How does anaerobic fermentation affect coffee flavour?

Key takeaways

• Anaerobic fermentation is defined less by excluding oxygen than by closing the vessel, since every open fermentation turns anaerobic within hours anyway.
• Flavour comes from microbial succession. Yeasts move first, lactic acid bacteria follow, and ester formation continues into the drying phase.
• Anaerobically fermented green coffee is more fragile than conventional lots, so water activity and hermetic storage matter as much as the tank protocol.

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Experimental processing has become increasingly prominent in specialty coffee. Anaerobic fermentation produces some of the most distinctive and expressive lots on the market. Drinkers associate these coffees with tropical fruit, floral aromatics, wine-like complexity, and creamy textures. Producing those characteristics, however, requires considerably more control than simply placing coffee in a sealed tank.

Few people understand that distinction better than Felipe Sardi, co-founder of Colombian producer La Palma & El Tucán.

His approach challenges some common assumptions about anaerobic fermentation, particularly the importance placed on oxygen and fermentation time. Instead, he argues that raw material, temperature, microbial activity, drying, and careful measurement ultimately determine whether experimental processing enhances or overwhelms a coffee. Read on for his insights.

What is anaerobic fermentation in coffee processing?

Anaerobic fermentation means fermenting in a closed vessel rather than an open one. Every coffee fermentation turns anaerobic within hours, so the real variable is the vessel, not the absence of oxygen.

The term “anaerobic fermentation” suggests a process that takes place without oxygen. However, Felipe explains that this definition can mislead because conventional coffee fermentation also becomes anaerobic relatively quickly.

“Every coffee fermentation becomes anaerobic at some point,” he says. “In an open tank the microorganisms consume the available oxygen within hours and the rest of the process happens without it.”

The more useful distinction, therefore, is between open and closed fermentation vessels.

“In an open fermentation you are mostly a spectator. In a closed one you choose the starting conditions, you keep the CO2 that the fermentation produces, and you suppress the populations that need oxygen to work.”

Closing the vessel changes which microorganisms can thrive. Acetic acid bacteria require oxygen, so limiting it favours yeasts and lactic acid bacteria. According to Felipe, this helps explain why controlled closed fermentations can produce coffees with rounder, softer characteristics rather than excessive sharpness. Peer-reviewed work on self-induced anaerobic fermentation records the same microbial shift, and links it directly to measurable changes in cup quality.

However, the process begins long before coffee enters a fermentation tank. Felipe considers the cherry’s quality and ripeness the single most important variable.

“Fermentation amplifies what is already there. It does not correct anything.”

Temperature comes next, because it influences both fermentation speed and which microbial populations dominate. Whether the coffee ferments as whole cherry or depulped also matters. Whole cherries retain mucilage beneath the skin, which makes for a slower and potentially more intense process. Depulping exposes the mucilage and can create a faster process that is easier to control.

Water provides another variable. In submerged fermentation, Felipe explains, it can act as a buffer, helping even out temperature and gradients throughout the tank.

Interestingly, fermentation time itself sits relatively low on his list.

“A hundred and twenty hours at eighteen degrees and a hundred and twenty hours at twenty six degrees are not the same process.”

At La Palma & El Tucán, data governs anaerobic fermentation rather than a predetermined number of hours. “We log pH, temperature, redox potential, conductivity, dissolved solids and salinity every four hours, and we cut the fermentation when the curve tells us to, not when the clock does.”

How does anaerobic fermentation shape flavour and cup profile?

Microbial succession creates the flavour. Yeasts move first and produce alcohols, lactic acid bacteria follow as the pH drops, and the esters behind tropical fruit and floral notes form from both.

The distinctive flavours associated with anaerobic coffees come largely from that succession of microbial activity.

“What you are tasting is mostly the result of a succession,” Felipe explains. “Yeasts move first, converting sugars into ethanol and CO2 and producing higher alcohols along the way. As the oxygen disappears and the pH drops, lactic acid bacteria take over.”

The interaction between these compounds contributes directly to flavour. Alcohols produced by yeasts can combine with organic acids to form esters. Those compounds sit behind many of the tropical fruit, red fruit, and floral characteristics in expressive fermented coffees. Research on the volatile compounds produced during anaerobic fermentation tracks exactly that build-up, and connects it to higher sensory scores.

Importantly, this development does not necessarily stop when coffee leaves the fermentation vessel.

“Part of that ester formation continues into the drying phase, which is why I insist that drying is part of the fermentation and not a separate step that happens afterwards.”

Organic acids also influence the sensory experience. Felipe describes lactic acid as contributing softer acidity and a creamy, almost dairy-like weight, while malic and citric acids can create brighter sensations. Excessive acetic acid, meanwhile, creates sharpness and can eventually produce undesirable vinegar characteristics.

However, acidity cannot be understood by measuring the amount of acid alone. The mineral composition of the coffee affects how acidity registers on the palate by influencing its buffering capacity.

“That is why the same process on two different lots of the same variety can give you two different sensations of brightness,” Felipe says. At La Palma & El Tucán, the team refers to this interaction as a “mineral economy”.

