Fermentation has become one of the most talked about subjects in specialty coffee.
Pick up a bag of coffee today and you might see terms such as anaerobic fermentation, wild fermentation, yeast inoculation, lactic fermentation or extended fermentation alongside the more familiar washed and natural processes.
It can make fermentation sound like a relatively new development in coffee.
It isn't.
Coffee has been fermenting for a very long time.
What's changing is our understanding of fermentation, the level of control producers can have over it and, increasingly, our ability to explore what fermentation actually contributes to the coffee we drink.
That's the idea behind The Fermentation Project, a global coffee tasting experiment created by James Hoffmann and coffee fermentation specialist Lucia Solis.
One coffee has been processed four different ways.
The intention is simple: change the fermentation, keep as much else as possible the same, and taste what happens.
But before we get to the four coffees, it's worth understanding what coffee fermentation actually is.
What Is Coffee Fermentation?
Coffee fermentation is a biological process involving microorganisms, particularly yeasts and bacteria, and the sugars and other compounds present in coffee fruit.
It's worth remembering that what we call a coffee "bean" is actually a seed.
That seed begins its life inside a coffee cherry, surrounded by layers of fruit. Once ripe cherries are picked, producers have to process them so the seeds can eventually be dried, stored, exported and roasted.
Fermentation can form part of that journey.
Microorganisms naturally present on the fruit, in the environment and on processing equipment can metabolise available compounds in the coffee fruit and mucilage. In doing so, they produce other compounds including organic acids, alcohols and aromatic metabolites.
For much of coffee's history, however, producers weren't talking about fermentation because they wanted to create exotic flavours.
Fermentation had a much more practical job.
Why Is Coffee Fermented?
In traditional washed coffee processing, the skin and much of the fruit are removed from the coffee cherry, leaving the seeds covered in a sticky layer called mucilage.
That mucilage can be difficult to remove mechanically.
One traditional solution is fermentation.
The depulped coffee is held for a period of time while biological and enzymatic activity helps break down the mucilage. The coffee can then be washed before being dried.
Fermentation, in other words, has historically been a useful part of turning freshly harvested coffee cherries into stable green coffee.
And this is an important distinction.
Fermentation wasn't invented by specialty coffee.
The microorganisms weren't waiting for us to discover them.
They were already there.
What's changed is how deliberately we can interact with them.
How Does the Coffee Fermentation Process Work?
There isn't one universal coffee fermentation process.
Fermentation can happen under different conditions depending on the processing method, environment and decisions made by the producer or processor.
In a traditional washed process, coffee is normally depulped before fermentation. Microbial and enzymatic activity helps break down the remaining mucilage before the coffee is washed and dried.
With natural processing, the entire coffee cherry is dried with the seed still inside it. Microbial activity and fermentation can still occur as the fruit dries.
Other approaches intentionally alter variables such as time, temperature, oxygen availability or the microorganisms present.
This is where modern coffee processing becomes particularly interesting.
Instead of simply allowing fermentation to happen, a producer can begin asking:
Which microorganisms are present?
How long should the fermentation last?
What temperature is it happening at?
How much oxygen is available?
What happens if we introduce a specific microorganism?
Can we repeat the same process next harvest?
And ultimately:
What effect does any of this have on the coffee?
Has Coffee Always Been Fermented?
Fermentation has long been associated with coffee processing, particularly traditional washed processing.
What's relatively new is the way fermentation is being discussed and deliberately managed within specialty coffee.
Historically, fermentation could be treated primarily as a practical processing stage: allow the mucilage to break down sufficiently, wash the coffee and continue with drying.
The microorganisms responsible for that fermentation could vary enormously from place to place and batch to batch.
- Temperature changes.
- Weather changes.
- Cherry ripeness changes.
- Microbial populations change.
Fermentation therefore represents one of many potentially variable stages between harvesting a coffee cherry and producing stable green coffee.
Modern fermentation research and processing techniques are increasingly interested in understanding and controlling those variables.
What Is Controlled Coffee Fermentation?
Controlled fermentation is less about discovering that coffee ferments and more about understanding how it ferments.
Producers can monitor or manipulate variables such as fermentation time, temperature, oxygen exposure, pH and microbial activity.
