Anaerobic fermentation

What sealing the tank does

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In a 2022 study in Paraná, Brazil, about 175 kilograms of pulped Catuaí coffee went into each plastic tank with 30 litres of water, and each fermentation was run three times. Each tank was closed with a screw cap fitted with an airlock, which lets the carbon dioxide made by the fermentation out and stops air getting in. The authors call this self-induced anaerobiosis: no gas is added, and the oxygen runs out because the microbes use it up and the fermentation's own gas displaces the rest.

That is the plainest meaning of the word on a label. Nothing is removed from the tank and nothing is pumped in. Carbonic maceration, where the vessel is filled with carbon dioxide from outside, is a different thing and is covered on the page about experimental processing.

What that study measured

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The tanks were fermented for 24, 48 and 72 hours, and sampled at the bottom, middle and top. The population the study found was not the one usually reported for open wet fermentation: it had a high prevalence of bacteria of the family Enterobacteriaceae and of filamentous fungi, and a low prevalence of lactic acid bacteria and yeasts. The authors suggest the lack of oxygen as the likely reason and say further study is needed to show it.

The kinds of microbe present differed little between the three layers of the tank, but what they did differed a good deal, in how fast sugar was used and in what was produced. The authors raise stirring as a way to even it out, and say that needs study.

Two certified tasters scored the roasted coffees. All three scored 84 or above. The 48 and 72 hour coffees were described as more complex and intense than the 24 hour one, with differences in aroma, flavour, acidity and body, and none in sweetness, cleanness or uniformity.

How far that goes

CoffeeHQ explanation

Not far. It is one variety at one farm, with two tasters, and it compared three lengths of sealed fermentation with each other. It had no open tank beside the sealed ones, so it shows that time in a sealed tank changed the coffee, and cannot show what sealing did.

A later study does put an open tank beside a sealed one. In 2025 Colombia's national coffee research centre fermented Castillo coffee, pulped and as whole fruit, in open tanks and in sealed tanks with an airlock, for up to 192 hours in a room held at 20 degrees Celsius. Sealing changed the microbes, and not as the first study would predict: the authors associate sealing with more lactic acid bacteria and less yeast activity, and in the open tanks acetic acid bacteria rose to between a fifth and more than a half of the bacterial reads. Three trained tasters scored the coffees, and the scores did not differ by condition, by time or by whether the fruit had been pulped; all but one sample fell between 80.75 and 85 points. What the authors found to matter most was time, especially beyond 72 hours, and it mattered to the soundness of the beans and the survival of their embryos, not to the scores. Neither study measured the oxygen in its tanks.

It also has a detail worth noticing. A sealed fermentation of this kind had few yeasts and few lactic acid bacteria at the start, though one family of lactic acid bacteria became more frequent in the 48 and 72 hour tanks. Another trial read for these pages connects added yeasts with higher scores. A sealed tank is not automatically a more controlled or a cleaner fermentation. It is a different one.

What the word does before you taste

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A 2026 experiment gave 180 speciality coffee drinkers in São Paulo the same coffee three times, in bags that differed only in what they said about processing. With the word fermentation on the bag, people expected the coffee to be the most acidic and liked it least, and said the same after tasting it. A bag marked carbonic maceration, a term the authors call vague, raised curiosity instead. The study also found that newer process names were expected to be more expensive and more exotic than washed and natural.

The tasting did not put the word anaerobic on a bag, though anaerobic fermentation was among the newer names the same people expected to be more exotic and more expensive. Its point carries all the same: part of what a process name does happens before the coffee is in the cup.

What the word anaerobic leaves unsaid

CoffeeHQ explanation

How an anaerobic coffee tastes. The trade describes intense, fermented-fruit character; the one study read found more complexity with more time and compared nothing with an open tank.

That a sealed tank is free of oxygen, or for how long. The study read did not report oxygen measurements, and a label says nothing about them.

How long, how warm or in what vessel any producer ferments. None of that is standardised, and a bag rarely says.

What to do next

• Experimental processing — Anaerobic, carbonic maceration, thermal shock, co-fermentation and infused coffee, as words first and science second.
• Coffee fermentation — What ferments in an ordinary open tank, for comparison with the sealed one.
• Tasting Lab — Taste a coffee blind before reading its bag, and see how much of the description you would have written yourself.
• Become a coffee expert — A structured programme from the plant and its processing to roasting, water and tasting.