Modern Experimental Coffee Processing: Anaerobic, Carbonic Maceration, and Beyond

Summary

Coffee producers have long processed coffee in three main ways: washed, natural and honey. Since the 2010s, specialty producers have also experimented with controlled fermentation, especially in sealed, low-oxygen tanks. The best-known methods are anaerobic fermentation and carbonic maceration, a technique borrowed from winemaking. This article explains what these methods are, how they entered specialty coffee, and what published research has found so far. For the three traditional methods, see Coffee Processing: Washed, Natural, and Honey Methods Explained.

Anaerobic Fermentation

In anaerobic fermentation, coffee is fermented in a sealed container with little or no oxygen. Coffee Review, which tested a group of such coffees, describes the main variations:

Anaerobic natural: the beans stay inside the whole fruit through both fermentation and drying.
Anaerobic washed: the skin and fruit flesh are removed after an initial anaerobic fermentation, before the beans are dried.
Carbonic maceration: the fermentation tanks are injected with extra carbon dioxide to cut off oxygen further.

A widely studied version is self-induced anaerobic fermentation (SIAF). In a 2022 study published in Foods, Brazilian researchers sealed coffee in tanks fitted with an airlock, which lets the carbon dioxide made by fermentation escape while keeping air out. They fermented the coffee for 24, 48 and 72 hours. The longer fermentations produced more of several key flavour compounds and a wider range of sensory profiles than the 24-hour fermentation. The researchers also found that the microbes dominating these tanks were very different from those in traditional coffee processing.

A second 2022 study in Foods compared SIAF coffees grown at different altitudes and processed as natural or pulped natural. It found that lactic acid bacteria of the genus Leuconostoc predominated in both processes at every altitude, and that the highest sensory scores came from natural-processed coffee from the highest farms.

Watch: Coffee Fermentation Process: Aerobic & Anaerobic Explained — Charcoal Coffee

Carbonic Maceration and the 2015 World Barista Championship

Carbonic maceration came to wide attention in coffee at the 2015 World Barista Championship in Seattle. The winner, Saša Šestić of Ona Coffee in Canberra, Australia, used a Sudan Rume coffee from Colombia that he said had been processed by a new method called washed carbonic maceration. He was the first Australian to win the title.

In an interview after his win, Šestić said the idea grew out of conversations with his head roaster and trainer about carbonic maceration in wine, which led to discussions with winemakers, brewers and coffee producers, and finally with the Colombian farmer who processed the coffee.

Researchers have since begun to study the method. A project funded by the São Paulo Research Foundation in Brazil reported that, in preliminary tests, carbonic maceration raised the cupping score of one coffee from 78 to 86 points.

Adding Yeast or Other Ingredients

Some producers add a selected yeast to the fermentation. A 2025 study in Food Chemistry compared natural self-induced anaerobic fermentation with the same method using the yeast Saccharomyces boulardii, over 24 hours. The natural fermentation showed greater microbial diversity, while the added yeast gave a more controlled fermentation. Both methods produced coffee that met the specialty standard of 80 points or more, with the natural fermentation scoring slightly higher at a light roast.

Other coffees are fermented with added fruit or other ingredients, sometimes called co-fermented or infused coffees. Where the line falls between special processing and infused coffee is a subject of discussion in the specialty industry.

Key Facts

Anaerobic fermentation: fermentation in a sealed container with little or no oxygen
Carbonic maceration: fermentation in tanks injected with extra carbon dioxide; borrowed from winemaking
2015 World Barista Championship: won by Saša Šestić with a washed carbonic maceration Sudan Rume from Colombia
SIAF research (2022): longer fermentation (48 and 72 hours) produced more flavour compounds and more varied sensory profiles than 24 hours
Yeast research (2025): natural fermentation and fermentation with Saccharomyces boulardii both reached specialty grade

Frequently Asked Questions

What is the difference between anaerobic fermentation and carbonic maceration? Both ferment coffee with little oxygen. In carbonic maceration, the tanks are also injected with extra carbon dioxide.

Who brought carbonic maceration to wide attention in coffee? Saša Šestić, who won the 2015 World Barista Championship with a Colombian coffee processed by washed carbonic maceration.

Does longer fermentation change the coffee? In a 2022 study, fermenting for 48 or 72 hours produced more of several flavour compounds and a wider range of sensory profiles than fermenting for 24 hours.

Are co-fermented or infused coffees labelled? Practice varies, and where the line falls between special processing and infused coffee is still discussed in the industry.

Sources

Global Coffee Report, "Australian wins 2015 World Barista Championship", 13 April 2015. Supports: Saša Šestić of Ona Coffee, Canberra, as the 2015 champion in Seattle; the Las Nubes Sudan Rume coffee from Colombia; washed carbonic maceration.
European Coffee Trip / magazine.coffee, "Exclusive Interview with the World Barista Champion Sasa Sestic", 28 April 2015. Supports: the first Australian champion; the origin of the idea in carbonic maceration in wine and the conversations that followed.
Coffee Review, "Anaerobically processed coffees". Supports: the definitions of anaerobic natural, anaerobic washed and carbonic maceration.
da Silva Vale, A., and others, "Understanding the Effects of Self-Induced Anaerobic Fermentation on Coffee Beans Quality: Microbiological, Metabolic, and Sensory Studies", Foods 12 (1): 37 (2022/2023). Supports: the airlock tank; 24, 48 and 72 hours; more metabolites and more varied sensory profiles with longer fermentation; different dominant microbes.
Martinez, S. J., and others, "Molecular, Chemical, and Sensory Attributes Fingerprinting of Self-Induced Anaerobic Fermented Coffees from Different Altitudes and Processing Methods", Foods 11 (24): 3945 (2022). Supports: Leuconostoc predominating; the highest scores for natural coffees from the highest altitude.
Pholtaisong, J., and others, "Natural versus Saccharomyces boulardii self-induced anaerobic coffee fermentation", Food Chemistry 488 (2025). Supports: the comparison of natural and yeast-inoculated fermentation; both at specialty grade.
São Paulo Research Foundation (FAPESP), grant 18/09186-3, "Study of coffee fermentation by carbonic maceration". Supports: preliminary tests raising one coffee from 78 to 86 points.
Coffee+ podcast, episode with Saša Šestić. Supports: discussion of the boundaries between special processing and infused coffee.

Claims this encyclopedia could not source

The following statements appear in published material about experimental processing. This encyclopedia could not trace them to a reliable source:

That experimental coffees sell for two to ten times the price of traditionally processed coffee from the same origin, for example about $5 against $30 a pound wholesale.
That Saša Šestić popularised anaerobic fermentation. (The documented account is that he used washed carbonic maceration.)
That Wilford Lamastus of Elida Estate in Panama developed an "Anaerobic Slow Dried" method with a drying period of about 60 days.
That named producers in Costa Rica and Colombia pioneered particular methods, including a "Dynamic Process" credited to Felipe Ospina.
That a particular Ethiopian lot was fermented at 15–18°C for seven days.
That a 2024–2025 study showed cold fermentation produces more complex flavour compounds than warm fermentation.
That the Specialty Coffee Association has called for clearer disclosure standards for co-fermented coffee.
That Puerto Rican farms in Yauco, Adjuntas and Maricao have produced or are preparing experimental lots, and that Puerto Rican coffee has a particularly dense bean structure.

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