Coffee Fermentation: What is it and how does it affect your cup?

Some people hate coffee fermentation. Some people love it. But one thing’s for sure: it’s the new trend in coffee processing.

So, what actually is fermentation? How can it be used to improve coffee quality? And is it even possible to process coffee without it?

What Is Fermentation?

Let’s take a trip back to our high school science class: fermentation is a chemical reaction. The combination of yeast, bacteria, and other microorganisms causes a substance to break down into other simpler substances. Normally, the substances that get broken down are sugars. As this happens, they tend to release heat. Also, different kinds of enzymes can catalyze this event.

Or, to put it more simply, fermentation is a natural change that happens when you put sugar and water together – and coffee cherries are full of both. And so, just after the cherries are picked (or sometimes before, depending on the humidity), the fermentation process will start.

The thing is, fermentation can improve a coffee’s flavor or ruin it. It’s just a matter of how you deal with it.

What Does Fermentation Have to Do With Coffee?

Fermentation is a key part of post-harvest coffee processing. It can happen in one of two ways:

• Aerobic: This is what happens when oxygen is available. Engineering this kind of fermentation is simple: just leave the recently picked cherries in a tank or a container and let the microorganisms work. Monitor the time and temperature to help you control and analyze it.

• Anaerobic: In this case, coffee cherries are laid in a tank (before or after pulping) and covered in water. That allows different microorganisms to work.

So, what’s the difference? The anaerobic processes are more homogeneous and easier to monitor, and the aerobics are more heterogeneous and more complex to monitor.

But you don’t have to just pick one or the other. Coffee farmers experiment with both aerobic and anaerobic processing and sometimes they even “start with the aerobic process and finish with the anaerobic process.”

There are many approaches to fermentation and the more they experiment, the more we learn about coffee quality.

How Does Fermentation Affect Coffee Quality?

Since fermentation is so complex, there are many different potential outcomes. Poor, uncontrolled fermentation can lead to moldy or even chemical flavors in coffee – which is why it’s so important that the producer understands the process, monitors it, and works according to best practices.

Because when fermentation is successful, it can enhance a coffee’s best attributes.

O’Coffee experiments with fermentation to “amplify [their] range of products and be able to provide [their] clients with coffees with distinct flavors, exotic coffees… Basically refining the sweetness, acidity, and body of these coffees, and also adding distinguished sensorial notes, like fruits, caramel, chocolate, and others.”

And as Dr. Britta Folmer writes in The Craft and Science of Coffee, “[Removing mucilage via] underwater fermentation is said to emphasize acidity and aroma and to dismiss some astringency. Pulped naturals or honey processed coffee consists of a process combining the wet and dry methods. Mucilage is not or only partially removed that may allow for some degree of limited fermentation at the drying stage. This can generate some special sweet flavors, closer to a natural process.”

But producers have to be careful. An over-extended fermentation time can be linked to a substantial loss of sensory quality… attributes like acidity, body and sweetness can be significantly diminished.

How Exactly Does Fermentation in Coffee Happen?

Producers can choose many different ways to ferment their coffee. As a newly trending topic, there is still plenty to learn and experimentation can help improve quality.

At O’Coffee, they follow these processes:

• “For the natural processed coffees… Those coffees are sent to fermentation tanks with a capacity for 5,000 liters, made out of masonry and coated in tiles.” And this is when aerobic fermentation takes place.

• “For the pulped coffees, they pass through ecological depulpers, without the presence of water, preserving the maximum amount of mucilage attached to the parchment. They are also driven to these tanks and remain there for variable amounts of time – it varies according to the environment temperature.”

So once the cherries are in the tanks, what next?

“For all fermentation conditions, aerobic, anaerobic, and mixed, the time can vary from 16 to 25 hours, in which we consider the process done when we have a brix reading [indication of probable sugar content] of 8°Bx (8 grams of sucrose per 100 grams of sample) and controlling the pH to approximately 4.5, not allowing this value to be lower than that.”

And, of course, every difference in the method produces a different result. “The results are, on average, better for the natural processed coffees, if compared to the pulped ones. We are able to raise these coffees’ scores by about three [cupping] points, on average,” 

“It’s important to highlight that it’s not only about the grade; the complexity of these coffees is also enhanced. The sensorial description of these coffees become richer and more complex.”

Consistency & Fermentation: Friends or Enemies?

But great coffee isn’t just high-quality: it’s also consistently high-quality. This adds security for coffee buyers and roasters as well as for producers.

You cannot always predict the results of experiments with coffee fermentation. However, there are ways to improve repeatability.

Producers should understand the processes behind fermentation, so that they can make informed decisions. They should be trained in quality analysis, such as cupping, so that they can evaluate the impact of their experiments – and change them, if they need to.

They should know their processes and follow them precisely; this will help ensure both quality and consistency. They should make sure equipment is clean. And they should record data both during and after fermentation, so that they can understand, control, and repeat the processes.

Coffee producers highlight the importance of being aware of brix, pH, fermentation time, temperature, and more – and then, finally, cupping the coffees. The more information they have, the easier it is to use fermentation to achieve consistently high-quality coffee.

Because done badly, fermentation can spell disaster for producers. But done well, it can lead to delicious, distinctive coffees that consumers love.

And after all, fermentation is inevitable. It’s simply a matter of choosing whether to limit it or embrace it.