Double fermentation coffee is coffee that goes through two separate fermentation stages after pulping instead of one. In the classic version, pulped beans ferment in their own sticky mucilage, are washed, and then ferment or soak again in clean water before drying. The second stage gives a cleaner cup, brighter but rounder acidity, and more sweetness than a single fermentation.
The method is best known from Kenya, where it is part of the reason Kenyan coffees taste so clean and juicy. Producers elsewhere, including in Indonesia, now use two-stage fermentation to add control and clarity. This guide explains how double fermentation coffee is made, how it differs from other fermentations, what the research says, and how to evaluate and brew it.
Last updated: 06 October 2026
What Is Double Fermentation Coffee?
A single-fermentation washed coffee is pulped, left to ferment until the mucilage loosens, washed, and dried. Double fermentation coffee adds a second controlled stage after the first wash. The term is used in two ways:
- Classic two-stage washed (Kenyan style). First fermentation in mucilage, wash, then a second soak or fermentation in fresh water, wash again, dry.
- Modern two-stage fermentation. Any process with two deliberately separate fermentation steps, for example whole cherry fermented first, then pulped coffee fermented again under different conditions such as sealed tanks.
When a spec sheet says “double fermented”, ask which version, how long each stage ran, and whether the tanks were open or sealed. Our guide to coffee processing methods covers the single-fermentation baseline.
How Double Fermentation Coffee Is Made, Step by Step
- Selective picking. Only ripe cherry; under-ripe cherry ferments unevenly.
- Pulping. Skin removed, mucilage left on the bean.
- First fermentation. Pulped coffee rests in a tank, usually without added water, for roughly 12 to 36 hours depending on temperature, until the mucilage breaks down.
- First wash. Mucilage residues washed off in channels with clean water.
- Second fermentation or soak. Parchment goes back into tanks under clean water for a further period, often 12 to 24 hours.
- Final wash and grading. Floaters and defects removed.
- Drying. On raised beds or patios to around 11 to 12 percent moisture, then hulled.
The exact times depend on altitude and temperature. Colder highland stations ferment more slowly, which is one reason high-grown double fermentation coffee can be so clean.
What Happens to Flavour During the Second Fermentation
The first fermentation does the heavy lifting: microbes break down mucilage sugars into acids and alcohols. The second stage, in clean water, does three useful things:
- Removes residues. Leftover sugars and fermentation by-products that could later read as muddy or sour are washed out.
- Allows controlled further development. A short second fermentation lets lactic acid bacteria keep working without the heavy sugar load, which favours rounder lactic acidity over sharp acetic notes.
- Evens out the lot. Beans that fermented at different speeds in the first tank have time to catch up.
The usual result in the cup is greater clarity, juicier acidity, and more defined sweetness. Our post-harvest fermentation guide explains the microbiology in more detail.
Want a cleaner, brighter Indonesian lot for filter? See our wholesale pricelist or contact our team for samples of washed and controlled-fermentation coffees.
What the Research Shows
Most research on fermentation looks at single fermentations, but one study is often quoted in discussions of a second, controlled fermentation step. Wang and colleagues, writing in LWT – Food Science and Technology (2019), fermented already-processed, sterilised green beans from Indonesian semi-dry processed Arabica with the lactic acid bacterium Lactococcus lactis subsp. cremoris for 24 hours at 30°C, with and without 0.5 percent added glucose.
| Treatment | What changed | Cup effect after roasting |
|---|---|---|
| With 0.5% glucose | Total furfurals about 1.4 times higher than untreated | Enhanced caramel aroma, acidity and sweetness preserved |
| Without glucose | Pyrazines about 2 times higher than untreated | Stronger nutty, roasted notes |
| Both treatments | Guaiacol reduced | Less smokiness than the untreated beans |
Two cautions. This was a lab fermentation of green beans after normal processing, which is a related idea rather than the Kenyan two-stage process at the wet mill. And it was a small controlled study, not a farm trial. Furthermore, it does show clearly that a deliberate extra fermentation with selected bacteria can move flavour in predictable directions.
Double Fermentation vs Other Processing Methods
| Method | Key feature | Typical cup |
|---|---|---|
| Single washed | One fermentation, then wash | Clean, bright |
| Double fermentation | Two fermentation stages with a wash between | Very clean, juicy acidity, defined sweetness |
| Anaerobic | Sealed tanks without oxygen | Intense fruit, winey |
| Aerobic fermentation | Open fermentation with oxygen | Fruit-forward, lively |
| Salt water fermentation | Salt added to the fermentation water | Experimental; aims for rounder acidity |
| Carbonic maceration | Whole cherry in CO₂ | Fruit-forward, wine-like |
Double fermentation coffee is the most conservative of the controlled methods. It aims for clarity rather than novelty, which makes it easier to repeat from harvest to harvest.
