Coffee and fermentation. Everything you need to know.

Fermentation as a tool

This offers new opportunities 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 should become rarer, and new aroma profiles the norm.

What we should remember so far:

• Coffee can, but does not have to be, fermented.
• The processes of "demucilaging" and fermentation are often mistakenly used synonymously.
• Here, we distinguish between the mechanical detachment of mucilage (demucilagination) and the biological detachment by yeasts and bacteria.
• Fermentation is often seen as a risk – in cuppings, "ferment" is often referred to as a flaw – many cuppers still describe coffees that are somewhat fruitier as ferment – yet 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 present on the fruit and in the air.
• Since fermentation has long been considered a mechanical and less an aroma-defining process, there is still a lack of diverse research results today.

2. Microbe-Climate instead of Micro-Climate

To demonstrate taste differences in coffees, we also usually cup coffees from very different regions. The differences are particularly well highlighted when the coffees are side by side. This primarily shows regional differences, but explains less the role that the processing of coffee can have on flavor development.

Most differences between coffees are usually explained by variety, climate, and prevailing local conditions. However, we likely overlook the contribution of microbes (yeast and bacteria) which can significantly influence flavor formation during the pulping 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, the slimy 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.

Mainly, we are 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. This changes the smell, color, pH value, 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 closer to a cowshed, there will be completely different microorganisms in the air than if the plantation is directly next to a stream. Likewise, riper cherries harbor more bacteria and more yeasts on their surface.

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

The microorganisms are located on and in the fruit and increase with the stage of ripeness. They become active immediately after harvesting (or by damage to the cherry) and initial signs of unplanned fermentation can already occur at this point, for example, if 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, pickup truck beds, fermentation tanks, etc., therefore always means a change in the microbial climate. Every contact with air, surfaces or skin (i.e., essentially always), changes the balance of the yeast-microbe cocktail.

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

If the goal is for coffee fermentation to always proceed completely uniformly, then it is important not only to consider all processes from the pulper onwards, but also from the bush to the pulper.

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 these only make up 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 pulped by a pulper, both seeds are exposed with a surrounding mucilage layer (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 water escape. Thus, the cherry can only lose water at this point. Immediately after pulping, the layer now surrounding the coffee seeds contains about 84% water. Where there is a lot of water, we also have high water activity, i.e.: dissolved substances in the water move and react quickly. At this point, the first metabolic reactions already take effect.

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

The largest part of the 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, no longer reactive material.

The Pectin

Pectin is the actual binding material that holds the mucilage together. We know pectin primarily from foods such as apples, quinces, or tomatoes, which naturally have high pectin levels.

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

But what really ferments 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, this yields about 120 kg of mucilage still attached to the seeds. The drying process begins, and if we completely disregard the water, we are left with 50% sugar: fructose, glucose, and sucrose. Of these, only glucose and fructose ferment.

In summary:

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

Take-Home Message:

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

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3. Fermentation as a Flavor Enhancer

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

However, the term fermentation in the literature is usually only used for simplified demucilagination; its influence on taste is hardly considered. Yet, the aroma precursors are already formed in the green coffee. The taste of the finished beverage is therefore largely predetermined in the green coffee.

• In other words: the quality and complexity in roasted coffee depend largely on the quality of the green 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. Through roasting, we influence the quality of the roasted coffee, but only to the extent that the green coffee allows.

How to ferment well?

By now, it has become clear that the fermentation of mucilage under controlled circumstances can make coffee even better – but it never has to.

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

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

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

When is coffee fermented? And when not?

We now know that fermentation is not mandatory for coffee. Larger Beneficios are geared towards efficiency and must process a lot of coffee in a short time, i.e., receive cherries, determine quality, machine sort, pulp, weigh, and dry.

One of the largest Beneficios I have 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 pulped until it lands in the dryer, only six minutes pass. There is no time for fermentation in between. Nor does there have to be, as today's pulping machines (despulpadoras) work with extreme precision and the demucilagers (desmucilaginador) remove the mucilage (almost) completely.

