Coffee and fermentation. Everything you need to know.

Fermentation as a Tool

This offers producers new possibilities: once the fermentation process has been carefully analyzed and understood, they can apply this knowledge to bring more consistency to coffee quality. Defects should become less common and new aroma profiles should become the norm.

What we should remember so far:

• Coffee can, but doesn't have to, be fermented
• the processes of "demucilation" and fermentation are often incorrectly used synonymously
• We distinguish here between the mechanical removal of mucilage (demucilagination) and the biological removal by yeasts and bacteria
• fermentation is often seen as a risk – in cuppings, "ferment" is often spoken of as a defect – many cuppers still refer to coffees that are somewhat fruitier as ferment – but in doing so, they deny a new reality: specialty coffees are becoming increasingly fruit-forward
• when coffee ferments, it usually does so through natural, wild yeasts that are already settled on the fruit and also present in the air
• Since fermentation has long been viewed as a mechanical rather than an aroma-defining process, there is still a lack of diverse research results today

2. Microbe-Climate instead of Micro-Climate

To show flavor differences in coffees, we usually taste coffees from very different regions. The differences are best discovered when the coffees are side by side. This way we primarily show regional differences, but explain less the role that processing the coffee can have on flavor development.

Most differences between coffees are usually explained by variety, climate, and prevailing local conditions. But in doing so, we often overlook the contribution of microbes (yeast and bacteria) that can have a major influence on flavor development during the depulping process.

So let's ask ourselves: How exactly does coffee fermentation work?

We now know that during coffee fermentation, the sugars and starches of the mucilage, that is, the slime layer surrounding the seed, are broken down by yeasts and bacteria (microorganisms) and converted into acids or alcohol.

This process always requires both actors: yeast and bacteria, because the latter work with the yeast, form enzymes, and begin to degrade the sugars in the mucilage.

We're mainly dealing with lactic acid bacteria here, which are particularly active in mucilage fermentation. Fats, proteins, and acids are also degraded and converted into alcoholic acids. The smell, color, pH value, and composition of the mucilage thus change.

An Unstable Yeast-Bacteria Cocktail

The ratio of yeast to bacteria is never stable in the yeast-bacteria cocktail. Even within the same plantation, there can be significant differences in the yeast-bacteria balance.

If, for example, part of the plantation is closer to a cow barn, then quite different microorganisms are swirling in the air than if the plantation is directly next to a stream. Also, riper cherries harbor more bacteria and more yeasts on their surface.

More ripeness = more sugar = more yeasts and bacteria = more potential fermentation

The microorganisms are found on and in the fruit and increase with the ripeness stage. They become active immediately after harvest (or through damage to the cherry) and first signs of unplanned fermentation can occur now if, for example, the ambient temperature is high, the cherries are not of uniform quality, or they are stored in a place with many bacteria.

Intermediate storage of ripe cherries in dirty baskets, buckets, truck beds, fermentation tanks, etc., always means a change in the microbe climate. Every contact with air, surfaces, or skin (which is basically always), changes the balance of the yeast-microbe cocktail.

A reality – fermentation tanks are often not clean and not covered. Uncontrolled fermentation can quickly occur here.

If the goal is for coffee fermentation to always proceed completely uniformly, then it's not just about looking at all the processes from the depulper and beyond, but also from the bush to the depulper.

In a Nutshell

• The fermenting organisms use the pulp as an energy source (carbon and nitrogen) and produce high levels of ethanol, acetic acid, and lactic acid
• The riper the cherries, the more of their own yeasts and bacteria they produce
• The yeast-bacteria cocktail in the coffee cherry is never stable

Which Part of the Coffee is Actually Fermented?

Only the reducing sugars: glucose and fructose. And they make up only about 20% of the mucilage.

The short answer

But what happens to the rest? The slightly longer answer:

As soon as the cherry has been depulped through a depulper, both seeds lie exposed with a slime layer (mesocarp) surrounding them. It's this highly sugary slime that can be "fermented away." But let's take a closer look at what this slime is made of.

Only during depulping of the coffee cherry does water come out. Thus, the cherry can only now lose water. Immediately after depulping, the layer surrounding the coffee seeds now has about 84% water. Where there's a lot of water, we also have high water activity, that is: dissolved substances in the water move and react quickly. At this point, the first metabolic reactions are already occurring.