Specific microorganisms can also be deliberately introduced. A process known as inoculation.
Instead of relying entirely on whichever microorganisms happen to dominate a fermentation naturally, a producer might introduce a selected yeast or bacterial culture.
That can provide another level of control over the fermentation.
And importantly, control doesn't necessarily mean making coffee taste stranger.
It can mean making processing more predictable.
If producers better understand what is happening during fermentation, there is potential to reduce processing risk, improve consistency and make desirable results easier to reproduce.
That brings us to someone particularly important to The Fermentation Project.
Who Is Lucia Solis?
Lucia Solis is a coffee fermentation and processing specialist whose career began not in coffee, but in wine.
She studied Viticulture and Enology at the University of California, Davis, specialising in microbiology and fermentation science, before spending nine years working in wine production in California's Napa Valley.
In 2014, her focus shifted to coffee.
Since then, Lucia has worked with coffee producers around the world, applying principles from microbiology and fermentation science to coffee processing.
A particularly interesting part of her work is the idea that fermentation shouldn't simply be viewed as a tool for creating increasingly dramatic flavours.
For producers, fermentation is also a question of risk, consistency and control.
Understanding the process can help create repeatable protocols at the wet mill rather than leaving one of coffee production's most variable stages entirely to chance.
That thinking is central to The Fermentation Project.
How Does Fermentation Affect Coffee Flavour?
This is where things get complicated.
Fermentation can alter the chemical environment surrounding the coffee seed. Different microorganisms and fermentation conditions can produce different metabolites, including organic acids and volatile compounds.
Those changes can ultimately contribute to differences in aroma, acidity, sweetness, body and flavour.
But there's a problem when we try to taste those effects.
Imagine drinking an intensely fruity fermented coffee from Colombia and comparing it with a clean washed coffee from Guatemala.
The coffees taste completely different.
Was fermentation responsible?
Maybe.
But the coffees might also have different varieties.
Different growing conditions.
Different altitudes.
Different climates.
Different levels of cherry ripeness.
Different drying conditions.
Different storage.
Different roasting.
There are so many variables between the two cups that attributing a particular flavour solely to fermentation becomes incredibly difficult.
And that leads to one of the most interesting questions in specialty coffee:
What does fermentation actually taste like when everything else is kept as similar as possible?
What Is the James Hoffmann Fermentation Project?
The Fermentation Project is a global tasting experiment created by James Hoffmann and Lucia Solis in collaboration with Caravela Coffee.
Instead of comparing unrelated coffees processed in different ways, the project starts with the same coffee and creates four different versions of it.
The coffee comes from Finca San Miguel Urias in Guatemala's Antigua Valley, owned by Hector and Lilian Leal and operated under Rueda Farms.
The coffees were processed at Caravela's El Jaguar washing station under the guidance of Lucia Solis.
The objective is to make processing, and particularly microbial fermentation, the variable we can explore.
For coffee drinkers, that creates something we rarely get.
A control.
What Are the Four Coffee Fermentation Methods in the Project?
All four coffees begin with the same starting material.
From there, their processing diverges.
1. Mechanically Demucilaged Coffee
The first coffee acts as the control.
Rather than using a fermentation stage to help remove the mucilage surrounding the coffee seed, the mucilage is mechanically removed.
This is incredibly important to the experiment.
Without it, we could taste three differently fermented coffees and identify differences between them, but we still wouldn't know how those coffees compare with the starting material without a fermentation treatment.
The mechanically demucilaged coffee gives us that reference point.
2. Wild Coffee Fermentation
The second coffee undergoes a 48-hour wild fermentation.
Rather than deliberately introducing a selected microorganism, the fermentation is driven by the microbial community naturally present around the coffee and its processing environment.
Wild fermentation is particularly interesting because it connects the experiment with the way fermentation has traditionally occurred in coffee processing.
No selected starter culture.
The microorganisms already present drive the process.
3. Lactobacillus Fermentation
The third coffee also undergoes a 48-hour fermentation, but this time it is inoculated with Lactobacillus.
Lactobacillus refers to a group of lactic acid bacteria capable of metabolising sugars and producing organic acids, including lactic acid.
