What is anaerobic fermentation in coffee?
What is anaerobic fermentation in coffee?
June 20, 2024

You may have noticed that we’ve just added a new Brazilian coffee to our offer list that has been processed using anaerobic fermentation. As this is our first coffee that’s been processed using this method, we thought it would be good to take some time to look at it in a bit more depth. This process imparts a unique flavour profile to coffee, so if you like it you’ll know what to look out for in the future!


It was a stroke of luck that we found this coffee. It was sent to us by mistake by one of our green suppliers, but we cupped it anyway, loved it and had to order some! Safe to say we were really drawn to those unique flavours. We think that anaerobic fermentation has preserved the sweetness and body of this Brazilian coffee, while adding lovely layers of complexity.


So what is anaerobic fermentation in coffee processing? 


Coffee cherries are processed using three main methods: washed, natural and honey processing. But more recently growers have been experimenting with new processing techniques such as anaerobic fermentation. 


There’s a bit of a debate to be had around this. Purists might argue, why mess with the inherent natural flavours of the coffee that are the result of their origin? But all processing methods have an impact on the final flavour profile of coffee, others will argue, so why not experiment, especially if you end up with amazing tasting coffee, right?


Fermentation in coffee processing


Fermentation is a process where microorganisms causes a substance to break down into other simpler substances, normally sugars. In coffee cherries, fermentation takes place when yeast and bacteria in the cherry mucilage start to convert acids and sugars into organic acids, CO2 and other compounds. In coffee, fermentation has two purposes, firstly to clean the mucilage and fruit from the coffee bean and secondly to add flavour.



With aerobic fermentation, coffee cherries are either laid out on patios or raised beds or put in water tanks to ferment. The key thing is the process takes place in the presence of oxygen.


With anaerobic fermentation, the producer deprives the coffee’s surrounding environment of oxygen by using a sealed container. The oxygen is released via a one-way valve. Anaerobic coffee processing can use natural, washed or honey depending on whether the coffee is fermented with the cherry on, de-pulped or in mucilage.




Anaerobic fermentation, which originally comes from the wine-making world, allows coffee to ferment slowly and this can result in a more complex flavour profile, accentuating the natural flavour of the coffee, bringing out distinct flavours and adding sweetness, acidity and body. Because anaerobic fermentation takes place within a sealed environment, it is also easier for the growers to control the process. They can control the length of the fermentation and also the temperature, all of which have an impact on the final flavours.


Fermentation is highly skilled. Get it wrong and you can end up with some very funky, sour tasting coffee. But this one from Brazil is a fantastic example of a highly skilled producer getting it exactly right.