Anaerobic Coffee Processing

Anaerobic processing means fermenting coffee cherries or parchment inside a sealed, low-oxygen container instead of the open tanks or patios traditional washed and natural processing use. As 1Zpresso’s guide explains, producers load fruit into an airtight tank where oxygen gets displaced, either naturally by the CO2 fermentation itself produces, or by active injection, and that low-oxygen environment favors different microbes than open-air fermentation does. Cafe Imports describes the practical setup: stainless steel, thick plastic, or sealed GrainPro bags, often fitted with a one-way valve that lets built-up CO2 escape without letting outside air back in. Carbonic maceration, the specific variant Klatch Coffee used on its Coffee Review-scored Colombia Monteblanco lot, adds a second phase: after an initial whole-fruit ferment in a closed CO2-injected tank, the skins come off and the beans ferment again in their own skins and juice before washing.

The coffee world didn’t invent this: fermenting fruit under CO2 is standard practice in winemaking, and it reached specialty coffee’s biggest stage when Australian barista Sasa Sestic served a cappuccino built on a 50% natural process, 50% washed carbonic maceration Sudan Rume, grown at Finca Las Nubes in Colombia, to win the 2015 World Barista Championship in Seattle, as Sprudge reported at the time. That win is generally credited with pulling the technique out of small-scale experimentation and onto competition-circuit and specialty shelves, where anaerobic and carbonic maceration lots have stayed a fixture ever since.

The two Coffee Review lots behind this page’s flavor profile show what the low-oxygen environment does to the cup: Olympia Coffee’s Ethiopia Gatta Anaerobic reads clove, cocoa, tangerine zest, honey, and orchid, while Klatch’s Colombia Monteblanco Carbonic Maceration reads rose, strawberry, cocoa, pink lemonade, and yogurt, both scored 94 and both leaning toward the intense floral and fruit-forward register that made the anaerobic style a talking point in the first place. Coffee Review’s own wider survey of 90 anaerobic coffees, averaging 89 with a 79-95 range, is the honest caveat: this style raises the ceiling more than it raises the floor, and a badly managed ferment produces exactly the boozy, medicinal off-notes that a well-managed one turns into vivid fruit and spice.

Because the fermentation is happening around the bean, not inside it, anaerobic processing changes sugars, acids, and aromatic precursors, not the bean’s underlying chemistry, which is also why the intensified flavor needs a gentler hand at the brewer: over-extracting an already hyper-soluble bean is what tips this style from vivid into unpleasant.