Fermentation in coffee: experiments

published on June 22, 2024 — by Marc-Alexandre Emond-Boisjoly — 20 mins of reading —

As we discussed in our previous article, fermentation is a crucial and necessary step in the transition from coffee cherry to green bean.

For several years, producers have been experimenting with controlled fermentation. They add fermentation stages, use external yeasts, mushrooms, juice from other coffee cherries, dried fruits, do prolonged fermentation, etc. These stages allow the best of each coffee cherry to be revealed, but also (and above all) to create new tasting profiles while raising the selling price.

In this last article, we will see the most popular new experiments at the moment, their impacts on the final cup, their costs and the risks incurred by producers.


The cherry is stored for a few hours just after picking, without being damaged.

This pre-fermentation allows internal fermentation to begin (as for a natural). If this pre-fermentation is properly controlled, ideally between 24 and 48 hours depending on the climate and the thickness of the cherry layer, it can add sweetness and more intense fruit notes to the cup [1].

Gros plan sur le développement de champignons de Koji sur des grains de café

Inoculated fermentation

Selected bacteria or yeasts are introduced during fermentation

Fermentation inoculation, using a starter culture, is relatively inexpensive and easy to implement, and its benefits are great. It reduces fermentation time, increases its efficiency, limits the development of undesirable fungi and increases the final quality of the cup in terms of flavors and aromas.

In addition, the choice of the starting culture makes it possible to guide the development of certain aromas and therefore provides a level of prediction on the quality of the final product [2, 3, 4].

Experiments on the use of the starting culture have been carried out for many years. Farms now have many proven techniques at their disposal that they can implement relatively quickly.

  • Using coffee cherry juice

Coffee cherries, when fermented whole, produce juice. This juice contains a high concentration of yeasts and bacteria already present on cherries

This juice can come either from the same batch (for example, during fermentation in humid tanks, the juice produced by the macerating cherries is reintroduced into the same tank) or from a different batch (for example, the juice resulting from the carbonic maceration of a different batch) [5].

  • Use of yeasts isolated but already present in the cherry or grain

Certain yeasts naturally present and having an impact on the final quality of the product are isolated and cultured to increase their concentration. This culture will force the spontaneous fermentation of these yeasts, which will make it possible to standardize the process and reduce the acidity of the final product.

  • Use of external yeasts, bacteria or fungi

Experiments were carried out using yeasts traditionally used in wine and beer, which results in more fruity, floral coffees with tea notes [6]. Other experiments were carried out using cultures of koji, a fungus traditionally used to make soy sauce and sake. The result is a silkier cup, which may be reminiscent of miso soup [7].


Addition of external aromatic elements

This type of fermentation is currently very controversial, mainly because the transparency in the processes is not always total.

The coffee bean, generally during a washed process, will be brought into contact with an aromatic element such as strawberry, yellow fruits or cinnamon, during anaerobic fermentation. Just as with the use of coffee cherry juice, this co-fermentation will allow the bean to absorb aromatic elements, but this time they would not have developed naturally. This will increase the value of the coffee and its aromatic profile.

Grande cuve pour la fermentation anaérobique du café

Long fermentation

Fermentation longer than average

The term “long fermentation” will greatly depend on the region of production and the climate when processing the coffee. It is mainly a much longer fermentation than usual. For some regions, this is only more than 36 hours, for others it corresponds to fermentation for more than 96 hours.

The term “long fermentation” can also be ambiguous because the complete pulping process is not always specified: is it an extended anaerobic fermentation in a washed process? or a longer but correctly managed natural process? or a relatively long pre-fermentation before a honey process? or even a colder and longer fermentation?

In any case, a really long fermentation (more than 96 hours) does not always bring relevant results, or can even start to develop defects like fungus (there are obviously experiments which prove the opposite [8] ). Since fermentation is the breakdown of elements into other compounds, once everything has been broken down and transformed, there is nothing left to transform unless other elements are added.

That said, a properly controlled long fermentation can finish transforming the yeasts and bacteria present to achieve more pronounced flavors, closer to red wines or very intense fruits. Unfortunately, the process is not always stable and can present risks for the producer.

