What is Anaerobic Fermentation Processing? Differences Between Anaerobic Natural, Anaerobic Washed, and Double Anaerobic Methods

Anaerobic Fermentation Processing: A Revolutionary Coffee Method

Anaerobic Fermentation has become extremely popular in the coffee world in recent years, primarily because Australian competitor Sasa Sestic, the 2015 WBC champion, sparked this trend. Even at the 2018 WBC competition, five of the top six finalists used coffee beans processed with "anaerobic fermentation," which shows how special this method is!

In the processing of raw coffee beans, "fermentation" is inevitably encountered and deeply influences the flavor profile of the coffee. Fermentation is generally divided into two types: "aerobic" and "anaerobic." The biggest difference between the two is that the anaerobic process is more uniform and controllable, whereas in aerobic conditions, there are too many environmental variables that are difficult to monitor. Generally, traditional processing methods mostly use "aerobic" fermentation, and anaerobic fermentation processing has only gradually developed in recent years.

Anaerobic fermentation processing, also known as carbon dioxide maceration, is inspired by winemaking techniques, hence it's also called the wine processing method. "Grapes are placed in carbon dioxide gas, putting them in an anaerobic state. This way, grapes can convert their own fructose into alcohol even without yeast, resulting in special aromas, and the brewed wine has soft aromas and rich mouthfeel." When applied to raw coffee bean processing, freshly picked coffee cherries are placed whole in sealed metal containers, injected with carbon dioxide to expel oxygen, creating an anaerobic fermentation environment. This slows down the decomposition rate of sugars in the coffee bean's mucilage and the decrease in pH value, allowing the beans to develop higher sweetness and more distinctive flavors.

The coffee beans used by Sasa Sestic himself at the 2015 WBC were Rume Sudan variety from Colombia's Las Nubes farm. According to him, the anaerobic fermentation method used for this coffee bean involved placing mature coffee cherries in sealed metal containers (stainless steel barrels), injecting carbon dioxide to expel oxygen, allowing fermentation and decomposition to occur in an anaerobic environment. This method enables effective control over factors such as pH value, gas composition, humidity, temperature, and even the types and quantities of bacteria involved in fermentation, developing better sweetness and more balanced flavors.

Variations of Anaerobic Processing

However, currently, the processing methods and raw bean processing claiming to be anaerobic fermentation vary among different processing facilities. After all, this type of processing is still in its development stage, so there is no standard or absolute version. What we mostly hear about now is based on Sasa's coffee bean processing method at that time, or modifications of this version. For example, recent experimental processing methods like anaerobic natural, anaerobic washed, and double anaerobic have emerged one after another, which is very exciting. Adding anaerobic processes to natural or washed processing methods has become the common anaerobic natural and anaerobic washed processing methods we see today.

Anaerobic Natural Processing

Anaerobic natural processing involves placing freshly picked coffee cherries in sealed barrels for low-temperature fermentation. In the anaerobic environment, the decomposition rate of mucilage decreases, while the pH value decreases more slowly, extending the coffee's fermentation time and developing more sweetness. The entire anaerobic environment must maintain strict temperature control, averaging between 10 to 15 degrees Celsius. After more than three days of anaerobic fermentation, the coffee cherries in the barrel undergo natural sun-drying treatment. Finally, the dried coffee cherries have excess parts removed to extract the coffee beans.

For example, the FrontStreet Coffee Colombia Flower Moon Night coffee beans on FrontStreet Coffee's menu are processed using the anaerobic natural method. This FrontStreet Coffee Colombia Flower Moon Night coffee bean achieves richer mature fruit acidity and fermentation aromas due to anaerobic natural processing. After roasting, it presents aromas of cranberry, strawberry jam, chocolate liqueur, and cream, along with rich fermentation notes.

Anaerobic Washed Processing

Anaerobic washed processing involves dry fermenting freshly picked coffee cherries for 12 hours, then mechanically removing the skin and pulp, and placing the coffee beans in sealed fermentation barrels for anaerobic fermentation for 24 to 48 hours. After fermentation, the coffee beans in the barrel undergo drying treatment. Coffee beans processed this way present very rich fruit sweetness. FrontStreet Coffee Colombia Rose Valley coffee beans use anaerobic washed processing, and when pour-over brewed, you can experience the aromas of peach and roses.

Double Anaerobic Washed Processing

Double anaerobic washed processing involves two anaerobic steps. Coffee cherries are processed immediately after picking, placing them in a sealed anaerobic environment for the first anaerobic fermentation. After fermentation, the coffee cherries are processed similarly to washed processing to remove skin and pulp, then the coffee beans with mucilage are placed again in a sealed anaerobic environment for the second anaerobic fermentation. Finally, the fermented coffee beans are dried until the moisture content reaches 11% to complete the processing.

Named double anaerobic washed due to its characteristic of undergoing two anaerobic fermentations, FrontStreet Coffee Colombia Paradise Farm Sakura coffee beans use double anaerobic washed processing, presenting flavors of spiced tea, citrus, dark cocoa, and mint due to this processing method.

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