If we now subtract the moisture and look at the remaining components of the mucilage, we get the following picture.

The largest part of the mucilage consists of pectin (33%) and reducing sugars (glucose and fructose). 20% are sucrose/table sugar, that is, complex, non-reducing sugars. The remaining 17% consists of ash, non-reactive material.

The Pectin

Pectin is the actual binding material that holds the slime together. We know pectin mainly from foods like apples, quinces, or tomatoes, which naturally have high pectin values.

Through the metabolism of yeasts, they begin to process the sugar in the mucilage and produce enzymes that drive the decomposition of pectins = pectinolysis. Pectins are polysaccharides, that is, macromolecules, and therefore not water-soluble. However, they can be cleaved with little water, the so-called hydrolysis. The pectins remain in the fermentation tank in the water (here suspension).

What actually ferments is not the whole mucilage, not the pectin, but only glucose and fructose, the simple sugars.

If we depulp 1 ton of ripe Arabica cherries, we get about 120 kg of mucilage still attached to the seeds. The drying process begins and if we completely ignore the water, we're left with 50% sugar: fructose, glucose, and sucrose. Of these, only glucose and fructose ferment.

That means in summary:

• 5% of the whole fresh mucilage are simple sugars. And only these 5% are actually fermented.
• The pectin is repelled from the coffee seeds through hydrolysis, but does not dissolve in water.

Take-Home Message:

Only 5% of the mucilage are directly responsible for the formation of so-called aroma precursors during fermentation in raw coffee. But these 5% are powerful: with improper fermentation, defects like stinker beans can form. With controlled fermentation, however, specific flavor notes can be deliberately emphasized or even newly created.

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3. Fermentation as a Flavor Enhancer

It's remarkable: 5% of the mucilage are directly responsible for the formation of so-called aroma precursors (aroma precursors) in raw coffee during fermentation.

However, the term fermentation in the literature is mostly only used for the simplified demucilagination, and the influence on flavor is hardly considered. Yet the aroma precursors (so-called aroma precursors) are already formed in the raw coffee. The taste of the finished beverage is therefore largely already determined in the raw coffee.

• In other words: the quality and complexity in roasted coffee largely depends on the quality of the raw coffee.

Roasting itself is responsible for the formation of volatile aromas, which are mostly created through a complex series of Maillard reactions, caramelization, and other thermal reactions. By roasting, we influence the quality of the roasted coffee, but only insofar as the raw coffee allows.

How is Good Fermentation Done?

By this point, it became clear that fermentation of the mucilage under controlled circumstances can make a coffee even better – but never has to.

How well fermentation is done depends on a variety of external circumstances. First, there must be awareness that this is not just a mechanical, but a biological process.

For good, controlled, and stable fermentation, the following criteria are essential:

• Climate
  • Ambient temperature
  • Humidity
  • Sunlight on the fermentation tank
• Water (when coffee is covered with water)
  • Water temperature
  • Water quality
• Coffee cherries
  • Quantity
  • Uniformity in ripeness
  • Deformed cherries
  • Rotten cherries
  • Foreign objects
• Hygiene
  • Depulper
  • Channels
  • Fermentation tank
  • Stirring rods

When is Coffee Fermented? And When Not?

We now know that fermentation in coffee is not mandatory. Larger beneficios are designed for efficiency and must process a lot of coffee in a short time, meaning receive cherries, determine quality, machine sort, depulp, weigh, and dry.

One of the largest beneficios I've visited since then was in Veracruz, Mexico. During peak season, up to 140 tons of cherries are processed there. Per day. From the moment the cherry is depulped until it lands in the dryer, just six minutes pass. There's no time for fermentation in between. Nor does it need to be, since today's depulping equipment (despulpadoras) work extremely precisely and the demucilators (desmucilaginador) remove the mucilage (almost) completely.

• So it's economic reasons that make more complex fermentation on a larger scale hardly possible.
• But there are also climatic or local reasons: is there enough water, space, channels, containers, etc. on the premises?
• Plus historical reasons? Is there even awareness of fermentation?
• In northern Nicaragua (Nueva Segovia), reposa, storing cherries overnight until depulping the next day, is widespread. In the coffee zones in the middle of the country, Jinotega and Matagalpa, this is hardly applied.

