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.
Behind the slightly intimidating terminology is a real shift in how coffee is processed. Anaerobic fermentation gives producers another way to manage flavour development, and it's behind some of the most complex, surprising coffees we've tasted. It works alongside washed, natural and honey processing rather than replacing them. Think of it as a modifier.
Not every anaerobic coffee tastes wild or funky, either. Plenty are surprisingly clean, with the process bringing out sweetness, fruit or aromatic detail rather than pushing things into unfamiliar territory. The range is huge, which is part of why it's worth understanding properly.
So this is a proper deep dive: sealed tanks, airlocks, microbiology, winemakers, an argument about what to call it all, and the World Barista Championship routine that put fermentation in the spotlight.
First, a quick word about fermentation
Coffee processing involves a mix of enzymatic and microbial activity, but the extent and role of fermentation vary from one method to another. In washed processing, fermentation is often used to help break down the sticky mucilage surrounding the seed. In natural and honey processing, microbial activity can also take place while the fruit or mucilage dries around the seed. How much fermentation occurs depends on moisture, temperature, time and the microorganisms present.
In many traditional processing systems, fermentation has mainly been managed as a practical step: helping remove mucilage and avoiding unwanted flavours, rather than deliberately steering the cup towards a particular profile. Producers have long known that fermentation affects flavour, though, and better monitoring and more controlled techniques give them more ways to understand and influence it.
Anaerobic fermentation is one of those approaches.
So what does 'anaerobic' actually mean?
In everyday terms, anaerobic means that something can happen without oxygen. In coffee, the term usually describes a fermentation stage carried out under oxygen-restricted conditions, often in a closed or sealed vessel. The aim is to manage the amount of oxygen available while the coffee ferments.
The usual 'sealed tank, one-way valve' explanation only tells part of the story. There isn't one standard anaerobic recipe. Producers may ferment whole cherries or depulped coffee, with or without added water. Some close a vessel and let oxygen levels change as fermentation develops. Others introduce carbon dioxide to push oxygen out, use an airlock to limit air getting in as gas escapes, or control the process in other ways. A sealed vessel can still contain oxygen at the start, and sealing it doesn't automatically make its contents oxygen-free.
In some processes, microorganisms and plant material use up oxygen as they become active, while the carbon dioxide produced during fermentation changes the atmosphere in the vessel. In others, the producer takes oxygen out before fermentation gets going. Which route a producer takes depends on what they're after and the kit they have, and not everyone uses an airlock.
There are parallels with brewing and winemaking, where producers also manage fermentation conditions and may choose to use selected cultures. The details aren't identical, though: coffee cherries and mucilage are different raw materials from grape must or brewing wort, and the microbial communities and practical goals differ too.
Managing oxygen can influence which microorganisms thrive and how they behave, but there's no simple rule that open-air fermentation means one set of microbes and anaerobic fermentation another. Yeasts and bacteria can be present in the same process, and the balance between them depends on the coffee, temperature, pH, time, water availability and other conditions. These organisms produce a range of organic acids, alcohols, esters and other compounds that can change the coffee's chemical composition, and so its flavour.
Fruit-forward aromas in some fermented coffees are associated with volatile compounds such as esters. But no single compound, microorganism or oxygen level guarantees a particular tasting note.
A closed vessel gives producers more opportunity to monitor and manage time, temperature, pH and oxygen. Some experimental systems also control pressure or use selected microbial cultures. That doesn't make every sealed-tank process precise by default: control depends on the equipment, the measurements taken and how consistently the process is run. Drying beds and open tanks have their own variables, including ambient temperature and weather, which can be harder to standardise.
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 conversation about whether 'anaerobic' is the right word for some of the processes being described. It's a fair question, and it comes down to the difference between a type of metabolism and the conditions in an environment.
