Anaerobic Natural. Anaerobic Washed. Anaerobic Honey. If you spend any time browsing specialty coffee – whether that's in one of our cafés, on our shelves, or on a menu somewhere else entirely – you'll have run into the word 'anaerobic' by now. It's everywhere. It also sounds like something from a GCSE science textbook, which doesn't help.
But behind the slightly intimidating terminology is a genuinely fascinating shift in how coffee is processed. Anaerobic fermentation gives producers a level of control over flavour development that simply wasn't available a decade ago, and it's responsible for some of the most complex, surprising coffees we've tasted. It's not a replacement for washed, natural, or honey processing – it's something that works alongside them. A modifier, not a category of its own.
It's also worth saying upfront: not every anaerobic coffee tastes wild or funky. Plenty are surprisingly clean, with the technique amplifying clarity and sweetness rather than pushing things into unfamiliar territory. The range is enormous, which is part of why the category is worth understanding properly.
What follows is a proper deep dive: sealed tanks, airlocks, microbiology, winemakers, a naming controversy that turns out to be more than just linguistic, and a World Barista Championship that helped popularise it all.
First, a Quick Word About Fermentation
Before we talk about what makes anaerobic fermentation different, it helps to understand that all coffee undergoes some degree of fermentation. It's not optional. The moment a cherry is picked from the tree, microorganisms – yeasts and bacteria – start breaking down the sugars and acids in the fruit's mucilage. This is fermentation, and it happens whether you want it to or not.
In traditional processing, this happens in the open air. Washed coffees ferment in open tanks of water. Naturals ferment slowly as they dry on raised beds with the whole cherry intact. Honey processed coffees sit somewhere in between, fermenting with varying amounts of mucilage left clinging to the seed. In each case, much of the metabolic activity occurs in low-oxygen microenvironments within the mucilage itself, but the surrounding environment is oxygen-rich, and that influences which microorganisms thrive and what compounds they produce.
For most of coffee's history, fermentation was treated as a functional step – something that needed to happen to remove the fruit from the seed – rather than a deliberate flavour tool. Producers managed it, certainly, but the focus was on not letting things go wrong rather than actively steering the outcome.
Anaerobic fermentation flips that thinking on its head.
So What Does 'Anaerobic' Actually Mean?
The word comes from Greek: 'an' (without) + 'aero' (air) + 'bios' (life). In the context of coffee, anaerobic fermentation means that the fermentation stage takes place in a sealed environment with no exchange of oxygen with the outside air.
Here's where it gets more interesting than the usual "sealed tank, one-way valve" explanation suggests. In practice, most producers doing anaerobic fermentation don't start with an oxygen-free environment at all. They immerse their cherries – either whole or after pulping – in water, inside a sealed container fitted with an airlock (essentially a one-way valve, the same kit homebrewers use). That water contains dissolved oxygen, and the air above it (before the lid goes on) contains more. That initial oxygen is actually needed: the small population of yeasts and bacteria living naturally on the coffee cherries uses it to grow and multiply. Only once they've built up a healthy population do they consume the remaining oxygen and switch to anaerobic metabolism, fermenting the sugars in the mucilage and producing CO2, organic acids, and alcohols. The CO2 accumulates in the headspace and helps limit further oxygen ingress through the airlock, and the environment becomes – and largely stays – oxygen-free for the rest of the process.
This is almost exactly how beer and wine fermentation works. Brewers and winemakers don't sterilise oxygen out of their wort or must before pitching yeast; they rely on the yeast to consume that oxygen during its initial growth phase before settling into anaerobic fermentation proper. The parallels with winemaking aren't just conceptual – the underlying microbiology is genuinely shared.
The sealed environment matters because removing oxygen shifts the balance of which microorganisms thrive. In open-air conditions, yeasts are typically more active, producing one set of acids and flavour compounds. In lower-oxygen environments, lactic acid bacteria often play a more prominent role alongside yeasts – producing lactic acid in meaningful quantities rather than the acetic acid more commonly associated with exposed fermentation. Different metabolic pathways generate different alcohols, esters, and organic acids, all of which are absorbed by the coffee seed and ultimately influence what you taste in the cup. In fact, many of the fruit-forward aromas associated with anaerobic coffees come from esters – compounds formed when alcohols produced by yeast interact with organic acids during fermentation. Those esters are a big part of why anaerobic naturals can smell so vividly of mango, pineapple, or ripe berries.
