What are the Characteristics of Anaerobic Coffee Processing? An Introduction to Anaerobic Washed Coffee Processing

What Is Anaerobic Coffee Processing?

Anaerobic processing — fermentation carried out in a sealed, oxygen-free environment — is one of the most talked-about innovations in modern specialty coffee. It grew out of the broader "wine-like" processing movement, which borrows techniques directly from winemaking. When Colombian producers first commercialized these wine-inspired methods, only eight estates in the country managed to bring such coffees to market. Based on how those eight estates work, the family is usually sorted into three branches: acetic acid fermentation (aerobic), lactic acid fermentation (anaerobic), and mixed fermentation combining both.

The core problem anaerobic processing solves is control. With traditional processing, the degree of fermentation is hard to rein in — it drifts with the weather and the microbes on hand. Sealed fermentation lets producers steer quality by monitoring pH, temperature, and humidity, and because the vessel is closed, aromatic compounds are far less likely to escape into the air.

How the Fermentation Actually Works

It starts in the field. Pickers harvest only fully red, sound cherries, and the lots are graded before fermentation: unripe cherries must make up less than 2% of the batch, defective beans less than 3%, and floaters less than 5%.

The selected cherries then go into sealed vessels — Australian barista Sasa Sestic famously used metal containers for his competition coffees, and that is the style most producers copied. The critical detail is a one-way valve or fermentation lock, like the airlock on a wine carboy: carbon dioxide escapes through it while oxygen cannot get back in, and the pressure and gas concentration inside the tank stay under control. Whether the result reads as acetic or lactic fermentation depends on how much oxygen remains, with acetic lots tending toward bright, clean flavors and lemony acidity.

FrontStreet Coffee: Colombian Pink Bourbon, 50-Hour Anaerobic Washed

This coffee is unusual on two fronts at once: the variety is the rare Pink Bourbon, and the process is an exclusive take on anaerobic fermentation.

Pink Bourbon is a hybrid of Red and Yellow Bourbon — a natural mutation that has become a cult favorite. It is extremely scarce, found almost exclusively in Colombia and Guatemala, and yields are very low, so it is worth grabbing whenever you see it. At cupping table after cupping table it showed astonishing sweetness and depth; importer Dimitry Boroday was so impressed that he bought the farm's entire 2018 harvest on the spot.

The farm is Finca La Granada, run by Colombian grower Gabriel Castaño. It sits in Colombia's second-highest coffee-growing region at up to 1800 meters, where warm days and cool nights create the wide diurnal temperature swing that lets cherries ripen slowly and pack in sugars.

Although this coffee is called anaerobic, its execution differs from the usual approach. Conventional anaerobic fermentation uses rigid sealed containers — plastic barrels or stainless-steel tanks — which are expensive, limited in capacity, and can leave coffee tasting flat and interchangeable, as if it came off an industrial line. Instead, food-grade plastic sheeting is used to build a sealed, oxygen-free environment around drying beds or fermentation tanks, chosen to suit the local climate. Without thick steel acting as a barrier, the coffee still breathes in the character of its surroundings while fermenting. The method is cheap, too: a modest investment buys the innovation across a much larger processing area, so the coffee keeps its personality at a lower cost than tank-based anaerobic lots.

In the cup: a strong jasmine fragrance, the sweet-tart juiciness of Sunkist orange, and a finish of bergamot, apricot, and green tea. Gentle and smooth throughout — remarkably restrained for a heavily fermented coffee.

FrontStreet Coffee Recommendation

Colombia's range is remarkably wide — from bright, fruit-forward lots to deep, heavy-bodied ones — which is why the country works equally well as single-origin pour-over or as the backbone of an espresso blend. If you want a classic expression of the region, FrontStreet Coffee's washed Huila comes from the hills of southern Huila province, planted above 1500 meters where the country's major rivers converge and water is plentiful. Roasted medium-dark, it highlights black chocolate and nutty sweetness with a full body that suits pour-over, French press, moka pot, and espresso alike.

For a taste of Colombia's experimental side, FrontStreet Coffee's Sidra (Xijua) from Finca Frontera is the one to try: anaerobic natural processing coaxes out intense tropical fruit, and at a medium-light roast you get passion fruit, grape, apricot, and dried-fruit sweetness whether you brew it as pour-over, cold brew, drip bag, or moka pot. And for home espresso machines or moka pots, our baristas recommend the FrontStreet Coffee house espresso blend — Brazilian cream-and-nut sweetness married to Colombian dark chocolate bitterness, pulling a fine golden crema that stands up beautifully to milk in a latte.