Every coffee is fermented. That is not a stylistic choice — it is what happens when you take a sugary fruit off a tree in a warm climate. Microbes arrive within hours whether anyone planned for them or not.
The only real question is whether the producer is steering.
The default: open fermentation
In conventional washed processing, cherries are pulped and the beans sit in open tanks or channels with water, exposed to air, for twelve to thirty-six hours. In conventional natural processing, whole cherries go straight onto a patio or bed and ferment slowly while they dry.
Both are open to the atmosphere, and that determines the cast of characters. With oxygen available, aerobic bacteria and wild yeasts from the air, the equipment and the fruit skin all compete. Acetobacter — the genus that turns wine into vinegar — is prominent, and its main product is acetic acid.
A little acetic acid reads as clean brightness. More of it reads as sharp. Too much reads as vinegar, and that is the classic over-fermented defect.
The other problem with open fermentation is that it is a lottery. The microbial population depends on what was in the air, on the last batch's residue, on the weather. Same farm, same day, different tank, different result.
What sealing the tank changes
Anaerobic fermentation puts the cherries — whole, pulped, or somewhere in between — into a sealed vessel and removes oxygen. Usually the CO₂ produced by the fermentation itself purges the headspace, sometimes helped by a one-way valve that lets gas out and nothing in.
Three things follow.
The population narrows. Without oxygen, obligate aerobes cannot grow. What thrives instead is yeasts (which ferment happily without air) and lactic acid bacteria. Acetobacter is largely shut out.
The acid profile shifts. Lactic acid instead of acetic. Lactic acid is perceived as rounder, creamier and softer — it is what makes yoghurt taste the way it does rather than sharp. Cups from anaerobic ferments frequently get described as "smooth" or "syrupy" for exactly this reason.
Ester production goes up. Yeasts under anaerobic conditions produce ethanol, and ethanol plus organic acids gives you esters — the compound class responsible for most fruit aromas in nature. Ethyl butanoate reads as pineapple. Isoamyl acetate reads as banana. This is the machinery behind the intense tropical fruit character of a well-made anaerobic coffee, and none of it was added.
And it is controllable. A sealed tank has a temperature, a duration, a pH curve and a starting population. Those can be measured, logged and repeated. That reproducibility is arguably a bigger deal than the flavour, because it turns processing from weather into technique.
Why Vietnam moved so fast
Anaerobic processing arrived in specialty coffee via Latin America in the 2010s, and it arrived in Vietnam almost immediately afterward — with far less resistance.
The reason is unflattering to everyone except the Vietnamese producers. A Panamanian Gesha farm has an enormous reputation invested in terroir transparency; heavy processing intervention is viewed there as covering up what the buyer is paying for. Vietnamese Robusta had no such reputation. There was nothing to protect, and the species was widely dismissed anyway.
So producers in Lâm Đồng, especially around Nam Ban, went straight at it — and then went further, into co-fermentation with whole fruit.
Robusta turns out to be an unusually good candidate. It has high chlorogenic acid, which anaerobic fermentation appears to moderate; it has a heavy body that carries delicate fruit notes without them evaporating; and it has almost no acidity of its own, so fermentation-derived brightness reads as a gain rather than a distortion.
Telling good from bad
Anaerobic processing is a technique, and techniques can be executed badly. Overrun the fermentation and you do not get a more intense version of the good thing — you get a different, worse thing.
Signs it went well: clearly identifiable fruit rather than generic "funk"; sweetness that carries through the finish; a rounded, creamy acidity; no sharpness; and a cup that still tastes like coffee underneath the fruit.
Signs it went too far: vinegar. Solvent or nail-polish notes. A boozy, winey quality that overwhelms everything else. Anything vegetal or rotten. A cup where you cannot find the coffee at all.
That last one is a matter of degree and taste, but it is worth being honest about: some producers push fermentation until the coffee is essentially a fruit drink. Whether that is a triumph or a failure depends on what you wanted.
Buying and brewing
On a bag, look for specifics. "Anaerobic, 72 hours, sealed tank, sun-dried 18 days on raised beds" tells you the producer was measuring. "Specially fermented" tells you nothing.
In the cup, give anaerobic coffees slightly cooler water and a slightly coarser grind than you would use otherwise — the fermentation has already done part of the extraction's work, and over-extracting a heavily fermented coffee pushes it straight into the boozy register. Our guide to brewing fermented coffees has the detail.
And taste it black first. The fruit is the reason it exists.
