Disentangling Diversity in the Coffee Fermentation Process

By Catalina Aranda
Catalina Aranda is a Biotech Engineer and Ecology Researcher that focuses on the patterns of microorganisms in ecosystems.

Every cup of coffee holds a world of complexity: the soil that nourished the coffee beans, the shade of the trees around them and the people who grew them. All these variables come together to create a unique cup of coffee, just like a cup from Colombia.  

Colombian specialty coffees are as diverse as nature in this wonderful land. One key characteristic for this coffee is the fermentation process used traditionally according to the environmental conditions there, but revisited and converted to ensure a particular cup.  In this article, we will go deeper into the fermentation process.

What Is Fermentation in Coffee?

Fermentation may remind us of a bad hangover with beer or maybe a bread recipe. However, not many would relate them with coffee. Technically, fermentation consists in the anaerobic oxidation process where complex substances are broken down to simpler molecules and energy production. In other words, sugars in a fruit are cleaved into “lighter” compounds like ethanol with an important condition, absence of oxygen. A community of microbes (yeast and bacteria) oversee this transformation process.

However, this conception may differ from the traditional usages in coffee process where “fermentation” refers to all steps in the process where microbes act over the sweet part of the coffee cherry (mucilage) whether oxygen is present or not. Commonly, these two conceptions involve the breakdown of mucilage, the formation of acids and esters, and ultimately the cup profile. Hereafter we refer to the latter conception. 

Fermentation and Consistency in Specialty Coffee

A desirable trait in specialty coffee sought by traders is consistency, the maintenance of the cup profile over time. To achieve this, producers are reviewing in increasingly detail each step and variable. Temperature of fermentation, type of water used, and the species of yeast inoculated are examples of details followed and controlled. 

Keep it in mind, the environment plays a crucial yet unpredictable role. We could say that producers are, in a way, trying to delimit the chaos of the process. This is the variability that makes each specialty coffee unique. 

The Three Layers of Diversity in Coffee Fermentation

1. Microbial Diversity

Yeast and bacteria are the main actors in this process. Whether naturally occurring or artificially added, they are responsible for the transformation that forms the fermentative community. 

In their way to live, this organism consumes the available nutrients, specially, but not only, sugars. When sugars in the coffee mucilage are broken down by yeasts and bacteria, they produce by-products like organic acids, alcohols, and esters. The balance between these compounds defines how a coffee tastes: its acidity, body, and aromatic complexity.

In the fermentative community, one of these by-products may be a waste or a tasty food depending on the microbe capacity to “digest” it. This interaction is known as cross-feeding forming a complex community network. Here lies the first layer of diversity, the identity of each microorganism, its ability to consume different nutrients and the community composition are crucial factors to the resulting cup.

The balance to be found resides in controlling this community. Native microbes provide a unique terroir shaped by the wild. However, it is difficult to control in practice, so the producers can inoculate specific and known microbes to assure reproducibility of their coffees.

2. Substrate: The Coffee Cherry Mucilage

The material upon which microbes work their magic is the mucilage of the coffee cherry. This is where fermentation begins, and its speed rate depends on the environmental conditions such as temperature and humidity. At this point, producers try to avoid uncontrolled fermentation by processing the cherry faster as possible in controlled conditions. 

Since each coffee tree species and varietal contains different amounts and composition of mucilage, the second diversity layer involves the type of the coffee tree harvested and following process selected.

3. Environmental Conditions

Fermentation does not occur in isolation; it’s a living process that responds to its surroundings.

• Oxygen gives life yet, in fermentation, not for all microbes. The community of microbes working on the coffee changes over time, and one of the main factors guiding that change is how much oxygen is available. Some microbes need oxygen to process sugars; others thrive with just a trace of it, while a few can’t survive in its presence at all. Each type converts sugars into different by-products, so the balance of oxygen shapes which microbes dominate, and ultimately, the flavor that ends up in the cup.
  

• Temperature defines the rhythm of fermentation. Every microbe has its comfort zone: too cold, and the process slows to a crawl; too warm, and it may race off in unexpected directions. 

Humidity, altitude, pressure, pH, even the water origin contributes to the reaction. The combination of these environmental conditions drives the process in different pathways which constitutes a third layer of diversity which is controlled by strict process protocols and specialized machinery. 

The Controversy: Co-Fermented Coffees

Co-fermented coffees are the new actors in the coffee stage. But are they true coffees? That is a hot debate among coffee lovers and experts. 

The term is not formally defined in the industry. Reviewing the literature, we could say that the prefix “co-“ means together, so in essence, co-fermentation are elements being fermented together. In this sense, every additional element in any step of the way, results in a co-fermented coffee. 

As we have discussed, under diversity light, co-fermented ones are just as complex as traditionally fermented ones. The same layers of microbial, chemical and environmental diversity are working together to craft a speciality cup. 

The real controversy relies on clarity and transparency. Then we wonder, where is the limit where producers should disclose their know-how? Are only natural processes the true coffee? But how would we enjoy a consistent cup of coffee if producers are not acting over the process? These are questions that still need to be addressed by our industry, ideally through open discussion and shared standards that can help balance creativity with honesty, and nature’s chaos with craft.

Curious about fermented coffees? Catalina spent time at Finca El Paraíso, where the analysis shared in this blog took place. The result: thoughtfully fermented lots, now available in our shop.