Key takeaways

Co-fermented coffee is fermented with something that is not inherently part of the coffee: fruit, juice, must, essential oils or botanical products. Oscar Hernández excludes yeast additions from that definition.
Oxygen, temperature, pH and time are all interconnected, and together they determine the flavour outcome. Oscar prefers around 17 to 20°C for balance in co-fermented coffee.
Co-fermented coffee fermented for more than four days has consumed much of its residual sugars and lipids, so it tends to last longer. Shorter ferments need hermetic protection.

Nothing divides the specialty coffee world like co-fermentation. Producers value the potential for flavour innovation. Critics question whether the resulting profiles represent the coffee or something else entirely. What is missing is a clear account of what co-fermented coffee really entails. That means the biochemistry, flavour outcomes, and what it means for roasters sourcing and storing high-intervention lots.

Oscar Hernández, director of Finca Nogales in Colombia, has spent years experimenting with these processes. He has pioneered their Eccentric line of botanical co-fermented coffees alongside a range of other experimental post-harvest processing protocols. They have recently planted a second farm, called Nakura, at 2,100 metres in a protected Colombian natural reserve. His scientific background informs his perspective on co-fermentation. Read on for his insights.

What is co-fermented coffee?

Co-fermented coffee is coffee fermented with an added substrate that is not inherently part of the coffee, such as fruit, essential oils or botanical products.

“To co-ferment, as the word suggests, means fermenting with something else – in this case, fermenting with fruits, flavouring agents, essential oils, natural botanical products, or other compounds,” Oscar explains. “Ultimately, I believe co-fermentation means fermenting with something that is not inherently part of the coffee.”

He draws a deliberate line around yeast additions. “I cannot say that co-fermentation means fermenting with yeasts, because you cannot talk about fermentation without talking about yeasts. It is impossible to separate the two.” The addition of commercial yeast strains to a coffee fermentation is, by this logic, a matter of fermentation management rather than co-fermentation. What makes co-fermentation distinct is the introduction of an exogenous substrate. Fruit, juice, must, essential oil, or botanical products each interact with the fermenting mass to shape the flavour profile.

Over 800 volatile compounds form during fermentation through microbial metabolism. Lactic acid bacteria, yeasts, and other microorganisms metabolise the sugars in the mucilage, producing esters, organic acids, and alcohols that directly determine what ends up in the cup. Co-cultures broaden volatile range compared to single-species fermentations, including compounds associated with fruity and caramel-like aromas. The introduction of a fruit substrate or botanical product into that environment adds further fermentable material and aromatic precursors, extending the range of possible flavour outcomes.

Oscar is careful about why this matters in practice. Adding a botanical product to a finished espresso or filter coffee is not the same as incorporating it during fermentation. “When these products are properly incorporated during fermentation, they integrate much better and lose that overly artificial sensation, becoming part of a much more complex coffee.” Acid production, metabolic activity, and structural changes occurring inside the fermentation chamber bind the introduced compounds into the coffee’s chemistry. A post-roast addition cannot replicate that.

How does co-fermentation affect coffee flavour?

Co-fermented coffee gets its flavour from oxygen, temperature, pH and time together, and none of those variables works in isolation from the others.

Oscar’s response to which fermentation variables matter most: all of them.

“You cannot separate one from the others; they are all interconnected.” Oxygen, temperature, pH, time – each influences the others and all determine the flavour outcome. Fermentation, in Oscar’s framework, requires anaerobic conditions. “Fermentation has to take place in the absence of oxygen. When oxygen is involved, we are dealing with oxidation or decomposition.” The anaerobic environment forces alternative metabolic pathways. These produce CO₂, ethanol, glycerol, and volatile esters that form the basis of the cup’s aromatic profile, much as carbonic maceration does.

Temperature is the most tractable lever. Higher temperatures favour bacterial activity, and bacteria produce acids. This drives pH down and produces sharper, more pronounced acidity in the cup. Lower temperatures favour yeasts. Yeast metabolism generates alcohols and acids that encourage ester formation. Esters are directly responsible for the fruity, floral characteristics associated with well-fermented specialty lots. Higher ester levels consistently distinguish fermented coffees from washed alternatives. Yeast-inoculated lots score highest in controlled trials, and fermentation length also changes how roasters should profile the lot.

Oscar’s preferred temperature range is around 17 to 20°C. “This can create more balance in the coffee.” The operative word is balance. Co-fermented coffees are not always about intensity, when the substrate and the variety are well matched. “If you know how to do it, you can make sure that neither flavour overwhelms the other or competes with it, resulting in a very balanced coffee.”

