THE PROCESS IN MOTION

“Anaerobic”: what really happens?

Start the scene or choose a stage. The animation simplifies the mechanism; what follows gives the details and the available references.

▶ Play the process01The tank is closed, that is all02The microbes eat the oxygen03CO₂ takes the place04Carbonic maceration, gas first

The tank is closed, that is all

Anaerobic means “without air,” and one imagines a tank from which the oxygen has been driven out. This is almost never the case: the tank is closed, and the air that was inside stays there.

Read the stages without animation

1. The tank is closed, that is allAnaerobic means “without air,” and one imagines a tank from which the oxygen has been driven out. This is almost never the case: the tank is closed, and the air that was inside stays there.
2. The microbes eat the oxygenThe microorganisms consume this air in a few hours. During that time there is oxygen, therefore respiration, and a chemistry that is not the one advertised. The right term is self-induced fermentation, in full self-induced anaerobic fermentation, SIAF.
3. CO₂ takes the placeThe carbon dioxide produced finishes driving out what remained and leaves through the valve when the tank has one. Acetic acid bacteria, which live only with oxygen, disappear first; the tank keeps its volatile compounds and drifts less in temperature. The pressure reached is almost never published.
4. Carbonic maceration, gas firstOne case really stands apart: the tank is filled with carbon dioxide before it is closed, with whole, intact cherries. It is then the air that comes out, pushed by the gas, and the fruit itself works from the inside. Most coffees sold under this name, however, have never seen a gas cylinder.

To remember: What is called anaerobic begins in the presence of air and ends without it; only carbonic maceration introduces the gas before closing.

In history: Carbonic maceration is borrowed from Beaujolais winemaking.

Read the full article and its sources ↓

Anaerobic means “without air.” When a coffee is sold as anaerobic, one imagines a tank from which the oxygen has been driven out.

That is almost never what happens. The tank is simply closed. The air that was inside stays there. The microorganisms consume it within a few hours, and the medium becomes poor in oxygen because they have eaten it, not because it has been removed. The carbon dioxide produced afterward takes its place and finishes driving out what remained.

The difference is not one of vocabulary. During the first hours, there is air, therefore respiration, therefore a chemistry that is not the one advertised. What is called anaerobic is in reality a fermentation that begins in the presence of air and ends without it, and the right word exists: it is called self-induced fermentation.

One case really stands apart, carbonic maceration, borrowed from Beaujolais. There, the tank is filled with carbon dioxide before it is closed, with whole, intact cherries. This is no longer the same thing: the fruit itself works from the inside. Most coffees sold under this name, however, have never seen a gas cylinder.

What closing really changes

Closing a tank produces four measurable effects, only one of which concerns oxygen:

• Oxygen depletionBy microbial consumption, within a few hours. Acetic acid bacteria, strictly aerobic, are the first eliminated, which rules out the risk of a vinegary drift.
• PressurizationCarbon dioxide accumulates. Some tanks are fitted with a valve, others not, and the pressure reached is almost never published even though it changes the solubility of the gas and the diffusion toward the bean.
• Confinement of volatilesWhat is produced no longer evaporates. This is undoubtedly the most underestimated effect: the closed tank retains what an open tank loses to the air.
• Thermal stabilityA closed tank exchanges less with the outside, so it drifts less, which makes the process more reproducible. This is probably a large part of its industrial success.

The acronym SIAF, for self-induced anaerobic fermentation, correctly names what happens and should replace “anaerobic” wherever the tank has not been purged. A true carbonic maceration is recognized by three requirements: whole, unwounded cherries, an introduced carbon dioxide atmosphere, and the fruit's intracellular metabolism as the main driver.

Study on coffee quality improvement by self-induced anaerobic fermentation: Microbial diversity and enzymatic activity, Food Research International, 2023; plate IX, section “Anaerobic and SIAF”

Five oxygen regimes instead of two

The plate sets only two states against each other, with air and without air, and that is already better than what is usually read. Microbiology distinguishes five, and this scale explains field observations that the binary split leaves without a cause, starting with the fact that a closed tank does not house the same flora at the top and at the bottom.

• Strict aerobeLives only in the presence of oxygen, because it needs it as the final acceptor of its electrons. Acetic acid bacteria are the example, and that is why they disappear within a few hours from a closed tank.
• Strict anaerobeLives only in its absence, oxygen being toxic to it, because it inactivates enzymes that this type of organism cannot protect. The Clostridia, which make butyric acid, are in this case, and that explains why they are never found in a well-stirred open tank.
• Facultative anaerobeAccommodates both regimes and switches from one to the other depending on what is available. Yeasts and enterobacteria belong here, and it is this flexibility that makes the composition of a tank change without the population changing.
• MicroaerophileTolerates oxygen only at low concentration, lower than in ambient air. Such an organism therefore lives far from the surface, in an intermediate layer, and it disappears from an open tank just as much as from a perfectly purged one.
• AerotolerantIs not affected by oxygen but does not use it. Lactic acid bacteria are the example, and it is the key to their success: they occupy the top and the bottom of a tank indifferently, where their competitors are confined to a layer.

The interest of this scale is that it turns a tank into a landscape rather than a switch. Hackler and Särchen in fact describe this stratification for carbonic maceration: under the pressure of the gas produced, the cherries at the bottom burst and ferment submerged, those at the top stay intact, and conditions differ at each height. The sheet on lactic fermentation measures exactly this gradient without being able to explain it: five point zero six grams per liter of lactic acid at the bottom of a one-hundred-seventy-five-kilogram tank, against two point one eight at the top, more than double.

The consequence for practice is clear and worth writing down. If a tank is a stack of oxygen regimes, then stirring is not a convenience of hygiene, it is a process variable on a par with duration or temperature. Stirring homogenizes the medium, therefore brings the layers closer, therefore brings the floras closer; not stirring produces, in one container, several different fermentations of which the cup will take the average.

None of the sources consulted publishes a stirring protocol with its frequency and measured effect, even though it is the least costly gesture of the whole process.

Oxygen regimes: Hackler and Särchen, glossary, pp. 112 to 118, and chapter 4, p. 46. Stratification of the tank: chapter 3, p. 33. Measured gradient: sheet 14, Brazilian trial on a 175 kg tank.