From Bean to Brew: How Microorganisms Impact Coffee Quality

Across the world, coffee stands out as one of the most popular beverages, and its consumption continues to grow, especially as it serves as a vital commodity in international markets ^[2]. Recently, there has been a surge in the demand for better coffee quality, characterised by rich aroma and flavour profiles. These desired traits are deeply rooted in factors such as the plant origin, their cultivated genotypes, and the process undertaken ^[1].

How Fermentation Enhances Coffee Quality

Once coffee is harvested the primary processing stage begins, there are many methods that can be used, although the fundamental one is through fermentation ^[2].

During fermentation, Indigenous microbiota begin to grow and implement metabolic activities within the coffee beans. This fermentation process involves both yeast and bacteria but can occasionally include filamentous fungi, however this is less common ^{[2] [3]}. The diversity of microorganisms involved originates from those native to the coffee fruit and includes various environmental factors such as machinery, plant sediments, and even insects ^{[4] [5]}.

Yeasts and Bacteria in Coffee Fermentation

There are different fermentation methods, including wet, dry, and semi-dry processing, each allowing for the growth of various yeast species. The most common yeast species detected in these processes include Saccharomyces, Candida, Hanseniaspora, S. cerevisiae var. ellipsoideus, Starmerella, and Torulaspora delbrueckii ^[2]. These yeasts play numerous roles, particularly through their metabolic activities, where they act as alchemists by consuming the sugars present in the coffee pulp. In this process, they convert these sugars and produce a range of secondary metabolites, acids, esters, aldehydes, ketones, and alcohols ^{[2] [7]}.

The Importance of Mucilage Degradation for Coffee Quality

A polysaccharide substance called mucilage coats the outer walls of coffee beans. This mucilage is eliminated during fermentation, thanks to the microbial metabolites of Lactic Acid Bacteria. These metabolites also aid in metabolising sugar citrate and amino acids, which then enhances the coffee quality and favours.^{[6] [8]}. The degradation or removal of mucilage is just one of many essential roles that microorganisms play, they also help inhibit the growth of mycotoxin-producing fungi while producing enzymes like pectin lyase, polygalacturonase, and pectin methyl esterase, which are essential to enable the degrading of pectin substances ^{[1] [7]}.

Enzyme Actions During Coffee Fermentation

The enzyme pectin lyase breaks down pectin through a process called Trans-elimination, which releases unsaturated galacturonic acids. Although this is an essential step, Polygalacturonase is also required, as it catalyses the hydrolysis of α-1, 4 glycosidic bonds into pectic (polygalacturonic) acid. Additionally, the enzyme pectin is responsible for de-esterification of the methoxyl group, Pectin, resulting in the formation of methanol and pectic acid^[7].

Microbial Growth in Coffee Quality

Despite the advantages diverse microorganisms for the enhancement and enrichment of desirable coffee, there are potential drawbacks. Notably, only a small fraction of the true metabolic activities can be defined or measured. If fermentation exceeds beyond the recommended time, adverse effects can occur, which may reduce the final quality through off-flavours and potential contamination due to the growth of certain cultures.

Microorganisms play a crucial role in coffee quality, from the fermentation process to the final cup. By breaking down sugars and mucilage, and important enzymes, they enhance the flavour, aroma, and overall characteristics of beans. However, managing the balance of microbial activity is key to control the quality. It’s worth appreciating the unseen work of these microorganisms that directly influence the coffee we enjoy every day.

References

1. https://doi.org/10.1155/2019/4836709
2. https://doi.org/10.1016/j.afres.2022.100253
3. https://doi.org/10.1016/j.foodres.2024.113972
4. https://doi.org/10.3390/molecules28166092
5. https://doi.org/10.3390/foods13060839
6. https://doi.org/10.1002/yea.3888
7. https://doi.org/10.3390/microorganisms8081142
8. https://doi.org/10.1080/10408398.2015.1067759