The other thing a sealed tank gives producers is the potential for far greater control. Temperature, pH, pressure, fermentation duration – all of these can be monitored and adjusted in a way that's simply not possible when coffee is sitting out in the open, exposed to whatever the weather and local microflora decide to throw at it. In some cases, producers are now experimenting with pressurised fermentation, where elevated CO2 pressure further influences microbial behaviour and compound formation. The results are still variable, and failure rates remain high even for experienced producers – but the direction of travel is towards more precision, not less.
Hold On – Should We Be Calling It 'Anoxic' Instead?
If you've been keeping an ear to the ground in specialty coffee circles, you might have noticed a growing conversation around whether 'anaerobic' is even the right word for what we're describing. It's a fair question – and, as it turns out, it's not just about linguistic precision. There's a meaningful process distinction hiding underneath it.
Start with the linguistic side. Fermentation, by definition, is an anaerobic process. It's the metabolic pathway that microorganisms use to convert sugars into acids, gases, and alcohol in the absence of oxygen. Even when coffee ferments in an open tank exposed to the air, the actual fermentation – the microbial activity happening inside the mucilage layer – is occurring without oxygen. As fermentation consultant Lucia Solis has pointed out, calling fermentation 'anaerobic' is a bit like calling water 'wet'. 'Anoxic' – meaning an environment without molecular oxygen – is the more scientifically accurate term for what the coffee industry usually means when it says 'anaerobic'. Our friends at Assembly Coffee have written thoughtfully on this distinction.
But here's where it gets more interesting. When you look at how a handful of cutting-edge producers are actually working, 'anoxic' isn't just a more accurate word for the same process – it describes a genuinely different technique.
The method we described above – immerse the cherries, let the native yeast and bacteria populations grow on the initial oxygen before switching to anaerobic metabolism – is how most producers doing what they call 'anaerobic' fermentation operate. It relies on the microbes that happen to be present on the fruit to do the work, and it uses fermentation itself to create the oxygen-free environment.
A true 'anoxic' approach, in the way some producers now use the term, starts from a different premise entirely. Adrian Quetzal at Finca El Paraíso in Colombia – one of the most technically ambitious producers working today – begins his fermentations with a low-pressure CO2 flush that displaces oxygen from the tank before any microbial activity can begin. No oxygen is present at the start, which means the small native population of yeasts on the cherries can't grow the way they would in the immersion method above. So Adrian's team grows their own – cultivating yeast in a lab from cherries harvested on their own farm, building up a large, known population, and pitching that culture into the tank in one go. Again, the parallel with brewing is direct: this is essentially how a modern brewery or winery operates, using a selected and propagated yeast strain rather than hoping for the best with whatever's floating around.
That's a very different process from the more common immersion approach, and it produces very different results – not just in the cup, but in terms of what the producer is actually controlling. Anaerobic (immersion-style): let the ambient microbiology do its thing in a controlled environment. Anoxic (Adrian-style, and a handful of others): significantly reduce the influence of ambient microbiology and replace it with a selected, lab-cultivated culture. Complete sterility isn't really achievable even with a CO2 flush – it's a question of dominance rather than total exclusion – but the practical effect is that the producer, not chance, decides which microbes do most of the work.
Does this matter for how we label coffees? For now, most producers, roasters, and customers use 'anaerobic' as a catch-all, and we understand what's meant in context. But as more producers adopt genuinely distinct methods, the language will have to catch up. Our approach is straightforward: we follow the producer's lead. If they call their coffee anaerobic, that's what goes on our bag. If they call it anoxic, we go with that. The people who built the tanks, monitored the pH, flushed the CO2 and decided when to stop fermentation are best placed to name what they've done.
How Anaerobic Fermentation Works with Other Processing Methods
This is where a lot of the confusion around anaerobic coffee tends to live. Anaerobic fermentation isn't a processing method in the same way that washed, natural, or honey are. It's a controlled fermentation phase that happens before or during one of those established methods.
