Experimental coffee processing: the words explained

Steps added to a process, not processes

CoffeeHQ explanation

Every coffee still ends up as one of the older kinds: dried in the cherry, dried in its mucilage, or washed and dried in parchment. The newer words describe an extra step, almost always a fermentation managed in some unusual way, placed before or inside one of those. That is why they stack: an anaerobic natural is whole cherries fermented in a closed tank and then dried as a natural.

It follows that one of these words tells you less than washed or natural does. It names an intervention and leaves out everything that decides its effect: how long, how warm, in what vessel and on what fruit.

The words, one at a time, and how well each holds up

CoffeeHQ explanation

Each entry says what the word is used for, what was read, and gives a verdict: defensible, borrowed, shorthand or poorly defined. The verdicts are CoffeeHQ's. They judge the word as a description, not the coffee.

Anaerobic

Defensible as a description of the vessel, poorly defined as a description of the chemistry. In the research read it means a sealed container, usually with a one-way valve. None of those studies reports how much oxygen was in the tank; the word records that air was shut out, not that oxygen was measured at zero. One of them notes that yeasts need small amounts of dissolved oxygen even in such conditions.

Self-induced anaerobic

Defensible. A research term for a sealed vessel in which the microbes themselves use up the oxygen and the carbon dioxide they produce escapes through an airlock. It names a mechanism and a piece of equipment, which is more than most of these words do. It still says nothing about time, temperature or what was put in the tank.

Semi-anaerobic

Poorly defined on a bag. A Colombian research paper of 2025 uses it for an ordinary open tank, on the reasoning that microbes consume the oxygen between the seeds and leave the depths of the mass short of it. On that definition the traditional washed fermentation is already semi-anaerobic, and the word describes almost every tank there has ever been.

Sealed or closed-tank

Defensible, and the plainest of the set. It says what was done and claims nothing about what resulted.

Carbonic maceration

Borrowed. A winemaking term for fermenting whole fruit under carbon dioxide. In coffee research it is used for whole cherries in a vessel filled with the gas from outside. One study read did that with small sealed bags of Catuaí cherries in Espírito Santo, Brazil, and followed the fungi in them: which were present changed with the temperature and the hours, and yeasts predominated. It compared no bag without the gas and reports its tasting in another paper, which was not read. Whether the grape process and the coffee one share more than a name was not shown in anything read.

Carbonic

Shorthand. Used alone it can mean carbon dioxide was injected, or only that a tank was sealed and its own gas built up, which is the self-induced method above. The two are different interventions under one word.

Controlled fermentation

Poorly defined. Every fermentation is controlled in some respect and none in all. The word is informative only when it is followed by what was controlled: the temperature, the hours, the vessel, the culture.

Yeast-inoculated, or cultured

Defensible. A chosen yeast or bacterium is added instead of relying on what arrives on the fruit. One trial read found a large change in the chemistry of the green coffee and higher scores for flavour, aroma and acidity from three tasters. The word is complete only with the name of what was added.

Extended or long fermentation

Defensible but relative. An ordinary fermentation left for longer than is usual at that mill. Two trials read found the cup changed with time, towards fruitier and more acidic in one and more complex and intense in the other. Without the hours and the temperature the word cannot be compared between two bags.

Double fermentation, fruit reserve

Shorthand for holding whole cherries, often in an open tank, before pulping and fermenting again. The Colombian study of 2025 tested this and its authors say that fermenting whole fruit in an open tank is not advisable beyond 24 hours, because the physical quality of the beans and the survival of their embryos suffered; the tasters' scores in that study did not fall.

Thermal shock

Poorly defined. Usually described as moving the coffee between hot and cold water during processing. No study of it was found, and nothing is said here about what it does.

Co-fermented

Defensible as a description when it names what was added. Fermented together with something that is not coffee, such as fruit, spices or their juice. No controlled study of it was found; the section on added substances below sets out what can and cannot be said.

Infused

Poorly defined. Used both for the same thing as co-fermented and for flavouring added to green coffee after processing. The two are different, and the word does not say which is meant.

Anaerobic does not mean zero oxygen

Sourced

The word invites a picture of coffee fermenting in the complete absence of oxygen, and of that absence as the cause of a flavour. Neither part of the picture is in the studies. The Paraná study closed its tanks with a screw cap and an airlock that let carbon dioxide out and kept air from coming in. The Minas Gerais study did the same and describes the valves as maintaining anaerobic conditions. The Colombian study used sealed tanks with an airlock valve. In none of the three is there a measurement of oxygen in the tank at any time.

What the design does establish is direction. A sealed tank starts with the air that was in it and in the fruit. Microbes respire that oxygen away and nothing replaces it, so the tank becomes poorer in oxygen than an open one, how quickly and how completely being unmeasured. The Colombian paper's own scheme makes the comparison a matter of degree: coffee spread thinly on a patio is aerobic, coffee in an open tank is semi-anaerobic because the oxygen between the seeds is consumed, and coffee in a sealed tank is anaerobic. An open tank and a sealed one differ at the surface and in what happens to the gas; below the surface both are short of oxygen.