By deliberately introducing the bacteria, the project can explore what happens when bacterial activity plays a more dominant role in the fermentation.
And because we have the mechanically demucilaged and wild coffees beside it, we have something meaningful to compare it against.
4. Yeast Fermentation
The fourth coffee undergoes another 48-hour fermentation, this time inoculated with Saccharomyces yeast.
Yeasts have been used by humans in fermentation for thousands of years and are fundamental to products including bread, beer and wine.
They're also naturally present within coffee fermentation environments.
Here, however, yeast isn't simply left to chance.
The fermentation is deliberately inoculated, allowing us to explore the effect of a yeast-dominated fermentation against the same coffee processed using the other three approaches.
Wild vs Inoculated Coffee Fermentation: What's the Difference?
This is one of the most useful comparisons within the project.
The wild fermentation relies on the microorganisms already present within the coffee and its environment.
The inoculated fermentations deliberately introduce a selected microorganism.
That distinction matters because a wild microbial population can contain numerous species competing and interacting throughout fermentation.
Inoculation attempts to influence that microbial environment more deliberately.
It doesn't mean that one approach is inherently better than the other.
Instead, the question becomes one of control and comparison.
Can inoculation produce a different sensory result?
Can it make fermentation more predictable?
How different will the yeast and bacterial fermentations actually taste?
And will we prefer them to the wild fermentation, or even to the coffee that didn't undergo a fermentation stage at all?
For once, we can taste those questions rather than simply discussing them.
Does Fermented Coffee Taste Better?
Not necessarily. It's subjective. However:
More fermentation doesn't automatically mean better coffee.
And a more complicated processing method doesn't automatically produce a better cup.
Fermentation can contribute desirable sensory characteristics, but poorly managed fermentation can also introduce undesirable flavours and reduce quality.
Equally, an exceptionally clean traditionally processed coffee can be far more enjoyable than an aggressively fermented experimental coffee.
The interesting question isn't:
How much fermentation can we add?
It's:
What is fermentation actually doing and is the result desirable?
The Fermentation Project gives us an unusually controlled way of exploring that question.
What We've Learned Roasting the Four Fermentation Project Coffees
Our involvement as one of the participating roasters gives us another opportunity to explore the experiment before the coffees even reach the cupping table.
We've already completed our first test roasts of all four coffees at Ómra.
And something surprised us.
Despite undergoing four different processing treatments, the coffees behaved remarkably similarly in our roaster.
Using the same roasting approach, all four test batches reached almost exactly the same endpoint at almost exactly the same time.
That doesn't tell us what they'll taste like.
But that's precisely what makes it interesting.
If their behaviour during roasting is remarkably similar, how significant will the differences become once they're brewed and tasted side-by-side?
We've also been measuring the coffees before and after roasting and documenting the physical differences we're seeing between each treatment.
We'll explore all of that separately as this series develops.
How Can You Taste the Effect of Coffee Fermentation?
This is ultimately the point of the experiment.
Rather than telling you what fermentation should taste like, you can experience the comparison yourself.
Place the four coffees beside one another.
Start with the mechanically demucilaged control.
Then taste the wild fermentation.
Then the Lactobacillus-inoculated coffee.
Then the yeast-inoculated coffee.
Look for differences in aroma.
Acidity.
Sweetness.
Body.
Flavour.
Aftertaste.
But don't go into it assuming the fermented coffees must be more complex or that the most dramatic coffee must be the best.
The interesting part is discovering what actually changes.
One coffee.
Four processing approaches.
One variable brought into focus.
And thousands of people around the world tasting the results together.
Explore The Fermentation Project
Ómra is one of the participating roasters in The James Hoffmann & Lucia Solis Fermentation Project 2026.
Our tasting set contains all four coffees:
- Mechanically Demucilaged
- Wild Fermentation
- Lactobacillus Inoculated
- Yeast Inoculated
Allowing you to taste the four processing approaches side-by-side.
Over the coming weeks, we'll be documenting each coffee individually, exploring wild fermentation, Lactobacillus and yeast inoculation in more detail, alongside what we're learning from roasting and tasting the coffees ourselves.
You can also read our complete guide to The Fermentation Project 2026 for more information about the project, coffees, global tasting and how to take part.