Special Coffee Processing Methods: What's the Difference Between Anaerobic and Oxygen-Free Coffee Processing?

Understanding Yeast in Coffee Processing

Yeast is essentially a facultative anaerobe, which means it can survive in both oxygen-rich and oxygen-poor environments. In an oxygen-rich environment it performs aerobic respiration, while in an oxygen-poor environment it carries out fermentation. Fermentation is not exclusive to yeast, however; lactic acid bacteria and acetic acid bacteria also drive different fermentation reactions.

Oxygen-Based and Anaerobic Fermentation

What is often called "oxygen-based fermentation" should more accurately be called respiration, since no true fermentation is involved. In an oxygen-rich environment, sugars are burned, carbon dioxide is released and energy is produced.

Anaerobic fermentation, also known as the red wine processing method, became a new trend after that year's WBC competition.

Anaerobic fermentation places raw coffee beans in specialised containers filled with carbon dioxide or other gases to create a relatively oxygen-free or oxygen-deficient environment. The anaerobic environment produces lactic acid, which makes the coffee's acidity milder and the taste smoother.

The Red Wine Processing Method

FrontStreet Coffee is here today to explain what the so-called "red wine processing method" actually is.

The red wine processing method for coffee beans, also called the wine-like processing method, draws its inspiration from winemaking. At present only eight farms in Colombia have successfully brought coffee processed this way to market. From the information those eight farms provide, we can roughly group red wine processing into acetic acid fermentation (aerobic fermentation), lactic acid fermentation (anaerobic fermentation) and mixed fermentation (mix fermentation = aerobic + anaerobic).

Advantages Over Traditional Processing

Traditional processing struggles to control the varying degrees of fermentation in coffee beans. The red wine processing method can instead ensure bean quality by controlling pH and even temperature and humidity. Enclosed fermentation also prevents aromatic substances from easily volatilising.

The Fermentation Process

Next, let us look at the specific fermentation process.

Farmers place selected coffee cherries into specific containers (when Australian barista Sasa competed, the beans he used were processed in metal containers). Whatever the container, it should have a device similar to a red wine fermentation lock or a one-way exhaust valve. This lets carbon dioxide escape through the device and controls the air concentration inside the container. At this point the cherries in the container undergo acetic acid fermentation, resulting in beans with relatively bright, clean flavours and citric acid notes.

According to Sasa's description during the competition, processors inject carbon dioxide into the container to prevent the formation of volatile acids through oxidation. This process is lactic acid fermentation, which produces malic acid and tartaric acid. Since tartaric acid is relatively stable, beans produced through lactic acid fermentation have a richer acidity with cheesy, nutty and creamy flavours.

FrontStreet Coffee Recommendation

FrontStreet Coffee's clearest example of a controlled fermentation is the Colombia Frontera Estate Sidra, which is processed as an anaerobic natural. Working in a low-oxygen environment, the fermentation converts more tropical fruit aroma, so at a medium-light roast you can expect passion fruit, grape, apricot and dried fruit across pour-over, cold brew, drip bags and moka pot.

If you want to taste how a more traditional process compares, FrontStreet Coffee's washed Colombia Huila comes from the country's best-known specialty region on the southern Central Cordillera, where plantations sit above 1,500 metres. Roasted medium-dark, it leads with black chocolate and nutty sweetness, medium oil and a full body, and it works for pour-over and French press as well as moka pot and espresso.

For espresso and milk drinks, the FrontStreet Classic Blend brings together Brazil's creamy nut aroma and balanced body with Colombia's dark chocolate bitterness. It extracts with a fine golden crema and stays rich and smooth in a latte, and it also serves as the base for an Americano, cappuccino or mocha.