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What Is Anaerobic Coffee?

Coffee fermentation vats.

What Is Anaerobic Coffee and Why Is It Gaining Popularity?

Coffee as a beverage has been around for centuries. Through the years, the ways we brew coffee have changed greatly. The same can be said for how producers process harvested coffee beans. One method that is gaining popularity among coffee connoisseurs is anaerobic processing.

Anaerobic coffee refers to beans that have undergone a special, oxygen-free fermentation process after they have been harvested. Typically, anaerobic processing uses airtight tanks into which coffee cherries are placed. The fermentation process can take 12 to over 100 hours.

Harnessing the power of naturally occurring microscopic organisms, anaerobic coffee may sound more like a science experiment than a coveted form of coffee. But from the looks of it, this unique processing method is here to stay, so here's what you need to know about it.

Fermentation and Coffee

In order to fully appreciate anaerobic processing in coffee, it is important to step back and understand the basics of fermentation. The art of fermentation has been around for thousands of years and has been used by civilizations around the globe.

Simply put, fermentation is a natural process by which microbes (i.e., bacteria and yeast) break down certain substances, say sugars, into simpler substances like carbon dioxide and alcohol.

This may sound a bit off-putting, but chances are, some of your favorite foods are fermented. For example:

  • Aged cheeses

  • Apple cider vinegar

  • Kimchi

  • Kombucha

  • Miso

  • Pickles

  • Sauerkraut

  • Sourdough bread

  • Yogurt

Believe it or not, fermentation can play a key role in the characteristics of your favorite coffee. To some degree, regardless of the processing method (i.e., natural, washed, semi-washed, etc.), all coffee beans undergo fermentation once they have been harvested.

Anaerobic Processing - What It Is and How It Works

Freshly picked coffee cherries naturally contain sugars and other substances that serve as fuel for microorganisms. When the two are brought together under the right circumstances, conditions are ripe for fermentation to occur.

The word anaerobic refers to the absence of oxygen. Thus, anaerobic processing requires that coffee cherries be put into an oxygen-free environment. In most cases, this means placing them in stainless steel vats or similar enclosures where oxygen cannot enter.

Fermented coffee cherries.

These are the key aspects of anaerobic processing:

  • Once coffee cherries are placed into the fermentation vat, the vessel is tightly sealed and nearly all the oxygen is removed.

  • A valve or similar device allows the operator to periodically release pressure caused by the build-up of carbon dioxide during the fermentation process.

  • Fermentation can last anywhere from 12 hours to over 100 hours, depending on conditions like climate and altitude.

  • Continuous monitoring throughout the fermentation process is key. If not enough fermentation occurs, target characteristics may not be achieved. If it goes too far, fermentation can result in harsh, vinegar-like qualities.

  • Some operators add other materials to the fermentation vat to develop unique flavors - these include pieces of fruit, spices, molasses, and even other coffee varieties.

Processing coffee cherries in this manner requires great skill to produce the desired result. So why go through this process?

It is said that anaerobic coffees present unique flavor characteristics including enhanced sweetness and fruitiness, smoother body, reduced acidity, and deeper finish.

But ultimately, as with so many things when it comes to coffee, you will have to be the judge.

What About Carbonic Maceration?

Another processing method that has been getting some attention lately is known as carbonic maceration. The basic concept is similar to anaerobic processing. Both methods rely on creating an oxygen-free environment allowing microorganisms to go about their fermentation business.

The main difference between carbonic maceration and anaerobic processing has to do with the state of the coffee cherries when they are placed into the fermentation vessel:

  • In anaerobic processing, the outer pulpy material is mostly removed prior to placing coffee cherries into the fermentation vat.

  • In the carbonic maceration method, coffee cherries are left largely intact, skins and all.

In terms of brew profile, carbonic maceration produces unique traits, including fruit-forward flavors, earthy notes, and buttery sweetness.

Final Thoughts

If you're the adventurous type and looking for something new to add to your brewing regimen, then anaerobic or carbonic maceration coffees might be for you. But before you rush out to stock up on them, just know that these processes are far from exact sciences.

Producers and roasters are still figuring out the nuances of letting microscopic organisms do their thing while keeping the fermentation process under control. But for now, the results are definitely promising so stay tuned.


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