Coffee and fermentation. Everything you need to know.

Fermentation as a tool

This opens up new possibilities for producers: Once the fermentation process has been thoroughly analyzed and understood, they can apply this knowledge to bring more consistency to coffee quality. Defects are likely to become fewer, and new aroma profiles will become the norm.

What we should remember so far:

• Coffee can, but does not have to, be fermented
• The processes of “degumming” and fermentation are often mistakenly used synonymously
• We distinguish here between the mechanical detachment of the mucilage ( demucilagination ) and the biological detachment by yeasts and bacteria
• Fermentation is often seen as a risk – in cuppings, the term “ ferment ” is often used as a mistake – many cuppers still refer to coffees that are a bit fruitier as fermented – but they are ignoring a new reality: specialty coffees are becoming more and more fruity
• When coffee ferments, it usually does so through natural, wild yeasts that are already present on the fruit and also 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

2. Microbe-Climate instead of Micro-Climate

To demonstrate the flavor differences between coffees, we also usually taste coffees from very different regions. The differences are particularly noticeable when the coffees are placed side by side. This primarily demonstrates regional differences, but less so explains the role that coffee processing can play in flavor development.

Most differences between coffees are usually explained by variety, climate, and prevailing local conditions. However, we often overlook the contribution of microbes (yeast and bacteria) during the pulping process, which can have a significant influence on flavor development.

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

We now know that during coffee fermentation, the sugars and starch of the mucilage , the mucous 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 interact with the yeast, form enzymes and begin to degrade the sugars in the mucilage.

We're primarily dealing with lactic acid bacteria, which are particularly active in mucilage fermentation. Fats, proteins, and acids are also degraded and converted into alcoholic acids. This changes the smell, color, pH, and composition of the mucilage.

An unstable yeast-bacteria cocktail

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

For example, if part of the plantation is located closer to a cowshed, the air will be filled with completely different microorganisms than if the plantation is located directly next to a stream. Ripe cherries also harbor more bacteria and yeast on their surface.

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

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

The temporary storage of ripe cherries in dirty baskets, buckets, pickup truck beds, fermentation tanks, etc., always means a change in the microbial climate. Any contact with air, surfaces, or skin (which is basically always) alters the balance of the yeast-microbial cocktail.

A reality – fermentation tanks are often not clean and uncovered. This can quickly lead to uncontrolled fermentation.

If the goal is to ensure that coffee fermentation always proceeds completely evenly, then it is important to consider not only all processes from the pulper and beyond, but also from the bush to the pulper.

Short & Sweet

• 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 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 only make up about 20% of the mucilage.

The short answer

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

Once the cherry has been pulped, both seeds are exposed, surrounded by a layer of mucilage (mesocarp). It is this highly sugary mucilage that can be "fermented away." But let's take a closer look at what this mucilage consists of.

Only when the coffee cherry is pulped does the water escape. This allows the cherry to lose water only at this point. Immediately after pulping, the layer surrounding the coffee seeds contains approximately 84% water. Where there is a lot of water, we also have high water activity, meaning that dissolved substances in the water move and react quickly. At this point, the first metabolic reactions are already taking effect.

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

The majority of mucilage consists of pectin (33%) and reducing sugars (glucose and fructose). 20% is sucrose, i.e., complex, non-reducing sugars. The remaining 17% consists of ash, a non-reactive material.

The pectin

Pectin is the binding substance that holds the mucus together. We know pectin primarily from foods like apples, quinces, and tomatoes, which naturally contain high levels of pectin.

Through the yeast's metabolism, they begin to process the sugar in the mucilage, producing enzymes that promote the decomposition of the pectins ( pectinolysis) . Pectins are polysaccharides, i.e., macromolecules, and therefore insoluble in water. However, they can be separated with a small amount of water, a process known as hydrolysis. The pectins remain suspended in the water (here, in suspension) in the fermentation tank.

But what is actually fermented is not the entire mucilage, not the pectin, but only glucose and fructose, the simple sugars.

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

In summary, this means:

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

Take-home message:

Only 5% of the mucilage is directly responsible for the formation of so-called aroma precursors in green coffee during fermentation. But this 5% is significant: improper fermentation can lead to defects such as stinker beans. With controlled fermentation, however, flavor notes can be specifically emphasized or even created.

