Metagenomic, metabolomic and sensorial characteristics of fermented Coffea arabica L. var. Castillo beans inoculated with microbial starter cultures

Coffee is one of the most important, and most widely consumed drinks around the world. Fermentation is a key step in determining the quality of the final cup of coffee. Recently, certain coffee producers have begun using microbial starter cultures during fermentation to improve sensory profiles. In the present study, freshly harvested Arabica coffee beans were processed through two different wet fermentation methods, in which one was inoculated with starter cultures, to determine the effect on the sensorial profile of the resulting cup and the corresponding metabolomic variation and microbial communities involved in the fermentation process. A quantitative descriptive analysis was applied to measure certain sensory attributes, including odor, flavor, and texture, in order to describe the sensorial profiles of the coffee. Moreover, the metabolomic profiles of the green and roasted coffee beans were studied with an untargeted metabolomic analysis. Finally, the bacterial and fungal composition were followed throughout the wet processing of the beans implementing DNA extraction and high-throughput sequencing. The results showed that the inoculated coffee scored higher in all sensorial attributes when compared to the standard fermentation, and was characterized by higher degrees of sweetness, less acidity and bitterness, and more notes preferred by consumers. Over a hundred unique metabolites were identified that differed significantly between fermentation methods for the green and roasted coffee beans. Higher abundance of compounds like sucrose, mannitol, methyl phenylacetate, as well as organic acids like malic, citric, and quinic acid, in the inoculated coffee could explain the desirable organoleptic properties found in the final beverage of the starter culture inoculated process. The inoculated fermentation was also characterized by high abundances of lactic acid bacteria and yeasts of the genus Kazachstania. These microorganisms are associated with the production of desirable metabolites in coffee fermentations, such as lactic, acetic, isobutyric and hexanoic acids. Consequently, coffee producers should be aware of the safety and benefits of using starter cultures to improve the quality of their coffee. Future studies should focus on microbial identification at a deeper level and the correlation between certain species and their resulting metabolic compounds, where specific bacterial and fungal groups could be isolated and grown to be used as commercially available starter cultures for wet processed coffee fermentation.