Fermentation is a naturally-occurring process where sugars are metabolized or broken down by microorganisms into ethanols, acids, and other simple substances. It happens when there are water and sugar present together in organic matter and is affected varyingly by external factors such as contact with oxygen or higher temperatures. Since most of the coffee fruit is basically water and sugar, fermentation is present in all the different coffee processing methods at different stages.
Traditionally with washed coffees, the de-pulped coffee beans are submerged in fermentation tanks for 12 to 72 hours before being put out to dry. Fermentation finishes the removal of the mucilage to expose the coffee bean. With natural coffees on the other hand, fermentation tends to happen inside the fruit mucilage surrounding the seed while the whole fruits are laid out to dry in the sun.
With the vast amount of knowledge that is being shared between the coffee industry worldwide, coffee farmers have learned ways to control and track the variables in fermentation in order to add value and enjoyable attributes to their coffee. For specialty coffee growers differentiation means a premium, and they are abandoning the mindset of yield in terms of quantity but instead are focusing on quality of the fruit.
Historically, fermentation was associated with natural coffees showing flavor notes of blueberry and strawberry but with modern fermentation techniques, even washed coffees result in some incredible flavor profiles of tropical fruits and an endless array of other aromas.
It is important to note that attributes are not being added to the coffee, they merely are being highlighted or extracted. There are more than 1000 chemical compounds in coffee and they all react in unique ways to the different microorganisms they come in contact with, whether they are introduced by mere chance such as native yeasts or if the producer chooses a specific yeast strain to obtain a specific result. After yeasts and bacteria metabolize the sugars they leave us with simple substances such as ethanol and acids.
Some of the acids resulting from fermentation are lactic and Acetic. Lactic acid like the name implies is characterized as yogurty, buttery, etc. Acetic acid gives us the essence of ripe fruit but too much of it may be vinegary, just like when a fruit is overripe (it’s all connected!). Another byproduct of fermentation is ethanol, you may have tasted this if you described a coffee as winey.
Admittedly this is more than a word on fermentation but this is just the tip of it, it is science at the end of the day. For a deeper dive into fermentation feel free to follow the links in this post and read Café Imports’ blogs on fermentation which you can visit here.