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What Does Fermentation Mean for Your Coffee?

Beer, chocolate, sourdough bread, yogurt... coffee.

by Ever Meister | September 08, 2020

Since some of the most delicious things on Earth are fermented — beer, chocolate, sourdough bread, yogurt, the list goes on and on — it should perhaps come as no surprise that fermentation has a tasty role to play in coffee production as well. While coffee's fermentation process won't get you buzzed and doesn't add any fizz to your morning brew, it can be both an important step for producers and a palate-pleasing one for you and me: a win-win situation.

Today, we're going to take a look at how fermentation fits into the coffee-making process, explore some of the techniques and technologies that producers use to control it, and even peruse a few options from your favorite roasters, so you can taste the impact that experimentation has from bean to bean.

Fermentation 101

In general terms, fermentation is the result of yeast and bacteria metabolizing and converting sugars and other compounds in organic material in order to create fuel for themselves and their little microscopic bodies. What's left behind from their feeding frenzy are a host of by-products, like acids and ethanol, which are what give fermented foods their character and taste; the process itself also can create a change in structure or texture—turning crisp cabbage into soft kimchi for example.

Yeasts and bacteria live on almost every surface, so there's not generally any need to introduce them or inoculate whatever it is you're trying to ferment. However, it is possible to curate the microorganisms involved, as certain strains can produce different results, such as higher fruity acids.

Coffee and fermentation

While coffee isn't a fermented beverage like kombucha or wine, it does see some raucous microorganism action during the post-harvest processing that happens at origin. As soon as those tiny beings find an entryway into the sweet coffee fruit, they'll start fermenting what's inside: That means that as soon as coffee's picked (which will create a small tear where the cherry detaches from the stem), it's fair game for hungry yeast and bacteria.

This process happens in the sticky fruit mucilage that surrounds the coffee seed, which eventually becomes the “bean” that we roast, grind, and brew. The thought is that these microorganisms create compounds through the fermentation process that then get absorbed into the cellular structure of the seed. When the roaster applies heat to those seeds, the compounds change into all kinds of interesting stuff, like sweetness and acidity. (It can also lead to terrible flavors, however, if it's left unattended to.)

The more practical purpose of fermentation for most producers, however, is that the process breaks down the fruit material that surrounds the seeds, making it easier and quicker to remove. This is a major component of the washed process, though fermentation happens in just about every coffee-preparation method: natural, washed, wet-hulled, or some experimental approach that hasn't been named yet.

Yeast and bacteria are present in the environment, and will make their way into the coffee cherries at the first opportunity to start metabolizing away. Fermentation will continue until the environment is inhospitable (when the fruit runs out of fuel or becomes too dry, for example).

Processing and fermentation

Often, we sort of interchange processing and fermentation, in part because the washed process typically utilizes a “fermentation tank,” some vessel or container where depulped coffee is stored for a while to enable microorganisms to access it. However, fermentation doesn't play by human rules, and neither starts when coffee enters the tank nor necessarily stops when it's taken out. In some regions of the world, coffee is picked and immediately put into sacks in its cherry; in others, coffee is depulped and spread out on patios with its mucilage still intact—and fermentation is happening in both processes. (See also this wonderfully informative blog post about coffee processing.)

There are countless variables at play in the way that fermentation happens — whether in a fermentation tank or not, the flavors it creates (or destroys), and the ways that a producer can modulate the process. The ripeness of the fruit, the ambient temperature as well as the temperature in the coffee as it's piled or soaked for fermentation, moisture content throughout the drying cycle, the types of microorganisms that are present — these are all things that a producer can have some degree of control over, and that can make a big difference in the finished product.

In Colombia, the national coffee institute developed a portable tool called a Fermaestro that helps producers gauge their fermentation: Depulped coffee seeds are piled inside a cone-shaped metal container, and the producer monitors the loss in volume as the sugars break down. Elsewhere, farmers might use Brix or pH meters to read the change in sugar content in a lot of coffee from the time it's first depulped until after a fixed period of fermentation has happened. Many producers use tradition, intuition, and experience to guide their fermentation; others turn to innovation.

“Anaerobic fermentation,” for example, is becoming more and more popular with experimentally minded producers: In this methodology, coffee is placed in an air-tight container rather than left exposed. This anaerobic environment keeps the temperature of the coffee lower as it ferments, slowing down the process considerably. (Note that fermentation is already an anaerobic process, meaning that yeast and bacteria don't need oxygen to be present in order to do what they do; the term refers to the environment where the coffee is held.)

Want to go deeper into a fermentation rabbit hole? Check out the work of Lucia Solis, a former winemaker who has switched from grapes to (coffee) cherries. Her website has a host of great information, as well as a link to her podcast, Making Coffee.

What about flavor?

The effects of fermentation on flavor are still somewhat contested in the coffee world, but it does seem apparent that there's a correlation between the process and what our palates experience.

For instance: In Huehuetenango, Guatemala, the cool overnight temperatures at high elevation make fermentation a longer process, sometimes taking as long as 72 hours in tanks. The best coffees from Huehue have a crisp acidity like tart apple, and articulate sweetness — generally more dynamic than those found at lower elevations or warmer climes in other regions of the same country.

In Costa Rica, honey processing is increasing in popularity, and producers will modulate the fermentation of the coffee mucilage by controlling the temperature and sun exposure to the seeds as they dry. More-fermented coffees like black honeys will have winey or rich berry notes, while less-fermented white or yellow honeys will taste lighter and more delicate — still fruity, but not quite a fruit bomb, so to speak.

Many coffees won't have specific details about the fermentation process on the bag, so matching a flavor and developing preferences can be tricky. But when you do find an offering that specifies things like the length of the fermentation, the type of vessel it was fermented in, even the Brix or pH reading of the finished coffee, make note and see if you can taste anything particularly special in the cup. You never know what you might find!