Double Fermentation Coffee in Indonesia
Most Indonesian Arabica is semi-washed and wet-hulled (Giling Basah), a process built for speed in a humid climate: parchment is hulled at 35 to 40 percent moisture. Double fermentation requires the opposite, more time and fully dried parchment, so it pairs naturally with full-washed, dry-hulled processing.
That means double fermentation coffee from Indonesia is almost always a small, washed, dry-hulled microlot from high-altitude areas such as Gayo or Java, not a standard export grade. The cup moves away from the classic earthy Sumatran profile toward cleaner citrus, stone fruit, and brown sugar. If you want to understand the contrast, cup it next to a semi-washed, wet-hulled lot from the same region; our wet-hulled process guide explains why the two taste so different.
A supplier note: unseasonal rain during the June to August Gayo drying window is the main risk for any washed microlot. Extra fermentation time means wet parchment sits longer, so ask for drying records and moisture at shipment, not just the fermentation protocol.
How to Buy and Brew Double Fermentation Coffee
When buying, ask for:
- Which double fermentation method was used, and the duration of each stage.
- Both Indonesian processing steps: cherry method and hulling method.
- Moisture at shipment and drying method.
- A cupping score for the specific lot on the SCA form.
- A 1 kg sample to cup against a single-fermentation lot from the same origin.
When brewing: double fermentation coffee is at its best as filter. Try a pour-over at 15 g to 250 g of water at 93°C for 2:30 to 3:00, roasted light to medium so the acidity stays bright. Our pour-over brewing time guide and French press ratio guide cover the details.
Frequently Asked Questions
What is double fermentation coffee?
Double fermentation coffee is coffee processed with two separate fermentation stages after pulping. In the classic Kenyan method, beans ferment in their mucilage, are washed, then ferment or soak again in clean water before drying. The second stage produces a cleaner cup with juicier acidity and more defined sweetness.
What does double fermentation coffee taste like?
Double fermentation coffee usually tastes very clean, with bright but rounded acidity, juicy fruit such as citrus or stone fruit, and clear sweetness like brown sugar or caramel. It tastes less funky than anaerobic coffee and cleaner than many single-fermentation washed coffees from the same farm.
Is double fermentation the same as anaerobic fermentation?
No. Double fermentation means two fermentation stages, traditionally in open tanks with a wash between them. Anaerobic fermentation means fermenting in sealed tanks without oxygen. A modern process can combine both, for example an anaerobic first stage followed by a second fermentation, but the terms describe different things.
Why is Kenyan coffee double fermented?
Kenyan washing stations developed two-stage fermentation to get a very clean, consistent cup from high-altitude coffee. The second soak removes residues from the first fermentation and evens out the lot. It is one reason Kenyan coffees are known for bright, juicy acidity and clarity.
Is there double fermentation coffee from Indonesia?
Yes, but in small quantities. Most Indonesian Arabica is semi-washed and wet-hulled for speed, while double fermentation needs fully washed, dry-hulled processing. Indonesian double fermentation coffee therefore appears as washed microlots from high-altitude regions such as Gayo or Java rather than as a standard export grade.
How should I brew double fermentation coffee?
Brew it as filter to show its clarity. Use a pour-over at around 15 g of coffee to 250 g of water at 93°C, finishing in 2:30 to 3:00, with a light to medium roast. Very dark roasts hide the clean acidity that double fermentation is meant to create.
Source Indonesian Fermented Lots from Indonesia Specialty Coffee
Indonesia Specialty Coffee supplies Indonesian green coffee across the full processing range, from classic semi-washed, wet-hulled Sumatra to full-washed, dry-hulled, honey, natural, wine, and controlled-fermentation microlots from Gayo and Java. Every lot is graded against SNI 01-2907-2008, cupped at 82 to 88 SCA points, and Halal certified, with Organic and Rainforest Alliance on request. We export from Medan, FOB Belawan.
Order tiers run 1 kg for a cupping sample, 60 kg for a microlot rate, 350 kg for standard wholesale, and container loads from 9 MT.
Moreover, ask which washed and fermented microlots are available this season. See our current wholesale pricelist or contact our team.