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3. Fermentation as a flavor enhancer

It is astonishing: 5% of the mucilage is directly responsible for the formation of so-called aroma precursors in the green coffee during fermentation.

However, the term fermentation is often used in the literature only for the simplified demucilagination process , with little consideration given to its influence on flavor. In this case, the aroma precursors are already formed in the green coffee. The flavor of the finished beverage is therefore largely inherent in the green coffee.

• In other words, the quality and complexity of roasted coffee depends largely on the quality of the green coffee.

Roasting itself is responsible for the formation of volatile aromas, which are largely created through a complex series of Maillard reactions, caramelization, and other thermal reactions. Roasting influences the quality of roasted coffee, but only to the extent that the green coffee allows.

How do you ferment well?

It has now become clear that fermentation of the mucilage under controlled conditions can make a coffee even better – but never necessarily.

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

The following criteria are essential for good, controlled and stable fermentation:

• climate
  • Ambient temperature
  • humidity
  • Sunlight on the fermentation tank
• Water (if coffee is covered with water)
  • Water temperature
  • Water quality
• coffee cherries
  • Crowd
  • Uniformity in maturity
  • Deformed cherries
  • Rotten cherries
  • Foreign body
• hygiene
  • pulper
  • Channels
  • Fermentation tank
  • Stirring sticks

When is coffee fermented? And when is it not?

We now know that fermentation isn't essential for coffee. Larger beneficios are geared toward efficiency and must process large amounts of coffee in a short period of time, meaning receiving cherries, determining their quality, mechanically sorting, pulping, weighing, and drying them.

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 every day. From the moment the cherry is pulped 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 pulping machines ( despulpadoras ) operate with extreme precision, and the desmucilaginators ( desmucilaginadors ) remove the mucilage (almost) completely.

• So it is economic reasons that hardly allow for more complex fermentation on a larger scale.
• But there are also climatic or local reasons: is there enough water, space, canals, containers, etc. on the site?
• Are there any historical reasons for this? Is there even any awareness of fermentation?
• In northern Nicaragua (Nueva Segovia), the reposa , or overnight storage of cherries until pulping the next day, is widespread. In the coffee regions in the center of the country, Jinotega and Matagalpa, this practice is rarely used.

The costs of wet processing with fermentation are quite high: it requires a lot of energy to operate the machines. The water must (or should) be recycled, and the channels, pulpers, and tanks must be cleaned thoroughly. Anywhere standing water poses a potential risk of contamination, which can negatively impact the flavor.

And what about the naturals ?

Dry processing ( sun-dried , natural ), in which the whole cherry is dried without pulping and still on the seed, is significantly cheaper on a large scale. However, the opposite is true for specialty coffees . Dry-processed coffees are generally more expensive, especially if they are of high quality. The labor involved in evenly drying the cherries is enormous.

Special fermentations are boutique goods

Anyone who has ever been to the coffee origins has surely seen various processing models. Personally, I always like to ask producers "why" they do this or that. Often, the answer is both meager and telling: "Because I've always done it that way." Meager, because the technical information is limited. Telling, because obviously, a lot of knowledge still needs to be transferred so that producers can get even more out of their coffee.

Of course, there are counterexamples. There are producers who can provide detailed information about every step of coffee cultivation and processing, and who know which external influences positively or negatively affect the taste. But they exist primarily in the specialty coffee world, perhaps even being a trained agronomist, having studied abroad, or engaging in an incredible amount of interaction with visitors like the rest of us.

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

One of many coffee realities

So when we talk about exceptional fermentation techniques, we are unfortunately still talking about a boutique item:

great, mostly expensive and rare.

However, at the Barista World Championships, for example, more coffees with special fermentation techniques are being used than ever before. On the barista stage, it seems almost standard practice these days to bring coffees with special fermentation techniques into the competition.

4. Fermentation with starter cultures

At the beginning of this article, we already mentioned this: what if we understand fermentation in detail and now influence it specifically with yeasts and bacteria? Still a possibility, or a monster?

Is coffee now facing the same fate as Chardonnay 20 years ago? Voluminous, intensely aromatic flavor profiles that are becoming increasingly similar, regardless of origin?

