Two bags from the same Colombian region sit side by side on a shelf. One tastes of caramel and red apple. The other tastes of cinnamon sweets and overripe strawberry, with something close to rum underneath. The difference is not the farm, the variety or the roast. It is a sealed tank.
Anaerobic fermentation coffee is coffee fermented with the oxygen shut out. The cherries go into a tank whole or depulped, the tank is sealed, and the oxygen inside is displaced or used up, often by the carbon dioxide the fermentation itself gives off. Without oxygen, a different set of microbes does the work, producing different acids and aromatic compounds than an open tank would, and the producer steers the result by controlling time and temperature, sometimes over several days rather than the hours a conventional ferment gets. Some go further and add chosen yeast or bacteria cultures, the way a brewer pitches a specific strain rather than trusting whatever is in the air.
Fermentation was always there
The word makes the process sound like an invention, and it is really a redirection. Nearly every washed coffee on earth ferments: James Hoffmann's The World Atlas of Coffee describes depulped seeds sitting in tanks while microbes break down the clinging fruit flesh, with heat speeding the process and producers pulling the coffee the moment the flesh lets go, because left too long it picks up off-flavours. In that world, fermentation is a removal step. It is watched nervously and stopped as early as possible.
Anaerobic processing flips the intent. The fermentation is no longer a chore on the way to drying; it is the flavour engine, run deliberately long under conditions chosen to favour the microbes the producer wants. Sealing the tank is what makes that control possible. An open ferment is at the mercy of ambient air, temperature swings and whatever organisms arrive. A closed one is closer to a recipe.
What it does to the cup, and how sure anyone actually is
The signature is intensity: loud tropical and berry fruit, sweet-spice aromatics, wine-like or faintly boozy notes, and a syrupy sweetness that conventional lots of the same coffee rarely show. On why, the honest answer is that the craft is running ahead of the science. Jessica Easto puts it plainly in How to Taste Coffee: the enzymes and yeasts at work during fermentation are thought to produce additional flavour compounds, producers are actively experimenting with yeast strains and lactic acid bacteria to encourage pleasant ones, and none of this has been scientifically studied in depth. Producers repeat what worked last harvest and adjust. When it goes wrong, it goes wrong recognisably: Easto notes that fermentation gone awry produces chemical notes and other off-flavours, which is why a bad anaerobic lot can taste of nail varnish or onion rather than merely tasting dull.
The label problem
"Anaerobic" on a bag tells you one fact: the tank was sealed. It does not tell you for how long, at what temperature, whether the cherries went in whole or depulped, or whether cultures were added, and each of those decisions moves the cup. Two anaerobic coffees can share nothing but the seal. The word has no legal definition and nobody audits its use, so treat it the way you treat every large-type claim: the small print on a bag is constraints and the large print is advertising. A roaster who publishes tank time and temperature is telling you something real. A bag that just says "anaerobic" is telling you the coffee is fashionable.
What it will not do
A sealed tank will not turn defective cherry into good coffee; it amplifies the fruit it is given, faults included. The word is not a quality grade or a freshness signal. And it will not deliver the taste-of-place experience that specialty marketing leans on, because a hard-driven ferment can bury origin entirely. Single origin already promises less than most buyers assume, and a heavy process flavour stacked on top can erase what little the phrase secured. Traditionalists dislike the category for exactly this reason, and they are not wrong about the mechanism, only about whether it matters to you.
How to approach one
What to do: work out whether you like process-forward coffee before you make a habit of paying its premium.
How: buy a single anaerobic lot from a roaster who states the tank time and temperature, and brew it next to a conventionally processed coffee you already know, same method, same ratio. Taste them against each other rather than in isolation.
Why it works: the sealed tank produces compounds a conventional process never makes, so the gap between the two cups is wide enough for an untrained palate to catch immediately. You are not testing your sensitivity. You are testing your preference.
How you know it worked: a few bags later, you can tell from the first sip whether a coffee was anaerobic before you turn the bag over, and you know whether that recognition makes you reach for a second cup or back toward the washed lot beside it. Either answer is a result. The only failure is paying the premium for years without ever finding out.