Coffee’s Fruity Aroma Also Depends on Fermentation and Washing

Coffee’s fruity aroma can change not only with variety and roasting, but also with post-harvest fermentation. In a 2019 study comparing processing conditions at a farm in Yunnan, China, coffee fermented for longer received higher ratings for fruity aroma, fruity flavor, and acidity than coffee processed with the standard fermentation period.

The research went beyond comparing cups. It followed the microbes surrounding the beans, the processing water, and the beans’ own metabolism through washing, soaking, and drying to investigate where compounds formed, moved, and were lost.

Comparing pulp, mucilage, fermentation time, and soaking

The study used Catimor, an Arabica variety, harvested at one farm in Yunnan. Two processing trials took place one week apart during a cold period, each using 2,000 kg of ripe coffee cherries.

The researchers first compared removing the skin and pulp while retaining the mucilage with a method that also removed the mucilage mechanically. For the former, standard fermentation lasted 36 or 48 hours, until the mucilage was removed in water, while extended fermentation continued to 84 hours. With mechanically removed mucilage, they compared a standard 12 hours with an extended 72 hours. The actual standard and extended durations therefore differed between methods.

After washing, the beans were either dried immediately or soaked in clean water for another 24 hours before drying. A control condition involved mechanically removing the mucilage and drying the beans without fermentation or soaking.

The team combined microbial culturing, microbial DNA analysis, and measurements of sugars, acids, amino acids, volatile compounds, and other components. The final beans were roasted to the same color and brewed under the same conditions. A trained panel of 12 assessed 27 attributes covering aroma, flavor in the mouth, and mouthfeel.

Longer fermentation increased fruitiness and acidity

Of the processing factors tested, fermentation duration had the broadest relationship with the composition of the final green beans and the cup’s sensory profile. Extended fermentation increased fruity aroma, fruity flavor in the mouth, and acidity, while reducing cereal-like and floral aromas.

Processing method also made a difference. Fermentation with mucilage retained produced higher fruity-aroma ratings and lower humus/earthy-flavor ratings than mechanical mucilage removal before fermentation. An additional 24-hour soak increased overall flavor intensity and reduced spicy aroma.

However, ratings across samples were close: the coffees did not become dramatically different products. The authors also viewed mechanical mucilage removal as an alternative capable of producing broadly comparable cup quality. Evaluating methods that could reduce water use or processing time was another important objective.

Lactic acid bacteria dominated around the beans

During underwater fermentation, lactic acid bacteria outnumbered other microbial groups. The process retaining mucilage after depulping was characterized by relatively abundant Leuconostoc, whereas the process mechanically removing the mucilage had relatively abundant Lactococcus. Some Lactobacillus also tended to increase during extended fermentation.

Water in the mucilage-retaining process contained more substrates such as sugars, acids, and amino acids, and accumulated more fermentation-related compounds such as lactic acid and mannitol. Components appear to have moved from the mucilage into the water while microbes simultaneously consumed and transformed them.

Analysis predicting gene functions also suggested differences in the microbial community’s enzyme composition between processing methods. However, the study did not add individual microbial species to test their contributions to aroma. It tracked changes in whole communities originating from the cherries and environment. Yeasts were also present, and their contribution was not ruled out.

The beans themselves remain active during processing

Coffee beans are seeds. While submerged, they do more than passively receive microbial products: they also change their own composition in response to a low-oxygen environment.

For example, aspartic acid in the beans decreased during fermentation, while succinic acid and alanine increased. The authors interpret these findings as consistent with seed metabolic pathways under low oxygen. Different patterns for sucrose and its constituent sugars, glucose and fructose, also suggested that sugar metabolism continued within the beans.

Composition continued to change during drying. Lactic acid and some other compounds decreased, while GABA and proline increased. The authors discuss a combination of seed responses to drying stress and compound breakdown or transformation. Composition immediately after fermentation therefore cannot be treated as identical to that of the finished green beans.

Washing and soaking reduced compositional differences

After washing, lactic acid, mannitol, and other compounds associated with the beans decreased. Further soaking tended to reduce compositional differences linked to fermentation duration or processing method. Both microbial products and water-soluble compounds originating in the beans or pulp could move into the surrounding water.

In a supporting experiment, pulp and thoroughly washed beans were soaked separately in water for 72 hours. Soluble compounds decreased in both materials and increased in the water, with particularly ready release from the pulp. Changes during fermentation therefore need to be understood in terms of diffusion as well as microbial activity.

The authors interpret the additional soak as a step that moderates fermentation-related differences and standardizes green-bean quality. However, compounds leaching out does not mean that the finished cup’s flavor becomes uniformly weaker. In this sensory evaluation, soaking actually increased overall flavor intensity and reduced spicy aroma.

Distinguishing green-bean chemistry from aroma in the cup

Analysis of green-bean volatiles found that extended fermentation increased some compounds described as fruity or floral. Yet sensory evaluation after roasting and brewing found less floral aroma. These are measurements at different processing stages, not contradictory reports of a single value.

Green-bean components react during roasting, and their extraction also differs during brewing. Increases or decreases inferred from instrumental peak areas cannot simply be equated with the strength of a particular aroma perceived by a person.

The findings are limited to one farm, one variety, and two trials during a cold period. Fermentation began at approximately 15°C, unlike the warmer conditions in some other growing regions. Effects depend on variety, resident microbes, temperature, water volume, and other factors. The results do not support recommending extended fermentation everywhere.

Summary

In this comparison using Catimor from Yunnan, extending fermentation increased fruitiness and acidity in brewed coffee and reduced cereal-like and floral aromas. Retaining the mucilage during processing and adding a water-soaking step also had distinct effects on aroma and flavor.

These outcomes involve overlapping contributions from communities dominated by lactic acid bacteria, the beans’ own metabolism, movement of compounds into water, and changes during drying. Considering fermentation, washing, and drying as a continuous process helps explain aroma in the cup and informs choices about processing conditions.

The research was supported by Vrije Universiteit Brussel, the Hercules Foundation, and Nestec S.A. The conflict-of-interest statement reports that two authors were employed by Nestlé at the time. The other authors declared no relevant commercial or financial relationships.

References

Zhang SJ, De Bruyn F, Pothakos V, et al. Influence of Various Processing Parameters on the Microbial Community Dynamics, Metabolomic Profiles, and Cup Quality During Wet Coffee Processing. Frontiers in Microbiology. 2019;10:2621. doi:10.3389/fmicb.2019.02621