When you see 'Anaerobic Natural' on a bag, it means the coffee underwent anaerobic fermentation as whole cherries in a sealed tank, and was then dried as a natural – with the fruit still intact around the seed. 'Anaerobic Washed' means the coffee was fermented anaerobically (usually after pulping), then washed clean of its mucilage and dried. 'Anaerobic Honey' follows the same logic: anaerobic fermentation followed by the honey process, where some mucilage remains on the seed during drying.
Each combination produces distinctly different results. An anaerobic natural will typically be the most intense and fruit-forward, because the cherry remains in contact with the seed throughout both fermentation and drying. An anaerobic washed coffee will often be cleaner and more structured, with the anaerobic step adding complexity and depth to what might otherwise be a more restrained profile. Anaerobic honeys tend to land in between – richer and sweeter than a straight washed coffee, but with more definition than a full natural.
The variables are enormous. Fermentation time can range from as little as 12 hours to well over a week. Temperature makes a significant difference – cooler fermentations tend to be slower and produce cleaner, more refined profiles, while warmer conditions accelerate microbial activity and can push intensity. Some producers monitor pH throughout, stopping fermentation when it drops to a target level (often somewhere in the high 3s). Others, as we've touched on, take it further still by pitching specific cultivated yeasts or bacteria to steer the flavour in a particular direction.
The result is that no two anaerobic coffees are quite alike, even when they come from the same farm. Which is part of the appeal – and part of the challenge.
Carbonic Maceration: Anaerobic's Flashier Cousin
If you've been paying attention to competition coffee or specialty menus over the past decade, you'll have come across another term that lives in the same family: carbonic maceration. It's related, but it's not quite the same thing. (We've written a dedicated deep dive on carbonic maceration if you want the full picture – what follows is a summary of how it relates to anaerobic processing more broadly.)
Carbonic maceration is borrowed directly from the wine world, where it's been used for decades to produce wines with vivid fruit character and softer tannins – Beaujolais Nouveau being the classic example. The technique was brought into specialty coffee by Saša Šestić, the Bosnian-Australian barista who won the 2015 World Barista Championship using carbonic-maceration-processed coffee from Colombia. That single competition routine didn't invent experimental fermentation in coffee, but it played a major role in popularising it, and arguably did more than any other single moment to bring the technique into mainstream specialty conversation.
Šestić had spent time with Australian winemaker Tim Kirk, learning fermentation principles that he then applied to coffee. The key insight was that by controlling oxygen levels and temperature during fermentation, you could enhance the clarity and distinctiveness of flavours already present in the coffee, rather than masking them.
So what makes carbonic maceration different from standard anaerobic fermentation? The main distinction is that in carbonic maceration, whole intact cherries are placed in a sealed tank and CO2 is actively pumped in from an external source to displace oxygen. The cherries aren't immersed in water, and they aren't pulped beforehand. That creates an extremely stable, CO2-saturated environment from the very start, and – crucially – it encourages intracellular fermentation: enzymatic activity happening inside the cells of the fruit itself, rather than just on the surface where external microorganisms are doing their work. That intracellular phase continues until the cherry skins eventually break down, at which point conventional microbial fermentation takes over.
You'll notice the overlap with the anoxic approach described earlier – both start by displacing oxygen with CO2 rather than relying on microbes to consume it. The difference is what happens next. Carbonic maceration keeps the cherries whole and relies on the fruit's own enzymes; Adrian's anoxic method pitches a cultivated yeast population into depulped coffee or cherries. Different tools, different outcomes.
At their best, carbonic maceration coffees have striking aromatics, elevated sweetness, and a clean, bright acidity that can feel almost sparkling. Where standard immersion-style anaerobic coffees might lean into heavier, more intense fruit or winey characteristics, carbonic maceration often produces something more refined and focused. For a closer look at how the process works step by step, head over to our carbonic maceration guide.
What Does Anaerobic Coffee Taste Like?