The cost of wet processing with fermentation is quite high: it requires a lot of energy to run the machines. The water must (or should) be recycled and the channels, depulpers, and tanks kept clean. Wherever water stands, there's a risk of contamination that can negatively affect the taste.

And What About the Naturals?

Dry processing (sun dried, natural), where the whole cherry is dried unpulped on the seed, is much cheaper on a large scale. But with specialty coffees, it's exactly the opposite. As a rule, dry-processed coffees are more expensive, if they're of high quality. The work that goes into evenly drying the cherries is enormous.

Special Fermentations Are Boutique Goods

Anyone who has been to the coffee origin themselves has certainly seen different processing models. I personally like to ask producers "why" they do this or that. But often the answer falls short and is simultaneously revealing: "because I've always done it this way." Short because the technical information content is low. Revealing because it's obvious that much knowledge still needs to be transferred so producers can get more out of their coffee.

Of course, there are counterexamples. The producer who can give detailed information about every step of coffee growing and processing, and knows which external influences can positively or negatively affect the taste, does exist. But he exists mainly in the specialty coffee world, maybe even trained as an agronomist, studied abroad, or has an enormous amount of exchange with visitors like all of us.

We must never forget that the vast majority of coffee producers neither make specialty coffee nor have a clean depulper at home, and they probably grow coffee because they've always done it.

One of many coffee realities

So when we talk about extraordinary fermentation techniques, we're also talking about a boutique item, unfortunately:

cool, usually expensive, and rare.

However, at the Barista World Championships, for example, there are more coffees using special fermentation techniques than ever before. On the barista stage, it almost seems like a standard today to bring coffees with special fermentation for competition.

4. Fermentation with Starter Cultures

At the beginning of this article, we already pointed out – what if we understand fermentation precisely and now deliberately influence it with yeasts and bacteria? Still a chance, or a monster?

Is Coffee Facing the Same Fate as Chardonnay 20 Years Ago? Voluminous, intensely aromatic flavor profiles that become increasingly similar, regardless of origin?

The short answer:

• there will be more intensely aromatic coffees
• every trend causes a countertrend
• Forecast – in three years we'll land in the middle: targeted fermentation that isn't perceived as such in the end product
• the coffee clientele probably forgives less than wine drinkers – if the coffee now tastes even less like coffee, then it's "not good coffee" anymore
• intensely aromatic coffees will probably never be the majority

And here now the longer answer:

Status Quo

We now know: coffee fermentation is mostly seen either as a pure mechanical function to loosen the mucilage, or as a risk pool: a collection of risks lurking on the path from cherry to dried coffee.

If we consider fermentation as a great unknown, then we exclude its ability to also positively change flavors. Often nowadays, a stinker bean in a cupping is directly linked to fermentation: "poorly fermented," "over-fermented," the mistake is in fermentation, not well done. But we need to be more precise here.

It's a reality that only in the rarest cases are "only the ripest cherries" (as baristas like to say at championships) picked.

The reality is: the large mass of delivered cherries is often inhomogeneous; unripe, half-ripe, well-ripe, very ripe, overripe; intact cherries (closed exocarp) and damaged cherries (e.g., burst from rain).

As soon as the cherry is open and comes into contact with air, this accelerates uncontrolled fermentation. So long before the cherries are depulped and then fermented in a tank, defects (off-flavors) can already be in the seed.

The explanation for why a bean can be "over-fermented" thus limps greatly and shows an attitude that fermentation alone is responsible for it. But again: the raw material, intact cherries, is a basic requirement for delicious, defect-free coffee.

Targeted Fermentation with Isolated Starter Cultures

The starter cultures responsible for fermentation (yeast and bacteria) as described above are already there – on and in the fruit, in the tank, in the air, on the skin, on leaves – everywhere. However, the composition of these microbes is never constant, it's influenced by rain, heat, sun, hygiene, in short: the environment of the coffee.

These fermenting organisms use the pulp as an energy source and produce high levels of ethanol, acetic acid, and lactic acid, which can lower the pH from originally 5.5-6° to about 3.7-4.6°.

If the factors pH, temperature, and sugar content are correlated in combination with variety and quality of the cherries, fermentation can be deliberately initiated. Isolated yeast starter cultures can help achieve a reliable and consistent result.

