August 16

Controlled Fermentations

An Overlooked Opportunity For Flavor Development

Fermentation is a topic very dear to me because many of my favorite beverages are fermented: wine, beer, distilled spirits, mead, cider, kombucha and coffee. Usually when I get to the end of the list with coffee, I get a sideways look and the question “coffee is fermented?” Well, yes and no. If you’re reading CoffeeTalk, you likely know coffee is fermented, but what does that really mean?

After the coffee is picked and depulped, fermentation is a step in the production of washed and semi washed coffees to remove the mucilage and leave a clean seed before drying the parchment. Unlike the other beverages I mentioned, the fermentation step is not strictly required because the mucilage can be removed through mechanical methods or dried in the natural process. While a fermentation is not required to process coffee, I believe it is an integral (and often overlooked) step to extract the maximum quality potential from the seed to the cup.

What is happening during fermentation? Yeast and bacteria naturally found in the soil, cherry skin, and mill environment metabolize the sugar in the mucilage and convert it – to carbon dioxide and alcohol in the case of yeast, and acids in the case of bacteria. Both yeast and bacteria produce enzymes to help break down available food sources – like the sugars found in mucilage. The coffee seed itself is not transformed by this process, but there is still a flavor impact from fermentation that can be noticed after roasting. Even though only mucilage is removed in the fermentation, the impact is obvious to anyone who has ever found an “over-fermentation” or “winey” defect in a cupping.

As my favorite Danish physicist Niels Bohr said, “the opposite of a profound truth may very well be another profound truth”. Very simply – if we can get undesirable flavors and aromas from poor fermentation, then the reverse is also true – desirable flavors can be maximized with controlled fermentation.

What does it mean to control the fermentation?

Wine is an example of a beverage that closely controls the fermentation process with carefully selected yeast strains to coax specific flavor profiles from grapes. If you want to bring out the tropical fruit in your Sauvignon blanc you would use a different yeast than if you’re looking for a floral flavor profile. Winemakers can choose from hundreds of different yeast strains to influence the specific flavor profile they are looking for.

Why aren’t we looking at coffee the same way? I have seen that the yeast strains have an effect on flavor profiles in coffee as well. In discussions regarding fermentation in Central America several producers detailed experiments, but rarely are they controlling the type of yeast that is actively in the tanks. Identifying the native yeast requires genetic testing and is very cost-prohibitive. Hence most trials involve manipulating parameters like time and temperature. This is a great step in coffee processing towards taking control of flavor development, but it is only scratching the surface of maximizing favor potential.

On the other side of the fermentation spectrum are producers who are experimenting with eliminating the fermentation step using mechanical demucilagers as water access becomes limited in producing areas. However, the tradeoff can be a reduction in flavor complexity. Producers moving away from fermentation indicated an interest in adopting a full natural process at their mills, but cited inconsistent flavors as too problematic to pivot away from washed processing.

Starting in 2014 I have conducted fourteen controlled fermentation trials in eight countries under very different circumstances. The trials in which the already present yeasts and bacteria were replaced with a controlled and intentional fermentation resulted in a markedly improved flavor profile in the best of cases and a more consistent flavor in the remaining instances. These trials are an excellent foundation on which to expand, and producer feedback is encouraging to continue calibrating the use of controlled fermentation to improve and stabilize coffee output. There’s plenty of hope for a happy union between the science of fermentation and the art of delicious coffee.

Yeast is not a magic solution that can always produce better flavors. It is still true that you’re only as good as your raw material. I do believe that controlled fermentations are key for quality and we can all agree that those seeking higher quality and flavor complexity need to begin at origin.

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