Coffee Processing

Cherry fermentation happens before the coffee has been depulped, while the seed is still contained inside the whole cherry.

The cherries might sit in a pile or bags while waiting to be processed. During this time, they bruise, heat up and begin fermenting. Because the skin, fruit and mucilage are all still present, there is a lot of fruit matter available to influence the fermentation.

Sometimes the cherries are sealed inside bags or tanks to ferment with minimal oxygen. This is often described as anaerobic cherry fermentation, although sealing coffee inside a bag does not necessarily create a truly anaerobic environment. It is more accurately an oxygen-restricted fermentation.

Cherry fermentation deserves its own heading because it happens at a different stage from dry or wet fermentation. The coffee has not yet been depulped and all of the cherry is still involved.

Aerobic fermentation means that oxygen remains available during fermentation. In coffee processing, this is often some form of cherry fermentation or dry fermentation in a pile.

The cherries may be left sitting together before depulping, or the coffee may be depulped and left with its mucilage attached. Oxygen remains present around the coffee and contributes to the fermentation.

These fermentations are not always perfectly or consistently aerobic. Even inside an open pile, the centre can become warmer and much more oxygen-restricted than the surface. Aerobic and anaerobic are often better understood as degrees of oxygen availability rather than two perfectly separate states.

Dry fermentation happens after the coffee has been depulped. The skin and most of the fruit have been removed, but mucilage remains attached to the seed. The coffee is then held in a pile or tank and allowed to ferment without water being added.

It can initially look as though the coffee is about to be laid out for honey processing, but it is instead held together and fermented in its mucilage before being washed.

This is common in Burundi. The depulped coffee may sit dry-fermenting inside an empty washing tank before water is added. It is then washed and continues through the fully washed process.

Dry fermentation tends to give coffee more dried-fruit or cooked-fruit characteristics. The acidity can remain, but the fruit becomes less fresh and a little fuller, fleshier and sweeter. We experience the acidity being mildly dialled down rather than completely lost.

Longer dry fermentation can intensify the coffee’s profile. This is one reason some washed coffees can present with far more fermentation character than the words “washed coffee” might suggest.

Wet fermentation happens when depulped coffee is fermented with water present.

Water limits the availability of oxygen, particularly as fermentation continues, but a tank of water is not automatically a completely anaerobic environment. The amount of oxygen still depends on the tank, how it is filled, whether it is open or sealed and how the coffee is being handled.

Extended wet fermentation can give coffee a more wine-like style of acidity. The acidity and sweetness become more integrated, rather than feeling like two separate parts of the coffee.

If it continues too far, the coffee can become olive-like and briny. Those are less desirable characteristics and can indicate that the fermentation has moved beyond the point where it was improving the coffee.

The water temperature, the amount of time spent in the tank and whether the coffee is flushed with fresh water can all change the result.

Anaerobic fermentation means that coffee is fermented in an environment with little or no available oxygen. It can happen while the coffee is still inside the cherry or after it has been depulped.

The important thing about anaerobic processing is that restricting oxygen can allow more time to pass without certain undesirable fermentations taking over. When the temperature is kept low, a producer can encourage particular types of fermentation to continue for longer.

Time and temperature are closely linked. At lower temperatures, fermentation slows down and can continue for longer. If the temperature is high, the time generally needs to be shorter. Otherwise, more acetic and undesirable characteristics can begin to develop, while the coffee’s complex acidity can diminish.

Anaerobic fermentation is not one flavour and it is not a complete processing method. Whole cherries, depulped coffee or coffee with mucilage attached can all be fermented in oxygen-restricted conditions. The word only describes one part of the environment in which the fermentation occurred.

Extended fermentation simply means that the producer has allowed a fermentation stage to continue for longer than would normally be expected.

It does not tell us whether the coffee was fermented whole or depulped, wet or dry, aerobically or anaerobically. It also does not tell us the temperature. All of those things are necessary if we actually want to understand what happened.

Longer fermentation can bring up deeper red, purple, dried or cooked fruit tones. It can also soften some of the greener or more vegetal characteristics found in particular varieties.

Longer is not automatically better. If fermentation continues too far, the coffee can become boozy, gamey, olive-like, briny or acetic. Temperature makes an enormous difference to how quickly those characteristics develop.

Carbonic maceration is adapted from winemaking. Whole, intact cherries are placed inside a sealed tank and the oxygen is displaced with carbon dioxide. Some fermentation then begins inside the intact fruit itself.

Maceration more generally means that fruit has been broken down or crushed, releasing its juices. If coffee cherries are macerated before being sealed away from oxygen, that is more accurately an anaerobic fermentation of macerated fruit than carbonic maceration in its strictest sense.

Carbonic maceration coffees tend to have an intense, sharp and concentrated flavour. We associate some of that intensity with the extended skin contact and the way the whole cherry ferments.

They can still retain plenty of acidity, but we often find the acidity more aggressive. The flavours become darker, more concentrated and sometimes more complex and grape-like. It is a distinct profile that can become more recognisable than the coffee’s variety or growing conditions.

In fully washed processing, the skin and fruit are removed before the coffee is laid out for final drying. Fermentation or mechanical demucilaging may be used to remove the remaining mucilage before the coffee is rinsed with clean water.

A very clean washed coffee is picked, depulped and placed into washing tanks within a short period of time. The mucilage is removed relatively quickly, the coffee is flushed with clean water and then laid out to dry. These fast times and fresh-water washes minimise fermentation.

When processing coffee this way, we are left with a more bare and authentic picture of its variety and growing conditions. It is like having a really great camera dialled in correctly and looking at a more raw result with minimal colour grading or editing.

