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Experimental Fermentation in Coffee Processing

In recent years, experimental fermentation has been a growing trend in the specialty coffee industry. We’ve all seen the terms “anaerobic”, “extended” or even “carbonic maceration” being used to describe the more controlled approach being championed by forward thinking producers and from the frequency of their occurrence It would be appear that these methods are becoming more mainstream.

Before we look a little closer at some of the different types of fermentation being experimented with at the moment it’s important to learn a little more about what fermentation actually is and how it has been traditionally used in the coffee world.

Fermentation is the chemical breakdown of a substance by bacteria, yeasts, or other microorganisms, typically involving effervescence and the giving off of heat. Fermentation plays a part in all methods of coffee processing but has a greater scope for experimentation and control within fully washed processes.

The traditional purpose of fermentation as used in washed coffees is the removal of the fruit from the seed whilst attempting to prevent any colouration of the flavour of the coffee. After the cherries have been pulped they are left in a tank to ferment, sometimes with water, until the fruit mucilage surrounding the seed is loose enough to remove easily. Although the fermentation is only concerned with the fruit it can still have an effect on the seed. For example, if the fermentation is too long this can lead to over fermented or “winey” flavours which are undesirable.

By definition, all fermentation is anaerobic because yeast and bacteria do not respire. When this term is used it is most likely to describe a process in which the oxygen has been removed from the fermentation environment. This could refer to something as simple as a fermentation under water or to a more complicated method such as fermentation in sealed containers, carefully monitoring and controlling the temperature and pH levels.

The duration of the fermentation required to remove the fruit will vary dependant on the altitude, environmental conditions, the strains of naturally occurring bacteria etc. Some producers try to push this as far as they dare in order to add more fruit characteristics to the coffee. Get it right and you end up with a coffee which tastes like an exceptionally clean natural process. Leave it too long and you end up with a cup which is overwhelmingly vinegary. Some producers also extend fermentation due to necessity. If a days harvest is not enough to fill the fermentation tanks they will “top up” with the subsequent days pickings. Each days pickings increase the pH levels which allows longer fermentation times without the acetic acid produced by bacteria at a low pH.

Carbonic maceration is a technique which is popular with the natural wine making community. Pioneered by Jules Chauvet, a winemaker trained in chemistry, it is widely associated with wines from Beaujolais. The sealed environment limits the introduction of new bacterias from the surrounding environment and allows for more control over temperature. Saša Šestić, a Serbian born Australian, was the first person to bring this process to the World Barista Championship in 2015. The coffee which he was using had been produced in Colombia with his collaborator Camilo Merizalde and the intention was to create more aromatic complexity and a lower concentration of acetic acid.

Carbonic maceration setup at Finca la Senda, Acatenango, Guatemala


Whole coffee cherries are loaded into a stainless steel container and sealed. This container is then flushed with CO2 from the bottom which forces all of the oxygen out through an airlock at the top of the container. The carbon dioxide gas permeates through the skins and begins to stimulate fermentation at an intracellular level. The entire process takes place inside each single, intact cherry. Once the coffee has reached its desired level of fermentation, the cherries can be processed via any chosen method as they are suitable for both dry and wet processes.

Another new trend in coffee processing is using specific strains of yeast to further control fermentation. There is some incredibly interesting work being done by Lucia Solis, a microbiologist and winemaker turned fermentation designer and consultant at Luxia Coffee Solutions. She is championing the use of carefully selected, laboratory grown strains of yeast in order to either intensify or subdue flavours and also control the fermentation time to mitigate adverse environmental factors. She makes it very clear that as we look to industries like wine for new techniques that there are some fundamental differences in the products which should be considered. With wine, the fermentation is necessary, the grape juice must be fermented to create alcohol and flavour. We directly consume the product of that fermentation whilst in coffee the fruit mucilage is fermenting around the seed before being discarded whereas the seed itself is not fermented.

There is no doubt that this is an area ripe for experimentation and can be beneficial to producers wishing to differentiate their products. However, these techniques can be comparatively labour intensive and there is a real risk of creating expensive products with no guarantee of a proportional increase in cup quality.