In microbiology, fermentation describes metabolic pathways that don't use oxygen as the final electron acceptor. 'Anaerobic' can describe organisms or processes that function without oxygen, while 'anoxic' describes an environment where molecular oxygen is absent. In coffee, though, 'anaerobic fermentation' has become the industry label for a fermentation stage carried out in a low-oxygen or oxygen-restricted environment. The terms overlap in everyday use, and the coffee industry hasn't settled on a single naming system.
Fermentation consultant Lucia Solis has helped popularise this distinction. Our friends at Assembly Coffee have written thoughtfully about why 'anoxic' can be a more precise term when the intention is to describe an environment without molecular oxygen, rather than the metabolism itself.
In practice, some producers do actively reduce oxygen before fermentation begins, for example by flushing a tank with carbon dioxide. They may then introduce a selected yeast or bacterial culture to guide the fermentation.
That's one way of doing it. 'Anoxic' describes the oxygen conditions, and the recipe is up to the producer. One might use a CO2 flush with a selected culture, while another relies on native microorganisms in an oxygen-restricted vessel. A CO2 flush doesn't make a tank sterile, either. It changes the atmosphere, and the microbes are still there.
Does this matter for how we label coffees? For now, most producers, roasters and customers use 'anaerobic' as a broad label, and its meaning is generally understood in context. As producers experiment with different methods, the terminology may become more specific. In the meantime, we use the producer's own description. They designed the process, watched the conditions and decided when to stop the fermentation, so they're best placed to explain 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. Washed, natural and honey each describe how the fruit comes off and how the coffee dries. Anaerobic describes a fermentation stage that can be combined with any of them.
When you see 'Anaerobic Natural' on a bag, it generally means the coffee underwent a low-oxygen fermentation stage as whole cherries, then dried as a natural, with the fruit still around the seed. 'Anaerobic Washed' means a low-oxygen fermentation stage was followed by removing the fruit and mucilage, washing as needed and drying the coffee. The fermentation may take place before or after depulping, depending on the producer's method. 'Anaerobic Honey' follows the same broad logic, but some mucilage stays on the parchment during drying.
These combinations can produce very different results, but the label alone won't predict them. An anaerobic natural isn't guaranteed to be more intense, and an anaerobic washed isn't automatically cleaner or more structured. Variety, ripeness, fermentation conditions, drying and roasting all contribute to what ends up in the cup.
Then there are the variables inside the tank. Fermentation can last from hours to several days, and some protocols run longer. Temperature affects the speed and course of microbial activity, but warmer doesn't automatically mean more flavour and cooler doesn't guarantee a cleaner profile. Some producers monitor pH, oxygen or temperature throughout; others use selected yeast or bacterial cultures to help guide the process.
So no two anaerobic coffees are necessarily 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 from the same family: carbonic maceration. It's related to anaerobic processing, but the terms aren't interchangeable. We've written a dedicated guide to carbonic maceration. Here's how it fits with anaerobic fermentation more broadly.
Carbonic maceration comes from the wine world, where it's used to make wines with bright fruit and softer tannins. The technique was brought into the specialty coffee spotlight by Saša Šestić, the Bosnian-Australian barista who won the 2015 World Barista Championship using a Colombian coffee described as carbonic-maceration processed. Experimental fermentation in coffee was already happening, but that routine put it in front of the whole industry.
Šestić had spent time with Australian winemaker Tim Kirk, learning fermentation principles that he then explored in coffee. His work encouraged producers to think more deliberately about how processing conditions can influence the cup, rather than treating fermentation only as a step to get through before drying.
In the stricter, wine-derived sense, carbonic maceration involves whole, intact cherries in a sealed tank with carbon dioxide introduced to create a low-oxygen environment. Keeping the cherries intact allows changes to take place inside the fruit, while microbial activity also contributes to fermentation. The balance between the two depends on the conditions and how the coffee is handled.
In coffee, the term has sometimes been used more broadly. Šestić's winning coffee was described as a washed carbonic maceration; in an interview about the process, he explained that the cherries were depulped before the controlled fermentation stage. So the name doesn't always mean a textbook whole-cherry process, which is one reason carbonic maceration and anaerobic fermentation overlap in real-world processing.