However, “washed” does not automatically mean minimal fermentation. A coffee may undergo cherry fermentation, extended dry fermentation or wet fermentation before its final wash. It is still classified as washed because of how the fruit is removed before drying, even when the earlier fermentation has drastically shaped its flavour.

This is something to keep in mind when buying coffee. Processing is often more complex than simply “natural” or “washed.”

Natural processing means that the coffee is dried inside the whole cherry.

A very clean natural coffee is laid onto raised beds soon after picking. The cherries are spread in thin layers, turned regularly and given plenty of airflow to minimise heat. Fermentation will still occur and the coffee will retain natural-process character, but careful drying reduces the chance of defects and undesirable fermentation flavours.

Natural coffees can take 40 days or longer to dry. During that time, the fermentation and prolonged contact with the fruit commonly increase sweetness and ripe-fruit character. The trade-off is that the acidity can become dulled. Much like dried fruit compared with fresh fruit, there can be less vibrancy.

Immature coffee seeds are also harder to identify while they remain inside the cherry. This can result in more quakers: unripe seeds that do not contain enough sugars to caramelise properly during roasting.

Natural processing is challenging. What we look for is ripe fruit and sweetness without sacrificing the coffee’s acidity or filling the lot with unwanted nasties. The very best natural coffees can be incredibly fruity while remaining clean, vibrant and expressive.

Honey processing happens when the coffee’s skin is removed but some of the mucilage remains attached while the seed dries.

Darker or black honeys generally have more mucilage left on, while lighter honey processes have progressively less. The names are not perfectly standardised between countries or producers, so they should be treated as a general indication rather than a precise formula.

There are also more experimental versions. Some coffees are partially dried with mucilage attached and then placed into water tanks. This can stop or redirect some of the fermentation occurring in the mucilage, creating a cleaner hybrid profile somewhere between honey and washed processing.

Some producers use acids to disinfect the coffee and mitigate particular fermentations beginning in the mucilage. Depulped coffee can also be sealed inside tanks, deprived of oxygen and filled with carbon dioxide. It is fermented with the mucilage still attached before being laid out to dry.

Honey processing is therefore not one fixed method. Producers can change how much mucilage remains, how it ferments and how the coffee is dried.

Yeast inoculation means that the producer deliberately jump-starts a particular fermentation rather than waiting for wild fermentation to begin through the yeast and bacteria naturally present on the cherries and in the surrounding environment.

The producer introduces a selected yeast, which may be a strain of Saccharomyces cerevisiae similar to those used in wine or champagne production.

Introducing a known yeast helps a particular fermentation establish itself quickly. It can make the process more predictable, help define the profile and reduce the opportunity for less desirable microorganisms to take over and create gamey or unpleasant flavours.

As the fermentation becomes established and the pH drops, the environment can become safer and more stable. Yeast inoculation is therefore not necessarily just about creating an obvious flavour. It can also be used to control and safeguard fermentation.

Co-fermentation is when something else is deliberately introduced during fermentation. This might be fruit, flowers, spices, fruit must or another prepared fermentation.

Sometimes producers add a fermentation made from the coffee’s own mucilage or juices. This is commonly called mosto. It is essentially a co-fermentation with the coffee’s own material and is sometimes kick-started using a selected yeast so that a particular fermentation becomes established.

That is not the type of co-fermentation we necessarily stay away from. Our concern is the addition of other fruits, flowers or ingredients that make the coffee stop tasting like itself and turn it into something designed to taste like something else.

We have shared coffees where added ingredients were used with a clear purpose. Rocket Flower, for example, was fermented with yeast and hop flowers to exaggerate floral character. There were undeniable notes of jasmine and the citric nature of bergamot, but the coffee still had the clarity and sweetness we associate with a washed coffee.

We are a people-first coffee company and enjoy seeing producers push coffee in curious and playful ways. But we generally prefer coffees that are less process-driven, where the variety, growing conditions and work of the producer remain visible.

We have not personally roasted any thermal-shock coffees, but the main purpose of the process is to stop fermentation at a precise point.

After fermentation, the coffee is exposed to hot water and then immediately cooled with cold water. This rapid change in temperature halts or significantly reduces microbial activity, preventing fermentation from continuing beyond the producer’s intended outcome.

Thermal shock is therefore less a style of fermentation and more a way of conclusively ending and controlling it.

A processing-forward coffee is one where flavours created during fermentation and processing become a major part of the cup. Sometimes those flavours sit beside the variety and terroir. Other times they take over almost completely.

There is a middle ground between a processing-forward coffee and a clean, variety-forward coffee. Some washed coffees carry funky, fermented character while still retaining clarity and a recognisable expression of the original coffee.

Heavier processing can bring out character, sharpen the focus or even add a new lens through which to see the coffee. That may be necessary for a producer to achieve their goal or reach a market willing to pay higher premiums.

When done averagely, however, it can look like one of those early, over-edited Instagram filters. The effect becomes more visible than the subject.

We tend to be drawn towards coffees with minimal fermentation and processing because they allow us to see the character of the harvest, the nuance of the terroir and the hallmarks of each variety. We love the simplicity of looking at a great coffee through a clear lens.

That does not mean experimental processing has no value. We like to plunge into that different spectrum of flavour from time to time, both to understand it and to see how those coffees react in the roaster.

But at the centre of what we do is still the coffee itself: the variety, the place it was grown, the season and the people who produced it. Processing can help reveal those things, or it can turn the coffee into something else entirely.