starches into fermentable sugars and produces glutamate, which is said to add complexity, structure, aroma, and sweetness. It's still used on only a handful of farms.
Co-fermentation. Cherries are fermented with fruit, spices, or other ingredients, sometimes sold as "infused" coffees. Because microbes transfer flavors as they work, producers can end up with notes like strawberry, cinnamon, or even chili without adding flavoring after roasting. It's controversial in some circles, and it raises the labeling questions below.
The debate: terroir, transparency, and repeatability
Experimental coffees have won competitions, brought in new drinkers, and earned many producers higher prices. They also raise fair questions.
- Does processing overshadow origin? When fermentation is intense, coffees from very different places can start to taste alike.
- What's actually in the bag? "Anaerobic" or "experimental" can cover very different practices. Good producers and roasters share process type, duration, and any additives.
- Can it be repeated? Temperature, humidity, cherry ripeness, and microbial populations all shift. Consistency takes careful monitoring, and one mistake can ruin a lot.
How to taste the difference
- Brew a washed and a natural coffee from the same country the same way.
- Compare body and sweetness first, then acidity, then aromatics.
- Try an anaerobic or co-fermented lot, and notice how far it moves from what you'd expect of the origin.
You don't have to pick a favorite. Washed coffees show structure, naturals bring exuberance, and experimental lots are a flavor adventure. Knowing what's behind the label helps you find what you like.
Final thoughts
Processing is where agriculture meets craft, and fermentation is the invisible engine inside it. The most interesting producers aren't chasing novelty for its own sake. They use these tools to bring out what's already good in their coffee, and they're honest about how they did it.
At Drip Coffee, we are keenly aware of how different fermentation techniques can influence the coffee; we deliberately select coffees where the fermentation profile does not overpower the cup, allowing the intrinsic flavors to shine through—something that is certainly possible even with experimental processing methods.