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Coffee Fermentation: Why’s Everyone Talking About Carbonic Maceration?

When most people think of fermentation, coffee processing isn’t usually the first thing that comes to mind. Kimchi, wine, yogurt— maybe Kombucha or other random beverages, but usually not coffee.

And that’s quite interesting because all coffee is fermented in one way or another. Whether it’s in a tank of water as it’s being washed, or in the cherry as it’s drying. Either way, there is at least a little fermentation happening. 

Today, let’s get psychedelic— let’s talk about coffee fermentation. What is it?

What Is Coffee Fermentation?

Fermentation is the breakdown of organic substances by various microorganisms. These microorganisms, like bacteria and yeast, convert one organic substance into a simpler one. That’s the elementary textbook definition. Ok. Got it.

But what does that mean in relation to coffee? 

Well, these microorganisms eat away at the sugars and other compounds in the fruit and the mucilage, breaking them down into byproducts like ethanol and various acids.

Then these byproducts are absorbed into the cellular structure of the seed. This leaves us with a coffee seed that is fermented to some degree, which becomes our green coffee beans, ready to roast and brew. 

Types of Fermentation Used for Coffee

There are two main types of coffee fermentation— Aerobic fermentation and Anaerobic fermentation.

Aerobic fermentation occurs when oxygen is available. A good example of the aerobic fermentation process is in natural coffees. 

The cherries are picked from the trees and laid out on patios or drying beds. The degree of fermentation that occurs depends on the temperature and the amount of time the cherries take to dry. 

Dry them in full sun and there will be less fermentation and less impact on the coffee because it will dry out much faster.

Keep them in the shade, and they will dry out slower, giving more time for the bacteria to break down the organic compounds. The longer the fermentation, the stronger the taste will be.  

Anaerobic fermentation happens in an oxygen-free environment. This oxygen-free environment might be a stainless steel tank purged with carbon dioxide or a tank filled with water. A good example of one of these processing methods is carbonic maceration.

Post harvest, the whole coffee fruit is thrown into a sealed stainless steel tank. Carbon dioxide is pumped into the tank. Because oxygen is lighter than C02, it is forced out of the tank via a valve at the top.

The bacteria can now get to work, eating away at the sugars and creating the good stuff that we want. 

Ethiopia Worka Chelbesa Anaerobic | Onyx | Trade Coffee
  • Light - medium roast
  • Fruity with notes of tropical fruit
  • Freshly roasted

Why Ferment It?

This does beg the question— why would you want to ferment a coffee? Or better yet, why would you want to drink one that has been fermented?

We use fermentation in coffee for two reasons— to add flavor and a unique quality to the beans or strip the bean clean. 

Depending on the type of fermentation chosen during production, a producer can create a clean coffee without any detectable fermented flavors or create a wild, bubblegum, fruity, sweet coffee with almost sparkling acidity.  

But what do we mean when we say ‘fermented flavors’? 

While it does vary depending on the type of fermentation that occurs, we can describe most coffees that use fermentation to their advantage as sweet and bright, with an incredibly fruity quality.

Naturals are often described as winey or boozey— certainly, flavors associated with fermentation (or possibly fermentation gone wrong, depending on who you ask).

Fermentation in coffee— some people love it, some people hate it.

Washed, Natural and Pulped

As I mentioned earlier, all coffee is fermented a little bit. 

Even those incredibly clean tasting high quality washed coffees have undergone a small amount of fermentation. But a washed coffee isn’t fermented to create different flavors.

Rather the fermentation helps the producer remove all of the sticky mucilage that remains attached after removing the fruit from the seed. 

Washed coffees are usually soaked in a tank of water for between 18 and 24 hours. During this time, microorganisms eat away at the sugars, creating enzymes that break down the sticky mucilage. When done right, it doesn’t create a detectable ‘fermented flavor’ in the coffee. This light form of fermentation allows the removal of all the mucilage.  

Pulped coffee has had its cherry-like fruit (the pulp) fully removed from the seed, leaving only the mucilage intact. From here, the coffee can be left in the sun or shade to begin the drying process, resulting in what is known as a honey processed coffee. This, too, is a form of fermentation. The bacteria will feed on the mucilage until the environment becomes too dry for it to survive. 

If we want to get crazy, the pulped coffee can be placed into a steel tank and undergo anaerobic fermentation

Natural coffee is pretty simple— in theory. It presents probably the most recognizable ‘fermented flavors’ in coffee. With naturals, the fruit is plucked from the trees, after which we can do a few different things. We can leave them to dry in the sun, or in the shade, producing a natural process coffee

We can also leave the whole fruit packed into small piles on a patio, creating more heat, more fermentation, and therefore a more boozey, fruity quality in the cup. 

Or, we can throw the cherries, fruit and all, into a stainless steel tank. Seal the tank and fill it with C02. Purge the oxygen, and we’ve started a process known as carbonic maceration. We’ll get to this fascinating technique in a few.

How long does it take for coffee to ferment?

Most washed coffees, with the least amount of fermentation, take between 18-24 hours, depending on the weather and the desired outcome of the producers. 

This leaves the bacteria enough quality time to get going, creating the enzymes that will break down the mucilage. But not long enough for much flavor to be imparted on the seeds. If it is left for too long, or the temperature is too high, we will start to see fermented flavors, some of which might not be good. 

The microorganisms that use the coffee’s fruit as fuel will continue to ferment until either all the fuel is used up or the environment becomes too dry for them to survive.   

