Understanding Specialty Coffee Flavors: The Role of Fermentation

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The processing of green coffee inevitably involves fermentation. Just as fermentation is essential to wine, fermentation in coffee directly affects the flavor and quality of the beans -- so much so that specialty methods such as "red wine treatment" and "carbon dioxide treatment" were developed specifically to push its effects further.

Fermentation is a chemical reaction. Yeast, bacteria, and other microorganisms break a substance down into simpler substances. Typically the substance being broken down is sugar, and the process releases heat. Different types of enzymes also act as catalysts along the way.

Put more simply, fermentation is a natural process: combine sugar and water and you set it in motion. Coffee cherries happen to contain both, so once the cherries are harvested -- sometimes even before harvest, depending on humidity -- fermentation has already begun.

The catch is that fermentation can both improve and ruin coffee's flavor. It all depends on how the processor manages it.

What is the relationship between fermentation and coffee beans?

Fermentation is a crucial step in green bean processing, and it happens in two ways:

Aerobic: Reactions that occur when oxygen is plentiful. Controlling this type is simple -- just place freshly harvested coffee cherries in a tank or container and let the microorganisms do their work, while time and temperature are monitored for later control and analysis.

Anaerobic: Here the cherries go into a tank filled with water, isolating them from the air, which lets a different set of microorganisms work. Basically, specialty coffee beans carry these characteristics: sweetness, acidity, body, and unique flavors such as fruit, floral notes, caramel, and chocolate.

Different growing regions exhibit different flavors.

In short: FrontStreet Coffee is a coffee research house dedicated to sharing coffee knowledge with everyone. We share everything without reservation simply to help more friends fall in love with coffee. Every month we hold three low-discount coffee events, because FrontStreet Coffee wants more friends to enjoy the best coffee at the lowest possible price. That has been FrontStreet Coffee's mission for the past 6 years.

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If fermentation is the flavor you are chasing, natural processing is the oldest and most widely used route to it: ripe cherries are dried whole in the sun until they reach the target moisture, then hulled. Because the fruit stays on the bean throughout drying, the cup gains more body and a riper, fruit-driven character. The method demands a run of clear days -- in Ethiopia it typically means 27-30 days on the beds.

FrontStreet Coffee's sun-dried Yirgacheffe Red Cherry is a good place to start. Only fully ripe red cherries go onto the raised beds, kept off the ground and turned by hand, and after a medium-light roast the cup shows elegant floral notes, bright acidity, tropical fruit, and a full fermented sweetness.

For a more intense expression, the Ethiopia Flower Queen is dried in Sidama with careful turning at high altitude. Medium-light roasted, it gives a light fermented note along with flowers, citrus, and mango, then honey sweetness, cocoa, and a hint of spice on a thick body with a long finish.

To see how far controlled fermentation can go, look at FrontStreet Coffee's Colombia double anaerobic natural -- the process also known as carbon dioxide maceration. The cherries are sealed in tanks and oxygen is displaced, which slows the breakdown of sugar and the fall in pH, yielding higher sweetness, more complexity, and a denser texture. Brewed, it shows passion fruit, apricot, dried fruit, grape, and an almond-like echo.