Both manage oxygen deliberately. Carbonic maceration, in its stricter sense, centres on intact cherries and a CO2-rich environment. Anaerobic processing is a broader label for low-oxygen fermentation and can involve whole cherries or depulped coffee, native or selected cultures, and different amounts of water.
At their best, carbonic maceration coffees have striking aromatics, sweetness and bright acidity, though there's no single flavour profile every carbonic maceration coffee shares.
What does anaerobic coffee taste like?
This is the question everyone really wants answered, and the honest response is: it depends. Anaerobic processing can influence and sometimes intensify a coffee's character, but what comes through depends on what's already there: the variety, growing conditions, altitude, ripeness at picking, and how fermentation and drying were managed.
That said, you might find tropical fruit like mango, papaya and passionfruit, or peach, plum and berries. Some coffees have a winey or richly aromatic quality, while others show floral notes such as jasmine or rose.
Acidity can vary, too. Some fermentations produce meaningful amounts of lactic acid, which can contribute to how acidity is perceived, but it's too simple to say that all oxygen-restricted coffees will taste softer, rounder or yoghurt-like. Other acids and aromatic compounds also matter, and the balance varies by process.
Body and sweetness can seem full or concentrated in some lots, while others are lighter and more delicate. At their best, anaerobic coffees are complex and surprising, the kind of cup that makes you stop mid-sip and think, 'Wait, this is coffee?'
And, to be fair, fermentation doesn't always go to plan. Some coffees develop sharp, vinegar-like, solventy or unpleasantly savoury flavours that overwhelm their better character. The causes vary, and those flavours aren't exclusive to anaerobic processing. Consistent results take careful selection, monitoring, hygiene and drying, alongside experience and a willingness to learn from the occasional batch that doesn't work out.
Science, money and a few reservations
Three things are pushing anaerobic fermentation forward. The science: a growing understanding of microbiology and fermentation chemistry gives producers more ways to investigate and influence processing. The market: drinkers and competition baristas want distinctive, memorable coffees. And the money: some experimental lots sell for higher prices, which can mean better returns for producers when the process works. Not always, though.
There's a tension here, too. Focus too hard on processing and it can overshadow what makes a coffee special in the first place: its variety, where it grows and the care taken in farming it. Is a great coffee great because of where and how it was grown, because of what happened after picking, or both? We see fermentation as one way of shaping or revealing a coffee's character. Good farming and careful processing come first.
Then there's access. Stainless steel tanks, temperature monitoring, pH meters, CO2 equipment and the knowledge to use them cost a lot to buy and to learn. Selected starter cultures and more advanced controls add another layer. Not every producer can afford to experiment, and not every experiment succeeds. For smallholders on tight margins, losing a lot to a fermentation or drying problem is a serious risk.
Even so, controlled fermentation is now an established part of specialty coffee. Producers from Beneficio San Pedro in El Salvador to Agricafe in Bolivia are experimenting with oxygen levels, time, temperature and microbial cultures, with methods and results varying widely. It's a growing set of tools that producers adapt to their coffee, resources and ambitions.
How to spot it (and what to expect)
When you see 'anaerobic' on one of our bags, you'll generally see it paired with another processing term, such as Anaerobic Natural or Anaerobic Washed. That pairing tells you the producer has identified a low-oxygen fermentation stage as part of the process, alongside the method used to remove the fruit and dry the coffee. It doesn't, by itself, tell you whether the cherries were whole or depulped during fermentation, whether water was added, or whether a selected culture was used.
If you're new to anaerobic coffees, go in with an open mind. Some are bold and expressive; others are clean, sweet and subtle. Let the cup cool as you drink it, because new aromas and flavours often turn up as the temperature drops.
And if you've ever wondered why two coffees from the same farm, same variety and same harvest can taste so different, fermentation may well be part of the answer.