How to roast anaerobically fermented coffee

Among the many different kinds of experimental processing methods, anaerobic fermentation is perhaps the most popular and intriguing. Interest in this processing technique is only growing, with more and more anaerobic fermented coffee available in cafés and roasteries around the world.

Moreover, of all the advanced processing methods, this particular technique has one of the biggest impacts on a coffee’s sensory profile – often resulting in more wine-like and fruit-forward flavours.

So to best highlight these attributes, roasters need to treat these coffees a little differently. To find out more, I spoke to Paul Harris, founder of The Village Snob, and Alejandro Sevillano, authorised SCA trainer and certified Barista Hustle coach.

You may also like our article on how to roast natural and honey processed coffees.

Anaerobic fermentation: What exactly is it?

In more scientific terms, anaerobic fermentation is a metabolic process that converts carbohydrates (sugars) into organic acids, gases, or alcohols under anaerobic conditions. 

“Anaerobic” simply means without the presence of oxygen. So in the context of processing, producers will ferment coffee in hermetically-sealed containers, such as large tanks, plastic barrels, or plastic bags. These containers can then be placed in temperature-regulated storage areas or water tanks.

As a result of the oxygen-free environment, microorganisms will break down the sugars at a much slower rate – creating a more complex and nuanced flavour profile.

Producers can choose to carry out anaerobic fermentation in different ways. For instance:

• The natural anaerobic fermentation process uses whole cherries which can be fermented for as long as 96 hours
• Washed anaerobic coffee is fermented with no skin, flesh, or mucilage on the beans

These variations of anaerobic fermentation will, of course, have a big influence on the overall flavour profile.

How does it affect flavour and mouthfeel?

Whether they were grown in the same country or are the same variety, no two coffees taste exactly the same. There are, however, some recurring and prominent flavour notes with anaerobic fermented coffees.

“These coffees can be more astringent on the finish as both filter and espresso,” Paul says. “Because of this, I think it’s important to rest fermented coffees for between 10 and 14 days after roasting. I would also rest denser beans for longer.”

Alejandro tells me the amount of acidity and mouthfeel also change during anaerobic fermentation.

“The increase in acidic compounds results in flavour profiles that you may not be able to create using other processing methods,” he explains. “As a wine and whisky lover, the boozy mouthfeel of anaerobic fermented coffees also reminds me of certain alcoholic spirits.”

How to get the best results from these coffees

With anaerobic fermentation creating (sometimes significantly) different results in the cup to a washed processing method, for instance, it’s important for roasters to know how to best highlight these nuances.

“I find you need to be more delicate with the roasting process,” Alejandro says. “I would aim to keep below a certain Development Time Ratio (the period of time spanning just after first crack and the end of the roast), such as 16 or 17%, and an end temperature under 93°C or 200°F.”

Given that they often have more “funky” flavour profiles, it isn’t always necessary to follow strict protocols when roasting anaerobic fermented coffees, but to follow more general guidance.

For example, because the sugars break down much slower with this processing method, the flavours are a lot more complex and layered. To showcase these qualities in the best way possible and bring out more sweetness and acidity, it is recommended to roast for slightly longer.

“To control the levels of acidity, you need to develop the roast profile enough to achieve more balance at the end,” Alejandro adds. “I would also try to find the ‘sweet spot’ for each batch size depending on the capacity of the drum. For instance, normally I would roast at 80% capacity, but for anaerobic fermented coffees, I would drop the batch size.

“This would allow more heat to circulate around the drum to avoid the overdevelopment of sugars,” he continues.

Balancing mouthfeel & body

The texture of anaerobic fermented coffee is one of its most interesting aspects. Many people often describe the body as being heavy, and the mouthfeel also being creamier and silkier than other processing techniques.

Some of the ways in which roasters can best highlight these characteristics is to control the timing of first crack or to prolong the development phase.

Alejandro emphasises, however, the processing method itself will have the biggest impact on mouthfeel. 

“Some people talk about heavy fermentation affecting the aftertaste and that the lingering flavours can be unpleasant,” he says. “It ultimately depends on how long it has been fermented for. From my experience, around 48 hours is optimal.”

Tips for roasting anaerobic fermented coffee

It takes skill and understanding to develop optimal roast profiles for different coffees. But when it comes to anaerobic fermentation, there are a few useful tips for roasters to know.

“My biggest piece of advice is to find out as much information as you can about the specific fermentation process used so you can get an accurate idea of what flavour profile the producer wanted to create,” Alejandro suggests. “Maybe they tried to highlight specific qualities – to elevate the lactic acid content or to keep the Brix level above a certain point.

“Following this, you should share this information – as well as specific roast profiles you use – with people along the supply chain,” he adds. This could include wholesale clients or end consumers, who in turn can select the best brewing method for different anaerobic fermented coffees.

“There are many roasters selling anaerobic fermented Colombian coffees at the moment, so there is a lot of information already out there,” Alejandro adds. “So I suggest starting with a coffee that is already popular and following general guidance – then you can eventually expand your horizons to include different coffees.”

As roasters become more and more familiar with different types of anaerobic fermentation, the industry’s knowledge of how to roast these coffees will only expand.

Following general guidelines and commonly recommended advice is the first step. But just like when carrying out the processing methods themselves, innovation and trial-and-error are ultimately the key to success.

Enjoyed this? Then read our article on how to roast different origins.

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