Anaerobic fermentation does not stop fermentation. It seals coffee away from the air while that fermentation keeps running, and when nobody adds a culture to start it, the organisms that take over are not the ones most coffee writing credits. A 2022 study in the peer-reviewed journal Foods, led by Alexander da Silva Vale with coffee-fermentation researcher Gilberto Vinícius de Melo Pereira on the team, sampled a sealed, self-induced anaerobic tank at 24, 48 and 72 hours and found it dominated by Enterobacteriaceae and a fungal family called Nectriaceae, consistent from the top of the tank to the bottom. Not lactic acid bacteria. Not yeast, the way a glass of wine or a loaf of sourdough would lead you to expect. Nectriaceae in particular is a family most people only run into in a plant-pathology paper, not a coffee one, since it also holds several fungal genera known for infecting crops rather than fermenting them.
Pulling the oxygen out does not pause the fermentation that was always going to happen on that fruit. It changes the contest. Plenty of microbes living on a coffee cherry can work with or without oxygen, but they compete on different terms once it's gone, and whichever ones handle a low-oxygen environment best get the run of the tank for however long the lid stays sealed. That is the whole mechanism behind every anaerobic lot, and it is also why two sealed tanks from two different farms, started with two different populations already living on the fruit, can finish in two different places.
We already publish the general mechanics of this, in our piece on carbonic maceration: coffee, whole or depulped, goes into a sealed container that excludes outside air, and the carbon dioxide that builds up is a byproduct of a fermentation the microbes were already running. Cafe Imports has documented a concrete version of it at La Chumeca in Costa Rica, where producer Martín Ureña ferments coffee in stainless tanks for about 96 hours, tracking the sugar content with a refractometer from roughly 22 to 25 °Bx at harvest down to about 16, and watching the tank's temperature for a spike that would mean air got in and an ordinary, oxygen-using fermentation started anyway. In his own words: "if there is a significant increase in the temperature inside the barrel it means that an aerobic fermentation is happening." What none of that trade writing says, because the microbiology wasn't the point of it, is which organisms are actually doing the work once the lid goes on.
Why Doesn't the Popular Lactic Acid Bacteria Story Hold Up Here?
It holds up fine elsewhere. We've written before about how an open-air natural dry runs on a very different cast: Enterobacteria moving in during the first 24 hours before yeast takes over for most of the two- to three-week dry, in our piece on honey-processed coffee, which traces a parallel succession for a semi-dry lot. That's a real, measured result, and it's a different fruit, a different amount of oxygen, and a different clock than a sealed tank runs on.
A sealed anaerobic tank removes the one variable that natural and honey drying still have in common with each other: air. da Silva Vale's team found that stripping it out does not hand the fermentation to lactic acid bacteria, the organisms that get credited for the "bright, lactic acidity, yogurt, sour cream" character our own processing-method page lists among an anaerobic lot's common notes. It handed it to Enterobacteriaceae and Nectriaceae instead, through 72 hours, with longer fermentation (48 and 72 hours, against 24) producing more of the esters, like ethyl acetate and amyl acetate, that read as fruit and flower later in the cup, and more acetic acid and pyrazine once the beans were roasted. The flavor story holds. The cast producing it is not the one the flavor story implies.
Does That Mean the Tasting Notes Are Wrong?
No, and this is worth being precise about, because it would be easy to read da Silva Vale's result as a correction to the cup rather than to the explanation. A lactic, yogurt-like note in an anaerobic coffee is a real, reported sensory result, not a mistake. What's wrong is the shorthand that gets built on top of it: the assumption that because the note tastes lactic, lactic acid bacteria must be running the fermentation that produced it. Flavor and mechanism are two different kinds of evidence, and a study that samples the tank directly is better evidence about the mechanism than a cupping note is, however reliable that note turns out to be about the cup.
Why Doesn't Inheritance Carry One Right Now?
Plainly: we don't have an anaerobic-processed coffee on the shelf at the moment. We've bought three, a Colombian, a Costa Rican and a Java lot, and all three are archived rather than in current rotation. That's not a verdict on the process. Every lot we add gets cupped and scored before it earns a spot on the shelf, the same way every coffee we sell publishes its score, and a longer, sealed, uninoculated fermentation carries more batch-to-batch variance than an open one, the same variance Martín Ureña is managing by hand with a refractometer and a thermometer rather than a recipe. When a lot like that doesn't cup consistently enough, we pass, and the cost of passing is a coffee we can't offer right now, not a position against the process itself.
That's a real cost, not a hedge. A roaster who buys anaerobic lots on reputation alone can end up with a bag that reads beautifully on the farm's own cupping sheet and falls apart by the time it reaches a customer's cup three or four months later, because a fermentation this variable is also a fermentation this hard to repeat lot to lot. Scoring every bag before it's for sale means some genuinely interesting coffee never makes it to the shelf, and I'd rather say that plainly than let an empty collection page speak for itself.
Key Takeaways
- Anaerobic processing seals coffee away from oxygen before drying. It does not pause fermentation, it changes which organisms dominate it and for how long.
- A 2022 peer-reviewed study of a sealed, uninoculated anaerobic tank found it dominated by Enterobacteriaceae and the fungal family Nectriaceae, not the lactic acid bacteria popular explanations usually credit.
- That's a different microbial story from an open-air natural or honey dry, where Enterobacteria give way to yeast within the first day or two.
- We don't currently carry an anaerobic lot. The three we've bought didn't stay in rotation, which is a cupping-and-buying outcome, not a position against the process.
If you want to taste what open-air fermentation does to a cup without a sealed tank anywhere near it, our natural-processed El Salvador Finca San Luis is on the shelf now, cupping 86.8. Read more on how fermentation shapes the rest of our shelf in the honey-processing and carbonic-maceration pieces linked above, and when a sealed anaerobic lot cups well enough to earn a place on our single-origin shelf, it will carry its own published score, the same as everything else we sell.