Lactic fermentation

Lactic fermentation is more widespread in the food industry for its preservation characteristics. In addition to extending the shelf life of foods, lactic acid bacteria improve the flavors, aromas and texture of foods and beverages.

In the field of coffee, we seek this fermentation to bring roundness, creaminess to the cup as well as “umami”. But it is very complicated and very expensive to create an environment favorable to the development of lactic fermentation for coffee:

  • these bacteria are naturally present in coffee cherries, so lactic fermentation occurs naturally
  • these are bacteria that can grow equally in the presence or absence of oxygen, but grow better in an anaerobic environment (less competition with other bacteria) [9]
  • this is not the only fermentation that will develop in an anaerobic environment unless you treat the cherries before fermentation and inoculate the fermentation with lactic acid bacteria
  • these bacteria are expensive, fragile and must be packaged in a humid and cold environment (unlike yeasts which are dry) which increases their cost

Additionally, current research has not demonstrated that lactic fermentation alone provides specific attributes to coffee [10].


These new fermentation methods, controlled in time and temperature, ensure consistency in the product and allow it to develop very specific and controlled flavors.

Although very beneficial, these processes must be closely monitored because fermentations that are too long can cause the formation of unwanted flavors and negate the efforts to obtain a better quality cup. In addition, implementing more complex steps has a significant cost for the producer: it is an investment in money, but also in training, research and development time and therefore a risk. significant losses while it takes time to find the method best suited to the variety, climate and infrastructure.

The use of controlled fermentations in the process of pulping the coffee cherry risks remaining something “exotic” in the world of coffee, even specialty, which still leaves plenty of room for a “simple” natural or washed. The important thing in using these new methods is the transparency of the process in order to correctly inform the consumer.

Thanks to Macarena, Sebastien Ramirez for the photos



[2] “A Review of Coffee Processing Methods and Their Influence on Aroma”, B. Girma, A. Sualeh,International Journal of Food Engineering and Technology2022; 6(1): 7-16,https://www.researchgate.net/profile/Abrar-Sualeh/publication/359746797_Bealu_Girma_Abrar_Sualeh_A_Review_of_Coffee_Processing_Methods_and_Their_Influence_on_Aroma/links/624ca2bdd726197cfd3e1403/Bealu-Girma-Abrar-Sualeh- A-Review-of-Coffee-Processing-Methods-and-Their-Influence- on-Aroma.pdf

[3] “Challenges in Specialty Coffee Processing and Quality Assurance”, P. Poltronieri, F. Rossi,Challenges2016,7(2), 19,https://www.mdpi.com/2078-1547/7/2/19

[4] ”Coffee and Yeasts: From Flavor to Biotechnology”, LL Ruta, IC Farcasanu,Fermentation2021,7(1), 9,https://www.mdpi.com/2311-5637/7/1/9


[6] “A Consumer Assessment of Fermented Green Coffee Beans with Common Beer/Wine Yeast Strains for Novel Flavor Properties”,N. Calderon,G.Zhujun Jiang,PA Gibney,R. Dando,Fermentation,2023,9(10), 865.,https://www.mdpi.com/2311-5637/9/10/865





[11] “Method of Technological Change: Experimental methods in coffee post-harvest processing”, Timothy H. Heinze, April 2021,https://www.researchgate.net/profile/Timothy-Heinze-3/publication/351460696_Method_of_Technological_Change_Experimental_methods_in_coffee_post-harvest_processing/links/60996970299bf1ad8d907b9a/Method-of-Technological-Change-Experimental-methods -in-coffee-post-harvest-processing. pdf

[12] “Influence of fermentation conditions on the sensorial quality of coffee inoculated with yeast”, MC Batista da Mota, NN Batista, MH Sances Rabelo, DE Ribeiro, F. Meira Borém, R. Freitas Schwan,Food Search International, Vol 136, October 2020,https://www.sciencedirect.com/science/article/pii/S096399692030507X

[13] “Coffee bean processing: Emerging methods and their effects on chemical, biological and sensory properties”, NA Febrianto, F. Zhu,Food Chemistry, Volume 412, June 30, 2023,https://www.sciencedirect.com/science/article/pii/S030881462300105X

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