In the past few years, there has been an increasing presence of fermentation processed green coffees on our radar. And while some may be quick to turn down these coffees due to their very distinctive profiles, it is undeniable that the quality of fermentation processed coffees have improved by leaps and bounds since it appeared on the scene.

Done right, fermentation processes singlehandedly have the capability of augmenting a coffee’s quality, producing deep, complex profiles by working alongside the natural components of mucilage and fruit of the coffee cherry. To learn to appreciate the uniqueness that comes from these types of processes, it helps to first begin by understanding the act of fermentation and what it does to these coffees.

Fermentation is a chemical reaction using microorganisms to metabolise organic compounds into simpler substances. In the context of coffee processing, fermentation takes place on the cherry and mucilage of the coffee seed, breaking them down into simpler products, which are then absorbed by the coffee seed.

There are two types of fermentation: aerobic and anaerobic.

Aerobic fermentation occurs when oxygen is readily available during the process, and the key to controlling the rate of this fermentation is the duration and ambient temperature of the mixture.

Anaerobic fermentation occurs in the absence of oxygen, and is usually achieved by placing coffee cherries in an enclosed tank to limit the inflow of oxygen. At times, inert gasses such as nitrogen and carbon dioxide are introduced to flush out the oxygen in the vat, giving rise to some commonly heard coffee processes: nitro-flush, and carbonic maceration.

The use of the label “anaerobic fermentation” and the like might seem like new buzz words that are often used to connote wild and exotic tasting coffees. However, that cannot be further from the truth.

In reality, there is an extent of fermentation present in all of the coffees that we drink, from the fruity naturals, to the delicate washed coffees – in fact, the first incidence of fermentation begins when there is a point of entry for microorganisms (such as yeast and bacteria) to begin metabolising sugars and other compounds. When coffee cherries are picked and separated from the stem at harvest, this point of entry is created, kick-starting the process of fermentation.

However, the main bulk of fermentation occurs during the processing phase of coffee production. In fully washed coffees, fermentation is necessary for the non-mechanical, complete removal of mucilage as the washed coffee seeds sit in water tanks, while on the other side of the processing spectrum, natural coffees undergo fermentation as the cherries are being left to dry out under the sun.

That being said, coffees from some fermentation processes seem to be distinguishable and easily identifiable by their unique aroma, while in others, this process of fermentation is almost imperceptible in the final cup profile.

A lot of this has to do with getting the right amount of fermentation when processing the coffee. A coffee producer’s experience with such processing methods, and excellence in production quality standards are both key factors to achieving the ideal extent of fermentation.

For each different type of processing, as well as the various ripeness levels at which coffee cherries are harvested, the variables of the fermentation process will need to be adjusted accordingly. A good understanding of one's own coffee beans are as important as the scientific fundamentals in fermentation processing. In this sense, fermentation is truly an art as much as it is a science.

There are many components to nail with fermentation when processing coffees.

Firstly, one has to know how much to ferment the coffee. Excessive fermentation might result in the production of undesirable compounds, with the resultant effect of coffees that are muted in flavour and sweetness, or at its worst, simply taste bad, like rotten fruit. On the other hand, insufficient fermentation of coffee cherries often leaves a layer of mucilage on the coffee bean highly conducive for fungal and bacteria growth, resulting in the spoilage of the bean or growth of moulds.

Secondly, the rate of fermentation of each coffee cherry is likely to differ across cherries. When comparing the two types of fermentation, it can be said that anaerobic fermentation is often more homogenous (with a similar rate of fermentation), due to a controlled setting. Aerobic fermentation is often considered to be more heterogeneous (with greater variance in rate of fermentation).

Achieving good and consistent fermentation requires careful selection of coffees of similar quality and ripeness, as well as cleanliness and consistency in the processing steps. Added elements of precise data collection, such as time, temperature, and fluctuations in Brix levels during fermentation, also play a significant role in replicability.

In recent times, some new innovations such as the Bio-Innovation process from La Palma Y El Tucan, or yeast fermentations, bring about even greater control by selectively introducing the microorganisms present in the fermentation process.

As the coffee industry continues to develop and experiment with fermentation of different sorts, new trends continue to appear in unexpected places.

The first unconventional approach takes the form of an additional fermentation of green coffee beans. In 2020, the concept of controlled fermentation of green coffee beans was shown to successfully modify and enhance the flavour complexity of resultant coffees. This works primarily through the selective use of a concoction of microorganisms to target certain acids for beneficial outcomes in fermentation, as well as producing specific compounds such as various acetate esters.

A second approach is that of introducing additives during the fermentation stage of coffee processing. The most popular example is cinnamon processed coffees, where cinnamon sticks are added to the fermentation tank to impart flavours to the coffee seeds.

In our current third wave of specialty coffee, where many people endeavor to taste the origin and terroir and the unique properties of the coffee, these new boundaries of processing represent a new direction that is tangential to the original, for better or worse, as it acts as more of an addition of flavours that works to elevate the perceived quality of the original coffee beans.

In truth, the complex flavours that we enjoy in our specialty coffee could not have been achieved without the help of fermentation in some form or another. But just like the range of coffees that are available to us today, the extent and type of fermentation can vary so widely that the exact same coffee could taste completely different, depending on the process undertaken and extent of fermentation.

Hence, it is important as coffee drinkers to understand for oneself what fermentation truly is, and unpack the actual meaning behind the various labels used to describe the processing methods for the coffees that we drink, to understand what makes up the amazing combination of flavours that sit in the cup in front of us.