Anaerobic fermentation also has implications for roasting. By modifying sugars and amino acids within the coffee, processing changes the raw material on which Maillard reactions act during the roast. In other words, fermentation does not simply add aromatic compounds. It changes how the coffee subsequently develops in the roaster, which is why many roasters adjust their approach for fermented lots.

How can producers balance anaerobic fermentation with terroir?

Producers balance the two by deciding how far the process should push. If you can no longer tell the variety apart after processing, the technique has overtaken the place.

Not all anaerobic coffees are intensely fruity or obviously fermented. They can range from subtle expressions to extraordinarily powerful profiles.

According to Felipe, microbial populations, temperature, whether fermentation is native or guided, and drying all influence where a coffee falls on this spectrum.

“Drying is where you either lock the profile in place or let the reaction keep going for another week without supervision.”

For producers, the challenge is deciding how much influence processing should have over the coffee’s inherent character. Felipe’s position is clear. Maximum intensity should not automatically count as success.

“We apply a simple test on our own cupping table, which is whether we can still tell the Geisha from the Sidra after processing. If we cannot, the process is driving the car and the coffee has become a technique rather than a place.”

La Palma & El Tucán therefore separates its processing into different intentions. Some coffees prioritise terroir and variety, while others provide room for experimentation and greater risk. Keeping those approaches distinct allows experimentation without disguising it as an expression of origin.

This also helps explain why approaches vary between producers. Farms with distinctive varieties, high elevations, or exceptional growing conditions may process more conservatively, because they have desirable characteristics they want to preserve. More expressive processing, meanwhile, can sometimes increase the appeal of coffees with less distinctive underlying profiles.

As Felipe acknowledges, “a very expressive process can also hide mediocre coffees.”

The greater achievement is consistency. Reproducing an exceptional anaerobic fermentation across changing varieties, weather conditions, and an entire harvest requires far more skill than producing one impressive experimental lot. “That is where the craft actually lives,” Felipe says.

Why does anaerobic fermentation demand careful storage?

Because anaerobic lots are more fragile, not less. The volatile esters that justify the premium are the first compounds to disappear once oxygen, heat, or moisture reach the green coffee.

Careful processing means little if the resulting green coffee deteriorates before it reaches the roaster. In fact, Felipe cautions that experimental coffees may require more attention during storage rather than less.

“These coffees are more fragile than conventional lots, not less,” he says. “You have a seed with a more complex chemistry, more residual compounds, and a set of volatile esters that are precisely what you paid the premium for. Those are also the first things you lose.”

Oxygen, heat, and moisture pose three significant threats. Oxygen encourages lipid oxidation and the deterioration of delicate aromatics, while higher temperatures accelerate chemical reactions. That makes conditions during international shipping and warehousing particularly important, and green coffee freshness declines faster than many buyers assume.

Water also requires more sophisticated monitoring than simply measuring moisture percentage.

“Moisture content is the standard number, but water activity is the one that predicts stability,” Felipe explains. Coffees with identical moisture readings can behave differently depending on how evenly they dried.

He recommends letting green coffee stabilise before packing, and measuring water activity as well as moisture. A hermetic barrier liner should then serve as the baseline for valuable experimental lots. Higher-value coffees can justify vacuum packing in smaller quantities, while exceptional lots may benefit from freezing.

“On our own farm we vacuum pack in bricks and freeze the top lots, and the difference a year later is still incredible.”

However, protective packaging cannot compensate for improper processing.

“Hermetic packaging does not fix a coffee that was packed too wet. It seals the problem in with the coffee, and then you have an anaerobic environment doing something you did not choose.”

Ultimately, preserving anaerobic fermentation lots requires the same philosophy as producing them. Control the variables rather than relying on labels or assumptions. As Felipe puts it, “Packaging protects a stable coffee. It does not stabilise an unstable one. Get the drying right first, and then protect it properly all the way to the roaster.”

Anaerobic fermentation only delivers value if the volatile compounds it creates survive the journey from origin to roastery. This is where packaging becomes a sourcing decision, not an afterthought.

MTPak Coffee‘s PureLock Hermetic Bags offer moisture and oxygen barriers designed for high-value lots, helping buyers protect the premium they paid. This ensures anaerobically fermented lots arrive tasting exactly as promised, ready for the first cup.

For more information about protecting anaerobic fermentation lots, contact our team.

FAQs

What is anaerobic fermentation in coffee?
Anaerobic fermentation means fermenting coffee in a sealed vessel rather than an open tank. Every fermentation becomes anaerobic within hours as microorganisms use up the oxygen, so the meaningful choice is the closed vessel and the starting conditions it lets you set.

How does anaerobic fermentation change the flavour?
It works through microbial succession. Yeasts convert sugars into ethanol and higher alcohols, then lactic acid bacteria take over as the pH drops. The resulting esters carry the tropical fruit, red fruit, and floral notes these coffees are known for.

Why does anaerobically fermented green coffee need hermetic storage?
Its volatile esters and residual compounds stay chemically active after processing. Oxygen, heat, and moisture degrade them faster than in conventional lots, so hermetic packaging protects the character that justified the premium in the first place.