What is Anaerobic Processing? How Does it Differ from Natural and Washed Methods? What are the Characteristics of Anaerobically Fermented Coffee Beans?

What is Anaerobic Processing?

In 2015, Mr. Sasa Sestic won the World Barista Championship with an anaerobically processed bean, throwing open the door to a whole new era of coffee processing. Since then, anaerobic methods of every kind have appeared in abundance. Whether at coffee competitions or in neighborhood cafes, anaerobically processed beans are everywhere.

Look around, though, or scroll through online platforms, and you will see that anaerobically processed beans do not enjoy the universal acceptance that traditionally processed beans do. There are two reasons. One is that the flavor of anaerobic processing is too distinctive, and not everyone takes to it. The other is that many people do not really understand how the process works, which easily leads to misconceptions. That is why anaerobically processed coffee is often a controversial subject. Today, FrontStreet Coffee explains what "anaerobic processing" actually is.

Understanding Anaerobic Processing

"Anaerobic processing," also called "anaerobic fermentation," places coffee beans, meaning whole coffee cherries, in an oxygen-free or oxygen-depleted environment for a period of fermentation. In that environment the fermentation can be controlled, which yields distinctly different and complex flavors. A quick note on fermentation itself, since it is both very common in daily life and extremely important: fermentation is a biochemical process in which microorganisms convert one substance into others through their life activities. The fermentation of coffee beans, for example, converts the sugars in the mucilage into alcohol and carbon dioxide.

It is worth understanding, however, that anaerobically processed beans are not the only ones that ferment. Traditional methods such as natural, washed, and honey processing all involve fermentation too. The difference lies in the degree, method, and direction of that fermentation compared with anaerobic processing, and the flavors shift accordingly. Why does washed coffee taste cleaner than natural coffee? Quite simply, because the fermentation in washed processing does not go as deep as in natural processing, so fewer substances are converted, giving a cleaner taste but with less complexity and fewer layers.

The Evolution and Requirements of Anaerobic Processing

Back to the main thread. Although Mr. Sasa Sestic popularized anaerobic processing, he was not its founder. Anaerobic techniques were first used in winemaking, then applied to coffee by the Costa Rican coffee farmer Luis Eduardo Campos, and finally carried into world competition by Sasa Sestic.

Many people imagine that anaerobic fermentation simply means putting coffee beans into a sealed container and letting them ferment on their own. The reality is not that simple. Anaerobic fermentation needs more than a closed space; it requires a monitorable, completely opaque, and fully sealed container, and the ambient temperature where the sealed barrels are kept must be strictly controlled. Only then can producers manage the fermentation level inside the barrels and steer the coffee flavors toward their target.

To achieve that, producers must not only use carbon dioxide to push out oxygen before fermentation but also constantly monitor temperature and pH changes throughout the process. Once the sugars in the mucilage and slime are fully broken down, the container has to be opened immediately to stop fermentation, and the remaining steps must be carried out quickly to avoid over-fermentation and its negative consequences. So anaerobic fermentation is not simply about putting beans into a sealed space; it places higher demands on the producer than other methods. In return, it gives coffee entirely new flavors distinct from traditional processing.

Besides those new flavors, anaerobically processed beans also develop fermentation flavors to varying degrees depending on the fermentation level, which is another major characteristic. When those fermentation flavors become too heavy, though, the coffee can easily take on a strong "soy sauce" taste. The reason many people dislike anaerobically processed coffee is precisely that this soy sauce note overwhelms everything else.

Variations of Anaerobic Processing

The names you see on the market, such as anaerobic natural and anaerobic washed, all refer to beans that underwent anaerobic fermentation in addition to a traditional flow such as natural or washed processing. Processes with "double" in the name indicate that the beans went through two rounds of anaerobic fermentation during processing. In any case, merchants simply abbreviate the general processing flow into the processing name, and we only need to read them literally.

FrontStreet Coffee Recommendation

If the bold, fermentation-driven flavors of anaerobic coffee are not quite what you want, FrontStreet Coffee's washed selections show what clean, transparent processing can do. The Ethiopia Yirgacheffe Gudingding Cooperative is a great example: because it is washed, it brings out the maximum jasmine florals and bright citrus acidity, and a medium-light roast leaves it with green tea and honey tones. It suits pour-over, cold brew, espresso, and more, making it a flexible everyday bean.

For a washed coffee with more bite, the Kenya Little Tomato is one of the best-known examples, since Kenya excels at double washed processing: repeated soaking and rinsing push the juice-like character to the extreme, yielding notes of cherry tomato, dark plum, snow pear, black plum, and yellow sugar, with a sweet-and-sour, juicy body. And if you want washed processing at its most balanced, the Jamaica Blue Mountain No.1 was born of it, since Jamaica's island climate and shorter sunlight pushed producers to wash the beans and dry them fast; the result is a dense, clean cup with hazelnut, cream, chocolate, and caramel notes and no single flavor dominating or missing.