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The Modern Approach to Coffee Fermentation

When you start your journey with coffee you learn about the natural (dry) and washed (wet) process. It’s the first time when on the cupping table you try to describe the sweetness, body and acidity of the coffee. With time, you start to realise that there are a lot of inconsistencies in traditional processing techniques. Today we know that most of these inconsistencies come from fermentation. Sasa Sestic (2015 World Barista Championship, ONA Coffee and Project Origin founder) explains that “Fermentation is where we have increased and also decreased quality of the coffee”.

Fermentation in coffee refers to the microbial reaction of yeasts and bacteria breaking down the sugars in mucilage. This process produces acids which will later add complexity and depth to a coffee. This reaction has been studied and developed in coffee-growing regions everywhere, and great care is taken to understand and master this process. Producers utilize a wide variety of methods to control or enhance fermentation in their coffees.

There are different types of fermentation around the world but two, in particular, deserve attention: dry and wet fermentation.

Dry fermentation and flavour profile:

This is where we pulp and place the parchment in concrete tiled pools. But it comes with a challenge: controlling temperature. Since temperature affects the rate of fermentation it can negatively affect consistency and flavour. If we spend too much time fermenting the coffee, alcohol acidity starts dominating fermentation. It results in notes of vinegar and dry and metallic characters.

Wet fermentation and flavour profile:

Described as double washed, double fermented. We pulp cherries, we cover the parchment in water. Water helps to extend fermentation time and will result in a softer body and complex acidity. But it comes with challenges as well: inconsistent temperature and water quality.

Sasa has been one of several industry leaders experimenting with processing methods, from his new ice processing to his famous carbonic maceration. The CM (carbonic maceration) process does not replace traditional methods; rather, it adds another step in processing. For example, the CM Selections coffees are still identified as being natural, washed etc. but as they also go through this maceration process, we need to include that information too. So instead of being just washed, we say that a CM Selections coffee is carbonic macerated (washed).

Coffee cherries are picked perfectly ripe, hand sorted and floated to remove unripe and over-ripe cherries. The washed CM Selection coffees are then pulped, before being placed in temperature and humidity controlled tanks flushed with carbon dioxide (CO2) to remove oxygen from the tank. Natural CM Selection coffees are placed in the tanks and after fermentation the coffee is dried on African beds for 12-18 days before being stored to rest before dry milling.

Sasa is determined to measure everything: pH, carbon dioxide, the temperature of fermentation. It helps to replicate the profile every time. Last year during the World of Coffee in Budapest he presented the Fermentation Monitoring Curve that helps him measure all data. Controlling fermentation help us to replicate flavour profile in coffees. Imagine that one day we will be able to control the fermentation process in a very similar way that we can control the roasting process.


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