"Wasn't There Already Research on This?"

On a small scale, there are several experiments on this, but they're still manageable. During the literature research for this blog series, I read what I believe to be the most relevant articles, but that was "only" about 20. If you compare the literature on flavor formation through roasting, we're well over 100 that have been published.

"And Who Uses Yeasts for Controlled Fermentation?"

In a conversation with Lucia Solis, she said that it's much more than you'd actually think. She has conducted experiments on more than 45 beneficios in eleven different countries over the last three years.

"Only many don't like to talk about it, as if it were something forbidden."

Lucia Solis, Coffee Fermentation Designer

But let's not forget: with cheese, with wine, with olives, with salami, with beer, with bread – yeast is used in so many places.

Coffee Collaborative Source (CCS) reports in a newsletter from Carmo Coffee from Brazil, who also conducted larger experiments with yeasts. One experiment is said to have been rated at 93 points – which in the specialty world is the jackpot.

It's interesting to read that Brazil "neither had the energy nor the need to ferment coffee this way, since it's time and resource intensive." "Brazil's coffee production has always been focused on volume and uniformity." Yet in this highly developed coffee industry with a focus on efficiency and innovation, such experiments met with great interest.

"Why Isn't This on the Broader Market Yet?"

We can certainly discuss that in the comment section. It's not that this isn't being done. "One" just doesn't like to talk about it. Apparently, many more coffee producers use yeasts than is known. If there are multiple manufacturers of yeast cultures, then there are also multiple markets.

Another question would be:

Are roasters ready to expand the story of coffee by another chapter?

And are consumers then ready to take in this new chapter? Stoll Roastery from Zurich launched a coffee from Burundi in spring 2019 that was treated with a yeast called "Cima" from Lalcafé. Stoll describes this clearly on the packaging and was the first roaster in Switzerland to make this public.

Controlled fermentation with the help of starter cultures can create standardized quality. This can mitigate risk for the producer. Starter cultures can be yeasts or bacteria. Both can be purchased in isolated form.

In various scientific experiments, which have been conducted mainly in Brazil, naturally occurring yeasts in isolated form were used: mainly those of the strains pichia and saccharomyces cerevisiae.

In another experiment (Pereira et al. 2014) 144 wild yeast species were identified on and in the cherries. However, not all serve the purpose of deliberately influencing flavor. As a rule, the most promising ones are isolated and multiplied – "although a broad microbial diversity is generally observed, only a few numbers of species are usually selected. Thus, most of these indigenous microorganisms are probably not necessary to obtain a final product with high quality", Pereira 2016.

When the yeast is added to the depulped coffee, the fermentation process usually starts faster and more intensively.

• The pH drops,
• as does the sugar content, since it enters metabolism with the yeast.
• The temperature rises through the activity.

Temperature, sugar content (brix), and pH already tell us a lot about what's happening during fermentation and which direction it will develop.

Fermentation creates flavor notes that wouldn't otherwise be in the coffee. You best taste and understand this when you have the same coffee in different processing methods in front of you. However, it's rather rare to have this opportunity for direct comparison. Has Bean from England, for example, maintains very close contact with their producers and thus has the ability to get different processing methods. It's worth it to order various coffees from the same farm.

Multiple studies show that the peak of yeast activities is reached at about 40 hours, so it metabolizes many bacteria and then almost completely dies off (48 hours). As described, not every yeast has the same properties. There are different strains, of which there are again different types.

Like with wine or beer, there are also different types of yeasts with coffee. In other experiments from Brazil (Ribeiro et al. 2016), different yeasts were used and observed over time.

The studies concluded that

• The flavor differs significantly depending on the type of yeast used. Always in comparison to the control sample, which was fermented with wild yeast
• The yeasts used produced different levels of acetic acid, alcohol, or lactic acid
• The same yeast didn't attach equally well to different varieties
• In a comparison of yeast-inoculated top coffee vs. sample without yeast, both samples scored equally well (89 points on the SCAA scale), but with different strengths

Particularly points 3 and 4 are exciting and important messages.