This is the question everyone really wants answered, and the honest response is: it depends. Anaerobic processing amplifies and transforms, but what it amplifies depends on what's already there – the variety, the terroir, the altitude, the ripeness at picking, and the specifics of how the fermentation was managed.
That said, there are some general tendencies. Anaerobically fermented coffees are often characterised by intense, vivid fruit notes – think tropical fruits like mango, papaya, and passionfruit, or stone fruits like peach and plum. Berry flavours are common, particularly in anaerobic naturals. There's frequently a winey quality – a richness and depth of flavour that draws easy comparisons to red wine or port. Floral notes can be heightened too, with jasmine, rose, and honeysuckle turning up with surprising frequency.
The acidity in anaerobic coffees often has a distinctive character. Because lactic acid bacteria play a bigger role in oxygen-free environments, the acidity tends to be softer, rounder, and sometimes described as creamy or yoghurt-like – quite different from the bright, citric acidity you might associate with a clean washed coffee.
Body tends to be full and sometimes almost syrupy, with a sweetness that can feel more concentrated than in conventionally processed coffees. At their best, anaerobic coffees are complex, layered, and genuinely surprising – the kind of thing that makes you stop mid-sip and think, 'Wait, this is coffee?'
At their worst – and this is worth acknowledging – they can taste over-fermented, boozy, or aggressively funky in a way that overshadows whatever character the coffee had to begin with. Specific defects to watch for include butyric acid notes (rancid, sour-milky, sometimes compared unkindly to baby sick), phenolic off-flavours, and the overproduction of ethanol that pushes a coffee into outright boozy territory. Getting it right requires precision, experience, and a willingness to accept that the occasional batch won't work out.
The Bigger Picture: Why It Matters
Anaerobic fermentation sits at the intersection of several things that are reshaping specialty coffee. There's the science – a growing understanding of microbiology and fermentation chemistry that's giving producers tools they simply didn't have a decade ago. There's the market – a demand from consumers (and competition baristas) for ever more distinctive, memorable coffees. And there's the economics – because these coffees can command significantly higher prices, the technique offers producers a pathway to better financial returns, provided they can manage the additional investment in equipment, knowledge, and labour.
It's not without tension, though. Some in the industry worry that the focus on processing innovation can overshadow what makes a coffee inherently special – its variety, its terroir, the care taken in cultivation. There's a legitimate question about whether a coffee is exceptional because of where and how it was grown, or because of what happened to it in a tank after picking. The most thoughtful producers tend to see anaerobic fermentation as a way of revealing what's already there rather than imposing something new – enhancing the coffee's character rather than replacing it.
There's also the accessibility question. Stainless steel tanks, temperature control, pH meters, CO2 flushing equipment, and the knowledge to use any of it effectively represent a significant investment. Lab-cultivated yeast is a level up again. Not every producer can afford to experiment, and not every experiment succeeds. The risk of losing an entire lot to a fermentation gone wrong is real, and for smallholder farmers operating on tight margins, that risk isn't trivial.
But the trajectory is clear. What started as a niche technique used by a handful of competition-focused producers has become part of the mainstream specialty coffee vocabulary. The equipment is becoming more accessible, knowledge is being shared more openly, and the range of producers working with anaerobic methods continues to grow. From Colombia and Brazil to Ethiopia, Costa Rica, and beyond, anaerobic fermentation is no longer the future of specialty coffee processing – it's very much the present.
How to Spot It (and What to Expect)
When you see 'anaerobic' on one of our bags, you'll always see it paired with another processing term – Anaerobic Natural, Anaerobic Washed, and so on. That pairing tells you two things: the fermentation was done in a sealed, oxygen-free environment, and the coffee was then finished using that particular processing method.
If you're new to anaerobic coffees, expect something a bit more intense and flavour-forward than you might be used to. They tend to reward a slower, more considered approach to brewing – give them time to open up as they cool and you'll often find new layers of flavour revealing themselves. If you're already a fan of complex, fruit-driven coffees, anaerobic processing is likely to be right up your street.
And if you've ever wondered why two coffees from the same farm, same variety, and same harvest can taste so dramatically different – well, now you know. It's what happens in the tank that makes all the difference.