• Thus, there are economic reasons that hardly allow for a more complex fermentation on a larger scale.
• But there are also climatic or local reasons: is there enough water, space, channels, containers, etc., on the premises?
• Added to this are historical reasons: is there even an awareness of fermentation?
• In northern Nicaragua (Nueva Segovia), reposa, the storage of cherries overnight until pulping the next day, is widespread. In the coffee zones in the center of the country, Jinotega and Matagalpa, this is rarely applied.

The cost of wet processing with fermentation is quite high: it takes a lot of energy to operate the machines. The water must (or should) be recycled, and the channels, pulper, and tanks cleaned thoroughly. Everywhere where water stands, there is a potential risk of contamination, which can have a negative impact on taste.

And what about the Naturals?

Dry processing (sun dried, natural), in which the entire cherry is dried unpulped on the seed, is significantly cheaper on a large scale. However, for specialty coffees, the opposite is true. Usually, dry-processed coffees are more expensive, provided they are of high quality. The effort invested in uniformly drying the cherries is enormous.

Special fermentations are boutique products

Anyone who has been to a coffee origin will surely have seen different processing models. I personally always like to ask producers "why" they do this or that. Often, however, the answer is meager and at the same time telling: "because I've always done it that way." Meager, because the technical information content is low. Telling, because it is obvious that 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. The producer who can give detailed information about every step of coffee cultivation and processing, and knows which external influences positively or negatively affect the taste, exists. But he exists mainly in the specialty coffee world, may even be a trained agronomist, has studied abroad, or has a tremendous 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 pulper at home, and they probably plant coffee because they have always done so.

One of many coffee realities

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

great, mostly expensive and rare.

However, at the World Barista Championships, for example, more coffees with special fermentation techniques are used than ever before. On the barista stage, it almost seems to be standard today to use coffees with special fermentation for the competition.

4. Fermentation with starter cultures

At the beginning of this article, we already pointed out – what if we understand fermentation precisely and now specifically influence it with yeasts and bacteria? Still an opportunity, 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 counter-trend
• forecast – in three years we will land in the middle: targeted fermentation that is not perceived as such in the final product
• the coffee clientele probably forgives less than wine drinkers – if coffee tastes even less like coffee, then it is no longer "good coffee"
• intensely aromatic coffees will probably never be the majority

And now for the longer answer:

Status Quo

We now know: coffee fermentation is mostly regarded either as a purely mechanical function for detaching the mucilage, or as a hazard pool: an accumulation 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 today, however, a "stinker bean" in a cupping is directly linked to fermentation: "poorly fermented," "over-fermented," the fault lies in fermentation, not well done. But here we need to be more precise.

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

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

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

The explanation of why a bean can be "over-fermented" is therefore greatly flawed and shows an attitude that fermentation alone is responsible. But again: the starting material, i.e., intact cherries, is a basic prerequisite for a delicious, defect-free coffee.

Targeted fermentation with isolated starter cultures

The starter cultures (yeast and bacteria) responsible for fermentation are already present as described above – 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 is influenced by rain, heat, sun, hygiene, in short: the coffee's surroundings.

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 value from an initial 5.5-6° to approx. 3.7-4.6°.

When the pH value, temperature, and sugar content are correlated with the variety and quality of the cherries, fermentation can be initiated in a targeted manner. Isolated yeast starter cultures can help achieve a reliable and consistent result.

"Hasn't research already been done on this?"

On a small scale, there are several experiments on this, but they are still manageable. During the literature search for this blog series, I read the (to my knowledge) most relevant articles, but these were "only" about 20. If you compare the literature on flavor formation through roasting, we are well over 100 that have been published.

"And who uses yeasts for controlled fermentation?"

In a conversation with Lucia Solis, she said that there were many more than one might think. In the last three years, she has conducted experiments in eleven different countries at more than 45 Beneficios.

"But many 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, in wine, in olives, in salami, in beer, in bread – yeast is used in so many places.

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

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

"Why isn't this already on the mass market?"

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

Another question would be:

Are roasters ready to extend the history of coffee by another chapter?

