Fermentation plays a critical role in developing the golden brown beverage we love. Similarly to wine, fermentation in coffee is part of what gives coffee its acidity and sweetness. Every coffee you’ve ever drank has gone through some type of fermentation post-harvest.
In fact, a lot of our favorite foods and drinks are fermented in some way, shape, or form: chocolate, wine, beer, bread, kimchi, kombucha, etc.
Now, we aren't saying coffee is a fermented beverage, but rather, we are talking about the process the coffee fruit undergoes between the time the cherry is picked and when the coffee is ready to roast.
Fermentation describes the process of the metabolization of sugars and other compounds done by microorganisms like yeast and bacteria (the good kind). The microorganisms eat and break down compounds like sugars and convert them into fuel the microorganism can consume for energy. What's left behind after this are by-products like ethanol and acids. This gives coffee its acidity, complex taste, and interesting characteristics.
When we think about fermentation in coffee, we consider a lot of variables:
Different types of bacteria and yeast populations vary depending on location, climate, and ecosystem health. These different types also consume different compounds in the coffee cherry and produce different byproducts. At the end of the day, the microorganisms will consume anything they can until there is no more food for them to survive.
For example, with naturally processed coffees, where the coffee cherry is kept intact while the coffee dries, the microorganisms will consume the fruit material until there is no more more fruit material left to consume or the environment becomes too dry. With washed process coffees, on the other hand, fermentation may occur all the way through the drying process. To better explain, we need to differentiate between aerobic and anaerobic fermentation:
This is fermentation that occurs with the presence of oxygen. The process is fairly simple, leaving the coffee cherries in a tank while monitoring time, temperature, brix (measurement of probable sugar content). Aerobic fermentation is harder to monitor due to the all of the uncontrollable variables like temperature, humidity, rain, etc.
Opposite of aerobic fermentation, anaerobic occurs with the absence of oxygen. The fermentation process in general is already anaerobic, meaning it doesn't need oxygen to do its job. Rather, anaerobic fermentation is beneficial because it allows a higher degree of control with variables and allows a slower fermentation, which provides drastically different flavors in the final product. Variables that are controlled and monitored are oxygen, CO2, pH, brix, and temperature.
And there are 4 processing methods that combined with fermentation affects the outcome of the final coffee product:
Occurs inside the fruit mucilage surrounding the seed and under the pulp. Fermentation will take place as long as there is fuel available to the microorganisms (e.g. sugar, moisture, acids, etc); the seeds typically become inhospitable to microorganisms when they reach ~11% moisture.
This process provides full bodied, slightly boozy, gently acidic coffees with notes of blue and black fruits and baked goods.
The coffee cherry’s skin or pulp is removed within 24 hours of harvest. A percentage of what’s left behind remains on the seed while it dries, with the amount left behind (25- 90%) driving the style and flavor of the final coffee. The mucilage ferments on the bean between 18 and 25 days, and poses less spoilage risk than a natural process while achieving similar flavor results.
This process provides creamy, silky, and sweet coffees with flavors like caramel, cooked sugar, jam, blue fruits, and baked goods (pie, pancakes, muffins)
Within 24 hours of harvest, cherries have their outer skin and most of the fruit flesh removed mechanically with a de-pulper machine. The beans then undergo fermentation in an open tank for around 18 to 36 hours before going to dry on raised beds or patios. This method is a conventional form of processing used all over the world for its ease and efficiency.
This process provides clean, clear, acidic, and bright profiles with a lighter body than other processes. The flavors in the cup are highly dependent on the terroir (the natural environment in which the coffee is produced).
Compared to the 3 processes above, which all go through aerobic fermentation, anaerobic fermentation yield drastically different results in the flavor of the cup. Anaerobic fermentation produces distinct acids, like lactic acid, which give the coffee a distinctly unique flavor. Anaerobics are placed in sealed tanks, removed of any oxygen and monitored on a regular basis for approximately 24 hours to a week. You can perform anaerobic fermentation with washed, honey, and natural processed coffees.
This process provides funky, fruity, boozy, and wild profiles. The slower fermentation provides a sweeter, more complex, fruit forward cup and tends to bring out more tropical fruit notes like pineapple, citrus, mango, papaya, and strawberry, to name a few.
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