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Anaerobic and carbonic maceration in coffee - where the wild, fruity flavours come from

Coffee that smells of strawberry, rum, wine or tropical fruit so intensely that it is hard to believe it is still coffee? Behind such profiles increasingly stands anaerobic fermentation, and in the variant with whole cherries, carbonic maceration. It is a new wave of experimental processing, in which the producer seals the coffee in an airtight tank without oxygen and precisely steers the fermentation. The result is flavours wild, fruity and often surprising, far from classic coffee. For some it is the future of speciality, for others an excess that blurs the flavour of origin. Here is a guide to anaerobic fermentation: what it is, how the lack of oxygen and CO2 work, where these intense flavours come from and why it stirs debate.

What anaerobic fermentation is

Anaerobic fermentation is the fermentation of coffee in an environment without oxygen. In classic processing coffee ferments in contact with air, that is aerobically. In anaerobic fermentation the producer seals it in an airtight tank, cutting off the access of oxygen - and this completely changes the course of the process and the microorganisms working in it. Without oxygen different strains of yeast and bacteria develop, giving different flavour compounds than aerobic fermentation. This is why anaerobic coffees have such a distinct, often more intense profile. The key idea is simple: cut off the oxygen, change the microbiology, change the flavour. It is a tool of control with which the producer deliberately steers fermentation in a particular direction. Understanding that it is about an environment without oxygen is the starting point for the rest. We cover processing in general more in coffee processing.

How the oxygen-free environment is made

The lack of oxygen in the tank is achieved in two ways. The first is actively pumping in carbon dioxide from a cylinder: CO2 is heavier than oxygen, so it displaces it from the tank through a valve, creating an oxygen-free atmosphere. The second, simpler, is to simply seal the coffee in: the initial respiration of the fruit and aerobic microbes uses up the residual oxygen, and the CO2 released by the starting fermentation itself creates an oxygen-free, slightly pressurised environment. In both cases the effect is the same - the coffee ferments under a blanket of CO2, with no access to air. The tank often has a one-way valve, letting out excess gas but not letting oxygen back in. It is simple but effective engineering, which turns ordinary fermentation into a precisely controlled process. Steering this environment is the heart of the method.

Carbonic maceration - whole cherries

Carbonic maceration is a particular variant of anaerobic fermentation, borrowed directly from winemaking. Its essence: whole, intact coffee cherries are put into the tank, which is then filled with CO2. In the sealed, oxygen-free environment fermentation starts inside the untouched cherry, before its skin bursts - it is an enzymatic fermentation, happening in the cells of the fruit. The enzymes inside drive the transformation of sugars, break down malic acid and create a whole range of higher alcohols and aromatic precursors, with a low level of ethanol, around 2 percent. It is literally the same technique used to make fruity, light wines by carbonic maceration. In coffee it gives an intensely fruity, floral and winey character. Carbonic maceration is the most winemaking of the methods of coffee processing.

The role of temperature and time

In anaerobic fermentation the producer steers the flavour mainly through temperature and time. The tanks are often held around 30-32 degrees, to keep the enzymes active and drive the transformations, but some chill to 18-22 degrees, to get a subtler, more delicate profile. The maceration itself usually lasts from five to ten days, sometimes longer. The higher the temperature and the longer the time, the more intense the fermentation and the stronger, wilder the flavours. Shorter and cooler gives a cleaner and more restrained profile. This means the producer holds precise dials with which to design the final flavour of the coffee. This control is the main advantage of the method: instead of relying on chance, one can aim at a particular effect. This is why anaerobic fermentation is a tool of deliberate experiment, not a lottery.

A table: aerobic versus anaerobic

Let us gather the differences in one place:

The table shows the heart of it: cutting off the oxygen changes the microbiology and gives a different, usually stronger and more fruity profile, with greater control for the producer.

Where the intense flavours come from

The intense, fruity flavours of anaerobic coffees come directly from the different fermentation. Without oxygen different yeasts and bacteria work, producing compounds that are scarce in classic processing: fruity esters, higher alcohols, lactic acid. Hence the frequent descriptions: strawberry, blueberry, tropical fruit, rum, wine, cinnamon. The acidity is often smooth, lactic, and the sweetness boosted. Carbonic maceration adds to this a winey, floral character from the transformations inside the cherry. This is why these coffees can taste more like a fruit liqueur than classic coffee. Important: these flavours are not added from outside but arise naturally during the steered fermentation. It is the effect of microbiology, not flavourings. For many it is fascinating how far the flavour of coffee can be shifted by changing the fermentation environment alone.

Why it stirs debate

Anaerobic fermentation divides the coffee world. Supporters see in it creativity, new flavours and a tool that raises the value of coffee from lesser-known regions. Critics have two objections. The first: such intense fermentation can blur the flavour of origin and variety - all coffees start to taste similarly fruity, regardless of terroir. The second: sometimes the profile is so wild and fermented that it departs from what many consider good coffee, towards novelty for novelty’s sake. There is also a technical risk: uncontrolled anaerobic fermentation easily gives faults. This is why the method demands enormous precision. It is not an objectively better processing, but a different one - a matter of taste and intent. The debate is about whether to highlight terroir or to create new flavours.

How to sense it in the cup

You will often recognise an anaerobic coffee by its unusual intensity and fruitiness. If a coffee smells and tastes clearly of a particular fruit - strawberry, blueberry, mango - or has notes of rum, wine, cinnamon or fermentation, that is a good clue that it is anaerobic or carbonic maceration. The profile is often sweeter, smoother in acidity and more winey than classic washed coffee. Sometimes the fermentation is so clear that the coffee smells almost alcoholic or pickled. It is worth comparing the same variety in a washed and an anaerobic version, to feel how enormous a difference the fermentation alone makes. Over time you will start to recognise the characteristic wild fruit of anaerobic coffees already in the aroma. We cover classic processing more in natural processing and honey.

The essentials in brief

Let us gather it up. Anaerobic fermentation is the fermentation of coffee in a sealed tank without oxygen, and carbonic maceration is its winemaking variant with whole cherries flooded with CO2. The lack of oxygen changes the working microbes, and these give different, intense flavour compounds: fruit, rum, wine, lactic acidity. The oxygen-free environment is made by pumping in CO2 or by airtight sealing, and the producer steers the effect with temperature (usually 18-32 degrees) and time (a few to a dozen days). The flavours are not added but arise naturally. The method gives great control and new profiles, but stirs debate: it can blur the flavour of origin and depart from the classic. Now you know where the wild, fruity flavours in coffee come from and why anaerobic fermentation so divides.

Note every coffee in GustoNote - including the processing and the fermentation notes you sense. Over time you will start to recognise the wild, fruity character of anaerobic coffees and tell it from the classic.