On Point 3:

In the experiment, the coffees were fermented for 12 days with the same amount of yeast. The coffees were of the varieties Mundo Novo (MN) and Ouro Amarelo (OA). The populations of the yeast cultures were different at the end of fermentation: only 25% "residual yeast" in MN and still 74% in OA. This means that MN had a more intensive metabolism.

In the sensory evaluation, the following picture emerged:

• The addition of a specific yeast to Mundo Novo reduced the coffee's flavor by more than 4 points
• With yeast, Mundo Novo had a much higher metabolism – but this doesn't mean that high metabolism always has to have positive effects on flavor
• The opposite is the case with Ouro Amarelo: the addition of yeast could improve the coffee by almost 2 points

We learn:

• Yeast is not yeast
• The choice of yeast is decisive for the sensory quality of the coffee
• The variety probably has a greater influence on fermentation potential than previously assumed

How to proceed? This question is actually almost inappropriate, because: from which point are we actually proceeding now? Does it remain that we continue to "play" with fermentations, or really experiment?

More Experiments – Structured

Earlier we mentioned various research approaches. Here we need to distinguish between those who experiment on the farm and those who underpin it scientifically.

The vast majority of research has been conducted in Brazil to date. The reasons for this are obvious: as the world's largest coffee producer, there's great interest in strengthening its position through innovation and making good quality better, and poor quality palatable. The driving forces today are still universities, which work together with producers to create new knowledge. Private initiatives, at least known ones, are still in the minority.

Outside of Brazil, we mainly read about innovative private beneficios, or even more rarely about exporters (Caravela, Project Origin) conducting fermentation experiments. These experiments result in special coffees that can then be marketed as "Carbonic Maceration," "Black Diamond," "Beaujolais Method," "Lactic-Acetic," or XY-Fermentation.

Competitive Advantage and/or Communication

So we see two approaches:

• the structured, reproducible, aimed at creating knowledge and therefore inclusive type of research that should be made accessible to a majority of producers
• the private, usually driven by "experimental enthusiasm" type, which creates new, even more complex flavors for exclusive markets (e.g., 90+, La Palma y el Tucan)

The pioneers among the experimentalists are creating a market that has been growing enormously for some years. The findings remain exclusive because one can create a competitive advantage with them in the increasingly differentiating raw coffee world.

It's rare for boutique producers to show their cards about what exactly was done during coffee processing. For fermentations to be reproducible requires a lot of investment in this process. Therefore, it's no wonder when a special method is understood as intellectual property.

Intellectual Property or Freely Accessible Knowledge?

At the same time, there's so much potential showing where a large mass of producers could arrive if standards could be formulated through science. A first look at the current state of research showed that today's results are very locally shaped. What would be needed are more general, universally applicable recipes for producers.

For the vast majority of coffee producers to gain access to this knowledge requires more research and more communication about it. We've described how strongly the coffee variety, a highly local property, influences flavor. The local color will continue to exist and it should be difficult to imitate a flavor profile by copying the methods.

5. ZHaW Zurich x Kaffeemacher – Research on Finca Santa Rita

When we say more research and communication is needed, we want to be an example with our Finca Santa Rita. We want to promote exchange. It's the core of our vision to create more intensive exchange across the entire supply chain. We want to run a farm that can also become a place of exchange for producers in the region. A place where something can be learned – for example, about fermentation.

The first concrete step toward "creating knowledge" we started in 2018, when it became clear that we had received research funding from InnoSuisse for a feasibility study together with ZHaW. Many thanks here to the networking through Peter Braun from Swiss Food Research.

Together with the team of Susanne Miescher-Schwenniger from ZHaW Food Biotechnology, Susette Freimüller Leischtfeld, and Barbara Beck, as well as the team of Chahan Yeretzian from the Coffee Excellence Center and Sebastian Opitz, we were able to present the first results in June 2019.

Barbara Beck, as a biotechnologist, presented her results, which she was able to achieve based on her field research in January 2019 on Finca Santa Rita. She focused on the development of yeast populations and their behavior in our two standard processes, which we apply on Santa Rita – the tradicional and reposo methods.

Sebastian Opitz, as an analytical chemist, investigated the influence of fermentation methods on taste. The treated raw coffee beans as well as coffee cherries from Santa Rita served as the basis for his findings.

We're delighted with this collaboration and proudly present here the first results of this multi-disciplinary experiment.