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 indispensable stage in the transition from coffee cherry to green bean.

In recent years, producers have been experimenting with new methods of controlled fermentation. They are adding fermentation steps, using external yeasts, mushrooms, juice from other coffee cherries, dried fruit, and extended fermentation. These techniques not only bring out the best in each coffee cherry, but also create new tasting profiles, while increasing the selling price.

In this article, we'll explore the most popular experiments today, their impact on the final cup, and the associated costs and risks for producers.

Pre-fermentation

After picking, the cherries are stored undamaged for a few hours.

This pre-fermentation initiates an internal fermentation process similar to that of natural coffees. If properly controlled-ideally between 24 and 48 hours, depending on climate and 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 set up, and its benefits are great. It reduces fermentation time, increases efficiency, limits the development of undesirable fungi and enhances the cup's final quality in terms of flavor and aroma.

In addition, the choice of starting culture helps to orientate the development of certain aromas and therefore gives a level of prediction on the quality of the final product [2, 3, 4].

Experimentation on the use of starter crops has been going on for many years. Farms now have a large number of tried and tested techniques at their disposal, which 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 the cherries.

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

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

Certain naturally occurring yeasts that have an impact on the final quality of the product are isolated and cultured to increase their concentration. This culture will force these yeasts to ferment spontaneously, thus standardizing the process and reducing the acidity of the final product.

  • Use of external yeasts, bacteria or fungi

Experiments have been carried out using yeasts traditionally used in the wine and beer industries, resulting in fruitier, more floral coffees with notes of tea [6]. Other experiments have been carried out using koji cultures, a fungus traditionally used to make soy sauce and sake. The result is a silkier cup, reminiscent of miso soup [7].

Co-fermentation

Adding external aromatic elements

This type of fermentation is currently highly controversial, mainly because process transparency is not always total.

The coffee bean, usually in a washed process, is brought into contact with an aromatic element such as strawberry, yellow fruit or cinnamon, during an anaerobic fermentation. As with the use of coffee cherry juice, this co-fermentation will enable the bean to absorb aromatic elements, but this time it would not have developed naturally. This increases the coffee's value and aromatic profile.

Grande cuve pour la fermentation anaérobique du café

Long fermentation

Longer than average fermentation

The term "long fermentation" will depend enormously on the region of production and the climate in which the coffee is processed. It mainly refers to a much longer fermentation than usual. For some regions, it's just over 36 hours, for others it's over 96 hours.

The term "long fermentation" can also be ambiguous, as 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 a colder, longer fermentation?

In any case, a really long fermentation (over 96 hours) doesn't always bring a relevant result, and may even start to develop defects such as fungus (there are, of course, experiments to prove the contrary [8]). As fermentation is the degradation of elements into other compounds, once everything has been degraded and transformed, there's nothing left to transform unless more elements are added.

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

Lactic fermentation

Lactic acid fermentation is most widely used in the food industry for its preservative properties. In addition to extending the shelf life of foods, lactic acid bacteria improve the flavor, aroma and texture of foods and beverages.

In the field of coffee, this fermentation is sought to bring roundness and creaminess to the cup, as well as "umami". But it's very complicated and costly to create a favorable environment for the development of lactic fermentation in coffee:

  • These bacteria are naturally present in coffee cherries, so lactic fermentation occurs naturally.
  • these bacteria can grow equally well in the presence or absence of oxygen, but do 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 the cherries are treated before fermentation and inoculated with lactic acid bacteria.
  • these bacteria are expensive, fragile and must be conditioned in a cold, humid environment (unlike yeast, which is dry), which increases their cost

What's more, current research has not shown that lactic fermentation alone provides coffee with any specific attributes [10].

Conclusion

These new time- and temperature-controlled fermentation methods ensure product consistency and the development of specific, controlled flavors.

Although highly beneficial, these processes need to be closely monitored, as excessively long fermentations can lead to the formation of undesired flavours, and negate efforts to achieve a better-quality cup. What's more, the implementation of more complex stages comes at a considerable cost to the grower: not only is it an investment in terms of money, but also in terms of training, research and development time, and therefore the risk of significant losses while the most suitable method is found for the variety, climate and infrastructure.

The use of controlled fermentations in the process of pulping coffee cherries is likely to remain something of an "exotic" in the world of coffee, even specialty coffee, leaving plenty of room for a "simple" natural or washed coffee. What's important in the use of these new methods is the transparency of the process in order to properly inform the consumer.

Thanks to Macarena, Sebastien Ramirez for the photos

References

  1. Fantine. "Coffee Fermentation: Things You Didn't Know."
  2. Girma, B., & Sualeh, A. (2022). "A Review of Coffee Processing Methods and Their Influence on Aroma." International Journal of Food Engineering and Technology, 6(1), 7-16. Link to article
  3. Poltronieri, P., & Rossi, F. (2016). "Challenges in Specialty Coffee Processing and Quality Assurance." Challenges, 7(2), 19. Link to article
  4. Ruta, L. L., & Farcasanu, I. C. (2021). "Coffee and Yeasts: From Flavor to Biotechnology." Fermentation, 7(1), 9. Link to article
  5. Coffee Green Beans. "Mosto Fermentation Process."
  6. Calderon, N., Jiang, G. Z., Gibney, P. A., & Dando, R. (2023). "A Consumer Assessment of Fermented Green Coffee Beans with Common Beer/Wine Yeast Strains for Novel Flavor Properties." Fermentation, 9(10), 865. Link to article
  7. Coffee Green Beans. "Step-by-Step Fermentation with Koji."
  8. Barista Hustle. "I Would Ferment Five Hundred Hours."
  9. Barista Hustle. "The Possibly Lactic Process."
  10. Fantine. "Lactic Acid Fermentation Coffee 101."
  11. Heinze, T. H. (2021). "Method of Technological Change: Experimental Methods in Coffee Post-Harvest Processing." Link to article
  12. Batista da Mota, M. C., Batista, N. N., Sances Rabelo, M. H., Ribeiro, D. E., Meira Borém, F., & Schwan, R. F. (2020). "Influence of Fermentation Conditions on the Sensorial Quality of Coffee Inoculated with Yeast." Food Search International, 136. Link to article
  13. Febrianto, N. A., & Zhu, F. (2023). "Coffee Bean Processing: Emerging Methods and Their Effects on Chemical, Biological and Sensory Properties." Food ChemistryVolume 412. Link to article
Tags: cerise de café fermentation procédé Troisième Vague
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