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

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

In various scientific experiments, mainly conducted in Brazil, naturally occurring yeasts were used in isolated form: primarily 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 of them serve the purpose of specifically influencing taste. Generally, 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 yeast is added to pulped coffee, the fermentation process usually starts faster and more intensely.

• The pH value drops,
• the sugar content also, as it enters the metabolism with the yeast.
• The temperature rises due to the activity.

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

Fermentation creates flavor notes that would not otherwise be present in coffee. This is best tasted and understood when the same coffee is presented in different processing methods. However, it is rather rare to have this opportunity for direct comparison. Has Bean from England, for example, maintains very close contacts with its producers and thus has the opportunity to obtain different processing methods. It is worthwhile to order various coffees from the same farm.

Several studies show that the peak of yeast activity is reached at about 40h, meaning they metabolize many bacteria and almost completely die off shortly thereafter (48h). As already described, not every yeast has the same properties. Different strains exist, of which there are again different types.

As with wine or beer, there are different types of yeast for coffee. 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 in comparison to the control sample, which was fermented with wild yeast.
• The yeasts used produced varying levels of acetic acid, alcohol, or lactic acid.
• The same yeast did not "dock" equally well with different varieties.
• In a comparison of top-quality coffee inoculated with yeast vs. a sample without yeast, both samples scored equally well (89 points according to 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 the same amount of yeast. The coffees were of the Mundo Novo (MN) and Ouro Amarelo (OA) varieties. The populations of yeast cultures were different at the end of fermentation: only 25% "residual yeast" in MN and 74% in OA. This means that a more intense metabolism took place in MN.

The sensory evaluation showed the following picture:

• The addition of a specific yeast to Mondo Novo reduced the taste of the coffee by more than 4 points.
• Although Mondo Novo had a much higher metabolism with yeast – this does not mean that a high metabolism always has positive effects on taste.
• Conversely, in the case of Ouro Amarelho: the addition of yeast made the coffee almost 2 points better.

So we learn:

• Yeast is not just yeast
• 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: from what point are we even moving forward now? Will we continue to "play" with fermentations or really experiment?

More experiments – structured

Further above, we mentioned various research approaches. Here, we must distinguish between those who experiment on the farm and those who scientifically substantiate it.

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 is great interest in strengthening its position through innovation and making good quality better, and less good quality palatable. The driving forces today are still universities that create new knowledge in cooperation with producers. Private initiatives, at least known ones, are still in the minority.

Outside of Brazil, we primarily read about innovative privately owned Beneficios, or even more rarely about exporters (Caravela, Project Origin) that conduct 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 thus inclusive type of research that should be made accessible to a majority of producers
• the private, mostly "experimentation-driven" 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 tremendously for several years. The findings remain exclusive, as they can create a competitive advantage in the increasingly differentiated world of green coffee.

Boutique producers rarely reveal what exactly was done during coffee processing. For fermentations to be reproducible, a lot of investment goes into this process. Therefore, it is not surprising when a special method is understood as intellectual property.

Intellectual property or freely accessible knowledge?

At the same time, this shows so much potential for what a large number of producers could achieve if standards could be formulated by science. An initial literature review of the current state of research showed that today's results are very strongly locally influenced. What is needed are more general, universally applicable recipes for producers.

However, for the vast majority of coffee producers to gain access to this knowledge, more research and more communication about it are needed. We have described how strongly the coffee variety, a highly local characteristic, influences the taste. Thus, the local flavor will continue to exist, and it is likely to be difficult to imitate a taste profile by copy-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 our Finca Santa Rita to be an example. We want to promote exchange. It is the core of our vision that we want to create a more intensive exchange throughout 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.

The first concrete step towards "creating knowledge" was started in 2018, when it became clear that a research contribution from InnoSuisse for a preliminary study together with the ZHaW was granted to us. Many thanks here to the networking by 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 achieved based on her field research in January 2019 at Finca Santa Rita. She focused on the development of yeast populations and their behavior in our two standard processes that we apply at Santa Rita – the tradicional and reposo methods.

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

We are thrilled about this collaboration and proudly present the first results of this multi-disciplinary experiment here.