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Anaerobic Fermentation Processing

By Carson Parodi on

This week, as we near the close of our coffee processing series, we address a process that is starting to gain more steam in the green coffee world: anaerobic fermentation. What is anaerobic coffee?

Aerobic v Anaerobic. In the most simple explanation, “aero” means “with air,” and "an-aero” means “without air.” In reference to coffee processing, nearly all fermentation methods (like those we’ve previously discussed) are conducted with air (oxygen).

Anaerobic Fermentation. Unlike aerobic fermentation, where beans are processed entirely outside, anaerobic fermentation processing is where coffee is fermented in an artificially created environment where air/oxygen is not present, typically using large airtight tanks (usually stainless steel).

How It’s Done. Coffee cherries are typically milled to remove skin and some mucilage (like honey processing), then placed inside the sealed tanks where, over the course of up to several days, the mucilage of the cherry is broken down through organic fermentation. The cherries begin to release CO2 and heat as pressure builds inside the tanks — some of which is allowed to release through one-way valves. Water is added when necessary to keep fermentation from overheating. Once again, if you’re familiar at all with beer or winemaking, the parallels of fermenting in stainless steel or oak tanks are unmistakable!

In this stressed, oxygen-deprived environment, only certain non-oxygen reliant microorganisms are able to get to work, turning the natural sugars into acids and ethyl alcohol — imparting a deep and unique flavor profile on the beans. After fermentation is complete, the beans spend several weeks carefully drying on raised beds.

Fermentation Control. One of the biggest benefits to anaerobic processing is that it gives farmers way more control over the nuances of the fermentation process. The nature of the tanks creates a more stable environment, where pH, CO2 levels, and temperature can be carefully managed.

In comparison, beans fermented using more standard aerobic processing methods (like a typical honey process, for instance) are subject to natural environmental swings and mold risk, threatening the consistency of the final product.

Adding Fruit. This is where much of the magical allure comes into play. Some farmers are beginning to add various citrus fruits to the tanks during the fermentation process in attempts to add more sugary compounds and impart more potent fruit flavors. It is hard to know with certainty which flavors in the eventual cup are inherent to the beans themselves, or a result of a citrusy addition, but the influence is striking.

Flavor. Coffees that are processed using anaerobic fermentation are typically sweeter and brighter, with complex fruit flavors. We’ve cupped some very intriguing Brazils that were fermented with tangerines and pineapples. You can likely expect to hear much more about these lots in the coming months/years...!

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