There is a convincing market argument for co-fermented coffees, which Oscar spells out. “When you give someone who is a beginner in the specialty coffee world a coffee in which they can clearly perceive specific flavour notes, they fall in love with coffee. I believe that is very positive for the industry.” The identifiability of co-fermented profiles functions as an entry point: clearly articulated flavour draws in consumers who might otherwise find specialty coffee abstract or inaccessible. This is important for the continued growth of the industry.

Oscar realises there are growing pains when introducing a new category of product. “What needs to exist behind all of this is transparency, at least to a certain extent.” He argues for transparency without demanding total disclosure to preserve some proprietary knowledge and processes at the farm level.

He also insists that the SCA cupping framework needs a specific methodology for co-fermented lots. Assessing them on the same form as conventionally processed coffees creates category confusion rather than meaningful evaluation. These are open conversations in the specialty coffee world. His position is that producers and roasters should have them more directly, and “ultimately, to enjoy these coffees”.

How should roasters store green co-fermented coffee to preserve its flavour integrity?

Roasters should store green co-fermented coffee in hermetic, oxygen-minimised packaging, because oxidation attacks the volatile compounds that fermentation created in the first place.

The same processes that make co-fermented coffees flavourfully distinctive also create specific needs for storage and transit.

Flavour degradation is usually a function of the amount of sugars and lipids remaining in the product. Oscar explains that a coffee fermented for more than four days has consumed much of its residual sugars and lipids during the fermentation process itself.

“Coffees that undergo fermentation for more than four days tend to last longer.” He offers a useful analogy: “It is like leaving apple juice and apple cider vinegar on your table. If you leave both there for three days, the apple juice will deteriorate, while the apple cider vinegar can remain stable for years because its sugars have already been consumed or transformed.”

A coffee with four or more days of fermentation is closer to the vinegar. One with minimal fermentation, a 24-hour washed lot or an unfermented Geisha, is comparable to the juice.

The implication for roasters is practical. “Acidity becomes more regulated when a coffee is fermented for more than four days. The acidity becomes more balanced and, in my experience, tends to improve.” The corollary also holds. A co-fermented lot without extended fermentation may be more vulnerable. So may one carrying residual botanical compounds with high aromatic volatility.

Lipid oxidation drives quality loss in green coffee storage. Oxygen exposure degrades unsaturated fatty acids, producing rancid aldehydes and destroying the volatile aromatic compounds that represent the flavour investment made at origin. Latent sensory damage accumulates in permeable bags before it becomes perceptible in the cup, which is why specialty green coffee needs better packaging. The window for protective action is narrower than one might expect. For co-fermented lots carrying a wider range of volatile compounds at higher initial concentrations, that vulnerability is proportionally greater.

Hermetic packaging limits oxygen ingress across the storage window in ways that permeable alternatives cannot fully achieve, including standard jute and GrainPro-style liners. For high-value co-fermented and experimentally processed green coffees, the case for airtight green coffee storage is not simply a matter of best practice. It is a logical extension of the investment already made in the fermentation process itself. The flavour compounds built over days of carefully managed fermentation deserve packaging capable of protecting them through the full transit from farm to roastery.

MTPak Coffee designs its PureLock hermetic bags for exactly this class of coffees: airtight, oxygen-minimised storage suited to specialty and experimentally processed green coffee.

Contact our team to discuss protecting your next co-fermented coffee lot in transit.

FAQs

Is co-fermented coffee the same as infused coffee?
It depends on who you ask. Oscar suggests the answer changes between individuals, roasters and cultures, so he wouldn’t give a simple yes or no. In Spanish, infusionar means extracting compounds with hot water, as when making a coffee, and that doesn’t accurately describe co-fermentation. In English, to infuse means to add external flavour to a substrate. Under that definition, Oscar says co-fermented coffee would count as infused.

Does adding yeast make a coffee a co-ferment?
Oscar says it does not. You cannot talk about fermentation without talking about yeasts, so he treats commercial strains as fermentation management. Co-fermented coffee requires a substrate that is not inherently part of the coffee.

Does co-fermented coffee keep as well as conventionally processed coffee?
It depends on fermentation length. Lots fermented for more than four days have consumed much of their residual sugars and lipids, and tend to last longer. Shorter ferments and volatile botanical compounds benefit from hermetic storage.