A Quick Guide to Anaerobic Coffee

You’ll often see comparisons drawn between anaerobically processed coffee and natural wine. But the parallels extend beyond the familiar flavor notes—chiefly, to the level of confusion surrounding the name and process that was also seen in the early days of the natural wine boom.

“Anaerobic processes have been used as a fermentation tool in the beer and wine industries for a very long time. The movement of this style of process into coffee is a natural transition, as coffee cherries and green beans are also fermented,” explains Andrea Allen, co-owner of Arkansas-based Onyx Coffee Lab. “There’s a myriad of different methodologies behind this process from farm to farm, country to country, but it produces wild and unexpected flavors in coffee.”

In our May/June 2024 issue, we asked Allen to recommend five anaerobically processed coffees from different roasters. The roundup highlights the spectrum of flavors and approaches to the process. But it also begs the question: What exactly is anaerobic coffee? Just like natural wine, there is no easy—or legal—definition.

Part of the Process

“Since about 10 years ago, we have been experiencing what I call the ‘processing revolution’ in which more and more methods are developed and branded by coffee producers,” says Mario Fernández, a technical officer with the Specialty Coffee Association and an eighth-generation family member of coffee growers in Mexico. “Because there is no standardization around coffee processing methods, there is no consensus about what any given processing method actually means. I think the term ‘anaerobic’ is probably one of the best examples of the confusion.”

What we now call the “natural” process has been used for thousands of years and simply involves letting the entire coffee cherry dry, typically in the sun.

It helps to first understand some of the basic and historic methods used to process coffee. What we now call the “natural” process has been used for thousands of years and simply involves letting the entire coffee cherry dry, typically in the sun. The “washed” method began being used in the 19th century, wherein the fruit of the coffee cherry was removed, and the beans were fermented in the remaining mucilage before being washed clean. In the ’90s, the “honey” process was created in which the coffee cherries are pulped and the beans in mucilage are dried immediately without fermentation.

What is the aim in processing beans one way or another? “A lot of different variations on these methods have been developed because producers have realized they have a way to impact the coffee flavor through processing,” says Fernández. “We need to start by acknowledging that coffee producers struggle a lot with low prices and low profitability. And many, many coffee farms are not profitable. They have very few opportunities to add value to their coffee.”

New Frontiers

Enter anaerobically processed coffee. “It’s another leap in the technique of coffee fermentation that both wows and delights lovers of specialty coffee,” says Allen of Onyx Coffee Lab. Thus it’s no surprise that more and more producers are experimenting with the process that often creates or emphasizes flavors similar to those found in ever-trendy natural wine. These flavors are created in the fermentation process, which, at its most basic level, occurs in a low- or no-oxygen environment (akin to carbonically macerated wine grapes), hence the anaerobic label.

“Usually these flavors for the anaerobically processed coffees are high on the fermented note. They show a lot of wine-like notes or sourness…”—Mario Fernández

“What producers are trying to achieve is a certain, distinct flavor,” says Fernández. “Usually these flavors for the anaerobically processed coffees are high on the fermented note. They show a lot of wine-like notes or sourness, for example.” But again, how this process is achieved varies widely between producers. Some may simply be sealing the fresh coffee cherries into plastic sacks to ferment before processing, while other producers are building specialty vessels and replacing the oxygen with an inert gas like CO2. The length of fermentation time will also vary, as does any following processes, such as whether the coffee is washed and how it is dried.

Fernández sees the process as having particular potential to add differentiating characteristics for producers working with typical, lowland-grown coffees. “It’s a legitimate way to look for added value in your coffee,” he says, but notes that the process would not be beneficial for all beans, especially if the fermented notes end up obfuscating the coffee’s naturally occurring desirable flavors.

“My belief is that we can get more sophisticated in the ways we process coffee to find a middle point between processing savvy that adds to the character of the product and also the character from the terroir,” says Fernández. “But I like the fact that we are living in a time of a lot of innovation at the processing level.”