The short answer:

• there will be more and more intensely aromatic coffees
• every trend causes a countertrend
• Forecast – in three years we will end up in the middle: targeted fermentation that is not perceived as such in the final product
• The coffee clientele is probably less forgiving than the wine drinkers – if the coffee now tastes even less like coffee, then it is “not good coffee” anymore
• Intensely aromatic coffees will probably never be the majority

And here is the longer answer:

status quo

We now know: coffee fermentation is mostly viewed either as a purely mechanical function to release the mucilage , or as a hazard pool: a collection of risks that lurk on the way from the cherry to the dried coffee.

If we view fermentation as a great unknown, we rule out its ability to positively alter flavors. However, today it's often the case that a stinky bean in a cupping is directly linked to fermentation: "poorly fermented," "overfermented," the fault lies in the fermentation, or the work wasn't done well. But we need to be more precise here.

It is a reality that only in very rare cases are “only the ripest cherries” (as baristas like to say at competitions) picked.

The reality is: the vast majority of cherries delivered are often inhomogeneous: unripe, half-ripe, well ripe, very ripe, overripe; intact cherries (closed exocarp) and damaged cherries (e.g., cracked by rain).

Once the cherry is open and comes into contact with air, this accelerates uncontrolled fermentation. Therefore, off-flavors (defects) can already be found in the seeds long before the cherries are pulped and fermented in a tank.

The explanation of why a bean can be "overfermented" is therefore seriously flawed and reflects a belief that fermentation alone is responsible. But again: the starting material, i.e., intact cherries, is a fundamental prerequisite for a delicious, defect-free coffee.

Targeted fermentation with isolated starter cultures

The starter cultures (yeast and bacteria) responsible for fermentation, as described above, are already present – ​​on and in the fruit, in the tank, in the air, on the skin, on the leaves – everywhere. However, the composition of these microbes is never constant; it is influenced by rain, heat, sun, hygiene—in short, everything surrounding 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 an initial 5.5-6° to approximately 3.7-4.6°.

By correlating the factors of pH, temperature, and sugar content, in combination with the variety and quality of the cherries, fermentation can be initiated in a targeted manner. Isolated yeast starter cultures can help achieve reliable and consistent results.

“Hasn’t research already been done on this?”

There are several small-scale experiments on this, but they are still manageable. While researching the literature for this blog series, I read the most relevant articles (to my knowledge), but they were "only" about 20. If you compare the literature on aroma development through roasting, we're looking at well over 100 published articles.

“And who uses yeast for controlled fermentation?”

In a conversation with Lucia Solis , she said there are many more than one might think. She has conducted experiments on more than 45 beneficios in eleven different countries over the past three years.

“It’s just that many people don’t like to talk about it, as if it were something forbidden.”

Lucia Solis, Coffee Fermentation Designer

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

Coffee Collaborative Source (CCS) reports in a newsletter that Carmo Coffee from Brazil also conducted larger experiments with yeast. One experiment reportedly received a score of 93 points—which, in the specialty coffee world, is a jackpot of millions.

It's fascinating to read that Brazil "had neither the energy nor the desire to ferment coffee in this way, as it was time- and resource-intensive." "Brazil's coffee production has always been geared toward volume and uniformity." Yet, especially in this highly developed coffee industry with its focus on efficiency and innovation, such experiments have attracted great interest.

“Why isn’t this available on the mainstream market yet?”

We're happy to discuss this in the comments section. It's not that this isn't being done yet. "People" just don't like to talk about it. Apparently, many more coffee producers use yeast than is generally known. If there are multiple producers of yeast cultures, then there are also multiple markets.

Another question would be:

Are roasters ready to add another chapter to the history of coffee?

And will consumers then be ready to embrace this new chapter? In spring 2019, the Zurich-based roastery Stoll launched a coffee from Burundi that was treated with a yeast called "Cima" from Lalcafé . Stoll clearly describes this on the packaging, making it the first roaster in Switzerland to do so publicly.

Controlled fermentation using starter cultures can create standardized quality. This can reduce risk for the producer. Starter cultures can be yeast or bacteria. Both can be purchased in isolation.

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

In another experiment ( Pereira et al. 2014 ), 144 wild yeast species were found on and in the cherries. However, not all of them are used to specifically influence flavor. Typically, 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 hight quality," Pereira 2016 .

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

• The pH value drops,
• the sugar content too, as this enters the metabolism with the yeast.
• The activity causes the temperature to rise.