Natural processing, for example, takes around 30 days. Because the seeds are wrapped in the moist shell of the cherry, drying takes much longer. This is on average and largely depends on weather and the climate at the processing facility.

When you taste a natural and compare it to a washed, you can taste how much fermentation has occurred during this time. Coffee farmers can control the amount of time it takes to dry in order to create the quality they want in the bean.

The longer it takes to dry, the more it will ferment.Things are a little different when we get to carbonic maceration and anaerobic processing. Both techniques use a sealed tank for the fermentation process.

Both of these methods are actually much more controlled because they remove many of the environmental factors that might affect fermentation. With the tank sealed off, void of oxygen, the controlled fermentation process can be slowed. The process might take 96 hours or longer, depending on the producer’s goal for the particular coffee. 

 Carbonic Maceration Coffee

The first time I ever tried a carbonic maceration coffee was at ‘The Cupping Room’ in Hong Kong, in 2017. At the time, I had never tried anything like it, even when compared to other high quality specialty coffees.

It was fruity like a natural, yet it was clean and complex. The acidity was so crisp and so fresh that it was almost sparkling. It blew me away. An incredibly high quality coffee.  

Carbonic maceration is actually nothing new. While it is fairly new in the world of coffee, it has been used for decades in wine production, particularly in the Beaujolais region of France. Carbonic maceration is used in wine to create fresh, lively, bubble-gum, and fruity wines that are perfect to drink shortly after production. 

But how did this technique come to be used for coffee processing?

Using carbonic maceration in specialty coffee really rose to prominence in 2015, when Australian pro, Saša Šestić, won the World Barista Championship using a coffee that employed this technique.

On the world stage, he raved about the technique— the sparkling citric acidity and the big stone fruit flavors it brought out. We can pretty safely assume that he wasn’t lying— he did win the competition that year!

How Does Carbonic Maceration Work?

First, the cherries are picked, then placed, intact, inside a stainless steel tank.

The tank is sealed. Carbon dioxide gas is pumped in from the bottom of the tank. As the C02 fills the tank, it forces the oxygen up and out via a one-way valve at the top. The coffee cherries are now in an oxygen-free or anaerobic environment. 

These stainless steel tanks are often kept in temperature-controlled rooms so the fermentation time and rate can be carefully controlled. The temperature the tank is kept at will drastically change the way the coffee will taste overall. 

For example, at between 4 and 8°C we will see loads of complexity, super juiciness, and a very clean acidity. That sparkling acidity that I keep mentioning— this is where it comes from.  

If a producer chooses to ferment at a higher temperature, say between 18 and 20°C, we will likely see a little less acidity. Instead, we’ll see much more focus on sweetness and fruity flavors. Super delicious. 

Once the producer is happy with the amount of fermentation that has occurred, the fruit is removed from the tank.

Now the producer must decide what to do next— to wash the beans clean and dry them as a washed coffee, or leave the pulp on and dry it as a natural.

Why Has Carbonic Maceration Become So Popular?

I remember when the natural processing method started becoming super popular with roasters and consumers.

Everyone was so stoked on this drink that tastes like all these crazy things coffee doesn’t usually taste like. From mangos to pineapple and jackfruit, or that crazy wine-like flavor, and with texture for days. People loved it and still do because it’s different, and it’s interesting (though to be fair, some people really don’t love it.) 

Carbonic maceration is kind of like the new natural. It’s exciting and yields a beverage that tastes so.


Any Coffee, Anywhere

So what beans are best suited to the process of carbonic maceration? 

I’ve seen great results from loads of different varietals and different production origins, from Uganda to Colombia. Because this process relies entirely on the facility that coffee farmers have available and the knowledge producers have regarding fermentation, It can seemingly be done in any country and with any varietal.  

There has even been some interesting research indicating that coffee fermentation may increase the quality of the Robusta variety significantly.

For beans whose flavor profile often consists of things like nuts and rubber— being able to impart fruity flavors will increase the coffee quality dramatically. This might be a game-changer for many coffee producers and the industry as a whole.

Problems With Carbonic Maceration

While carbonic maceration can be very controllable and consistent when perfected, it is still considered an experimental processing method when it comes to coffee. 

Small changes to a tank of coffee fruit can have a huge impact, either making or completely destroying the flavor. Temperature is largely what controls the fermentation processes. And getting this right comes down to both trial and error and the producer’s skills. A small change could lead to over fermentation, giving the cherry a very undesirable fermented flavor.   

These experiments come with not only more risk, but more work, higher production costs, and require more equipment.

This means that carbonic maceration and anaerobic coffees are usually more expensive than more traditional washed or natural and dry process coffees.

Carbonic Maceration VS Anaerobic Fermentation 

There’s a little confusion floating around as to the difference between carbonic maceration and anaerobic fermentation. I mean, the process is essentially the same— both use a tank to create an oxygen-free environment to ferment in. What’s the diff? 

When whole coffee cherries are placed in the tank, we call it carbonic maceration

When the seeds are placed in the tank without the skin and flesh, but only the sticky sweet mucilage, we call it Anaerobic fermentation.

Really, the two terms could be used interchangeably. In fact, the coffee used by Saša Šestić for his WBC performance was washed before it was processed using carbonic maceration. He referred to it as washed carbonic maceration.    

Coffee fermentation is a fascinating thing. And I think we are only just at the beginning of what is possible. Keep your eye out for these psychedelic tasting beans, and give them a try if you see them. You won’t regret it!