The microbes bear this out without settling it. Acetic acid bacteria, which need oxygen, were more abundant in the open tanks of the Colombian study and were under one per cent of sequences in the sealed tank in Paraná. But the same two studies disagree about lactic acid bacteria, and in the Colombian study the scores of three trained tasters did not differ between open and sealed tanks. So the supportable statement is modest: sealing changes which microbes are favoured. That it produces a particular flavour, and that it does so because oxygen is absent, has not been shown.

What is actually known

Sourced

Three things, from the trials read. Changing a fermentation changes the green coffee: its sugars, acids and the volatile compounds in it. The change is large enough to taste, on small panels, in the direction of more fruit, acidity or intensity. And which microbes do the work depends on the conditions, so that sealing a tank, adding a yeast or leaving mucilage on the parchment each produces a different population.

What is not known, from anything read here, is the step the labels imply: that a named technique produces a named taste. The studies are single sites with a handful of tasters, they compare a technique with a variant of itself more often than with an ordinary process, and they do not agree on what the words mean. The international guidance on processing makes the general point about the whole field, that claims linking a processing step to quality usually lack an objective comparison of the alternatives.

When something other than coffee goes in

Accounts differ

Co-fermented and infused coffees raise a question the others do not: whether what you taste came from the coffee. A coffee fermented with fruit can taste strongly of that fruit, and a buyer who is not told may take it for the character of the variety or the place.

How it might work. Two routes are plausible, and they are different. In the first, aroma compounds from the added material pass into the seed directly. The trials of ordinary fermentation show that the seed is not sealed off: in Yunnan, lactic acid and mannitol made in the tank accumulated inside the beans, and in the yeast-inoculation trial alcohols and esters produced by the yeast were found at higher concentrations in the green beans and some survived roasting. If a seed takes up what its own fermentation makes, it can presumably take up what a crushed fruit or a spice releases beside it. In the second route the addition feeds or seeds the fermentation: extra sugar, extra acid and the microbes living on the added fruit change what the fermentation produces. Both are proposed mechanisms. CoffeeHQ found no controlled study that added a fruit to one half of a lot and not the other and then measured the beans.

What the evidence is. No analytical study of co-fermented coffee, and no tasting trial of it, was found in an open journal. That is an absence in what was searched and not a finding that the method does nothing: the reports of its effect are tasters' and traders'. What cannot be said from them is how much of the result comes from the first route and how much from the second, whether the added character is stable in storage, or how a co-fermentation differs chemically from flavouring applied to finished green coffee.

How it is classified. The disagreement in the trade is about what counts as coffee's own character, and the one rulebook opened draws the line by stage. The 2026 rules of the World Barista Championship say that no additives of any kind may be added to coffee after it reaches the green coffee stage, which they define as seeds dried as part of the post-harvest process and free of all layers of fruit, and that this includes exposure to aromatic substances, flavourings, perfumes, liquids and powders. On that wording something added to the fermentation tank is before the line and something applied to green or roasted beans is after it. Other competitions have rules of their own; they were not opened and are not summarised here.

The practical position is simpler than the argument. Nothing read suggests such coffees are unsafe or improper, and nothing read shows they are better. Whether one is what you want is your decision, and you can only make it if the bag says what was added and when. A label that says co-fermented with a named fruit has told you. A label that says only infused, or gives a tasting note that happens to match an ingredient, has not.

What the word does to the taster

Sourced

A 2026 experiment in São Paulo gave 180 regular drinkers of speciality coffee the same coffee three times, brewed out of sight, from bags that differed only in the processing line: nothing, fermentation, or carbonic maceration. People expected the coffee marked fermentation to be the most acidic and liked it least, and reported the same on tasting it. The bag marked carbonic maceration raised curiosity and a sense of novelty. Asked about process names in general, the same people expected the newer ones to cost more and to taste more exotic than washed and natural.

The authors' conclusion is about expectation: the same coffee was liked more or less depending on a word. For anyone trying to learn what a process tastes like, that is the main obstacle, and the remedy is to taste before reading.

Questions worth asking of a bag

CoffeeHQ explanation

1. What was it before the extra step?. Washed, natural or honey. If the bag gives only the newer word, the older one is missing information, not implied.
2. Was anything added?. A yeast culture, fruit, or flavouring. A roaster who knows will usually say when asked, and one who does not know has told you something too.
3. How long, and in what?. Hours and vessel. These decide more than the name does, and a producer who controls them generally records them.
4. Would I have described it this way unprompted?. Taste it blind against a conventionally processed coffee, and write your notes before reading either bag.

These are questions, not a test. Plenty of good coffee is sold with none of them answered.

What nothing read can tell you yet

CoffeeHQ explanation

What carbonic maceration, thermal shock or co-fermentation does to the taste of coffee. The one study of carbonic maceration read followed the fungi and tested no alternative; no study of the other two was found.

How much oxygen is in a tank called anaerobic. No study read measured it.

Where any competition other than the one whose rules were opened draws its line on added substances.

What any competition allows. The rules were not opened.

That any of these methods is better or worse than conventional processing, safer or less safe, or worth its price.

A definition of any of these words. None has one that CoffeeHQ could find in a document.

What to do next

• Anaerobic fermentation — The one newer method with a study read in detail for these pages.
• Coffee fermentation — What an ordinary fermentation does, which is the thing all of these modify.
• Tasting Lab — A worksheet for tasting two coffees blind and writing your own notes before the bag's.
• Processing, from cherry to green coffee — The older methods these steps are added to, side by side.