Temperature, sugar content (brix) and pH value say a lot about what happens during fermentation and in which direction it will develop.

Fermentation creates flavors that wouldn't otherwise be present in coffee . This is best appreciated and understood when the same coffee is prepared in different ways. However, it's rare to have this opportunity for direct comparison. Has Bean from England, for example, maintains very close relationships with its producers and thus has the opportunity to obtain different processing methods. It's worth ordering different coffees from the same finca.

Several studies show that yeast activity peaks at approximately 40 hours, meaning it metabolizes many bacteria and almost completely dies shortly thereafter (48 hours). As already described, not all yeast has the same properties. There are different strains, each of which, in turn, has different species.

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

The studies concluded that

• The taste differs significantly depending on the type of yeast used, always compared 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 did not dock equally well with different varieties
• In a comparison of top coffee inoculated with yeast vs. sample without yeast, both samples performed equally well (89 points according to the SCAA scale), but with different strengths

Points 3 and 4 in particular are an exciting and important message.

Regarding point 3:

In the experiment, the coffees were fermented for 12 days with equal amounts of yeast. The coffees were of the Mundo Novo (MN) and Ouro Amarelo (OA) varieties. The yeast culture populations differed at the end of fermentation: only 25% residual yeast remained in the MN and 74% in the OA. This indicates that more intensive metabolism took place in the MN.

The sensory test revealed the following:

• The addition of a specific yeast to Mondo Novo has reduced the taste of the coffee by more than 4 points
• Mondo Novo had a much higher metabolism with yeast – but that does not mean that a high metabolism always has a positive effect on the taste
• The opposite is true for Ouro Amarelho: the addition of yeast improved the coffee by almost 2 points

So we learn:

• Not all yeast is the same
• The choice of yeast is crucial for the sensory quality of the coffee
• The variety probably has a greater influence on the fermentation potential than previously assumed

What's next? This question is almost inappropriate, because where do we even begin? Do we continue to "play" with fermentations or do we truly experiment?

More experiments – structured

We mentioned various research approaches above. Here we need to distinguish between those that conduct experiments on the farm and those that support them scientifically.

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

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

Competitive advantage and/or communication

So we see two approaches:

• the structured, reproducible, knowledge-creating and therefore inclusive type of research that should be made accessible to a majority of producers
• the private, mostly experimental style, which creates new, even more complex tastes for exclusive markets (e.g. 90+, La Palma y el Tucan)

The pioneers among experimentalists are creating a market that has been growing tremendously for several years. Their findings remain exclusive, as they can provide a competitive advantage in the increasingly diversified green coffee world.

Boutique producers rarely reveal what exactly was done during the coffee processing. A significant investment goes into this process to ensure reproducible fermentations. Therefore, it's not surprising that a specific method is considered intellectual property.

Intellectual property or freely accessible knowledge?

At the same time, however, this demonstrates so much potential for a large number of producers to achieve if standards can be formulated by science. An initial review of the current state of research revealed that current findings are very much local in nature. What is needed are more general, universally applicable recipes for producers.

However, for the vast majority of coffee producers to access this knowledge, more research and communication are needed. We have described how strongly the coffee variety, a highly local characteristic, influences the taste. Local flavor will therefore continue to exist, and it will likely be difficult to imitate a flavor profile by copying and pasting the methods.

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

When we say that more research and communication are needed, we want to set an example with our Finca Santa Rita . We want to promote exchange. It is at the core of our vision that we want to create a more intensive exchange across the entire supply chain. We want to operate 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.

We took the first concrete step toward "creating knowledge" in 2018, when we learned that InnoSuisse had approved a research grant for a preliminary study with the University of Applied Sciences Zurich (ZHaW). Many thanks to the networking provided by Peter Braun of Swiss Food Research .

Together with the team of Susanne Miescher-Schwenniger from the 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, a biotechnologist, presented her findings from her field research at Finca Santa Rita in January 2019. She focused on the development of yeast populations and their behavior in our two standard processes that we use at Santa Rita – the tradicional and reposo methods.

Sebastian Opitz, an analytical chemist, investigated the influence of fermentation methods on flavor. He used treated green coffee beans and coffee cherries from Santa Rita as the basis for his findings.

We are delighted about this collaboration and proudly present the first results of this multidisciplinary experiment.